Astrophysics at the Aspen Center for Physics

by J. Craig Wheeler, Michael S. Turner and Joshua A. Frieman

The Early Years: 1962-1979

by J. Craig Wheeler

Astrophysics was entangled in the DNA of the Aspen Center for Physics from the beginning, even if it was not immediately apparent. Although George Stranahan, the visionary founder of the Physics Center, practiced condensed matter physics during his research career, he read astronomy books on his own time during his stint at the Hotchkiss School and did astrophysical research as an undergraduate at Caltech, long before the Center was even a gleam in his eye. As a senior at Caltech, George worked with Leverett Davis, Jr. on measuring interstellar magnetic fields by detecting the polarization of dust–scattered starlight. Real astrophysics! In addition, one of the first attendees at the brand new, Navy–supported, Physics Division of the Aspen Institute for Humanistic Studies in 1962 was the renowned Hans Bethe. Bethe donated part of his winnings from his 1967 Nobel Prize for elucidating nucleosynthesis in stars to build the Bethe Building. (Click History on the left and “First 35 Years” by Jeremy Bernstein, Chapter II for more on Bethe.)

Author Craig Wheeler (left) and Stirling Colgate in Conundrum Hot Springs MANY years ago

There was no obvious work on astrophysics in that first year (Who knows what Bethe was thinking?), but there were smatterings of astrophysics in the first few years of the Center. In 1963, Al Schild from the new Center for Relativity at the University of Texas at Austin gave a seminar on the “Principle of Equivalence” on July 23 and Curtis Callan gave a seminar on “Cosmological Solutions with Inhomogeneous Matter Distributions” on August 27.

One of the clear contributions of the Center to astrophysics was to catalyze the transformation of hardcore physicists into astrophysicists when the lure became too enticing to ignore. Among the astrophysicists–to–be was David G. Ravenhall in 1962. By 1963 the list included Sid Bludman, Steve Frautschi (a bit of a reach, but his work on the Hagedorn equation of state had implications for cosmology and neutron stars), Daniel Z. Freedman, Al Petschek, David Pines, and Ray Sawyer, all of whom would contribute significantly to astrophysics when the time came.

By 1964, Bethe was on the Center's Advisory Committee. Joe Weber, a pioneer in the search for waves in the fabric of space predicted by Einstein, began the first of many visits, so the Center can rightfully claim to have a founding role in the effort to measure gravitational waves. Weber gave a seminar on July 15 entitled ”Gravitational Waves.” In addition to Bludman, on the list of attendees for 1964 was a young graduate student, perhaps one of the first non–PhDs to attend, Mr. Leonard Susskind. Perhaps Susskind should not be classified as an astrophysicist–to–be, but his career led to his deep studies of black holes as they play a role in the information crisis and to his role in expounding the hypothesis of a holographic universe, an area right on the dramatic boundary of physics and astrophysics.

The astrophysics report for 1965 is not in the ACP archives, but Pines recalls that Mal Ruderman gave a talk on neutron stars. The conclusion, at least according to Pines, was that the topic was interesting, but basically hopeless, because they would never be observed.

The report for 1966 shows more evidence for astrophysics and astrophysics–to–be. In addition to Weber and Pines, Ruderman, a Columbia University astrophysicist still studying neutron stars who was then at NYU, made his first recorded appearance. Ruderman gave a seminar on July 1 entitled “Some Astrophysical Many–Body Problems” and reported that he worked on cooling white dwarfs and talked with Pines about physics at high astrophysical densities 104 – 109 g cm–3. Pines himself was working on dilute solutions of He3 in He4 and gave a seminar on that topic on July 15.

In 1967, Weber gave a seminar on August 4 entitled “Gravitational Theory and Recent Experiments” and worked that summer on scalar–tensor theories of General Relativity, a hot topic of the time. Gordon Baym and Chris Pethick made their first recorded appearances. Baym gave a seminar on June 23 on his work with Pines, ”Dilute Solutions of He3 in superfluid He4.” Pethick worked on periodic ionic lattices in simple metals and ferromagnetic Fermi systems.

Neither Baym's nor Pethick's report for that year makes reference to any astrophysical topic. Pines apparently did not manage to turn in a report. Within a year, all three would be active, impassioned astrophysicists and the Aspen Center for Physics would make its first broad impact on the world of astrophysics. The Center was about to be transformed.

The superfluid was about to hit the vortex.

There is no archival report for 1968, but history records that Jocelyn Bell made her staggering discovery of “little green men.” These rhythmic radio pulses were soon established to be of astrophysical, not intelligent, origin. There was a flurry of activity in the attempt to explain these remarkable signals, as normal stars, white dwarfs (my favorite being laser emission from the perimeter of a white dwarf), were eliminated. The gang from Illinois had a hammer in the theory of superconducting fluids and Tommy Gold's (Gold was the first to propose that, or at least loudly broadcast, the idea that pulsars were neutron stars) suggestion that these new signals were from neutron stars looked like a nail. They were right.

The discovery of pulsars as rotating magnetic neutron stars and the rapid developments that arose from the discovery of glitches, leading to theories of superfluid cores, put Pines, Baym, Pethick, their collaborators, and the Aspen Center for Physics at the core of a huge new field of astrophysics. By 1969, Aspen was the intellectual center of this activity, drawing researchers from around the globe.

Pines notes that things were simpler back then. He began to organize a conference on pulsars in July of 1969 and it convened only a month later, in the week of August 19 - 23.-23. The attendees were the new who's who of the pulsar game: Gold, Bethe, Pines, Baym, Pethick, and Ruderman, among others.

Bethe had received a 1967 Nobel prize for his contributions in the theory of nuclear reactions, especially his discoveries concerning the energy production in stars. He gave a 1969 talk o n the properties of matter in neutron stars. He also noted in his report of that summer that “In private discussions I gave D. Ruderman (sic) some advice on calculating the superfluidity of neutron matter.” Earlier (August 7) he also gave a public talk on the anti–ballistic missile system. Bludman, newborn astrophysicist along with Pines, Baym, and Pethick, gave a seminar on ultradense matter. Frautschi reported that he enjoyed learning about pulsars and had some ideas about how “bootstrap” physics might apply to central regions of neutrons stars with excited hadrons. In a footnote, I, a new postdoc at Caltech, wrote a paper with Frautschi that year on a “bootstrap” equation of state, not knowing he was already indirectly under the sway of Aspen. I had not yet been to Aspen, actually had not even heard of it at that time, but Frautschi had participated and he was one of the people who implicitly spread the spirit of Aspen. The Aspen participants were out in the world influencing young people whether they advertised it explicitly or not.

Jim Hartle was also present in 1969, working on neutrino cosmology, neutron star matter, and symmetry violations. Weber continued his work on detection of gravity waves. Ruderman worked with Bludman on causality and instability in superdense matter, and, of course, Pines, Baym, Pethick, and Ruderman worked on superfluidity in neutron stars and the sudden change that had been observed in the period of the Vela pulsar.

Stirling Colgate had purchased a house on Galena Street, right off the ski slope in 1968. In that summer of 1969, he got wind of the pulsar meeting. He felt strongly that the proposed studies of neutron stars needed to be done in the context of supernova research and that astrophysics should be formally recognized as a component of the Center. He had lunch at the Weinerstube with Pines and Pethick and pressed his case. Some funding for the Center was now coming from the NSF and Colgate thought that the Center needed a presence in both astrophysics and biophysics to keep the NSF happy. Pines eventually bought the idea and presented it to a meeting of the ACP Board of Trustees and the proposal to include astrophysics as a formal subject of study at the Physics Center was accepted. Colgate, Al Cameron and George Field dedicated time over the next eight years obtaining funding from NASA for what would become the June astrophysics workshops.

That summer of 1969 also brought the first overt mention of afternoon volleyball.

1970 brought a diversity of activity that would mark the future of astrophysics at Aspen. Work on neutron stars was still prominent. George Greenstein gave a seminar on August 6 entitled “Pulsar Burps and Wobbles.” He worked on superfluidity in neutron stars, talked with Morrel Cohen, Alfred E. Glassgold and Bludman on the topic and with Steve Berry drafted a paper on the effects of an airplane falling on a neutron star, thus foreshadowing some later models for cosmic gamma–ray bursts models. Bludman reported working on the equation of state of superdense matter in neutron stars. Weber gave a seminar on July 29 on “Gravitational Radiation Experiments” and discussed problems of gravimetry with Jean–Paul Richard. Peter Goldreich worked on molecular masers in magnetic fields as related to OH radio sources in the Galaxy. He talked with Greenstein and Colgate, who, Goldreich remarks, “visited the Center in an unofficial capacity.” This is the first formal mention of Colgate in the record. Setsuo Ichimaru of Illinois worked on strong turbulence in plasmas and a model of pulsar radiation based on strong plasma turbulence. He gave a seminar on “Plasma Turbulence as a Possible Mechanism of Pulsar Radiation.”

In 1971 there was another Pulsar Study Group. Bethe reported that he “enjoyed particularly the talk by Sterling (sic) Colgate on Supernovas, and that by A. Cameron on conditions of a star preceding the supernova outburst.” Bethe gave a seminar on August 19 on the equation of state at extremely high densities. Gold had discussions with Bethe, Ruderman, Pines, and Baym on the difficulty of explaining starquakes and advocating the speed of light circle as the origin of pulsar radiation. He shaped a program to find high–speed pulsars. Ruderman worked on his paper on the nature of matter in fields of 1012 Gauss showing that atoms would link up in one–dimensional metals. A Classic! Ruderman also worked with Baym and Pethick on rotating superfluids, vortex lines, and the effects of external torques. Baym worked with Bethe and Pethick on the equation of state at high densities. Baym, Pethick and Bethe began work on the equation of state of a star undergoing a supernova explosion, a result of discussions with Colgate and Cameron, just the result Colgate had encouraged. Their work on the equation of state at high temperatures was another classic of the era. Sawyer, another newly converted astrophysicst, reported being influenced by Bethe's talk to work on multi–pionic interactions in nuclear matter at superhigh densities. Ed Groth worked on Fourier transform algorithms and application to pulsar searches. Ichimaru, who was in Tokyo by now, worked on radiation transfer in astrophysical plasmas. Bruno Coppi and Aldo Treves worked on plasma instabilities near speed of light cylinder. Bethe reported talking with Gerhard Borner on energy loss from pulsars and Parbahan Kabir on proton/nucleon collisions. Greenstein continued work on superfluidity in neutron stars. Jon Arons, then at the Institute for Advanced Studies, worked on absorption line spectra of QSOs and with Jerry Ostriker on a theoretical model for QSOs based on a central cluster of pulsars.

