Friday, March 14, 2014
Avery Dining Hall – Avery House

Workshop: Comedy as a Teaching Tool

Caltech's "Secrets" to Success

Everyone who really knows Caltech understands that it is unique among universities around the world. But just what makes Caltech so special? We've asked that question before, and the numbers don't tell the full story. So, is it our focus? Our culture? Our people?

The UK's Times Higher Education magazine recently tackled the topic, asking more specifically, "How does a tiny institution create such an outsized impact?" Caltech faculty share their perspectives in the cover story of the magazine's latest issue.

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Monday, May 5, 2014

Teaching Statement Workshop - 2-Part Event

Monday, May 12, 2014
Center for Student Services 360 (Workshop Space) – Center for Student Services

Teaching Statement Workshop - 2-Part Event

Friday, April 4, 2014
Center for Student Services 360 (Workshop Space) – Center for Student Services

Spring TA Training

Tuesday, April 1, 2014
Center for Student Services 360 (Workshop Space) – Center for Student Services

Spring Head TA Lunch

Galaxies on FIRE: Star Feedback Results in Less Massive Galaxies

For decades, astrophysicists have encountered a puzzling contradiction: although many galactic-wind models—simulations of how matter is distributed in our universe—predict that the majority of the "normal" matter exists in stars at the center of galaxies, in actuality these stars account for less than 10 percent of the matter in the universe. A new set of simulations offer insight into this mismatch between the models and reality: the energy released by individual stars within galaxies can have a substantial effect on where matter is located in the universe.

The Feedback in Realistic Environments, or FIRE, project is the culmination of a multiyear, multiuniversity effort that—for the first time—simulates the evolution of galaxies from shortly after the Big Bang through today. The first simulation to factor in the realistic effects of stars on their galaxies, FIRE results suggest that the radiation from stars is powerful enough to push matter out of galaxies. And this push is enough to account for the "missing" galactic mass in previous calculations, says Philip Hopkins, assistant professor of theoretical astrophysics at the California Institute of Technology (Caltech) and lead author of a paper resulting from the project.

"People have guessed for a long time that the 'missing physics' in these models was what we call feedback from stars," Hopkins says. "When stars form, they should have a dramatic impact on the galaxies in which they arise, through the radiation they emit, the winds they blow off of their surfaces, and their explosions as supernovae. Previously, it has not been possible to directly follow any of these processes within a galaxy, so the earlier models simply estimated—indirectly—the impact of these effects."

By incorporating the data of individual stars into whole-galaxy models, Hopkins and his colleagues can look at the actual effects of star feedback—how radiation from stars "pushes" on galactic matter—in each of the galaxies they study. With new and improved computer codes, Hopkins and his colleagues can now focus their model on specific galaxies, using what are called zoom-in simulations. "Zoom-in simulations allow you to 'cut out' and study just the region of the universe—a few million light-years across, for example—around what's going to become the galaxy you care about," he says. "It would be crazy expensive to run simulations of the entire universe—about 50 billion light-years across—all at once, so you just pick one galaxy at a time, and you concentrate all of your resolution there."

A zoomed-in view of evolving stars within galaxies allows the researchers to see the radiation from stars and supernovae explosions blowing large amounts of material out of those galaxies. When they calculate the amount of matter lost from the galaxies during these events, that feedback from stars in the simulation accurately accounts for the low masses that have been actually observed in real galaxies. "The big thing that we are able to explain is that real galaxies are much less massive than they would be if these feedback processes weren't operating," he says. "So if you care about the structure of a galaxy, you really need to care about star formation and supernovae—and the effect of their feedback on the galaxy."

But once stars push this matter out of the galaxy, where does it go?

That's a good question, Hopkins says—and one that the researchers hope to answer by combining their simulations with new observations in the coming months.

"Stars and supernovae seem to produce these galactic superwinds that blow material out into what we call the circum- and intergalactic medium—the space around and between galaxies. It's really timely for us because there are a lot of new observations of the gas in this intergalactic medium right now, many of them coming from Caltech," Hopkins says. "For example, people have recently found that there are more heavy elements floating around a couple hundred thousand light-years away from a galaxy than are actually inside the galaxy itself. You can track the lost matter by finding these heavy elements; we know they are only made in the fusion in stars, so they had to be inside a galaxy at some point. This fits in with our picture and we can now actually start to map out where this stuff is going."

Although the FIRE simulations can accurately account for the low mass of small- to average-size galaxies, the physics included, as in previous models, can't explain all of the missing mass in very large galaxies—like those larger than our Milky Way. Hopkins and his colleagues have hypothesized that black holes at the centers of these large galaxies might release enough energy to push out the rest of the matter not blown out by stars. "The next step for the simulations is accounting for the energy from black holes that we've mostly ignored for now," he says.

The information provided by the FIRE simulations shows that feedback from stars can alter the growth and history of galaxies in a much more dramatic way than anyone had previously anticipated, Hopkins says. "We've just begun to explore these new surprises, but we hope that these new tools will enable us to study a whole host of open questions in the field."

