Happy Birthday to Vera Rubin, Discoverer of Dark Matter

23 July 201531 October 2015 / raminskibba / 1 Comment

Peering through their powerful telescopes, scientists observe a stunningly diverse array of phenomena, including comets, planets, stars, gaseous nebulae, novae, quasars, galaxies, and numerous other exciting things. But astrophysicists argue that these light-emitting objects only amount to a tiny fraction of the universe. According to the latest measurements from the European Space Agency’s Planck telescope earlier this year, they account for less than 5% of the universe’s matter and energy, while mysterious-sounding “dark matter” accounts for nearly six times as much. Nevertheless, dark matter cannot be seen and does not interact with normal matter, so how did astronomers figure out that so much invisible, intangible stuff exists out there?

Vera Rubin measuring spectra, circa 1970. (Credit: American Institute of Physics)

As I recently wrote in a post for the International Year of Light, the story of scientists’ discovery and exploration of dark matter began many decades ago. Physicists had long utilized Newton’s and Einstein’s gravitational laws to estimate our sun’s mass by measuring planets’ distances from it and examining how fast they travel around it. For example, Mercury is very close to the sun and orbits it much faster than Pluto, which takes 248 Earth-years to complete an orbit. (If you’re wondering, the sun has a mass larger than a trillion billion billion kilograms. That’s a lot!) Similarly, it turns out that one can make such calculations for stars within galaxies and infer the enclosed mass, but the results of the analysis are not so simple to understand.

Detailed image of the Andromeda Galaxy, recently surveyed by the Panchromatic Hubble Andromeda Treasury . (Credit: NASA, ESA, J. Dalcanton et al.)

In the 1960s and 1970s, American astronomer Vera Rubin measured and analyzed the precise velocities of stars in spiral galaxies and came to a startling conclusion. Most stars at outer radii orbit the center at surprisingly large speeds, much faster than they should be based on the mass of the stars themselves, but the galaxies do not tear themselves apart or fling their stars hurtling away. Studying galaxies, such as Andromeda, as a whole, she found that they rotate too quickly for their stars’ gravity to keep them intact. It was as if the galaxies contain and are surrounded by much more unseen dark matter, which gravitationally binds the galaxies together. Rubin’s crucial discovery has not yet received the recognition it deserves.

This critically important area of research came to be known as galaxy “rotation curves,” in which Rubin became an influential figure. Rotation curve measurements of spiral galaxies from two of her many highly-cited publications appear in the reference, Galactic Astronomy, which every respectable astrophysicist has on their bookshelf. Her measurements from hundreds of galaxies constitute strong evidence for the existence of massive clumps of dark matter extending to many thousands of light-years beyond the edge of the galaxies themselves. Astrophysicists also considered the alternative hypothesis that Newton’s gravitational laws need to be modified for objects separated by large distances, but that approach has been less successful and lacks support among the community.

Rotation curves of three spiral galaxies of varying brightness, adapted from an influential 1985 paper by Rubin. (Credit: Binney & Merrifield, “Galactic Astronomy,” Princeton, 1998.)

Vera Rubin turns 87 years old today. She continues her work at the Department of Terrestrial Magnetism at the Carnegie Institution of Washington, and she still publishes research in galactic astronomy. In addition, she writes popularly such as in Scientific American and Physics Today. Moreover, she inspires, supports, and encourages young people, especially women, in science. This includes her four children, all of whom have earned Ph.D. degrees in the natural sciences or mathematics.

In 1965, Vera Rubin was the first woman permitted to observe at Palomar Observatory. When she applied to graduate schools, she was told that “Princeton does not accept women” in the astronomy program; she went to Cornell instead. As she put it in a recent astronomical memoir, “Women generally required more luck and perseverance than men did.” In her 1996 book, Bright Galaxies, Dark Matters, she wrote

Since the 1950s, opportunities for women in astronomy have increased, but serious problems have not disappeared…The saddest part, of course, is that only about one-fifth of the women who enter college intend to study science. Lack of support and encouragement at an early age has by then taken its toll. A young woman who enters graduate school to study science is a rare creature indeed…but the colleges are often a part of the problem rather than part of the solution.