The diversity of astrophysical research at the Center also continued to expand that summer. Fred Lamb worked on spectral line formation in magnetic fields in the solar atmosphere and Ap stars. He worked on magnetic white dwarfs with Peter Sutherland and on stellar opacity sources in strong magnetic fields. He had discussions with Martin Rees on observational data on magnetic white dwarfs (Lord Martin as an observer!), with Gabriel Karl on molecular spectra in magnetic fields with Ichimaru on electron–ion correlations in magnetic plasmas. Remo Ruffini talked about research on black holes. Greenstein started work with Martin Olsson of Wisconsin on small black holes. Weber did a noise analysis of gravity wave experiments.

Geoffrey Bath of Oxford spent his time preparing the first–ever undergraduate lecture course on astrophysics at Oxford, and Greenstein worked on a new course on astrophysics for non–science majors. These efforts gave Aspen a role in pedagogical as well as research astrophysics.

Joel Primack, Freedman and Carl Akerlof all attended that summer. Though they would all make substantial contributions to astrophysics, they were still mired in particle physics at the time.

In the summer of 1971, Colgate, Cameron, and Field submitted the first proposal to NASA to fund an astrophysics workshop. The first NASA proposal netted $5,000 to support the workshop, to which NSF added another $3,000.

With this new NASA support, the first astrophysics workshop was held that was not devoted to neutron stars. This was also the first of the series of June astrophysics workshops, in this case two weeks from June 12–23. The topic was “The Physics of the Early Universe.” Thirty–five people attended, including Cameron, Colgate, Joe Silk, Chris McKee, and Dick McCray. The format of morning discussions on the patio was adopted. The indelible memory remains of Cameron ruling the patio in shorts and an Hawaiian shirt, a beer in one hand and a cigar in the other. The discussion leaders that year were Gary Steigman on the Hadron Era, Bob Wagoner on Cosmological Nucleosysthesis, Charlie Misner on Chaotic Universes, Ted Harrison on Galaxy Formation, Bill Saslaw on Dynamics of Dense Stellar Systems, Field on Intergalactic Matter, Jim Bardeen on Rotating Massive Objects, and Rees on Quasi–Stellar Objects.

McCray worked on a paper with Joseph Swartz and Bob Stein on filaments in fossil HII regions and with Jim Buff on a time–dependent model for the ionization structure of interstellar hydrogen and the exposure of the interstellar medium to a soft X–ray burst. Gordon MacAlpine gave a seminar on July 27 entitled “Photoionization Models of Quasars and Related Objects.”

Hartle gave a seminar on July 13 on the topic of “Tidal Friction in Black Holes.” He had conversations with Susan Keyser of Chicago State about general relativity and relativistic astrophysics.

Neutron stars remained an important theme. Ruderman worked with Jacob Shaham and Pines on a review of glitches and timing noise, on superfluids and vortices, on the stability of a vortex lattice in a superfluid, and on a model for the 35.7 day on/off period of Her X–1. Sachiko Tsuruta (at least two women that year!) worked on cooling of dense stars and the composition of matter at high densities. She reported conversations with Ruderman, Shaham, and Kayser and with “Dr. C. Wheeler; properties of ultradense stars and dense star models in these high densities.” There was no report from this C. Wheeler fellow.

In the summer of 1972, Colgate, Cameron, and Field wrote a renewal proposal to NASA that funded the 1973 June Workshop on Interstellar Physics. The span was increased from two to three weeks and the format of nine topics, three per week, was instituted to try to prevent the morning discussion sessions from expanding to fill the day and preclude informal research. In attendance were Field, McCray, Charlie Townes, Lyman Spitzer, Ed Purcell, Silk, Frank Shu, Alex Dalgarno, Goldreich, Dave De Young, Russell Kulsrud, Don Cox, J.Randy Jokipii, Phil Solomon, Paul Woodward, Peter Biermann, Teije de Jong, Catherine and Diego Cesarsky, Peter Martin, Peter Meszaros, John Kwan, Dan Watson, Bill Watson, Minas Kafatos, J. Mayo Greenberg, Telemachos Mouschovias, Reuben Ramaty, Marvin Litvak, Chun M. Leung, Donald W. Goldsmith, Steigman, Moshe Elitzur, Mike Oppenheimer, Jon Weisheit, John Black (as a grad student of Dalgarno), Saslaw, William Klemperer, Berry, Goldsmith, John Lyon, Humberto Gerola, Trinh Thuan, John Krizan, Jim Truran, Theodore Stecher, and Jim Felten among others.

Cross fertilization was in full swing. De Young worked on the interaction of galactic and intergalactic gas, and had discussions with Cox, Silk, and Field on those topics, with Jokipii on winds from galaxies, and on the motion of supernova remnants with Colgate. Shu had discussions with Cameron, Meszaros with Colgate. Jokipii talked with Ramaty about Sco X–1. Felten talked about cosmic pion decay with Goldsmith. Kwan talked about astrophysical masers with Litvak, DeJong, Townes and Goldreich. Spitzer allowed that he “expects other astronomers have benefitted from his discussions of Copernicus satellite.”

Gary Steigman recalls that in the summer of 1972, Dave Schramm was on his way East after receiving his PhD from Caltech. Steigman invited him and his family to stop off at Aspen and stay with him for a long weekend. This was Dave’s first time in “real” mountains, and it was then and there that the bug bit him, leading to his love of mountaineering and his commitment to the Center for the remainder of his all too short life. In their hike that weekend, Steigman met, for the first time, Bob and Elaine Williams who were in Colorado visiting family and just happened to be on the same trail. This may also have been Bob’s first introduction to the Center.

Gary Steigman and Holly, a welcome companion on many hikes.

In addition to cross–fertilization, intense collaborations were started or furthered. Cameron worked with Truran. Sutherland worked with Lamb. Sawyer worked with Sasha Migdal on pion condensation. In 1974, the summer featured the newly traditional June Workshop on the Physics of Galaxies organized by De Young. A 1974 letter from Heinz Pagels to the Executive Committee mentions 213 applicants. One of the new stars among them was Beatrice Tinsley. Colgate, DeYoung, and Pines, astrophysicists all, were on the Executive Committee. The age of astrophysics at the Aspen Center for Physics was in full swing.

The latter half of the decade of the 70's saw the flowering of the June astrophysics workshops sponsored by NASA as well as the NSF. The astrophysicists were banned to June, early in the summer, but they made the most of it. In this interval, it is somewhat difficult to point to specific developments that were uniquely tied to Aspen, but Aspen clearly nurtured breaking developments, drawing luminaries from throughout the world including the Soviet Scientist Program. This program was a proactive attempt by Aspen to get scientists from behind the iron curtain to Aspen. It was not limited to astronomers, but I think included some astronomers. To make this work, a coterie of dedicated astrophysicists devised and organized the workshops and wrote the proposals to fund them. Among them were Arons, Cameron, Colgate, De Young, Susan Lea, McCray, Steigman, Saslaw and Truran. At the same time, to avoid an “old boys network” aura, great effort was made to capture the most exciting astrophysics and to get new people to attend each year. Diversity would remain an issue (Susan Lea being the lone female in an organizational role in this era), but there was great success in the range of topics and the number of new, young, people introduced to the wonders of astrophysics on the patio of the Aspen Center for Physics.

The early 1970's had seen the success of Uhuru to open the X–ray sky to the startling understanding that the Universe was not a quiet place, but ablaze with violent activity. In particular there was the dramatic discovery of candidate neutron stars and black holes in binary star orbits. Aspen capitalized on the excitement of this burgeoning era with a workshop on Physics of Compact X–ray Sources organized by Cameron in 1975. Many of the principals of this new field attended: Russell Hulse, who would win the Nobel Prize in 1993 for his co–discovery of the binary pulsar that showed orbital decay at the rate predicted by Einstein, and Riccardo Giacconi, who would later win for work with Uhuru and other missions. Among others in attendance were Kip Thorne who was doing his pioneering work with Igor Novikov on the structure of accretion disks, Lamb, Ethan Schrier, Mel Ulmer, John Bahcall, Ostriker, Kevin Prendergast, Jim Pringle, McCray, Craig Wheeler, Pines, Rees, Sutherland, Truran, Paul Joss, and Stu Shapiro. The topics discussed were neutron stars and black holes, accretion disks, radiative processes, and common envelope evolution. The strange “dips” in the light curve of Hercules X–1 were broadly discussed by many people with Paul Boynton, who had done pioneering work on them. Work continued on neutron star interiors with Phil W. Anderson, who would become a Nobel Laureate in 1977, with Pines, Ruderman, Baym and Shaham all actively involved. Cameron talked about parallels between stellar disks and the proto–solar disk. Cox absorbed discussions of pulsations of magnetic white dwarfs by Pringle and supernova light curves by Arnett to consider what might be computed with facilities at Los Alamos. Bill Rose, Sumner Starrfield, Carl Hansen, Hugh Van Horn and Truran talked about the physics of nova and dwarf nova explosions. Conversations on binary evolution occupied H.–C. Thomas, Wheeler, Hulse, Mike Lecar, and Ostriker. Steigman was involved with his important translation of the book by Yakov B. Zel'dovich and Novikov on astrophysics: Stars and Relativity. First time attendee, John Stewart remarked that there were too many morning talks, a price paid for many years by packing so many astrophysicists onto the patio, all of whom had much to contribute.

