Imagine Magazine - Johns Hopkins - September/October 2011 - (Page 7)

PHOTO AND CAPTION: PD-HUBBLE Modeling the universe The main tools I use in my A powerful collision of galaxy clusters research are computer has been captured by NASA’s Hubble simulations. The first types of Space Telescope and Chandra X-ray Observatory. This clash of clusters simulations—of the evolution of provides striking evidence for dark the universe—are on a massive matter and insight into its properties. scale. Since there are hundreds of billions of galaxies in the universe, these simulations are naturally very large and complex. It’s a nice physics problem, though, because we have a pretty good handle on the initial conditions of the universe, and we know that most of the physics that is important is gravity. Gravity isn’t the only important force, but on large scales, gravity dominates. Because dark matter makes up about 85% of the mass of the universe, what happens on a large scale is dominated by dark matter and gravity’s effect on it. In these simulations, we can start with the very small fluctuations in the smooth early universe, add in gravity, and then predict how we expect dark matter to be distributed. We think there is a galaxy at the center of every “clump” of dark matter in the universe. It mapped out positions of about a million galaxies in almost 3D and got pictures of about a hundred million galaxies. When I make predictions about how galaxies should be distributed in space, I can then compare them with these observations and go back and refine my models. A day in the life When I was an advanced graduate student and post-doc, I was already doing computer simulations of the formation of the universe, so for the most part I spent my days writing code and programming the models. I also developed theoretical models for the formation of galaxies and larger structures. I still get to do a bit of this, but now I typically spend my time in meetings with my graduate students and post-docs discussing and planning our research. Or I spend time writing and editing papers and proposals. Cosmology is a field that relies on international collaborations, so I go to several international meetings a year, and some of the large projects I’m on have researchers from 10 countries involved. I travel a lot in the United States as well to give talks and to meet collaborators. That’s one thing I didn’t fully anticipate going into this field—just how much travel there would be. I find it fun and stimulating to talk to colleagues from around the world. A biographer of galaxies Figuring out how the distribution of dark matter is related to what we see with a telescope is much more challenging. In order to do this, I combine simulations of the evolution of mass in the universe with theoretical models for how individual galaxies form in the dark matter skeleton of the universe. You have to factor more physical processes into these models: how gas cools, how stars form, how stars eject energy into the surrounding gas when they explode. All these processes affect the subsequent evolution of the galaxies. It’s truly challenging to model the physics of an individual star-forming region while taking into account all these local and regional forces. Looking back to leap forward Telescopes can probe the universe now at greater distances and with greater precision than ever before, and that’s good news for cosmologists. I just came back from a meeting of the Dark Energy Survey, which is going to start collecting data in the next six months and will collect data for five years. The SDSS surveyed about a quarter of the sky and focused on galaxies that formed in the last billion years. The DES will go much deeper, tracing galaxies over the last 7 billion years, measuring the history of cosmic expansion. Our hope is that studying these galaxies formed so long ago will help scientists figure out what dark energy is, and why the expansion of the universe is accelerating. Model, meet data We compare our models to the actual state of the universe by using observations from massive telescopes. The more closely the models match the data, the more likely the underlying parameters we set in the simulation are to explain our own universe. We use data from a lot of sources, including deep-sky surveys that can observe galaxies formed more than 12 billion years ago. One of the most important current datasets is the Sloan Digital Sky Survey (SDSS). The largest survey ever of the local universe, the SDSS lasted 10 years and was just completed. Learn more about the big questions that drive Dr. Wechsler at http://risa.stanford.edu. Click the link for Outreach to see videos of and slides from her presentations for a lay audience, including her 2010 TED talk for YouthCastilleja. www.cty.jhu.edu/imagine imagine 7 http://risa.stanford.edu http://www.cty.jhu.edu/imagine

Table of Contents for the Digital Edition of Imagine Magazine - Johns Hopkins - September/October 2011

Imagine Magazine - Johns Hopkins - September/October 2011
Contents
Big Picture
In My Own Words
Physics Is…
Making the Team
Nuclear by Nature
Physics: The Next Generation
Telescopic Views
Exploring the Mysterious Lives of Stars
Around the Universe in Three Weeks
Selected Opportunities & Resources
Book Buddies
Meeting the (Economics) Challenge
Off the Shelf
Word Wise
Exploring Career Options
One Step Ahead
Planning Ahead for College
Students Review
Creative Minds Imagine
Mark Your Calendar
Knossos Games

Imagine Magazine - Johns Hopkins - September/October 2011