Now with many years of hard work and persistence, people are making gradual progress. For example, Meg Urry leads the American Astronomical Society, France Córdova is the director of the National Science Foundation, and Marcia McNutt now heads the National Academy of sciences. But much more work needs to be done to reduce gender inequality and underrepresentation throughout science research and education.

Many people argue that Vera Rubin would be a strong contender for a Nobel Prize in Physics, and I join that call. She has already won many other awards, including the National Medal of Science, but the Nobel would officially recognize her enormous contributions to astrophysics and her critical role in illuminating the way to dark matter. Considering that the 2011 Nobel Prize went to Saul Perlmutter, Brian Schmidt, and Adam Riess for discovering dark energy, it’s time for dark matter to have its day.

New Science at the American Astronomical Society Meeting

4 February 20156 March 2015 / raminskibba / Leave a comment

I’d just like to summarize some of the exciting new scientific results presented at the American Astronomical Society meeting in Seattle last month. I think it will be interesting to those of you science lovers who’re wondering what all the hubbub was about and for you astronomers who weren’t able to make it.

This is my third and final post in a series about the AAS meeting. The first two dealt with science policy, and diversity and sustainability. As I mentioned in a previous post, I enjoyed attending as both a scientist and science writer, and I was happy to personally meet the journalists writing excellent stories about the meeting (some of which I’ve linked to below).

I’ll start with some special sessions and other sessions focused on interesting science that included results I hadn’t seen before, and then I’ll end with some interesting plenary talks given by great speakers. It was a busy meeting and many of the sessions ran in parallel, so it’s inevitable that I missed some things and that this summary is incomplete. (Plus, I’m usually drawn to the sessions about galaxies, dark matter, and cosmology, and I often miss the other ones.) If you know of interesting announcements or talks that I missed here, you’re welcome to comment on them below.

Sloan Digital Sky Survey (SDSS)

After 15 years of great science, it was exciting to see the SDSS have its final public data release—until SDSS-IV data eventually come out, that is. At the press conference, Michael Wood-Vasey gave an overview, Constance Rockosi spoke about the data release, Daniel Eisenstein spoke about the Baryon Oscillation Spectroscopic Survey (BOSS), Jian Ge spoke about the Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS), and Steven Majewski spoke about the APO Galactic Evolution Experiment (APOGEE). According to Rockosi, more than 6,000 papers have been published using publicly released SDSS data. The SDSS has observed tens of thousands of stars, hundreds of thousands of quasars, and millions of galaxies.

In addition, members of the BOSS collaboration presented (nearly) final results at a session dedicated to the survey. If you’re interested, check out this article I wrote about it for Universe Today. (Thanks to Nancy Atkinson for editing assistance.)

Distribution of galaxies in a slice of the BOSS survey. (Courtesy: SDSS-III)

3D-HST

Researchers presented newly published results and interesting work-in-progress about the evolution of distant galaxies using spectroscopic data from the 3D-HST survey, which is led by Pieter van Dokkum (Yale Univ.) and Ivelina Momcheva (Carnegie Observatories), combined with imaging data from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), taking advantage of instruments aboard the Hubble Space Telescope. The figure below shows the spectral features of tens of thousands of galaxies, which indicate star formation activity, active galactic nuclei activity, and stellar age. If you’re interested, I wrote an article about some of these results for Sky & Telescope. (Thanks to Monica Young for editing assistance.)

Spectral features of high-redshift galaxies. (Courtesy: Gabriel Brammer, 3D-HST)

Andromeda Galaxy

In a session dedicated to Andromeda—known as M31 by astronomers—as well as in other related sessions, research scientists and Ph.D. students presented studies about the stars, globular clusters, molecular clouds, dust, structure, dynamics, surface brightness profile, and stellar halo of the galaxy. The continued interest in our fascinating neighbor is understandable; Andromeda’s only 2.5 million light-years away from our galaxy! Like our Milky Way, Andromeda is a spiral galaxy, and it’s the most massive galaxy in the Local Group.

