A Cosmic Conversation
In the last few decades, we have found answers to some of the biggest puzzles in astronomy and cosmology — thanks to the work of theoretical cosmologists like USC Dornsife’s Vera Gluscevic and Elena Pierpaoli, whose research aims to determine the composition and evolution of the universe. However, while cosmologists worldwide have made huge strides in recent years with the help of advanced probes such as satellites and space telescopes, many of the deepest mysteries of the universe still remain to be solved.
Gluscevic, Gabilan Assistant Professor of Physics and Astronomy, combines the tools of theoretical astrophysics, particle physics and astronomical data analysis to explore dark matter, dark energy and processes that shaped the universe before the time of the first stars. Her research involves coming up with new ways of using objects and phenomena that we see in our universe throughout cosmic history — observables from the cosmic microwave background (CMB) radiation that comes to us almost from the time of the Big Bang to populations of dwarf galaxies around the Milky Way — in order to test the fundamental fabric of nature.
The primary research interests of Pierpaoli, professor of physics and astronomy, are the CMB and the large-scale structure of the universe, in particular galaxy clusters. She uses observations to understand fundamental physical principles, such as how gravity works on large scales and the nature of dark energy and dark matter.
Here, Gluscevic and Pierpaoli explain their research and discuss some of the recent breakthroughs in our understanding of the universe as well as cosmic conundrums, such as dark
energy, dark matter and supermassive black holes, that still have cosmologists and astrophysicists scratching their heads.
“Very shortly after the Big Bang, we believe there was a period when the expansion of the universe was highly accelerated. Right after that, all the particles that we know about — even those that may make us up — were created.”
— Elena Pierpaoli, professor of physics and astronomy
A BRIEF HISTORY OF THE UNIVERSE
VG: As physicists and cosmologists, we use the tools of physics to try to understand the physical
laws that govern our universe on the largest of scales, through time and space. We know that our universe began with a Big Bang almost 14 billion years ago. The universe started off small and very uniform, free of any lumps, bumps or structure.
EP: Very shortly after the Big Bang, we believe there was a period when the expansion of the
universe was highly accelerated. Right after that, all the particles that we know about — even those that may make us up — were created.
VG: There were no galaxies at first, just clumps of matter that were denser than their
surroundings. From that primordial soup is where our own galaxies that we see around us today eventually originated, emerging billions of years ago from tiny kernels of dark and normal matter.
EP: However, some very big questions about the origins of the universe still remain. For example, we know that at the center of most galaxies — including our own — there is a supermassive black hole. Sagittarius A*, the black hole at the center of our own Milky Way, is equivalent to slightly more than 4 million solar masses. We don’t exactly know how such huge black holes form, but we think they are probably not primordial, but formed at later times during the history of the cosmos.
VG: When we look at the night sky, what’s incredible is that everything we see — all the stars, galaxies, us, the planets — accounts for maybe only a few percent of everything there is in
Cosmologists made a major breakthrough in the last few decades in understanding that our universe is made primarily of some new substances that we don’t yet understand — dark energy and dark matter. They’re very different from each other. We know they’re not made out of particles that we know and understand in particle physics and yet they decide what galaxies look like and what our universe is doing as a whole.
What I do is try to understand how we can use the smallest of galaxies that we see around us in the universe today to rewind this movie and understand what subatomic particles — particles that are even tinier than an atom — were doing at these very first moments after the Big Bang. In doing so, I try to understand dark matter and dark energy.
EP: As Vera said, the dominant energy density of the universe appears to be dark matter and dark energy. This is what we understand when we apply Einstein’s theory of relativity to interpret observations at all cosmological scales and distances. The validity of Einstein’s
theory has been tested in the past on scales from our Earth to the solar system. However, it’s conceivable that the same laws don’t hold as we move to larger and larger scales, so that we need to modify Einstein’s theory.
Technological advancements of the past decades allowed us to observe many more distant, extragalactic objects. Therefore, we now have the possibility of using these new and
powerful observations to revise and test the law of gravity on a very large scale — something that was previously impossible, even when I was in grad school, because our observations back then simply weren’t powerful enough. Such theoretical changes, of course, would also have implications for our understanding of the existence of dark matter and dark energy in our universe.
HOW TIME BEGAN
VG: We understand spacetime and its evolution thanks to Einstein’s general theory of relativity, which allowed us to figure out that there was a beginning of time. At that first moment, there was a massive expansion of the universe.
EP: It’s important to remember that the Big Bang isn’t why the universe formed, it’s how. If we imagine an explosion, we tend to think of some space where a bomb
explodes, while the Big Bang essentially created space.
VG: To make this easier to understand, let’s think of our universe as a two-dimensional universe — it’s really four dimensional, but that’s harder to envision — and the space is expanding like the rubber surface of a balloon expands while being inflated.
This doesn’t mean that our universe expanded into something else, into some space that existed before it and around it. Spacetime — all of it, what we see and what we cannot yet see — began at that moment, and has been stretching and growing ever since.
OBSERVING THE HORIZON OF THE UNIVERSE
EP: As Vera said, we know our universe began at a certain moment in time. According to our knowledge of physics, information can travel only as fast as the speed of light. This means we can only see up to a certain distance from us — what we call the horizon — which is the largest distance that could have been traveled by light from the beginning of time, in other words, from the Big Bang to now. This means there’s a limit to what we can assess
because we can see only a given volume around us and not the entire universe. We don’t know if the universe is infinite, or if it has other boundaries or peculiarities beyond the horizon: If these exist, we cannot see them.
“Dark energy is winning, it’s becoming the most dominant thing in our universe, the one that decides what our universe as a whole is doing.”