These results were submitted to the Monthly Notices of the Royal Astronomical Society on November 8, 2013 in a paper titled "Galaxies on FIRE (Feedback In Realistic Environments): Stellar Feedback Explains Cosmologically Inefficient Star Formation." In addition to Hopkins, other authors on the paper include Duìan Kereì, UC San Diego; José Oñorbe and James S. Bullock, UC Irvine; Claude-André Faucher-Giguère, Northwestern University; Eliot Quataert, UC Berkeley; and Norman Murray, the Canadian Institute for Theoretical Astrophysics. Hopkins's work was funded by the National Science Foundation and a NASA Einstein Postdoctoral Fellowship, as well as the Gordon and Betty Moore Foundation.

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Friday, January 24, 2014
Beckman Institute Auditorium – Beckman Institute

2014 Frontiers in Nano Science and Technology

John H. Schwarz Wins the Fundamental Physics Prize

John H. Schwarz, the Harold Brown Professor of Theoretical Physics at Caltech, and Michael B. Green of the University of Cambridge have been awarded the 2014 Fundamental Physics Prize in recognition of the new perspectives they have brought to quantum gravity and the unification of the fundamental physical forces of the universe. The prize comes with a $3 million award.

The prize was announced at an award ceremony at NASA's Ames Research Center in Silicon Valley on December 12. Alexander Varshavsky, Caltech's Howard and Gwen Laurie Smits Professor of Cell Biology received the 2014 Breakthrough Prize in Life Sciences at the same ceremony. Schwarz and Green were awarded the Physics Frontiers Prize earlier this year, which admitted them to candidacy for the Fundamental Physics Prize. The 2014 Physics Frontiers Prize was also awarded to Joseph Polchinski of the University of California, Santa Barbara, a Caltech alumnus (BS, 1975).

The Fundamental Physics Prize is awarded by the Fundamental Physics Prize Foundation, which was established in July 2012 by Russian physicist and Internet entrepreneur Yuri Milner to recognize groundbreaking work in the field. Previous winners include Caltech's Alexei Kitaev, the Ronald and Maxine Linde Professor of Theoretical Physics and Mathematics. He and the other recipients of the award—including theoretical physicist Stephen Hawking—served on the selection committee for this year's prize.

Schwarz and Green were honored for developing superstring theory during their collaboration between 1979 and 1986. Its predecessor, string theory, originated in the late 1960s in response to the discovery of many new particles via accelerator experiments. Theoretical physicists, says Schwarz, tried "to make order out of all this chaos" by postulating that the fundamental object of the universe is the string and that the various particles in the universe could be adequately described as different oscillation modes of the string. It was thought for a time that string theory would yield an explanation of the strong nuclear force that binds protons and neutrons together in an atom's nucleus (or even more fundamentally, the quarks and gluons that make up protons and neutrons). But then in the mid-1970s, quantum chromodynamics provided an excellent account of the strong nuclear force, and string theory fell out of favor among most theoretical physicists.

In 1974, Schwarz and his then collaborator, Joel Scherk, suggested a different possible use of string theory: a quantum theory of gravity and the unification of all the forces in nature. To follow up on this suggestion, Schwarz began his collaboration with Green in 1979, and together they created superstring theory, a version of string theory that relies on the property of supersymmetry to relate the two fundamental types of particles in quantum theory—bosons and fermions—to one another.

According to Schwarz, this is "a very ambitious project, and not something that's going to be completed in my lifetime." But, he says, "people are making lots and lots of progress. We keep discovering new things about superstring theory, which give us the sense that we're closing in on something really important." Indeed, experimental physicists working on CERN's Large Hadron Collider may soon be able to prove the existence of supersymmetry, which, says Schwarz, "wouldn't prove that superstring theory is right, but would be extremely encouraging."

This optimism regarding the ultimate success of superstring theory has not always been shared throughout the scientific community. When Schwarz and Green began their work together in 1979, it was, says Schwarz, "not particularly fashionable or popular." But in 1984, the pair's discovery of the so-called Green-Schwarz anomaly cancellation mechanism brought new excitement to superstring theory. "It has remained popular ever since—30 years later," Schwarz remarks.

Schwarz notes that he is especially honored to receive the Fundamental Physics Prize because "the people who were making the selection were other theoretical physicists who've already won the prize, and they are people that I respect and admire. Being chosen by them is particularly meaningful." Schwarz and Green are now eligible to serve on the selection committee for future Fundamental Physics Prizes.

Cynthia Eller
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Massive Galaxy Cluster Verifies Predictions of Cosmological Theory

First Detection of the Kinetic SZ Effect in an Individual Galaxy Cluster

By observing a high-speed component of a massive galaxy cluster, Caltech/JPL scientists and collaborators have detected for the first time in an individual object the kinetic Sunyaev-Zel'dovich effect, a change in the cosmic microwave background caused by its interaction with massive moving objects.  