Many of these AAS results came from the Panchromatic Hubble Andromeda Treasury (PHAT) Survey, which is led by Julianne Dalcanton (Univ. of Washington), who presented highlights in a press conference as well. Dalcanton and her colleagues released this PHAT panoramic image of the galaxy below, and it received well-deserved press attention, including in NBC and Sky & Telescope.

Map of Andromeda galaxy. (Courtesy: HST, PHAT)

Exoplanets

Many people were understandably excited about extra-solar planets, or exoplanets, detected by scientists with NASA’s Kepler space telescope. Every day of the meeting included talks and posters about the masses, abundances, dynamics, compositions and other properties of exoplanets as well as those of stars and supernova remnants examined with Kepler. In addition, astronomers’ announcement that they now have more than 1,000 confirmed exoplanets with Kepler and follow-up observations garnered considerable media attention (including these articles in Nature, BBC, and New York Times). They have at least 3,000 more planet candidates, and they will surely identify many more as Kepler continues its mission through 2016.

Of course, astronomers seek to find as many as possible Earth-like planets in or near the habitable regions orbiting Sun-like stars (often referred to as the “Goldilocks” zone). When these are successfully identified, the next step is to characterize their properties and try to assess the likelihood of life forming on them. Astronomers have found at least eight Earth-size planets in the habitable zone, including two of the newly announced ones, Kepler-438b and Kepler-442b. They also released these cool old-school travel posters. If you have a space ship that can travel 500 light-years a reasonable time, you should check out 186f on your next vacation!

Kepler’s alien planet travel posters. (Courtesy: NASA)

“Pillars of Creation”

On the 25th anniversary of the launch of the Hubble Space Telescope, astronomers released new images of the iconic star-forming region in the Eagle Nebula in the Serpens Cauda constellation, known as the “pillars of creation.” Journalists at Slate, CBS, and elsewhere shared these amazing images. At first I thought not much science was done with them, but by combining observations at visible and infrared wavelengths, astronomers can investigate what’s happening with the cold gas clouds and dust grains and assess how rapidly new stars are forming and where. For more, you can also see Hubble’s press release, which coincided with the press conference on the first day of the meeting.

Image of “pillars of creation.” (Courtesy: NASA and ESA)

Other Results

I saw many other interesting talks and posters at the meeting, but I don’t have the time/space to get into them here. On galaxies and the large-scale structure of the universe (which I’m interested in), I saw talks involving modeling and measurements with the Galaxy And Mass Assembly (GAMA) survey, the Six-degree Field Galaxy Survey (6dF), and I presented research using the PRIsm MUlti-object Survey (PRIMUS). But the SDSS dominated the field.

In addition, Joss Bland-Hawthorn, Sarah Martell, and Dan Zucker presented some impressive early science results from the GALactic Archaeology with HERMES (GALAH) survey of the Milky Way, which uses an instrument with the Anglo-Australian Telescope. (GALAH is named after an Australian bird.) Astronomers combine GALAH observations with astrometry from Gaia and over the survey’s duration will produce detailed data for 1 million stars in our galaxy! In particular, they utilize a technique called “chemical tagging” to study the abundances of at least 15 chemical elements for each star, allowing for studies of stellar dynamics and merger events from infalling “satellite” galaxies. I look forward to seeing more results as they continue to take data and analyze them; their first public data release is planned for 2016.

PLENARY SESSIONS

I’ll briefly describe a couple of the plenary talks below, but I missed a few others that sounded like they could be interesting, including “The Discovery of High Energy Astrophysical Neutrinos” (Kara Hoffman); “Gaia – ESA’s Galactic Census Mission” (Gerry Gilmore); and “The Interactions of Exoplanets with their Parent Stars” (Katja Poppenhaeger).