Anna Zytkow, a native of Poland, was at the time a postdoc with Thorne. They were doing their pioneering work on the structure of red giant stars with accreting neutron star cores. Kip loaned Anna his ageing Chevy Suburban to make the trip from Pasadena to Aspen. Kip said he thought it might break down, but that having a vehicle break down in remote circumstances and getting it repaired were a formative part of the American experience he hoped that Anna could share. She drove. It broke. She coped and arrived in Aspen to share the patio and the mountains.

The 1976 workshop was organized by Cameron and Truran on the topic of “Environments for Element Production.” This workshop brought together theorists and optical and infrared observers to discuss how measurement of the elemental and isotopic distributions in a variety of astrophysical objects could lead to deeper understanding of the Universe. Steigman talked about primordial nucleosynthesis. Ostriker talked about the possibility of zero–metallicity stars and the conundrum that there seemed to be a floor to the observed metallicity of stars. Dave Arnett talked about the advanced evolution of stars and the conundrum that stars of 4 to 8 solar masses (which Bohdan Paczynski has shown all converged to the same electron–degenerate structure) seemed to explode, not collapse. Later work showed that was a feature, not a bug, and the physics was closely involved with Type Ia supernovae. Dave Schramm talked about the origin of the r–process. There were also discussions of the convective Urca process, another development from Paczynski. There were many discussions of the role of neutrinos, especially neutrino scattering and neutrino degeneracy, in the context of stellar core collapse. The discovery that neutrinos could become degenerate, leading to trapping and adiabatic collapse was just breaking. This insight demanded a hot collapse and hence a hot nuclear matter equation of state, dramatically altering the understanding of the collapse ambience. Unfortunately, despite the qualitative change in physics, models of core collapse continued to fail to explode. Cameron talked about the newly discovered 26Mg anomaly in meteorites. The important implication was that if this isotope came form the relative short decay of 26Al, a nearby supernovae was probably required to inject this short–lived species into the proto–solar nebula.

Somewhere in this era, the Wheeler family invented the concept of marshmallow fights. This was done with uncooked marshmallows that did not hurt and left only a little powdery residue after the mayhem. This began as just a family affair, but one memorable evening, when they hosted many of the workshop attendees at a party, left an indelible image: dozens of world–famous astrophysicists thronging the living room and up the stairwell to the bedroom area, pelting one another with marshmallows in a happy burst of harmless violence. Stale marshmallows were found behind the sofa for weeks afterward.

1978 showed some breaking of the mold. Steigman organized a workshop on Particle Physics and Astrophysics under the auspices of the NSF program, rather than as a June Astrophysics workshop. A June workshop was organized by Colgate and De Young on Astrophysical Plasmas to discuss magnetospheres, particle accelerations, the origin of magnetic fields, nuclear fusion, and the application of plasma physics to planets, stars, and galaxies. A special astrophysics workshop on The Instruments of the 80's was organized by McCray and Saslaw.

The 1979 workshop on Stellar Collapse and Neutrino Physics was organized by Lamb. In his exit report, Wagoner notes that he talked to Ed Kibblewhite about his new techniques for batch processing photometry. Wagoner recognized that this was a key development in his goal to discover distant supernovae and use them to measure cosmological parameters. It would take two more decades, but these ideas would flower, with Type Ia supernovae replacing Type II as the tool of choice (Type Ia have complex physics, but uniform behavior; Type Ia have a simple composition, hydrogen, but the hydrogen lines arise in strongly in non–thermal equilibrium conditions, making them difficult to analyze in practice), leading to the discovery of the accelerating Universe, the pre–eminent physics problem of the day as the Aspen Center for Physics celebrates its 50th anniversary.

The Middle Years: 1980 to 1995

by Michael S. Turner

I came to Aspen for the first time in 1979. I was a young postdoc attending the NASA astrophysics workshop, that year entitled Stellar Collapse and Neutrino Physics. By then, astrophysics was a well-established part of the summer program and many astrophysicists attended on a regular basis, usually during the three-week June NASA workshop. The “astro-style” summer workshop was different, more formal, with hours of talks and individuals staying for a single week or two rather than the three- or four-week stays that physicists enjoyed. By the end of the middle years, that would change as astrophysics became fully integrated into the ACP.

In 1972 NASA began funding a workshop that provided the structure for the astrophysics program. There was an “Astrophysics Organizing Committee” (AOC) that oversaw the selection of the topic, wrote the proposal and selected the participants. The AOC operated as a “shadow government” for astrophysics. (So shadowy that almost all the records of its existence are oral; in going through the archives I found only one written memo, dated 1986 from David De Young to the ACP President Mike Simmons, which De Young signed as Vice Chair of the AOC).

The core group of astrophysicists who rotated in and out of the AOC, and took care of the program until astrophysics was mainstreamed included W. David Arnett, Al Cameron, Stirling Colgate, David De Young, George Field, Jay Gallagher, Susan Lea – the lone female – Dick McCray, Bill Saslaw, Gary Steigman, James Truran, J. Craig Wheeler, and Robert Williams. Their names appear on many of the NASA proposals, and these Aspen astro-pioneers got astrophysics established at the Center.

In the early years, the astro-pioneers had help and encouragement from astro-leaning physicists like Gordon Baym, David Pines and Hans Bethe. Bethe served as ACP Vice-President from 1972 to 1973; Stirling Colgate served as a Trustee from 1972 to 1978 and many members of the AOC ultimately went on to serve as members, trustees and officers of the ACP (e.g., Robert Williams served as Treasurer from 1985 to 1988, James Truran served as Vice President from 1985 to 1988, and David De Young as President from 2001 to 2004).

As the story has often been told, Aspen began as a particle-physics theory center; very soon, condensed matter physics was added (see other histories). The path to “full membership” for astrophysics would take longer. In the early years astrophysics at Aspen revolved around neutron stars, pulsars, and nucleosynthesis. The first two topics were triggered by the 1967 discovery of pulsars, and their quick identification as neutron stars. The science of neutron stars was rich with exciting physics -- superfluidity, ultra-high-temperature superconductivity (1012 K), nuclear physics and general relativity -- and the informal, interdisciplinary environment of Aspen was the ideal place to address it.

Later, when astrophysics began to make inroads into the mainstream in Aspen, cosmology was the vehicle. In 1978, Gary Steigman organized a physics workshop (as opposed to the yearly NASA workshop), entitled The Early Universe. Early-Universe cosmology brought together particle physicists and cosmologists. The physics at the intersection of particle physics and astrophysics/cosmology was ripe for breakthroughs and would be a major theme at the Center for the next 30 years. Today, particle astrophysics and cosmology is central to both astrophysics and particle physics, with the agendas of both fields having great overlap. Aspen played a significant role in making this happen.

David Schramm, who spent his first summer as a “physicist” in Aspen in 1978 (see below), was a pioneer in bringing particle physics and astrophysics/cosmology together in Aspen and around the world. Schramm and his “Chicago mafia” (myself included) made Aspen one of the incubators for this new field. Tragically, Schramm died in a plane crash in 1997 before particle astrophysics and cosmology achieved the central role that it has in both fields today. Nonetheless, during his more than 20 years at the Center, he helped elevate astrophysics to the prominence that it has today.

(Until the late 1980s, attendees were classified as either astrophysicists or physicists. In 1976 Schramm attended officially for the first time, as an astrophysicist. While at the ACP in 1976, he collaborated with Steigman and Gunn on his most influential paper – the 1977 Physics Letter B on the BBN limit to the number of neutrino species. This paper also helped to launch the field of particle cosmology.)

David N. Schramm in Aspen, circa 1995.

There has always been (and probably always will be) a spirited discussion about the role and importance of unstructured research vis-à-vis workshops at Aspen. However, there is no better example of how workshops bring new people and new fields to the Center than astrophysics. Beginning with the yearly NASA summer workshop, followed by a growing number of physics workshops in astrophysics and finally winter conferences, astrophysics grew to be one of the three scientific pillars of the Center. Once a new group discovers the ACP, it keeps coming back and astrophysicists were no different!

Starting around 1980, powered by an influx of new ideas from particle physics about the early Universe, cosmology went from a sleepy topic shunned by physicists to one of the most active and exciting areas of both physics and astrophysics. Activities at the Aspen Center for Physics played a key role in this cosmic revolution. Between 1980 and 1995 there were at least 15 summer workshops on topics at the intersection of particle physics and astrophysics/cosmology, another three particle physics workshops with significant overlap, and four winter conferences focused on cosmology (The Early Universe in 1986, The Physics of the CMB in 1990, Recent Advances in Cosmology in 1991 and the Cosmological Distance Scale in 1993). While a couple of the summer cosmology workshops occurred within the NASA astrophysics program, most did not. Not only did these activities help to move the field forward, but they also helped to launch (or to advance) the careers of a number of today’s leaders in cosmology – including Andy Albrecht, Marc Davis, Simon White, Katie Freese, Josh Frieman, Marc Kamionkowski, Lawrence Krauss, Rocky Kolb, Pierre Sikivie, Neil Turok and myself.

With the advent of grand unified theories (GUTs), there was the smell of big things in the air for particle physics – unification of the forces and particles, proton decay, magnetic monopoles and neutrino mass – and for cosmology -- baryogenesis, dark matter, inflation and monopoles. The 1980 workshop Cosmology and Particle Physics (organized by Gary Steigman and Frank Wilczek) featured talks on the baryon asymmetry of the Universe, superheavy magnetic monopoles, the quark/hadron transition and gravitational radiation. John Preskill laid out the monopole problem, the dramatic overproduction of magnetic monopoles in the standard hot big-bang cosmology. The monopole problem spurred Alan Guth to discover the inflationary universe scenario. The participant list included John Preskill, Frank Wilczek, Rocky Kolb, Keith Olive, James Fry, Robert Wagoner, and myself.

