What’s Out There?

What is the universe made of—and how do the push of dark energy and the pull of dark matter shed light on this age-old question? Our scientists are moving ever closer to finding out.

What is the universe made of—and how do the push of dark energy and the pull of dark matter shed light on this age-old question? Our scientists are moving ever closer to finding out.

By Douglas Birch
Illustrations by J.F. Podevin

Growing up in India, Tanvi Karwal would sometimes pester her father to take the family out to the countryside far from bright lights to watch meteor showers. “We would drive for an hour out of the city and take a blanket and put it on the ground, then just lie down and just watch the stars all night,” she says.

Her star-gazing started her thinking about the deeper mysteries hidden in the night sky, and she began reading physics books that her dad left lying around the house. She was struck by how little is known about what is out there. “There is so much more mystery in the cosmos concerning the fate of the universe, and how the universe got there, and the Big Bang, and dark energy, and it’s all really cool,” she says.

Now she is a second-year graduate student at the Henry A. Rowland Department of Physics and Astronomy, one of a cadre of Hopkins faculty and students looking for answers to some of the universe’s darkest secrets.

While Hopkins’ physics and astronomy faculty is relatively small compared with those at other major universities, it includes some of the most influential scientists pursuing the answers to these biggest of big-picture questions: What is the universe made of? What is the dark energy that starting about 6 billion years ago began to push space itself to expand at an ever-increasing rate? What is the dark matter that keeps the galaxies tightly bound to one another, and formed the earliest structures in the cosmos?

What is the universe made of?

There are billions of planets, stars, and galaxies in the night sky made up of the ordinary atoms that we can see with telescopes. But these are just the icing on the cake, accounting for less than 5 percent of the universe’s bulk.

About 24 percent of the cosmos consists of a substance called dark matter that is invisible to telescopes and is almost certainly not made of atoms. More than two-thirds of the universe, 71 percent or so, is something even more difficult to grasp, called dark energy, which is fueling the runaway expansion of space.

Rosemary Wyse photo

If we could understand what dark matter is, then that’s a huge leap forward in our understanding of the universe.”Rosemary Wyse

Dark matter in particular seems to be the gravitational glue that binds the galaxies together, and has helped shape our stellar neighborhood. “If we could understand what dark matter is, then that’s a huge leap forward in our understanding of the universe,” says Rosemary F. G. Wyse, professor of physics and astronomy, who practices galactic archaeology, using ancient nearby stars to infer how the Milky Way evolved. Discovering what the dark matter is made of and how it behaves would have a “tremendous impact,” she says, on questions about how, when, and why galaxies like ours form stars and why galaxies come in different shapes and sizes.

Charles Bennett, Bloomberg Distinguished Professor of Physics, who has spent his career mapping the large-scale structure of the universe and has won a long list of scientific prizes for his work, says dark matter is hiding everywhere astronomers look.

“You see some light and stars and things in the galaxy, but under that is this enormous sea of dark matter,” says Bennett, who also holds the Alumni Centennial Professorship. “And so, whatever size you think the galaxy is by looking at the light, it’s 10 times bigger because there’s this colossal cloud of dark matter that is just centered on them.”

To get an idea of the scale of the dark universe, consider our Milky Way galaxy, which appears to be about 2.3 million light years from its nearest neighbor, the Andromeda galaxy. While the two star clusters appear to be separated by a vast region of space, Bennett says, their haloes of dark matter touch one another.

Adam Riess, Bloomberg Distinguished Professor of Physics and Astronomy and the Thomas J. Barber Professor at Hopkins, won the Nobel Prize in 2011 for his role in the 1998 discovery that a baffling dark energy is stretching the infinitely elastic fabric of space at a faster and faster rate. And as the universe grows, the distances between galaxies increase. (Think of pieces of pepperoni on pizza dough, which separate as the dough is stretched.)

