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, portrait by Bernice Durand

David Schramm, portrait by Bernice Durand

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.)

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 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.

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.

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. The 1993 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.

This post was written in celebration of ACP’s 50th Anniversary