In 1982, the NASA workshop was entitled the Large-scale Structure of the Universe. Alex Szalay discussed the role of massive neutrinos in structure formation, Gary Steigman spoke on his theme topic – big-bang nucleosynthesis (BBN) – Bob Kirshner summarized our knowledge of the mean density of the Universe (definitely sub critical!); Schramm spoke about the age of the Universe and whether or not the different cosmic clocks were consistent, and before leaving for the Nuffield conference on the Very Early Universe, where the current inflationary paradigm would come together, I spoke about ideas from the early Universe that would impact structure formation. Among the other participants were Simon White, Joe Silk, Jerry Ostriker, Dick Bond, Katie Freese, Francois Bouchet, Adrian Mellot, Bernard Carr, Carlos Frenk, Avashai Dekel, and Ethan Vishniac. About this time, the idea of particle dark matter was beginning to take hold and impact the study of structure formation, first with eV-mass neutrinos (hot dark matter) and soon thereafter with WIMPs (cold dark matter). Over the next two decades, workshops at the Center would play a crucial role in developing the current paradigm for structure formation: inflation + cold dark matter.

That same summer Pierre Ramond organized a workshop entitled Grand Unified Theories. Some of the talks had cosmological connections: Sheldon Glashow’s on magnetic monopoles; Graham Ross’s and Stuart Raby’s on SUSY GUTS; Pierre Sikivie’s on cosmology confronting Higgslets (now known as axions and one of the leading dark-matter candidates); Michael Green’s on superstrings (this two years before the string revolution) and Marc Sher’s on inflationary cosmology.

In 1983 Gary Steigman, Frank Wilczek and I organized a workshop entitled Monopoles, the Early Universe and Particle Physics. This workshop came in the wake of Blas Cabrera’s detection of a magnetic monopole on Valentine Day’s in 1982, the advent of the first workable models of inflation (so-called new or slow roll inflation) and their prediction of almost scale-invariant density perturbations, new stringent limits on the magnetic monopole flux from catalyzed proton decay in neutron stars and the survival of astrophysical magnetic fields, and growing interest in cosmic strings as an alternative to inflation. All were hot topics at this workshop whose attendees included Katie Freese, Alan Guth, Jim Fry, Jeff Harvey, Rocky Kolb, Lawrence Krauss, David Olive, So-Young Pi, David Schramm, Gino Segre, Stephen Wolfram (soon to become the founder of Mathematica), Henry Tye, Paul Steinhardt, Robert Brandenberger, Mal Ruderman, Jacob Shaham, Neil Turok, and Erick Weinberg.

1983 Aspen Workshop on Cosmology and Monopoles.

One of the highlights of the workshop was Neil Turok’s defense of his PhD thesis on cosmic strings with David Schramm as an external member of his Cambridge committee. Neil and Andy Albrecht (both frequent Aspen attendees) would go on to lead the cosmic string revolution in cosmology.

In 1983, Dick Slansky organized a particle physics workshop entitled Beyond the Standard Model. While focused on more formal aspects of particle theory, many of the topics were and would be relevant for cosmology: supersymmetry (SUSY), SUSY GUTs, superstrings and anomalies. During one summer around this time (probably a few years earlier), Murray Gell-Mann, Pierre Ramond and Dick Slansky came up with the “see-saw” mechanism for neutrino mass. Their see-saw mechanism predicts “eV-range” neutrino masses, which is what has now been observed. Masses in this range make neutrinos of great interest for cosmology (eV masses correspond to percent level contribution to the critical density) and for astrophysics (neutrino oscillations in the Sun, and exploding stars). The see-saw mechanism is one of the many important scientific achievements that has taken place at the ACP.

The NASA astrophysics workshop in 1984 was entitled the Physical Basis of the Cosmological Distance Scale. Talks included Jim Peebles on “Cosmological models and the distance scale,” Bob Wagoner on “Novae/Supernovae as distance indicators,” David Schramm on “Nucleocosmochronology,” John Huchra on the “Tully-Fisher method,” and Robert Kennicutt on “How secure is the foundation of the distance scale.” Other participants included Marc Aaronson, Craig Hogan, Bohdan Paczynski, and Vahe Petrosian. (Vahe, arguably the best physicist volleyball player of all time, must have played volleyball that summer on our sand court and taught his colleagues a few things; there is no record of any injuries from his powerful spikes.)

The summer of 1985 saw two workshops on particle astrophysics and cosmology: Exotic Particles and Dark Matter (organized by Tony Zee and myself), whose attendees included Jim Bardeen, Jim Cline, George Blumenthal, Avashai Dekel, Ricardo Flores, Katie Freese, Josh Frieman, Jim Fry, Margaret Geller, Graciela Gelmini, Craig Hogan, John Huchra, David Koo, Jerry Ostriker, Heinz Pagels, Joel Primack, David Schramm, Alex Szalay, Brent Tully, Ethan Vishniac, and Neil Turok. (A few years earlier, Primack and Pagels had published their PRL suggesting the gravitino as a dark-matter candidate, likely the first mention of the lightest SUSY partner as a dark-matter candidate.) With the cold dark-matter (CDM) paradigm for structure formation just starting to gel, this meeting of theorists and observers, astronomers and physicists focused on CDM and its predictions.

The other astrophysics workshop that summer was Underground Physics, organized by Sid Bludman and Tom Gaisser. Around this time, the large underground detectors originally built to detect proton decay were being “re-purposed” for solar, supernova and atmospheric neutrinos and underground physics was becoming a “hot topic.” Two years later, neutrinos from SN 1987A would be detected by two such detectors, and a decade after that, data from similar detectors would definitively make the case for neutrino mass, based upon the deficit of solar and atmospheric neutrinos, establishing the first experimental evidence for physics beyond the standard model of particle physics.

The NASA astrophysics workshop in 1985 was entitled The Origins of Supernovae, and many of the leading theorists attended -- Hans Bethe, Bob Kirshner, David Branch and W. David Arnett. One of the topics – Type Ia supernova (SNeIa) – would soon be of great interest to cosmologists too. In 1998, using SNeIa as cosmic distance indicators, two teams (with many Aspen regulars on them) showed the expansion of the Universe was speeding up, not slowing down. This discovery would solve the glaring and perennial problem of inflation + CDM – not enough matter to provide the critical density predicted by inflation – by adding dark energy to the mix. The current cosmological paradigm, ΛCDM, is based upon inflation, dark matter and dark energy, with a critical density (spatially flat) Universe comprised of 72% dark energy and 28% dark matter.

In 1986, when the debate between inflation and cosmic strings, as the origin of the seeds of large-scale structure was heating up, David Schramm and Gary Steigman organized a summer workshop entitled Inflation, Cosmic Strings and Related Topics, where the battle lines were drawn and the combatants engaged. Inflation was becoming “the establishment picture” and the cosmic string picture was the exciting, new alternative. The issue would be settled in the late 1990s when the CMB anisotropy on the degree-scale ruled out large-scale structure seeded by cosmic strings in favor of inflation + cold dark matter.

With the detection of neutrinos from SN 1987A on February 23 - 1987 was a super year for astrophysics, and the Center held a workshop entitled Solar and Astrophysical Neutrinos. The attendees included the major figures in the field -- John Bahcall, Adam Burrows, David Schramm, Gene Beier, David Cline, Peter Rosen, Stirling Colgate, David Arnett, Lincoln Wolfenstein, George Fuller, and Jim Wilson. The detection of neutrinos from SN1987A confirmed a prediction made by Aspen astro-pioneer Stirling Colgate, namely that most of the energy released in the gravitational collapse of a massive star is carried off by neutrinos.

The topics at the 1988 workshop Dark Matter Particles and Their Detection (organized by David Seckel and myself) included dark matter annihilation signatures (neutrinos, photons and anti-protons), the latest calculation of neutralino interaction cross sections (Kim Griest), dark-matter detectors (Robert Lanou and David Cline), the effect of WIMPs on stars (Pierre Salati) and axions as dark matter (Pierre Sikivie). All of these topics have only grown in importance as we close in on identifying the dark-matter particle this decade.

The same summer, the NASA Workshop was entitled The Large-scale Structure of the Universe (organized by Schramm, Peebles and Joseph Silk), and among the participants were Jim Bardeen, Nick Kaiser, George Efstathiou, Mark Wise, and Marc Davis. The focus of the workshop was once again the cold dark-matter paradigm of structure formation, which was re-vitalizing the field of structure formation, both theoretically and observationally. Without a well-motivated paradigm to guide and motivate observers, the study of large scale experienced a lull in the late 1970s. CDM with its many predictions – and for a while competitors (HDM and cosmic strings) -- led to a renaissance in this subject that continues to the present.

Hiking has always been central to the ACP experience. Cosmologists were no difference in their interest in hiking and have some good stories to tell. In 1988, there was a famous "Death March" led by Marc Davis that went up and over Electric Pass and then down into Conundrum Valley, up to the hot springs for a soak and finally out the nine miles of Conundrum Valley along Conundrum Creek. At one point, Larry Widrow removed his boots for a creek crossing, slipped, and lost a boot in the rushing creek. Fortunately, Marc had an old pair of tennis shoes that Widrow could wear on the long way out.

Some of the survivors of the Death March, on top of Electric Peak: kneeling: Craig Wheeler, Marc Davis, Larry Widrow; standing: Simon White, Jens Villumsen, Ed Bertschinger, and unidentified. Others on the hike included Sergei Shandarin and John Huchra.

Edward (Rocky) Kolb and Erick Weinberg organized the 1990 summer workshop Cosmological Phase Transitions. Andy Albrecht, Pierre Sikivie and Ed Copeland spoke about cosmic strings and numerical simulations of the formation of string networks. Two new models of inflation were discussed: extended inflation (Paul Steinhardt), a model based upon Brans-Dicke theory, and natural inflation (Josh Frieman), a model where the inflaton was a Nambu-Goldstone boson. Neil Turok spoke about a new mechanism for baryogenesis.