Far in the future, all the galaxies beyond the Milky Way and its neighbors may disappear from view, leaving the Earth’s little corner of the universe a lonely outpost of light in an ocean of darkness.

Finding what dark matter is made of could produce a whole new field of fundamental physics, while understanding dark energy could help predict the cosmos’ ultimate fate.

David Kaplan photo

We’re discovering things that have weird properties and we don’t know what the hell they are.”David Kaplan

Both live on the frontiers of science, where evidence is sparse and theories run rampant. But there’s little doubt dark matter and dark energy are out there. David E. Kaplan, a leading theorist and professor of physics and astronomy at Hopkins, called them the “Wild West” of astrophysics. “We’re discovering things that have weird properties and we don’t know what the hell they are,” he says.

What is dark matter?

In the early 1930s, a prickly Swiss astrophysicist named Fritz Zwicky, working long nights at the Mount Wilson Observatory, realized the galaxies in the Coma Cluster were speeding around as if they contained a lot more stars than he could find. He concluded in a paper that they must contain some “Dunkle Materie,” or dark matter.

For decades, this dark stuff was believed to be dwarf stars, Jupiter-sized planets, and other humdrum objects built of the same kind of atoms that make pencils and periwinkles. But by the 1980s, new models of the Big Bang were raising doubts.

Charles Bennett says he was shocked when he went to lunch with a colleague at an astronomy meeting in Buenos Aires in July 1991, and learned that dark matter had to be made of an as-yet unknown material. “Most of the matter out there, it doesn’t have any light, it’s not made of atoms, it’s something else,” his colleague told him.

photo of Charles Bennett

You see some light and stars and things in the galaxy, but under that is this enormous sea of dark matter.”Charles Bennett

Bennett said it’s as though the universe is a field of icebergs, with only the tips poking above the surface. “At first I thought it was crazy,” recalls Bennett, smiling sheepishly during an interview in his office in the Bloomberg Center for Physics and Astronomy. “But the more you think about it, the more compelling it is.”

Bennett later served as principal investigator for NASA’s Wilkinson Microwave Anisotropy Probe mission (WMAP), which from 2001 to 2010 meticulously mapped the faint blush of radiation left over from the Big Bang, a relic of a time when the universe was a tiny fraction of its current age.

WMAP produced an exquisitely detailed map of the infant universe that serves as a crucial reference point for theories about its evolution and established the Standard Model of Cosmology. “We’ve been looking at the patterns in the sky, and with WMAP, we determined not only that dark matter is the dominant type of matter in the universe, but the precise measurements pinned down that dark matter comprises 24 percent of the universe,” he says.

For decades, teams of scientists have been hunting for a dark matter particle, which must be unlike any other particle known. Many were betting on one candidate in particular, the weakly interacting massive particle, or WIMP.

Predictions of the abundance of WIMPS that would have survived the harsh conditions after the Big Bang turned out to align uncannily with the observed abundance of dark matter today. The WIMP also meshed with the concept of “supersymmetry,” predictions that each of the 17 fundamental particles in the standard model of physics has one or more as-yet undiscovered partner particles.

The 2013 discovery of the Higgs boson, which gives matter its mass, by the Large Hadron Collider (LHC) outside Geneva, raised hopes that the WIMP would soon be discovered there as well. “Experimentally there has been an insane amount of effort to try to detect the WIMP particle,” says David Kaplan, because it seemed to fit so neatly into the picture science was building of the early universe.

But the WIMP still hasn’t been found at the LHC or anywhere else, and some scientists are beginning to doubt that it exists. So a number of physicists have shifted their focus to a particle called the axion.