Also, Paul Weissman (JPL/Caltech) gave an overview of the Rosetta mission and the comet C-G/67P, and Al Wootten (NRAO) gave an overview of many recent science papers using the Atacama Large Millimeter Array (ALMA). Rosetta and its lander Philae has run a few experiments already, and scientists with the mission have found that the bulk density of the nucleus is less than half the density of water ice and that its D/H ratio is different than the abundance ratio of the Earth’s oceans. More recently, Rosetta detected a crack in the “neck” of the comet, and they’ve abandoned an idea for a close flyby search for the lost lander, which might wake up in a few months when it receives more solar power. And if you’re interested in ALMA science, such as involving the gas kinematics of protostars and protoplanetary disks and the gas and dust clouds of distant galaxies, watch for proceedings from their recent Tokyo meeting, which are due to be published next month.

Cosmology Results from Planck

Martin White (UC Berkeley) gave an excellent talk about cosmological results from the Planck telescope, which he described as having the “weight of a heavy hippo and the height of a small giraffe.” Based on analyses of the power spectrum of the cosmic microwave background (CMB) radiation, so far it seems that the standard model of cold dark matter plus a cosmological constant (ΛCDM) is still a very good fit. Scientists in the collaboration are obtaining tighter constraints than before, and the universe still appears very flat (no curvature). They are planning a second data release this year, including more simulations to assess systematic uncertainties and more precise gravitational lensing measurements. White ended by saying, “I can explain to you what really well, but I can’t tell you why at all.”

White also hinted at, but didn’t reveal anything about, the joint analysis by Planck and BICEP2 astrophysicists. That analysis was completed recently, and now it seems that the detected polarization signal might be at best a mixture of primordial gravitational waves produced by inflation and of Milky Way dust, and they’ve obtained only an upper limit on the tensor-to-scalar ratio. Check out my recent article in Universe Today about this controversy.

Courtesy: ESA

Inflation and Parallel Universes

Max Tegmark (MIT) has talked and written about both inflation and the multiverse for many years, such as in a 2003 cover article and a recent blog post for Scientific American and in his book, “Our Mathematical Universe.” From the way he presented the talk, it was clear that he has discussed and debated these issues many times before.

Tegmark began by explaining models of inflation. According to inflation, the universe expanded for a brief period at an exponential rate 10^-36 seconds after the Big Bang, and the theory could explain why the universe appears to have no overall curvature, why it approximately appears the same in all directions, and why it has structures of galaxies in it. In one entertaining slide, he even compared the expansion rate of a universe to that of a fetus and baby, but then he said, “if the baby kept expanding at that rate, you’d have a very unhappy mommy.”

Expansion rates of a baby (human) and a baby universe

He subtitled his talk, “Science or Science Fiction?”, and that question certainly came up. Tegmark argued that inflation seems to imply at least some levels of a multiverse (see his slide below), which makes many astrophysicists (including me) nervous and skeptical, partly because parallel universes aren’t exactly testable predictions. But he made the point that some general relativity predictions, such as about what happens in the center of a black hole, aren’t testable yet we accept that theory today. He discussed “modus ponens” arguments: once we accept “if p then q,” then if p is asserted, we must accept q, whether we like it or not. In other words, if inflation generally predicts parallel universes and if we accept inflationary theory, then we must accept its implications about parallel universes. This is an important issue, and it’s another reason why BICEP2 and Planck scientists are trying to resolve the controversy about polarization in the CMB.

Predictions of different levels of the multiverse.

The Dark and Light Side of Galaxy Formation

Finally, in another interesting talk, Piero Madau (UC Santa Cruz), who was recently awarded the Heineman Prize for Astrophysics, spoke about galaxy formation and dark matter. In particular, he spoke about difficulties and problems astrophysicists have encountered while attempting to model and simulate galaxies forming while assuming a cold dark matter (CDM) universe. For example, he described: the cusp-core controversy about the inner profiles of dark matter clumps and galaxy groups; the problem of angular momentum, which is conserved by dark matter but not gas and stars; the missing satellites problem, in which more simulated dark matter subclumps (“subhaloes”) than observed satellite galaxies are found; and the “too-big-to-fail” problem, such that simulated subhaloes are much more dense than the galaxies we see around the Milky Way. These problems motivated astrophysicists to rethink assumptions about how galaxies form and to consider warm or self-interacting dark matter.

