A Weighty Matter

Squeezing the Universe into a Ball

I’m not a very practical person," remarks Penn cosmologist Chung-Pei Ma, her dark eyes shining like stars. It’s probably one of the great understatements of the millennium. An assistant professor of physics and astronomy, she studies the universe as a single object; her research ranges across the life history of the cosmos–origin, structure, evolution, and eventual fate.

To carry out her inquiry, Ma does things like weigh superclusters of galaxies–the way grocers might weigh a bag of coffee beans, except in her case the scale is concocted of observations and theoretical calculations woven together with computer simulation. A galaxy is an ocean of matter whose droplets consist of countless suns, and a supercluster is a configuration of maybe 10,000 galaxies all bound together by gravity into a massive structure. Superclusters, the largest cosmic objects besides the universe itself, can occupy regions of space from 50 to more than 100 million light years across and are set within voids of space so vast that some show up as pin pricks of light only through the most powerful telescopes. Ma and her Caltech collaborators estimate the Corona Borealis Supercluster, which is 600 million light years from earth and spans the night sky the width of ten full moons, weighs "at least" 30,000 trillion of our suns. "Weighing superclusters is arduous work," she confides as though talking about wrestling with a 50 pound sack of beans.

Cosmologists parse the universe the way physicists resolve matter into its component quarks, protons, and atoms. "One of the fascinating parts of cosmology," she observes, "is that it sits between astronomy and particle physics." Halfway between atoms and the stars, it’s a science that probes the depths of outer space as well as the microcosm of high-energy particles that ran rampant in the early universe. What counts as large and small, though, keeps shifting.

To cosmologists, the universe is a dappled thing. Our sun, 330,000 times more massive than the earth, is just one of countless stars that stipple the night sky. Billions and billions of them populate the great spirals of galaxies that wheel across the heavens. Draw further back, and these giant, spinning star systems are themselves mere points of light joined to other pin pricks in a stippled galactic structure astronomers call a cluster. Step back again, and the clusters of galaxies become flecks of graded densities that constitute a supercluster. In a photo of such a behemoth, the Milky Way–the galaxy to which our Sun gives its bit of illumination–would be impossible to resolve. How many of these prodigious gems dapple the vast and velvet-black void of space, no one knows for sure. But in recent times cosmologists have come to understand that the universe is much more than this unimaginable dance of lights.

Ma admits to being filled with wonder by the scale of it all. When she was nine years old, her violin teacher asked if she was serious about becoming a musician, because she would soon need to begin training at a high powered school like Julliard. Ma recalls answering, "Well, I want to be an astronaut, and I’m not sure why." Later she understood that her little-girl mind was casting about for some way to get at the "fundamental questions" that already intrigued her: What is the fate of the universe? How old is it? What is it made of? "It’s just awe inspiring," she quietly exults as though she had just put a chunk of chocolate in her mouth.

A frequent collaborator with observational cosmologists, Ma has looked through the world’s largest telescope at the Keck Observatory on the summit of Mauna Kea in Hawaii, but she prefers theory because, as she says, "It’s very neat. I’ve always liked math, and I like to do calculations. The beauty of theoretical work is that I can just sit here and think, or I go to the beach and lie there. I used to do it rollerblading on the Santa Monica beach when I was a postdoc [at Caltech]." Ma is a theoretical cosmologist. She squeezes the universe into a ball to roll it toward an overwhelming question: "Doesn’t it bother you that we don’t know what the universe is made of?"

In an age when space probes explore the farthest reaches of the solar system and observational technologies record events transpiring almost at the edge of the cosmos, it’s surprising that scientists don’t know what the universe is made of. Researchers can estimate how much luminous matter there is in a galaxy by measuring the light from its constituent stars and gas. The motion of celestial bodies provides another way to calculate mass. When astronomers tally masses based on the motion of stars turning on great galactic wheels, they’ve discovered there is far more mass in galaxies than is accounted for by calculations based on measurements of luminosity. The same holds true for superclusters. "The galaxies in clusters are swarming around so fast that some [scientists] believe the luminous matter makes up less than ten percent of the total mass," notes Ma. "These and other observations all point to a single fact: more mass is out there than we can see. It seems unreal, but the evidence for dark matter is so strong that there’s almost no question that it’s out there–and a lot of it–and we want to quantify it." She points out that over 90 percent of the universe may be made of "dark matter" that we can’t see.