The absence of detected fluctuations in the cosmic microwave background (CMB) temperature arising from the primordial density perturbations that seeded cosmic structure formation was becoming a concern in cosmology. [In fact, a silly book entitled The Big Bang Never Happened (by “Ph.D. astronomer” Eric J. Lerner) appeared that summer on Aspen bookshelves, and one of Lerner’s few scientific arguments against the big bang was the absence of detected CMB fluctuations.] At the same time, CDM and cosmic strings were making more and more quantitative predictions about large-scale structure and the level of the CMB fluctuations: at least 10-5, near the then-current upper limits. In 1991, Mark Dragovan, Jim Peebles and David Wilkinson organized a workshop, Cosmic Background Radiation, to discuss the state of affairs and plans for the future.

Timing is everything: that same summer, scientists – including Aspen regulars George Smoot and Ned Wright – were analyzing data from the DMR instrument on NASA’s Cosmic Background Explorer (COBE) satellite, and on 23 April 1992 they would announce the first detection of CMB anisotropy, confirming the gravitational instability theory of structure formation that Peebles had helped to invent and strengthening the foundations of cosmology (we haven’t heard from Lerner since!). The COBE detection, which led to a Nobel Prize for Smoot and John Mather, ushered in the era of precision cosmology.

Over the years, Aspen has hosted the visits of many Soviet scientists, providing a venue for maintaining scientific connections between the East and West during the Cold War. In the summer of 1991, the Soviet and US Academies of Sciences held an exchange program for young cosmologists in Aspen, organized by Alexei Starobinskii, David Schramm and Kip Thorne. The US delegation included Josh Frieman, Marcelo Gleiser, David Seckel, Jennie Traschen, Jane Charlton, Barbara Ryden, Hume Feldman, and Hardy Hodges. The Russian side featured three young scientists who would eventually spend much time in the US: Nick Gnedin (now at Fermilab and UChicago), the late Lev Kofman (who spent much of his career at CITA and the University of Hawaii), and Igor Tkachev (after several years at Fermilab, back in Russia and now a member of the Russian Academy of Sciences). Baryogenesis, large-scale structure, cosmological phase transitions, inflation, and the loitering universe (models with a cosmological constant) were among the topics discussed.

The 1992 summer workshop Massive Neutrinos in Particle Physics and Astrophysics featured talks on the effect of lepton-number violation on core collapse (David Seckel, Graciela Gelmini and George Fuller), solar neutrinos (Gene Beier, Hamish Robertson, Lincoln Wolfenstein, Robert Shrock, George Fuller, David Schramm, Lawrence Krauss, Adam Burrows, Peter Rosen, Steve Parke, Sergei Petcov, Boris Kayser and David Cline) and measuring the non-Gaussianity of the density field of the Universe (Robert Scherrer, Josh Frieman and Jim Fry).Today, non-Gaussianity has become a topic of great interest in cosmology as it provides a means of testing inflation and ruling out the simplest models.

That same summer Craig Hogan organized a workshop entitled Gravitational Lensing in Cosmology, and the attendees included Christopher Stubbs, Tony Tyson, Rachel Webster, Roger Blandford, Ramesh Narayan, Chris Kochanek, and Kip Thorne. While gravitational lensing had been first observed in 1979 (the twin quasar Q0957+561 discovered by Walsh, Carswell and Weymann), its use as a tool to study the Universe – searching for cosmic strings, mapping out dark matter and large-scale structure, determining the Hubble constant and constraining the cosmological constant – was still in its infancy and this workshop helped to shape the future of the subject. Today, both strong and weak gravitational lensing are powerful probes of the Universe.

The COBE team’s April 1992 announcement of the detection of anisotropy in the CMB the DMR instrument on angular scales of order 10 degrees transformed cosmology. Among other things, it provided a physics-based normalization for the power spectra of density perturbations predicted by inflation and cosmic strings. That in turn spurred new ideas about structure formation because the simplest version of inflation + cold dark matter (i.e., a critical-density Universe comprised only of CDM and baryons) so normalized did not fit all the data; new variants, including the hot + cold dark matter and ΛCDM arose. With the long-sought CMB anisotropy having been detected, the next step was the design of experiments with better angular resolution to map out the CMB anisotropy on all scales to test the different models of structure formation and reap the treasure trove of information therein. The1993 workshop entitled Large-scale Structure after COBE (organized by Nick Kaiser, Robert Scherrer and Wojciech Zurek) brought together theorists and experimentalists to assess where cosmology stood after the COBE detection and to discuss what future CMB experiments should be carried out. The design of WMAP, Planck and a host of ground-based experiments were informed by this workshop whose participants included Andrei Linde, Andy Albrecht, Marc Kamionkowski, Jim Peebles, Paul Steinhardt, David Wilkinson, Jim Bardeen, Robert Brandenberger, Josh Frieman, Hume Feldman, Andrew Liddle, Angela Olinto, Joel Primack and myself.

Running concurrently with the COBE workshop was a particle physics workshop entitled B+L Violation in Electroweak Theory, organized by Larry McLerran and Valery Rubakov. There were several joint sessions and the topic of electroweak baryogenesis was of interest to both the cosmologists and the particle physicists. The gradual acceptance that B+L violation within the standard model led to rapid B+L violating interactions in the early Universe changed the way baryogenesis was viewed, eventually leading to the current view that baryogenesis likely begins with a lepton asymmetry (leptogenesis) that is morphed into a baryon asymmetry by B+L violating interactions when the Universe was a picosecond old.

In 1994 David Weinberg and Adrian Melott organized a workshop entitled Gravitational Clustering in Cosmology. Numerical simulations of the formation of large-scale structure in the Universe were becoming increasingly predictive and the quality and quantity of large-scale structure data with which to compare were improving as well. (E.g., both the SDSS and 2dF Surveys were getting ready to begin.) The participants in this workshop, which included Neal Katz, Francois Bouchet, John Huchra, Angela Olinto, Gus Evrard, Lev Kofman, Anatoly Klypin, Carlos Frenk, Guinevere Kauffmann, and Simon White, were enthusiastic about the interactions that took place at the workshop, calling it “one of the best ever.”

The summer 1995 workshop, Inflation: From Theory to Observation and Back (organized by Katie Freese and myself), drew an impressive group of US “inflationists” (Albrecht, Linde, Guth, Frieman, Martin White, Arthur Kosowsky, Kamionkowski, Janna Levin) and members of the growing British inflation community (Andrew Liddle, David Lyth, and Malcolm Perry). There was intense discussion of open inflation (the growing evidence for a matter density less than the critical density was pushing some inflation proponents to relax the inflationary prediction of a flat Universe) as well as what CMB measurements on small angular scales (less than 1 degree) could reveal about inflation and the features of the Universe more broadly (e.g., determination of cosmological parameters including the curvature, composition and age of the Universe). Mixed dark matter (hot + cold dark matter) was a “hot” topic, because it was a variant of CDM (along with ΛCDM) that could fit all the data. Lisa Randall attended this workshop, and she and Guth worked together on a search for “natural” (i.e., well-motivated) particle physics models of inflation.

The two-week Big-bang Nucleosynthesis (BBN) workshop organized by Lawrence Krauss and Schramm) featured a spirited debate between the Chicago school (Schramm and Turner) and the Ohio school (Steigman, Scherrer, Walker and Olive) about whether or not there was a crisis involving the primordial Helium abundance, one of the key predictions of BBN. Arguing for a low value for the primordial helium abundance (around 24%) the Ohio school claimed BBN was in crisis because not all three neutrino species could be accommodated with such a low Helium abundance. The Chicago school argued that the evidence for a low Helium abundance was weak and there was no crisis (which turned out to be correct). Interestingly enough, there is a crisis today involving the primordial Helium abundance once again. This time, some of the same people who argued for a low primordial abundance are arguing for high primordial abundance, around 26%, which necessitates an additional neutrino species. The only certain thing is the importance of BBN to both cosmology and particle physics: It is the earliest quantitative test of the big-bang framework and a powerful probe of particle physics. And that BBN connection between particle physics and cosmology traces back to the paper Steigman, Schramm and Gunn wrote in Aspen in the summer of 1976.

Cosmology wasn’t the only area of astrophysics that flourished at the Center during the middle period. Core collapse, supernovae, neutron stars and nucleosynthesis continued to be major scientific themes, as evidenced by summer workshops and winter conferences on these subjects. Supernova 1987A, the brightest supernova in almost 400 hundred years and the first to be “seen” in neutrinos certainly had a lot to do with that. Other notable non-cosmology workshops included New Directions in Pulsar Physics (1984, organized by Jon Arons, Roger Blandford, David Helfand and Franco Pacini) and Gamma-ray Bursters (1994,organized by Don Lamb and Bohdan Pacyznski) held at the height of the debate about whether or not gamma-ray bursts were galactic or extragalactic events. Bohdan and Don would debate the subject in Washington, DC in April 1995 (Great Debate II, modeled after the famous 1920 Curtis-Shapley debate about the size of the Universe). Don would win the debate, but Bohdan had the right answer (which is much more important than winning the debate!). Two years later, the BeppoSAX satellite settled the question: GRBs are extragalactic. (The BeppoSAX detection of GRB afterglows revealed that distant galaxies host them.)

Arguably the most influential astrophysics workshop of the middle period was Adaptive Optics in 1992. The extensive classified work on adaptive optics was first shared with the astrophysics community in this workshop. The NSF Science Technology Center on Adaptive Optics at UC Santa Cruz traces its origins to discussions at this Aspen meeting, and adaptive optics has transformed astronomy by giving every ground-based telescope the potential to have the same visual acuity as a space telescope.