Madau ended by saying that evidence that the universe conforms to expectations of the CDM model is “compelling but not definitive,” and warm dark matter remains a possibility. Considering all of the exciting work being done in this field, this could be “the DM decade”…but then he said people have been talking of a DM decade for the past thirty years.

Astrophysicists Gather in Aspen to Study the Galaxy-Dark Matter Connection

8 September 20142 February 2015 / raminskibba / Leave a comment

I just returned from a summer workshop at the Aspen Center for Physics, and I enjoyed it quite a bit! The official title of our workshop is “The Galaxy-Halo Connection Across Cosmic Time.” It was organized by Risa Wechsler (Stanford) and Frank van den Bosch (Yale) and others who unfortunately weren’t able to attend (Andreas Berlind, Jeremy Tinker, and Andrew Zentner). The workshop itself was very well attended by researchers and faculty from a geographically diverse range of institutions, but since it was relatively late in the summer, a few people couldn’t come because of teaching duties.

Since I grew up in Colorado, I have to add that Aspen is fine and I understand why it’s popular, but there are many beautiful mountain towns in the Colorado Rockies. Visitors and businesses should spread the love to other places too, like Glenwood Springs, Durango, Leadville, Estes Park, etc… In any case, when we had time off, it was fun to go hiking and biking in the area. For example, I took the following photo after hiking to the top of Electric Peak (elev. 13635 ft., 4155 m), and lower down I’ve included photos of Lost Man Lake (near the continental divide) and the iconic Maroon Bells.

The Aspen Center for Physics (ACP) is a great place for working and collaborating with colleagues. As they say on their website, “Set in a friendly, small town of inspiring landscapes, the Center is conducive to deep thinking with few distractions, rules or demands.” As usual, we had a very flexible schedule that allowed for plenty of conversations and discussions outdoors or in our temporary offices. Weather permitting, we had lunch and some meetings outside, and we had many social events too, including lemonade and cookies on Mondays and weekly barbecues. It’s also family-friendly, and many physicists brought their spouses and kids to Aspen too. I’ve attended one ACP summer workshop on a similar theme (“Modeling Galaxy Clustering”) in June 2007, and it too was both fun and productive. Note that the ACP workshop is very different than the Madrid workshop I attended earlier this summer, which had specific goals we were working toward (and I’ll give an update about it later).

This year’s Aspen workshop connected important research on the large-scale structure of the universe, the physics of dark matter halo assembly, the formation and evolution of galaxies, and cosmology. We had informal discussions about the masses and boundaries of dark matter haloes in simulations, ways to quantify the abundances and statistics of galaxies we observe with telescopes and surveys, and how to construct improved models that accurately associate particular classes of galaxies with particular regions of the “cosmic web”—see this Bolshoi simulation image, for example, and the following slice from a galaxy catalog of the Sloan Digital Sky Survey:

While some of these issues have plagued us for years and remain unresolved, there are some subtle issues that have cropped up more recently. We (including me) have successfully modeled the spatial distribution of galaxies in the “local” universe, but now we are trying to distinguish between seemingly inconsistent but similarly successful models. For example, we know that the distribution of dark matter haloes in numerical simulations depends on the mass of the haloes—bigger and more massive systems tend to form in denser environments—as well as on their assembly history (such as their formation time), but these correlations can be quantified in different ways and it’s not clear whether there is a preferred way to associate galaxies with haloes as a function of these properties. For the galaxies themselves, we want to understand why some of them have particular brightnesses, colors, masses, gas contents, star formation rates, and structures and whether they can be explained with particular kinds of dark matter halo models.