MACS J0717.5+3745 is an extraordinarily dynamic galaxy cluster with a total mass greater than 1015 (a million billion) times the mass of the sun or more than 1,000 times the mass of our own galaxy. It appears to contain three relatively stationary subclusters (A, C, and D) and one subcluster (B) that is being drawn into the larger galaxy cluster, moving at a speed of 3,000 kilometers per second.

The galaxy cluster was observed by a team led by Sunil Golwala, professor of physics at Caltech and director of the Caltech Submillimeter Observatory (CSO) in Hawaii. Subcluster B was observed during what appears to be its first fall into MACS J0717.5+3745. Its momentum will carry it through the center of the galaxy cluster temporarily, but the strong gravitational pull of MACS J0717.5+3745 will pull subcluster B back again. Eventually, subcluster B should settle in with its stationary counterparts, subclusters A, C, and D.

Though subcluster B's behavior is dramatic, it fits neatly within the standard cosmological model. But the details of the observations of MACS J0717.5+3745 at different wavelengths were puzzling until they were analyzed in terms of a theory called the kinetic Sunyaev-Zel'dovich (SZ) effect.

In 1972, two Russian physicists, Rashid Sunyaev and Yakov Zel'dovich, predicted that we should be able to see distortions in the cosmic microwave background (CMB)—the afterglow of the Big Bang—whenever it interacts with a collection of free electrons. These free electrons are present in the intracluster medium, which is made up primarily of gas. Gas within dense clusters of galaxies is heated to such an extreme temperature, around 100 million degrees, that it no longer coheres into atoms. According to Sunyaev and Zel'dovich, the photons of the CMB should be scattered by the high-energy electrons in the intracluster medium and take on a measurable energy boost as they pass through the galaxy cluster.

This phenomenon, known as the thermal SZ effect, has been well supported by observational data since the early 1980s, so it was no surprise when MACS J0717.5+3745 showed signs of the effect. But recent observations of this galaxy cluster yielded some curious data. A team led by Golwala and Jamie Bock—also a Caltech professor of physics—observed MACS J0717.5+3745 with the CSO's Bolocam instrument, measuring microwave radiation from the cluster at two frequencies: 140 GHz and 268 GHz. Through a simple extrapolation, the 140 GHz measurement can be used to predict the 268 GHz measurement assuming the thermal SZ effect.

Yet observations of subcluster B at 268 GHz did not match those expectations. The trio of Caltech and JPL postdocs who had first proposed observations of MACS J0717.5+3745—Jack Sayers, Phil Korngut, and Tony Mroczkowski—puzzled over these images for some time. Trying to sort out the discrepancy, Korngut kept returning to subcluster B's rapid velocity relative to the rest of the cluster. Prompted by Korngut's interest, Mroczkowski decided one weekend to calculate whether the kinetic SZ effect might explain the discrepancy between the 140 GHz and 268 GHz data. To everyone's surprise, it could. In order to show this conclusively, the signals from dusty galaxies behind MACS J0717.5+3745 also had to be accounted for, which was done using data at higher frequencies from the Herschel Space Observatory analyzed by Mike Zemcov, a senior postdoctoral scholar at Caltech. The model combining the two SZ effects and the dusty galaxies was a good match to the observations.

The kinetic SZ effect, like the thermal SZ effect, is caused by the interaction of the extremely hot and energetic electrons in the gas of the intracluster medium with the CMB's photons. However, in the kinetic effect, the photons are affected not by the heat of the electrons, which gives a random, uncoordinated motion, but instead by their coherent motion as their host subcluster moves through space. The size of the effect is proportional to the electrons' speed—in this case, the speed of subcluster B.

Prior to this study of MACS J0717.5+3745, the best indication of the kinetic SZ effect came from a statistical study of a large number of galaxies and galaxy clusters that had been detected by the Atacama Cosmology Telescope and the Sloan Digital Sky Survey. This is the first time, Golwala says, "that you can point to a single object and say, 'We think we see it, right there.'"

"By using the kinetic SZ effect to measure the velocities of whole clusters relative to the expanding universe, we may be able to learn more about what causes the universe's accelerating expansion," Golwala explains. The next step in the process is the development of new, more sensitive instrumentation, including the new Multiwavelength Sub/millimeter Inductance Camera recently commissioned on the CSO.

The paper detailing these observations is titled "A Measurement of the Kinetic Sunyaev-Zel'dovich Signal Towards MACS J0717.5+3745," and appears in Astrophysical Journal. Sayers, Mroczkowski (now at the U.S. Naval Research Laboratory), Zemcov, and Korngut are the lead authors. Other authors from Caltech and JPL include Bock, Nicole Czakon (now at Academia Sinica in Taiwan), Golwala, Leonidas Moustakas, and Seth Siegel. Funding for the research was provided by the Gordon and Betty Moore Foundation, the National Aeronautics and Space Administration, the National Science Foundation, the Norris Foundation, the National Science Council of Taiwan, and the Academia Sinica Institute of Astronomy and Astrophysics.

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