Seeing Stars

There is a lack of baby photos of the Universe. Scientists know it was born nearly 14 billion years ago, but it didn’t start with a bang, as the name—Big Bang—suggests. Instead, the Universe was like a rapidly inflated balloon. And it was hot. Very hot.

But there is a basic question that everyone is still asking: Religion aside, will science ever explain how we got here? Cosmologists—astronomers who study the Universe’s lifespan, its birth, growth, and eventual death—have more questions. Is there one Universe, or many? What did the Universe sound like when it was born? What is the power of its black holes—could they eventually absorb us all? And finally—this force that started to push the Universe bigger and bigger—what is it?

One of the rising stars in the field of cosmology and astrophysics is Shirley Ho, an assistant professor at Carnegie Mellon University. This year, she was named an Emmy Noether Fellow, an international award for young faculty members seeking to do collaborative research. Her research on dark energy made her the co-winner of the 2014 Outstanding Young Researcher Award, given by the International Organization for Chinese Physicists and Astronomers.

Dark Energy

Ho’s drive comes from wanting to understand the hardest questions. 

She wants to figure out what dark energy is. She knows what it does—it’s some mysterious component in the Universe that pushes things apart.

“In the children’s museum, when you drop a coin and it goes around and around into the center—that’s gravity. Imagine a force that does the opposite. We’re trying to figure out what that is,” Ho says.

“Calculations have become pretty good at producing real Universes.”

Einstein didn’t know about dark energy. In fact, nobody knew about it until 1998, when exploding stars appeared dimmer than they should have. A team of scientists used telescopes across the world to compare notes on the fading stars and came to a landmark conclusion—the stars looked dimmer because the Universe was getting bigger; the stars were farther away than they used to be. If dark energy is pushing the universe to stretch, could it, like some scientists worry, cause The Big Rip, tearing our Universe apart?

“Something was pushing the Universe apart, and we didn’t know what it was. Einstein believed that the universe was static,” Ho says. Einstein had gotten some clues that the Universe was moving, but he found a way to justify his assertion, saying that space had an energy in and of itself that could resist expansion or contraction.

Ho wants to better figure out why the Universe is expanding the way it is. To do so, she is watching galaxies move with the expansion, looking for any change in how they relate to one another as they move. She hopes this can help her understand the properties of dark energy. Discovering more about dark energy could lead to undiscovered physics or new particles—or could challenge scientists to rework the theory of gravity.

How does she do it? A lot of time at the computer.

There are several sophisticated telescopes gathering data on millions of galaxies. The data are compiled into a digital catalog and given to scientists.

“People like to call this data mining or big data—really, I’m just looking to make sure I get all the interesting physics that you may not expect from the massive amount of data,” Ho says.

She thinks of it as giving each galaxy a barcode that she can scan and check in on. Over time, she watches how the barcodes relate to one another. This helps her with another piece of her work—trying to figure out just what the Universe was like fractions of a second after the Big Bang, another conundrum about dark energy—figuring it out may mean that scientists have to rework their entire theory of the Big Bang, so Ho and other cosmologists are testing carefully any information they have around the beginning of the Universe.

She has been able to confirm that the Universe did extend very quickly in a short amount of time, and after it did so it looked like a “cosmic soup” of particles.

She’s also looked into the sound waves of the early Universe, 400,000 years after the Big Bang. The sound waves are imprinted, drawing a circle-like object around each galaxy. Imagine if these circles were filled with water and that you dropped a rock into the center—those ripples have helped determine where future galaxies are formed, meaning each galaxy has a sphere of galaxies around it.

“These sound waves were the first cry of the Universe, when the baby was born,” Ho says. Watching how galaxies formed and moved over time further helps her understand whether the Universe’s speed of expansion has changed over time, pointing to the strength of the dark energy pulling the Universe apart.

Ho’s own passion for science was born at a young age—she remembers standing as a young girl in a public library in Hong Kong, reading a biography of Marie Curie. There was not much of a night sky in the city, but regular visits to the space museum helped her start to dream. At 18, she did her first astrophysics research project at the University of California–Berkeley while getting joint degrees in physics and computer science—two fields that can be typically dominated by men. She earned her PhD in astrophysical sciences from Princeton.