The main purpose of these workshops is to facilitate collaborations and inspire new ideas about (astro)physical issues, and it looks like we accomplished that. The previous workshop I attended helped me to finish a paper on analyzing the observed spatial distribution of red and blue galaxies with dark matter halo models (arXiv:0805.0310), and I’m sure that my current projects are already benefiting from this summer’s workshop. We seem to be gradually learning more about the relations between galaxy formation and dark matter, and my colleagues and I will have new questions to ask the next time we return to the Rockies.

Finally, here are those Maroon Bells you’ve been waiting for:

Exploring the “Multiverse” and the Origin of Life

13 August 20142 February 2015 / raminskibba / 2 Comments

After two weeks away from the blog, I’m back! At the end of July, I attended an interesting event at UC San Diego’s Arthur C. Clarke Center for Human Imagination. (Yes, that’s what it’s called!) The event was a panel discussion entitled, “How Big is the World?: Exploring the Multiverse in Modern Astrophysics, Cosmology, and Beyond” (and you can watch the event here). The three speakers included Andrew Friedman (postdoctoral fellow in astronomy at MIT), Brian Keating (professor of physics in my department at UCSD), and David Brin (Hugo & Nebula Award Winning Author).

The Clarke Center seems to be a unique place with an ambitious program that incorporates a variety of “transdisciplinary” activities. This event fits with their nebulous theme, and the talks and discussions frequently overlapped between science, philosophy of science, and science fiction. I think science and philosophy of science go well together especially when we’re exploring the edges of scientific knowledge, including cosmological astrophysics and the origins of human life. (See my previous post and this recent article on Salon.) Too often astrophysicists, myself included, become very specialized and neglect the “big questions.” Nonetheless, I think we should be careful when we traverse the border between science and science fiction: while it’s exciting to connect them and useful for public outreach, we should mind the gap.

Andrew Friedman focused on the “multiverse”. What is a multiverse, you ask? I’m not entirely clear on it myself, but I’ll try to explain. In the first fraction of a second of the Big Bag, the universe appears to have gone through a phase of accelerated, exponential expansion (called “inflation”) driven by the vacuum energy of one or more quantum fields. The gravitational waves that were recently detected by BICEP2 (in which Brian Keating was involved) appear to support particular inflationary models in which once inflation starts, the process happens repeatedly and in multiple ways. In other words, there may be not one but many universes, including parallel universes—a popular topic in science fiction.

Inflationary theory solves some problems involving the initial conditions of the Big Bang cosmology, but I’m not so sure that we have—or can ever have—evidence clearly pointing to the existence of multiverses. In addition, in my opinion, Friedman stretched the concept of “universe” to try to argue for the multiverse. He spoke about the fact that there are parts of the universe that are completely inaccessible even if we could go the speed of light, but that doesn’t mean that the inaccessible regions are another universe. It’s fun to think about a “quantum divergence of worlds,” as David Brin referred to it, but quantum mechanics (with the standard Copenhagen interpretation; see this book by Notre Dame professor Jim Cushing) don’t imply a multiverse either: Schrödinger’s live cat and dead cat are not in separate universes. As far as I know, I’m not creating new universes every time I barely miss or catch the train.

The speakers did bring up some interesting questions though about the “anthropic principle” and “fine tuning.” The anthropic principle is a contentious topic that has attracted wide interest and criticism, and if you’re interested, read this review of the literature by Pittsburgh professor John Earman. The anthropic principle is the idea that the physical universe we observe must be compatible with conscious life. It’s a cosmic coincidence that the density of vacuum energy and matter are nearly equal and that the universe’s expansion rate is nearly equal to the critical rate which separates eternal expansion from recontraction, and if the universe were significantly different, it would be impossible to develop conscious life such as humans who can contemplate their own universe. (In the context of the multiverse, there may be numerous universes but only a tiny fraction of them could support life.) It’s important to study the various coincidences and (im)probabilities in physics and cosmology in our universe, but it’s not clear what these considerations explain.