During this period, general relativity, especially its intersection with cosmology, astrophysics and particle physics was an active area at the Center as well, with workshops on Gravitational Radiation in 1980 (organized by Bob Wagoner and Dave Douglas), Baby Universes in 1989 (organized by Steve Giddings and Sidney Coleman, with a participant list that included Leonard Susskind and Stephen Hawking who gave a July 5th public lecture), Current Problems in Classical Gravitation (organized by Jim York, Richard Matzner and Tsvi Piran), Cosmic Censorship in 1992, Quantum Aspects of Black Holes also in 1992, Gravitational Problems in Relativistic Astrophysics in 1993, and Numerical Investigations of Singularities in General Relativity in 1994 (triggered by Matt Choptuik’s successful numerical modeling of black-hole formation from scalar fields). While relativity per se has not risen to be a primary activity at the Center, it continues to be a recurring theme.

To the chagrin of many of my astrophysics colleagues – especially the members of the AOC -- during my presidency (1989 to 1993) we mainstreamed astrophysics, abolishing the single guaranteed June astrophysics workshop in favor of astrophysics competing with particle physics and condensed matter physics for workshop slots. Likewise, the AOC disappeared (of course, like Skull and Bones, it may continue secretly to this day). Astrophysicists in increasing numbers were added as general members and trustees – Neta Bahcall, David De Young, Katherine Freese, Josh Grindlay, Craig Hogan, Angela Olinto, P.J.E. Peebles, Mal Ruderman, Joe Silk, J. Craig Wheeler, and David Wilkinson. In 1980, only two of the 23 members/trustees were astrophysicists; by 1995, almost one third of the nearly 80 general members were astrophysicists and the ACP had had its first astrophysicist as president. In the 1990s, several astrophysics workshops were held each summer, in the informal Aspen style with lots of interactions and few formal talks. In 1986, during the second year of the Winter Conference series, astrophysics joined as a permanent member. Some winters have had as many as three weeks devoted to astrophysics.

During the middle years, astrophysics at Aspen transitioned from a program built around a single, yearly NASA-sponsored workshop to an integral part of the Center’s scientific program. The early connections between astrophysics and condensed matter physics built around neutron stars remained, but new, stronger connections between particle physics and cosmology emerged and greatly enhanced astrophysics at Aspen. The Center can proudly claim that it helped to propel the field of particle astrophysics and cosmology to its current prominence by providing a summer home for workshops and research. Moreover, cosmology provided the vehicle for the Center to “go big” in astrophysics. During the modern era, astrophysics would grow to stand on its own, bringing the full breadth of contemporary astronomy to the Center and establishing new connections to the other disciplines.

The Modern Era: 1996 – 2012

by Joshua A. Frieman

By the mid-1990’s, astrophysics had become fully integrated into the activities of the Aspen Center for Physics, on a par with particle physics and condensed matter physics in terms of the number of members and officers and the number of and participants in workshops. To give one measure of the level of activity, from 1996 through 2011, the ACP hosted roughly 90 summer workshops and winter conferences on astrophysics and closely related areas, with several thousand participants.

The emphasis at the ACP has traditionally been in theoretical physics, albeit strongly informed by experiment and observation. The majority of members and workshop organizers have been drawn from the theoretical physics and astrophysics communities. This theoretical bent dates to the early history of the Center and also traces to the simple fact that theorists tend to have fewer external constraints than experimentalists on their schedules and thus more freedom to spend extended periods at the Center carrying out coffee-driven research.

However, the last 15 years have coincided with a golden age of observational astrophysics and cosmology, and the balance between theory and observation/experiment in astrophysics at the Center has shifted accordingly. For example, in the summer 2010 session, roughly half of the astrophysics participants were experimentalists/observers. The Center has played a central role in bringing theorists and observers/experimenters together in a series of astrophysics workshops that synthesize the observational results, delve into interpretations of new data sets, pose questions to be answered by new experiments, and help inform the design of new projects. Highlights of this period include the COBE detection of cosmic microwave background (CMB) anisotropy in the early 90’s (toward the end of the middle years), followed by the detection of the first Doppler peak in the CMB angular power spectrum by balloon-borne CMB experiments in the late 90’s, followed by the WMAP results in the early 2000’s, the discovery of CMB polarization by DASI, and large-scale Sunyaev-Zel’dovich surveys in the late 2000’s. Since 1993, the Hubble Space Telescope has generated spectacular astronomical discoveries and images treasured by millions. In the same period, NASA launched its other Great Observatories (Compton, Chandra, Spitzer), which have revolutionized studies over a broad range of electromagnetic wavelengths (gamma-ray, infrared, X-ray), complemented by smaller missions. On the ground, new sky surveys, epitomized by the Sloan Digital Sky Survey (SDSS), uncovered the large-scale structure of the Universe, the small-scale structure of the Milky Way, and the most distant quasars. In these years, the first generation of 6- to 10-meter telescopes (Keck, VLT, Magellan, Gemini, HET, among others) opened up new vistas in the distant and faint Universe. Atmospheric and solar neutrino experiments provided evidence for non-zero neutrino masses, the Auger array found that ultra-high energy cosmic rays correlate with active galaxies, and gamma-ray bursts with optical afterglows were discovered in the high-redshift Universe. Meanwhile, astronomers found supermassive black holes lurking in the cores of galaxies and hundreds of planets orbiting other stars. On the largest scales, observations of distant supernovae led to the remarkable discovery that the expansion of the Universe is speeding up. These results and more have been highlighted at a number of ACP astrophysics workshops.

Apart from exploring the theoretical implications of the latest observational discoveries, in these years the astrophysics program also provided an increasingly important venue for planning and development of major new astrophysical facilities and for the organization of very large data bases resulting from massive survey programs. Examples of workshops in this vein include: Wide, Fast, Deep Surveys (summer 2009), Astrophysics and Cosmology with the 21-cm Background (summer 2010), The Cosmic Microwave Background (CMB, winter 2008), The Cosmic Microwave Background after MAP (summer 2003), Taking Supernova Cosmology into the Next Decade (summer 2010), Galaxy Evolution from Large Surveys (summer 2006), Cosmic Voids (summer 2006), The Large-scale Distribution of Mass and Light in the Universe (winter 2004), Science with Wide-Field Survey Telescopes (summer 2001), Precision Measures of Large-scale Structure in the Universe (summer 1997), Structure Formation in the Era of Large Surveys (summer 2002), and New Astrophysics and Cosmology with a Virtual Observatory (summer 2001).

As noted above, cosmology became a core theme of the ACP in the middle years. While this trend has continued in the modern era, a highlight of this later period has been the tremendous expansion of the breadth of topics in astrophysics covered in both the summer and winter workshops. Examples include workshops on the formation and evolution of extrasolar planets, galaxy formation and evolution, and topics in high-energy astrophysics such as astrophysical jets, stellar winds, gamma-ray bursts, black-hole formation and evolution, and ultra-high-energy cosmic rays. There have been a series of workshops on gravitational wave detection (spearheaded by Syd Meshkov), a long-running series on non-linear dynamics in astrophysics (led by Robert Buchler and friends), and a number of workshops on the Local Group and on galaxy clusters. Along with this increasing breadth, the astro summer workshops broadened out beyond the traditional June time frame to meetings throughout much of the summer.

A natural consequence of this increasing breadth has been a growing focus on interdisciplinary topics both within astrophysics and between astrophysics and other areas, particularly particle physics. Examples of the latter are the series of workshops that have been held on neutrino physics and astrophysics, on the particle physics and astrophysics of dark matter and dark energy, and on early-Universe physics. As noted at several points below, this trend has led in a number of cases to stimulating joint sessions between coincident workshops on different topics. The overlap between astrophysics and other programs at the ACP continues the tradition established in the earliest days of astrophysics at the Center, when workshops and working groups studying neutron star interiors and pulsars drew heavily upon the expertise of condensed matter physicists.

In the early and middle years, workshop sessions were mainly held on the patio, and speakers used the advanced visual aid of the chalk board. With the recent explosion of observational results in astrophysics, the ability to display data in presentations increasingly became an imperative. With the opening of Flug Forum in the new Smart Hall in 1996, this became technically possible, but there remained for some time an official preference for the black board in order to preserve the informal nature of the workshop presentations. Nevertheless, the writing was, so to speak, on the wall. In what now seems like a throwback to another era, one workshop participant wrote in a 1999 exit report: “In the future, let us show some viewgraphs.” These days, Powerpoint and Keynote by and large rule the day in the astro workshops, and the most recent winter astro workshop included live video streaming via a laptop webcam, but the meetings have retained their informal character by allowing plenty of time for smaller group interactions.

In the modern era, astrophysics has played an important role as the Center sought to become more diverse by increasing participation by traditionally underrepresented groups. Women have co-organized many of the modern-era astro workshops. During this period there were 10 female astrophysics members of the ACP, and the first woman (and current) President of the ACP is astrophysicist Rosie Wyse.

1996 included a winter conference on Black Hole X-ray Transients and summer workshops on High-energy Neutrino Astrophysics (led by Tom Weiler), Galaxy Interactions at Low and High Redshift, (Josh Barnes) and Galactic and Cosmological Magnetic Fields. The latter, organized by Angela Olinto and Tanmay Vachaspati, in characteristic Aspen fashion brought together a broad spectrum of cosmologists and astrophysicists, from early Universe theorists to experts in MHD; refreshingly, 20% of the participants were female. To help celebrate the opening of Smart Hall, Chairman of the Board David Schramm and member Stephen Hawking delivered well-attended public lectures. In August, accomplished young musicians Gil and Orli Shaham, who had studied in Aspen during earlier summers when their astrophysicist father was participating at the Center, played a concert at the Aspen Music Festival (AMF) to benefit the newly established Jacob Shaham scholarships at the AMF and ACP. Along with physicist-born Stefan Jackiw, they will reprise their recognition of the Center in another benefit concert during the ACP’s 50th anniversary celebration in summer 2012.