At CMU, the cosmology department has one of the highest female-to-male ratios, illustrating the changing nature of the field. Ho keeps an international profile. She co-chairs committees for Sloan Digital Sky Survey (SDSS), employing one of the most advanced telescopes, which is used by more than 40 universities; the Dark Energy Spectroscopic Instrument dark energy group, which has members from more than 20 universities; and the Large Synoptic Survey Telescope Project, which has more than 40 university members from around the world.

Imagined Universes

Ho isn’t the only rising star at Carnegie Mellon. Tiziana Di Matteo, a CMU professor of physics, has created some of the largest simulations of the Universe in the world. Her simulations were the first to incorporate black hole physics. Her research has been featured on PBS’s Nova, in Astronomy Magazine, in Science News, on MSNBC’s Science and Technology, and in the Pittsburgh Post-Gazette.

Di Matteo uses theories to create her imagined Universes, where she predicts both what might happen in the future and what might have been.

“It’s fascinating that through the power of this beautiful theory we can predict the world and its reality. It can predict something that nobody has dreamed of discovering,” Di Matteo says.

The most famous theory drew her to the field in the first place—Einstein’s landmark theory of general relativity explained how objects behave in space and time, showed how light bent due to gravity, and, most importantly to Di Matteo as a student, proved that black holes existed well before they were actually physically proven to be a reality. She was hooked.

Di Matteo’s predictions can sometimes be initiated from someone on her CMU astrophysics and cosmology team, which covers a range of research, studying theoretical, computational, and observational cosmology. The team has access to worldwide X-ray satellites, the Hubble Space Telescope, and the SDSS.

If a person from the observational side of cosmology is looking at the Universe and wondering about, say, the properties of galaxies, they’ll come to her as a resource, asking, “Where did they come from? How did they form?”

While they are using telescopes to observe the real Universe, Di Matteo does two things—first she thinks up a theory to explain something, then she goes to her computer to run simulations of the Universe.

Di Matteo starts to create her model Universes by accessing a supercomputer—a computer so large, which holds so much data, that there are only a few in the United States. The computer is accessed by people all over the world and is massive in size because it holds so many details of the physics of the Universe, starting with the Big Bang and continuing until present day. However, access takes time. Scientists have to be approved by the National Science Foundation by showing that their calculations are worthy of the resources.

“If you’re looking at a galaxy, you can use that to build a picture of how the galaxy has formed stars over time.”

Computers have become so fast and sophisticated that Di Matteo calls her simulations “mock observations”—data that can be just as valuable as what is actually observed with the most powerful telescopes.

When Di Matteo adds her physics and equations to the Universe’s initial condition (the moments right after the Big Bang), she has a pretty good idea where it might be going next. She can show this prediction to other astronomers, who may be waiting for “real” data.

“Calculations have become pretty good at producing real Universes,” Di Matteo says.

Di Matteo also uses this model to study black holes—which can have a mass a billion times the size of the sun—sitting in the middle of galaxies. This makes her the go-to person for questions. Her CMU colleagues studying “real” data had seen that the size of a black hole was related to the size of the surrounding galaxies—but they didn’t know why.

“How do black holes know about the surrounding galaxy?” Di Matteo asks. “And why does every galaxy end up with a massive black hole in the middle?”

Di Matteo starts, again, with a theory, and then starts running simulations. She creates a fake star that becomes a black hole, swallowing matter around it. It grows bigger and bigger. She notices that in some regions of the Universe, which are very dense, that the evolution speeds up, the galaxy and black hole growing together. She can take this back to the observers and compare it to what they see through their massive telescopes.

Sometimes the observations of the real Universe guide the simulations, but sometimes it’s Di Matteo’s own curiosity and testing through her models that guide new observational tools.

“The things I see in simulations—maybe one day there will be an instrument that can test my predictions,” Di Matteo says.

Cosmic Structures

Complementing the research of Ho and Di Matteo is Rachel Mandelbaum, an associate professor at CMU who is one of the best young researchers using deflected light to try to understand dark matter. She is in the midst of a five-year, $750,000 grant from the U.S. Department of Energy to study dark matter and dark energy. She was also just one of 68 researchers nationwide to receive funding from the DOE’s Early Career Research Program.