— Vera Gluscevic, Gabilan Assistant Professor of Physics and Astronomy
Despite our limitations in only being able to observe what is inside the horizon, there is a lot that we can learn from the volume of the universe that is within our reach. Some of the observed radiation — specifically the CMB radiation — was emitted close to the farthest edge of the horizon. Because the CMB radiation reaches us from very large distances and therefore we know it was emitted very early on, it informs us about moments in the history of the universe that were very close to the Big Bang. In this sense, the CMB is the most
accurate probe of early universe physics, and studying it is very helpful in understanding what was happening back then.
One of the questions that we aim at understanding better from our study of the CMB is how inflation occurred and how it seeded the ripples in densities that later created the structures — galaxies, stars, etc. — we observe around us.
VG: USC Dornsife’s Department of Physics and Astronomy is an institutional partner in the international Simons Observatory Collaboration to build the next generation of CMB telescopes in Chile’s Atacama Desert. This array of new-generation telescopes will help us observe the CMB in much greater detail than we’ve ever been able to before. They may also provide information about possible new types of particles in our universe that we can’t see in any other way.
EP: And then there are other probes that are closer to us, typically, all the galaxies and structures that formed more recently. All those cosmological probes should point toward the same picture for the content, evolution and model of the universe, so part of our research is also to find the correct model that can match these very distant observables and those closer to us.
A BEAUTIFUL FOAM-LIKE STRUCTURE
VG: Our universe currently has a lot of structure. Stars group into galaxies that group into larger collections of galaxies, sometimes clusters of galaxies. If you zoom out and look at our universe on the largest of scales, it resembles this beautiful foam-like structure of matter that forms big bubbles, with walls and filaments stretching between them. Wherever these filaments of matter cross, that’s where you find most of the galaxies and clusters of galaxies.
In global terms, we understand well enough how this structure forms to be able to program a computer to reproduce our universe by telling it how gravity works and then letting it figure out what the structure of the universe looks like today. The result is a good match for our observations, which means that our theory of gravity works fairly well — so long as we input the right amounts of matter.
Although I would say the universe is extremely orderly and is described by several laws of physics that we understand, it’s also disorderly in the sense that these laws reveal that there’s much more matter in the universe than the stars and galaxies. They also reveal that the universe is doing this weird thing where it’s expanding faster and faster as time goes on, as if something is inflating it — something beyond the normal substances that we understand in standard physics.
THE DARK SIDE
EP: That brings us to dark energy — so-called because it typically doesn’t interact much with light, but it does not behave as dark matter in terms of ruling the universe’s expansion. It is the dominant component in the universe in terms of total energy density. At the moment, dark
energy makes up 70%, maybe 25% is dark matter, and the remaining 3% to 5%, that’s the stars, us, everything that we’re used to envisioning when we think about the universe. So, dark energy is a very big deal indeed.
Then there’s dark matter, about which we also know very little. In fact, we just know one type of dark matter, the neutrino component — a neutral particle with a very small mass that rarely interacts with normal matter — which we’ve been able to calculate comprises less than 10% of dark matter.
VG: Evidence for both dark energy and dark matter comes from observations. There is six times more dark matter than normal matter in the universe and we’re confident that it isn’t any of the normal stuff we understand in particle physics. We do know that dark energy and dark matter don’t require each other. There are theories, certainly, that try to link them together, but they behave so differently. Dark matter behaves like normal matter in that when the universe expands, we end up with a lower density of it. Whereas dark energy behaves dramatically differently.
Let me explain. So, we all know that gravity pulls things together. The more massive you are, the more gravity you exert on other things. If I throw a ball up in the air, it goes up high, turns around and comes back down again. That’s how gravity works. If I throw it really hard, it will go farther up. If I throw it with escape velocity — about 11 kilometers a second, the speed at which we launch rockets into interplanetary space — it’s going to break off from the
gravitational grip of the Earth and continue floating into space with a constant velocity. What it’s never going to do is break off from that gravitational grip and then speed up more and more as it goes farther and farther away. This would be crazy, and yet that is exactly what our universe is doing.
At best, it should be just expanding with constant velocity, and, instead, the bigger it is, the faster it’s expanding. This is what dark energy is doing — it’s making the universe expand faster and faster.
Another weird thing that we now understand about dark energy is that unlike normal matter — which decreases in density when the volume of space increases — the density of dark energy remains the same when the volume of space increases. It’s almost as if the more space there is, the more of the stuff — whatever it is — there is. And so, dark energy is winning, it’s becoming the most dominant thing in our universe, the one that decides what our universe as a whole is doing.
A MATTER OF SOUL
EP: Why did I become a cosmologist? It was the intellectual challenge that drew me into this field when I was young, and
even in my short life, I’ve lived through exciting changes. Thanks to the data we can now access, cosmology has become one of the fastest evolving fields in physics.
Sometimes I’m asked, ‘What would you say to people who argue this research is a waste of time, energy and money?’ It’s true there’s no direct practical application for what we study, but our research does answer fundamental questions of humankind, questions that have preoccupied people since civilization began: What’s in the universe? Where are we in the universe? Was there a beginning and will there be an end? And so, in that sense, it is important because there’s also the soul — not only practical things are relevant.
I would also argue that I don’t think Edison and Tesla were actually thinking of lighting up the whole planet when they were discovering electricity. So, we never know!
VG: I totally second Elena’s Edison and Tesla argument. In addition, our universe is so beautiful. At the same time, it’s completely mysterious. But while we don’t yet understand dark matter and dark energy, we do have the mathematical tools to explore and dig deeper and understand how our universe began, to know that there was a beginning and to figure out the age of it. It’s a whole field for exploration, so, to me, having those tools and that challenge is incredibly empowering.
After all, understanding the universe — who doesn’t want to do that?
Additional reporting by Stephen Koenig