1997 featured a winter conference on Observational Tests of Cosmological Models (organized by Craig Hogan and friends) and five (!) summer Astro workshops on Microlensing, Dark Matter, and Galactic Structure (Michael Turner, Charles Alcock, and Katie Freese), Formation and Evolution of Extrasolar Planets and Brown Dwarfs (Adam Burrows and Dave De Young), Nonlinear Dynamics in Astrophysics and Geophysics (Robert Buchler, Robert Rosner, and Schmidt), Precision Measures of Large-scale Structure in the Universe (Josh Frieman, Alex Szalay, Martin White, and Douglas Scott), and Gamma Ray Bursters (Don Lamb, Michael Briggs, and Igor Mitrofanov). The Microlensing workshop synthesized the exciting results from MACHO, EROS, OGLE, AGAPE, and other projects. While MACHOs proved not to be the dominant component of dark matter in the Milky Way halo, the new microlensing detection of an extrasolar planet discussed at the workshop helped spur a new subfield of research, one that it is hoped will come to fruition with a space-based microlensing survey as part of the WFIRST mission. The ACP workshops have often been extremely timely. The workshop on extrasolar planets brought together theorists and observers in this exciting new field, then still in its relative infancy---the boom in extrasolar planet research had begun with the announcement of the discovery of an exoplanet around a sunlike star less than two years earlier, in Oct. 1995. The exit report of J. Frieman, one of the organizers of the Precision Measures workshop, noted somewhat presciently that this “workshop brought together over 40 astrophysicists to discuss the latest results in large-scale structure and the CMB. It also provided a forum for discussion of methods recently developed to optimally process and analyze the very large data sets that will inundate researchers over the next decade. The symbiosis between these two sub-communities of cosmology was a highlight of the meeting.” At this workshop astronomers made progress on planning the DEEP red-shift survey with Keck. Another highlight was the presentation and discussion of results by two groups of theorists (Albert Stebbins, Robert Caldwell, Scott Dodelson, Bruce Allen, Lloyd Knox, Paul Shellard and Andy Albrecht, Richard Battye, and James Robinson) who showed that the cosmic string alternative to inflation as the origin of structure was ruled out by the early CMB detections of small-scale anisotropies. On a personal note, I had the pleasure of skiing up and down Mt. Sopris with Dave Schramm that summer, just a few months before he passed away; the sudden onset of a thunderstorm while we were on the summit inspired me to ski down faster than Dave, the only time I ever managed to do so. I also recall being dragged up the snow route on Maroon Peak by Greg Anderson, Lance Dixon, and Jack Gunion.

The 1998 winter conferences were on Gravitational Waves and their Detection and Universal Star Formation. The summer session featured the German American Academic Council (GAAC) Young Scholars Institute and workshops on Physical Applications of Radio Pulsar Timing; Star Formation, Interstellar Medium, and Evolution of Galaxies; Neutrino Physics and Astrophysics: from Solar to Ultra-High Energy; and Weighing Galaxy Clusters. The GAAC meeting (organized by Michael Turner, Josh Frieman, Simon White, and Gerhard Boerner) brought together 31 US-based and German young researchers in cosmology (and was followed the next year by a meeting in Munich of the same researchers) to stimulate cross-Atlantic interactions and collaboration. As another example of the timeliness of the ACP meetings, the neutrino workshop followed by only 10 days the announcement of atmospheric neutrino oscillations by Super-Kamiokande, the first evidence for non-zero neutrino mass. About a quarter of the participants at this workshop were experimentalists.

In 1999, the winter conference was on Cosmological Implications of the Local Group. The summer workshops were on The Cosmological Constant (organized by Turner and Lawrence Krauss), Type Ia Supernovae as Distance Indicators in the Universe (Don Lamb and others, the first of three on this topic over the years), and X-ray Astrophysics. Following the announcement of the discovery of cosmic acceleration by two supernovae groups—the High-Z Supernova Team and the Supernova Cosmology Project---in early 1998 (work that was awarded the 2011 Nobel Prize), the ACP has become an important venue for the discussion of the implications of these results and of planning for future projects to extend them; the first two summer workshops above are prime, early examples of this. In true ACP fashion, the supernova workshop brought together theorists and observers to ponder these epochal results. The two groups involved in the discovery were both represented and had a chance to interact informally and intensely and to work toward cross-checking their results. The fact that the Cosmological Constant and Supernova workshops overlapped in time allowed for a provocative joint session of the two. The Supernova workshop included early discussion of the SNAP proposal for a space-based dark-energy mission, which helped lay the groundwork for JDEM and now WFIRST. These meetings helped cement the consensus in favor of the now-standard Lambda-Cold Dark Matter cosmology.

Members of the High-Z Supernova Team at the 1999 Astrophysics Workshop on Type Ia Supernovae as Distance Indicators in the Universe. Left to right: John Tonry, Saurabh Jha, Nick Suntzeff, Adam Riess, Bruno Leibundgut, Brian Schmidt, Alex Filippenko, Robert Kirshner, Mario Hamuy. Photo taken by HZT member Mark Phillips.

The title of the 2000 winter astrophysics meeting, Way Beyond the Standard Models of Particle Physics and Cosmology (lead organizer Rocky Kolb), drives home the point that by that time there was a well-established standard model of cosmology. The summer workshops that year were on Astrophysical Dynamos (Eric Blackman, Stirling Colgate, Philipp Kronberg, Hui Li, and Chris Thompson), Collimation of Astrophysical Flows (Dave De Young, Don Lamb, Reva Williams, and Craig Wheeler), and The Dark Side of the Universe (Lars Bergstrom, Marc Kamionkowski, Krauss, Pierre Sikivie, and Turner). The Dynamos and Collimation workshops met simultaneously and interacted fruitfully. As Craig Wheeler, one of the Collimation organizers, noted, ``Another Collimation participant wrote, “My all-too-brief stay at the Center for Physics has been a scientific rebirth and resurrection for me.” The Dark Side workshop featured the tension between the DAMA detection of an annual modulation signal and the strong upper limits of the CDMS experiment, an unresolved saga that continues a dozen years later.

2001 again featured two winter astro conferences, one on Galaxy Formation and Evolution (led by Garth Illingworth), the other on Advanced Detectors and Astrophysical Implications (Syd Meshkov). The summer included Science with Wide-Field Survey Telescopes, New Astrophysics and Cosmology with a Virtual Observatory (Frieman, De Young, and Piet Hut), Gamma-ray Bursts in the Afterglow Era, and Compact Objects in Dense Star Clusters (Andrea Ghez, Josh Grindlay, Vicky Kalogera, and Fred Rasio). The first two of the summer workshops demonstrated the early impact of the 2000 Decadal Survey of Astronomy and Astrophysics, which had recommended the GSMT (30-meter class telescope), LSST, and a National Virtual Observatory.

The 2002 winter conference was on Ultra-high Energy Particles from Space (Martens, Olinto), with excitement building about the Pierre Auger project then under construction. The summer workshops were on Structure Formation in the Era of Large Surveys (Scott Dodelson, Andrew Hamilton, Chung-Pei Ma, John Peacock, Josh Frieman), Compact Object Populations in External Galaxies (Pepi Fabbiano, Josh Grindlay, King), Astrophysical Disks (Buchler, Colgate, Li, Rosner, and Doug Lin), New Dimensions in String Cosmology (Robert Brandenberger, Jim Cline, Brian Greene, and Paul Steinhardt), and Underground Science (Wick Haxton, Janet Conrad, Paul Langacker, and Bernard Sadoulet). The Structure Formation workshop focused on early results from the SDSS and 2dF surveys. The Underground workshop focused on neutrino physics and dark-matter detection and the case for a national underground laboratory, an issue that still resonates.

2003 included winter conferences on The Baryonic Universe and Exploring the Widest Gravitational Wave Frequency Range. Summer workshops were The Nuclear Physics of Core Collapse Supernovae (Adam Burrows, Jim Truran, Hendrik Schatz, and Michael Wiescher), Cosmology and Astrophysics with Galaxy Clusters (Joanne Cohn, Joe Mohr, Gus Evrard, and Erica Ellingson), Magnetic Reconnection (Benjamin Chandran, Rosner, Amitava Bhattacharjee, and Richard Fitzpatrick), and The Microwave Background after MAP (Wayne Hu and Arthur Kosowsky). The coincident Cluster and CMB workshops brought together cosmologists and astrophysicists for fruitful interactions to discuss optical, X-ray, lensing, and SZ measurements. The CMB summer workshop was again extremely well-timed and topical, following on the first-year WMAP results in February 2003.

The 2004 winter conferences were Binary Radio Pulsars, The Large-scale Distribution of Mass and Light in the Universe (Frieman,and Advancing Gravitational Wave Detectors (another Syd Meshkov production). The present author recalls being cleverly bamboozled into organizing the second of those workshops by Jim Peebles but extracting his revenge by having Jim give a public lecture at the Wheeler Opera House. (I note that the Opera House is not named after the author of ``the Early Years” section of this astro history, though that author does date from the same era; however there is a possible family connection to another Wheeler in Aspen history, known for his thieving of horses.) The summer workshops were on The Formation of Supermassive Black Holes (Mitch Begelman, Tom Abel, Martin Haehnelt, Andrew Hamilton, Andrea Ghez), Star Formation in Galaxies (Chris McKee, Tim Heckman, Andreas Burkert, Guinevere Kauffmann), and Cosmic Acceleration: Dark Energy or New Gravitational Physics (Nemanja Kaloper, Wendy Freedman, Scott Thomas, Josh Frieman). The Star Formation workshop brought together experts in star formation and in extragalactic astronomy, both theorists and observers, to understand the evolution of stars and galaxies. Leaders of ground- and space-based projects with Spitzer, Galex, HST, and Keck met to plan multi-wavelength surveys of distant galaxies to determine stellar and galactic evolution. The Cosmic Acceleration workshop helped frame the major question about the origin of accelerated expansion, focusing on theoretical approaches and how to test them with future surveys, a major theme of research for the coming decade.

