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.

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.

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.

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.

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 et.al.),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.

Physicists playing in the snow.

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.

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