Dark matter doesn’t absorb, reflect, or emit light. Scientists haven’t seen it, but they know it’s there because it has gravity, and that gravity has an effect on all matter (including galaxies) around it. But scientists don’t know much about it, despite the fact that it makes up about 85% of the matter in the Universe.

For Mandelbaum, the appeal of dark matter research can be traced back to first grade when she started organizing the details of her future. She wrote out her goals: She would be 4'11". She would have brown hair. And she would be a scientist. She was wrong about the first two—she has grown taller than her younger self expected, and she has red hair. But the last prediction was correct.

“I always wanted to know how the world works,” she says.

Her observational technique, called weak gravitational lensing, watches how light is deflected in distant galaxies by different matter. She can observe the impact of subtle changes in the appearances of galaxies due to gravitational lensing, the deflection of light by the gravitational influence of matter—including dark matter. So even though dark matter isn’t emitting light, she can see that it’s reacting to gravity.

“It’s one of the best ways to learn about the distribution of dark matter,” she says. Figuring out its distribution can help her figure out what dark matter is altogether.

Mandelbaum can study a lot of different things with weak lensing, ranging from dark energy to how galaxies relate to dark matter. If dark energy is causing the expansion of the Universe to accelerate, then it’s changing the way that galaxies are clustering due to gravity. Galaxies tend to clump together, but using lensing, Mandelbaum can watch their cosmic structures change.

In fact, it’s not a certain piece of dark matter floating here or there that concerns Mandelbaum —it’s the larger picture they’re in and how the pieces relate to one another.

“We’re looking at the degree of clumpiness,” she says. “Because of gravity, we know it’s not random.” That clumpiness likely helped form our own galaxy; its pull of gravity counteracted the expansion of the Universe—meaning it might have saved the Milky Way from staying in the hot-mess phase.

Mandelbaum can do her work in part thanks to the SDSS, utilizing the large telescope that has created the most detailed three-dimensional pictures of the Universe. “If you’re looking at a galaxy, you can use that to build a picture of how the galaxy has formed stars over time,” she says.

She also works with a camera called the Hyper Suprime-Cam, which is on an 8-meter telescope, compared to the 2.5-meter SDSS telescope. That width can collect more light, which lets Mandelbaum look at galaxies that are farther away.

The information captured by these telescopes is essentially organized by a software team into catalogs of images, which list things like all the galaxies, their positions, and the amount of light that comes from them. This information is the foundation for many observational research papers.

However, the work that Mandelbaum needs to do is critical to this first step of cataloging, and if there are mistakes, her conclusions may be wrong. So she has to serve as the Universe’s fact-checker. She meticulously checks correlations in the data, sometimes doing “null tests,” finding things that should equal zero if everything is done right. Her process to check all of the data can last up to a year.

For example, just like when someone walks in front of the camera as a picture is being taken, so too can things like dust in the atmosphere prevent a clear image. In a sense, Mandelbaum and her team are taking cosmic photobombs out of the picture, which is much more than just tinkering with code. To develop a solution, it isn't like wait-an-hour-on-hold IT call—it can take years.

All of this means that scientists are having to hedge their bets to draw conclusions from the data—there are a lot of partially written papers while everyone waits. Mandelbaum’s thesis took three years because the original SDSS data analysis took a long time to come together properly, and she had to work with raw telescope data to produce some of the results. But her patience and meticulous approach are getting her noticed—her mentor, James Gunn, a professor at Princeton who helped develop the SDSS telescope, praises her technique.

“It’s easy to take the data and misinterpret it.” he says. “I think she is one of the best people in the world at this business. Maybe the very best.”

Universal Story

So going back to that age-old question, How did we get here?, it’s clear we’re edging closer to some answers, thanks to the research of three scientists from Carnegie Mellon—Shirley Ho, Tiziana Di Matteo, and Rachel Mandelbaum—who are helping other scientists explain what has been unexplainable. As the three accomplished women distill their data and theories to create a picture of the Universe, they are telling the Universe’s life story.