The 2005 winter conference was on Planet Formation and Detection. Summer workshops included Physics of the s-process (Falk Herwig, Wiescher, and Rene Reifarth), Revealing Black Holes (Wlodmerz Kluzniak, Michael Nowak, Mariano Mendez, and Lev Titarchuk), LISA Data: Analysis, Sources, and Science (Vicky Kalogera and Alberto Vecchio), Supercosmology (Paolo Gondolo, Anupam Mazumdar, and Tong), and Ultra-high energy cosmic rays (Graciela Gelmini, Alex Kusenko, and Angela Olinto). The two Black Hole workshops in 2004 and 2005 spawned new collaborations to attempt a more complete understanding of the formation and evolution of black holes, from the formation of the first stars to the evolution of supermassive black holes (SMBHs). A number of numerical codes were developed to follow stellar-mass black-hole formation starting from Pop III stars, accretion, and the merging of black holes into SMBHs. The 2005 Ultra-high energy cosmic rays workshop featured a confrontation between the largest UHECR experiments at the time, HiRes and AGASA, on the existence of the GZK cutoff (the effect of interactions between extragalactic cosmic rays and the cosmic background radiation, named after Greisen, Zatsepin, and Kuzmin). The debate included an organized snowball fight at the ACP premises. This long dispute was solved in 2008 by more sensitive measurements of the HiRes and the Pierre Auger Observatories.

The 2006 winter conferences featured Cosmological Probes of Baryons and Dark Matter (Renan Barkana and Zoltan Haiman) and Local Group Cosmology (Joss Bland-Hawthorn). Summer workshops were on Cosmic Voids (Fiona Hoyle, Jim Peebles, Ravi Sheth, Rien van de Weygaert, and Michael Vogeley), Galaxy Evolution from Large Surveys (Mauro Giavalisco, Nick Scoville, Casey Papovich, Daniela Calzetti), Deconstructing the Local Group (James Bullock, Annette Ferguson, Kathryn Johnston, Andrey Kravtsov, Mary Putnam, Kim Venn, Yogesh Joglekar), and Magnetic Self-organization in Laboratory and Astrophysical Plasmas (Fausto Cattaneo, Steven Cowley). The Voids and Local Group workshops highlighted and discussed the formation of the rich structures that were being uncovered by surveys such as the SDSS, from the cosmic web of large-scale structure to very faint dwarf galaxies and tidal streams in the nearby Universe. The Magnetic Self-Organization workshop directly connected the astrophysics and plasma physics communities.

The 2007 winter meetings focused on Neutrinos in Physics and Astrophysics (Alex Kusenko, George Fuller), Clusters of Galaxies as Cosmological Probes (Tim McKay, William Forman),and Supernova 1987A: 20 Years After (Kurt Weiler, Stefan Immler, Dick McCray). The summer astro workshops were Implications of Swift’s Discoveries about GRBs (Don Lamb, Neil Gehrels, Anthony Piro), Modeling Galaxy Clustering (Andreas Berlind, Joanne Cohn, Martin White, Idit Zehavi), and Supernovae as Cosmological Distance Indicators (Wendy Freedman, Alexei Khokhlov, Josh Frieman). Swift is a NASA explorer mission that has observed hundreds of GRBs in X-rays and the ultraviolet. A search of the arXiv turns up 108 papers with ‘Swift’ and ‘GRB’ in the title, so it is clear that it has had an immense impact on the study and understanding of GRBs. Astrophysics workshops that are coincident with workshops in related fields often lead to new and surprising interactions. The workshop on Implications of Swift’s Discoveries about Gamma-Ray Bursts overlapped with the workshop Neutrino Physics: Looking Forward, and the organizers arranged a joint session to discuss neutrino emission from GRBs. The Galaxy Clustering workshop again highlighted the impact of the SDSS, bringing together insights from data and N-body simulations to constrain models of how galaxies occupy dark matter halos. The Supernova workshop continued the tradition of the 1999 Supernova cosmology workshop, bringing together different groups of observers and supernova theorists to discuss how to refine supernovae as cosmological probes; this workshop was timely, coming on the heels of early results from the first generation of large, post-acceleration-discovery supernova surveys such as the Supernova Legacy Survey, ESSENCE, the Carnegie Supernova Survey, the Nearby Supernova Factory, and the SDSS-II Supernova Survey.

In 2008, there were winter workshops on Cosmic Microwave Radiation (Eichiro Komatsu, Kris Gorski, Marc Kamionkowski) and The First Two Billion Years of Galaxy Formation (Guinevere Kauffmann, Rosie Wyse, Garth Illingworth), which highlighted the insights into the high-redshift Universe coming from large ground-based telescopes and HST. The summer workshops were Active Galactic Nuclei (Mara Salvato, Kauffmann, Crystal Martin), Characteristics and Habitability of Super Earths (Sara Seager, Fred Rasio, Eric Ford), and Gravitational Wave Astronomy (Kristen Menou, Graham Woan, Matthew Benacquista, Lee S Finn). The Super Earths workshop captured the excitement of the discovery of exoplanets several times more massive than the Earth in the habitable zones close to low-mass stars and the growing understanding of their atmospheres. Participants in the Gravitational Wave Astronomy workshop and in the condensed matter Complexity, Disorder, and Algorithms workshop arranged a joint session entitled Projective Dynamics: A possible algorithm for reducing the dimensionality in dynamics for protein folding, nano-scale magnets, and astrophysics. This kind of interdisciplinary activity is a hallmark of the unique, informal atmosphere of the ACP.

The 2009 winter workshops covered Understanding the Dark Sector: Dark Matter and Dark Energy (Marc Kamionkowski, Mark Trodden, Rachel Bean) and Thirty Years of Magnetars (Silvia Zane, Roberto Turolla, Gian Luca Israel). The summer workshops discussed Fingerprints of the Early Universe (Richard Easther, Hiranya Peiris, Liam McAllister, Will Kinney), Wide-Fast-Deep Surveys (Tony Tyson, Zeljko Ivezic, Josh Frieman, Michael Strauss), Testing General Relativity in the Cosmos (Rachel Bean, Wayne Hu, Bhuvnesh Jain), and Neutrino Physics on Earth, in Stars and in the Cosmos (Andre de Gouvea, Alex Friedland, Irina Mocioiu). The General Relativity workshop brought together astrophysicists and those working in experimental gravitation. The Wide-Fast-Deep and General Relativity workshops overlapped and held some productive joint sessions, as planned large surveys will be able to test modifications of General Relativity by probing the evolution of large-scale structure and the history of the cosmic expansion rate. Fingerprints of the Early Universe, organized by four young cosmologists, brought cosmology and particle physics theorists together to explore observational and experimental tests of the beyond-the-standard-model physics that governs the earliest moments of the Big Bang.

In 2010, the winter astro conferences were on Formation and Evolution of Black Holes (Fred Rasio, Vicky Kalogera, Stein Sigurdsson) and The High-Redshift Universe (Renan Barkana, Chris Carilli, Judd Bowman). The summer workshops included GeV and TeV Sources in the Milky Way (Stefan Funk, Roger Romani, Alice Harding, Elizabeth Hays), Astrophysics and Cosmology with the 21 cm Background (Judd Bowman, Asantha Cooray, Steven Fulanetto, Peng Oh), Taking Supernova Cosmology into the Next Decade (Mark Phillips, Nick Sunzteff, Michael Wood-Vasey, Isobel Hook, Greg Aldering), which as noted above can be considered the third in a series on this topic, and Star Formation in Galaxies (Elizabeth Tasker, Nick Gnedin, Mark Krumholz). The GeV and TeVSources workshop featured new results from Fermi and other high-energy astrophysics missions. The 21cm workshop explored the potential for near- and far-future radio facilities to constrain the epoch of cosmic reionization and the dark ages as well as the intermediate-redshift Universe and cosmology.

The 2011 winter workshop overviewed the latest results on Direct and Indirect Detection of Dark Matter (Matthew Buckley, Stefano Profumo, Simona Murgia). The summer workshops were Galaxy and Central Black Hole Co-evolution (Matthew Benacquista, Lee S Finn, J. Kelly Holley-Bockelman, Graham Woan), Stellar and Intermediate Mass Black Holes (Vicky Kalogera, Steinn Sigurdsson, Marta Volonteri, Fred Rasio), A Theoretical and Experimental Vision of Direct and Indirect Dark Matter Detection (Stefano Profumo, Louis Strigari, Devin Walker), and The Galactic Bulge and Bar (Ken Freeman, Jennifer Johnson, Dante Minniti, Michael Rich, Sofia Feltzing).

The astrophysics workshops have fostered a number of important advances that have set the course for new avenues of research. Recent examples of influential papers based on interactions at the ACP include: Luigi Guzzo and collaborators on the use of red-shift distortions as a new probe of cosmic acceleration (published in Nature), which is now being pursued by proposed ground- and space-based dark energy missions; Guinevere Kauffmann and collaborators on interaction-induced star formation and AGN activity; Ben Koester and collaborators, who put together the largest catalog of galaxy clusters, using the SDSS survey, which has been the basis of many subsequent studies of clusters and cosmology; Andy Howell and collaborators on correlations of progenitor age and metallicity with SN Ia luminosity and Nickel yield, which helped pave the way to the recent observation of SN luminosity-host galaxy correlations; and Julia Comerford and collaborators on inspiralling supermassive black holes as a new signpost for galaxy mergers.

As I write this in early February 2012, 60 cosmologists – from specialists in particle physics theory to those focused on observational astronomy – are in Aspen for a workshop on Cosmic Inflation. A telling indicator of the continued vibrancy of the ACP is the fact that only a handful of us at this workshop participated in ACP workshops before the modern era: the vast majority are young postdocs who are blazing new pathways in inflationary research, a field that the ACP helped nurture 30 years ago. The continuing excitement of astrophysics at the ACP is evidenced by the fact that there are three astrophysics winter conferences this year.