"Although we know a bit better about how much dark matter is out there," says Ma, "we don’t know for sure what it is." Cosmic sleuths divide dark matter into two classes: baryonic and nonbaryonic, or ordinary and exotic. The ordinary kind consists of nonluminous things like dead stars, black holes, and brown dwarfs, which are massive planet-like bodies that are not quite big enough to ignite the forces of stellar combustion. You can’t see them, but you could conceivably touch them. The exotic stuff is the dominant type of dark matter, and part of Ma’s research focuses on neutrinos as likely candidates to solve the "missing mass problem." Neutrinos are uncharged subatomic particles. She says they interact so weakly with ordinary matter that it would take a block of lead several light years thick to stop one. They’re ghosts of matter, about as close to nothing as you can get without actually arriving. You have to use a negative expression to make a positive assertion: The best, it seems, you can say about them is that they possess a "non-zero mass." They’re not nothing.

Neutrinos were part of the "cosmic soup" of photons, electrons, and quarks that populated the nascent universe. Just one second after the big bang, the fiery crucible that forged the universe, they escaped the thermal equilibrium and have been coursing the heavens for 15 billion years. Theoretical calculations indicate there are about 340 of these "weak and wispy" particles per cubic centimeter of space. Calculate the volume of a sphere based on the observable universe’s 30 billion light-year diameter; convert it into cc’s, and then multiply the figure by 340. The resulting mass of that invisible neutrino blizzard would exert a powerful, destiny-altering gravitational effect–"capable of speeding up the motion of stars in galaxies and slowing down the expansion of the universe," notes Ma.

There could be other unthought-of particles that make up dark matter, but Ma believes the relative abundance of neutrinos makes them likely suspects. Since the absolute mass of a neutrino is unknown, one of the questions she explores is: How much do these nearly-nothing entities weigh? "To approach this question," she explains, "we can’t do experiments. That is why we use simulations on supercomputers to model the universe. We make assumptions about the universe being made of certain kinds of neutrinos under certain kinds of conditions, and then we just calculate how a patch of the universe would evolve over time." To simulate gravity, the computer is programmed to calculate the force exerted on each particle by millions of others. The position and velocity of every particle is calculated and advanced in a process that’s repeated thousands of times. If the correct assumptions are entered into the program, the simulation should yield a universe that forms galaxies and superclusters the way the known universe does. She refers to the method as using data to "constrain" theory; the simulations allow the researcher to gradually constrain parameters until an acceptable model is arrived at. In effect, the simulations tell her how wispy the neutrinos would be, assuming the neutrino theory is correct. The results indicate that they can weigh no more than .0001 of an electron or about five electron volts.

Because neutrinos have a non-zero mass, they are subject to gravity, which pulls dark and luminous matter together into clusters. Superclusters hold within their gravitational fields most of the universe’s matter, and Ma uses computers to weigh how much dark matter these giant structures hide. In one simulation, 17 million particles were tracked through a cube of space that measured 2 billion light years across one face. When the exercise finally yielded a supercluster that "looked" sufficiently like one from the real universe, the resulting data showed these astronomical beasts to be formidable concealers of dark matter. "The implied mass density of the universe," she concludes, "is approximately 40 percent of the critical value for closure"–the sum total of all mass that would exert a gravitational force strong enough to slow down and turn back the expanding universe toward a "big crunch."

Earlier this year, Ma was named a Sloan Fellow and a Cottrell Scholar. Each year the Sloan Foundation awards grants to 100 young scientists who show exceptional promise of making fundamental contributions to new knowledge. Twenty-three Sloan Fellows have gone on to receive Nobel Prizes. Ma is the first member of Penn’s faculty to win the Cottrell, which is awarded annually to 18 young faculty for excellence in research and undergraduate teaching. She also won Penn’s Lindback Award for Distinguished Teaching this year. "Is Professor Ma from another galaxy?" asked one student in the University’s Undergraduate Course Guide. "She is great!"

"Perhaps sometime during the first quarter of the twenty-first century," she muses, "cosmologists will finally be able to tell the story of dark matter with an ending note." Based on present estimates from Ma’s and other cosmologists’ work, there is insufficient mass–both luminous and dark–to stop the expansion of the universe: it will continue swelling relentlessly and in time will be little more than a diffuse and burnt out cinder. "T.S. Eliot may be right after all," Ma recites, "‘This is the way the world ends/ Not with a bang but with a whimper.’" How fast the universe fizzles out depends on how much dark matter there is–how much the universe weighs. She is putting the whole cosmos on her scales too. You might say that Chung-Pei Ma has squeezed the universe into a ball and holds its fate in her hands.