David Brin spoke differently than the others, since he’s more a writer than a scientist, and his part of the discussion was always interesting. He frequently made interesting connections to fiction (such as a legitimate criticism of Walt Whitman’s “Learn’d Astronomer“) and he had a poetic way of speaking; when talking about the possibility of life beyond Earth, he said “If there are living creatures on Titan, they will be made of wax.” He also brought up the “Drake equation,” which is relevant in the context of the topics above. The Drake equation is a probabilistic expression for estimating the number of active, communicating civilizations in our galaxy. It involves a multiplication of many highly uncertain quantities (see this xkcd comic), but it’s nonetheless interesting to think about. The problem is that space is really big—”vastly, hugely, mindbogglingly big,” according to Douglas Adams—so even if there are Vulcans or Klingons or dozens or millions of other civilizations out there, it would take a really really really long time to find them and attempt to communicate with them. We could send people from Earth in a long shuttle ride to visit another civilization, but there’s no guarantee that humanity will still be around when they try to call back. It’s unfortunate, but this is the universe we live in.

Comparing Models of Dark Matter and Galaxy Formation

14 July 20142 February 2015 / raminskibba / 2 Comments

I just got back from the “nIFTy” Cosmology workshop, which took place at the IFT (Instituto de Física Teórica) of the Universidad Autonoma de Madrid. It was organized primarily by Alexander Knebe, Frazer Pearce, Gustavo Yepes, and Francisco Prada. As usual, it was a very international workshop, which could’ve been interesting in the context of the World Cup, except that most of the participants’ teams had already been eliminated before the workshop began! In spite of Spain’s early exit, the stadium of Real Madrid (which I visited on a day of sightseeing) was nonetheless a popular tourist spot. I also visited the Prado museum, which had an interesting painting by Rubens involving the Milky Way.

This was one of a series of workshops and comparison projects, and I was involved in some of the previous ones as well. For example, following a conference in 2009, some colleagues and I compared measures of galaxy environment—which are supposed to quantify to what extent galaxy properties are affected by whether they’re in clustered or less dense regions—using a galaxy catalog produced by my model. (The overview paper is here.) I also participated in a project comparing the clustering properties of dark matter substructures identified with different methods (here is the paper). Then last year, colleagues and I participated in a workshop in Nottingham, in which we modeled galaxy cluster catalogs that were then analyzed by different methods for estimating masses, richnesses and membership in these clusters. (See this paper for details.)

This time, we had an ambitious three week workshop in which each week’s program is sort of related to the other weeks. During the first week, we compared codes of different hydrodynamical simulations, including the code used by the popular Illustris simulation, while focusing on simulated galaxy clusters. In week #2, we compared a variety of models of galaxy formation as well as models of the spatial distributions and dynamics of dark matter haloes. Then in week #3, we’re continuing the work from that Nottingham workshop I mentioned above. (All of these topics are also related to those of the conference in Xi’an that I attended a couple months ago, and a couple other attendees were here as well.)

The motivation of these workshops and comparison workshops is to compare popular models, simulations, and observational methods in order to better understand our points of agreement and disagreement and to investigate our systematic uncertainties and assumptions that are often ignored or not taken sufficiently seriously. (This is also relevant to my posts on scientific consensus and so-called paradigm shifts.)

Last week, I would say that we had surprisingly strong disagreement and interesting debates about dark matter halo masses, which are the primary drivers of environmental effects on galaxies; about the treatment of tidally stripped substructures and ‘orphan’ satellite galaxies in models; and various assumptions about ‘merger trees’ (see also this previous workshop.) These debates highlight the importance of such comparisons: they’re very useful for the scientific community and for science in general. I’ve found that the scatter among different models and methods often turns out to be far larger than assumed, with important implications. For example, before we can learn about how a galaxy’s environment affects its evolution, we need to figure out how to properly characterize its environment, but it turns out that this is difficult to do precisely. Before we can learn about the physical mechanisms involved in galaxy formation, we need to better understand how accurate our models’ assumptions might be, especially assumptions about how galaxy formation processes are associated with evolving dark matter haloes. Considering the many systematic uncertainties involved, it seems that these models can’t be used reliably for “precision cosmology” either.