A representation of the evolution of the universe over 13.77 billion years. The expansion of the universe (vertical direction) is illustrated as time passes to the right (horizontal direction). The far left depicts the beginning of the universe. The current thinking is that random microscopic quantum fluctuations inflated exponentially to astronomical sizes in a period called “inflation.” Then, the expansion rate of the universe slowed dramatically and continued for 13.77 billion years. The rate of expansion slowed due to the force of gravity from matter (mostly “dark matter”) in the universe. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The afterglow light seen by WMAP (and other telescopes) was emitted about 375,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light; it also forms a backlight for later developments of the universe. We still do not know what the dark matter or dark energy are—we only know the physical effect of their presence—and we do not know if inflation actually happened and if it did, exactly what kind of inflation happened. These remaining mysteries are among the most important questions in physics and cosmology and their answers are being actively sought by experiments and telescopes.
[Image and text: NASA/WMAP Science Team]

Could the axion be the answer?

In 1977, physicists trying to understand the behavior of quarks, the building blocks of protons and neutrons, came up with a hypothetical particle that was named after a 1960s Colgate-Palmolive detergent because it cleaned up a stubborn problem in physics.

While some scientists have searched for the axion, it did not get anywhere near the attention that the WIMP has drawn.

That’s changing. Today, for example, Kaplan is collaborating on the hunt for ultra-light, axion-like dark matter candidates in collaboration with a group of scientists at the University of Washington. The group’s experiment, called Eöt-Wash, aims to detect the hypothetical particle using a bell-shaped version of the torsion balance, an instrument that measures small forces acting on a pendulum.

Comparing the axion to the WIMP is like comparing a pebble to Jupiter. While the WIMP is thought to be heavier than the proton, the axion could be trillions of times lighter. Kaplan says that if it exists, the particle is likely to be so abundant that “bazillions” are passing through detectors at any given moment. The trouble, he says, would be to detect its passing amid the background buzz of electromagnetic signals.

If the WIMP turns out to have been a dead end, some physicists would mourn. “The WIMP was really such a great hypothesis,” because it opened new frontiers in physics, says Marc Kamionkowski, William R. Kenan, Jr. Professor in the Department of Physics and Astronomy and a pioneer in WIMP theory. “If it doesn’t work out, if dark matter turns out to be something else, it will be a real shame. It will be a lost opportunity.”

Kaplan takes a somewhat different view. “It’s awesome, it’s a fantastic time,” he says, because new avenues of dark matter research are opening up. “I think some people are gloomy about the demise of the WIMP. Not me. This is the way it should look, with people pushing in all directions.”

How fast is the universe expanding?

Back in the late 1990s, Adam Riess led one of two teams of astronomers trying to determine the rate at which the expansion of the universe was presumed to be slowing after the Big Bang.

He used images from the Hubble Space Telescope to measure the speed with which stars and galaxies were moving away from one another. To fix their age and velocity, he and his colleagues looked for a type of supernova, an exploding star that can give off more energy in its final two minutes than the sun will in its lifetime.

As they studied the images, Riess and his colleagues realized that the cosmos wasn’t slowing its expansion as energy from the Big Bang dissipated. It was actually speeding up at an ever-increasing rate and had been for about half its 13.8 billion-year lifetime.

Adam Riess photoCould dark matter be big black holes?”Adam Riess

Bennett’s WMAP mission later measured this expansion at 43.5 miles per second about 3.26 million light years away from Earth, and double that speed every 3.26 million light years beyond that. (A plane traveling 43.5 miles a second could fly from Baltimore to Los Angeles in about a minute.)

On one recent afternoon, Riess was sitting in his office at the Bloomberg physics building, listening to the music of Simon and Garfunkel and James Taylor and studying Hubble images of supernovae.

Taking a break, he said that the prevailing assumption is that dark energy is the cosmological constant, which started as a fudge factor invented by Einstein to tidy up the general theory of relativity but that physicists later decided could have a physical basis.

The cosmological constant suggests that dark energy is a property of empty space, which under quantum theory isn’t truly empty but frothing with so-called “virtual” particles and antiparticles that pop into and out of existence too fast to be glimpsed.

The idea is that dark energy drives the accelerated expansion of space. The more space expands, the more dark energy it creates and the faster the galaxies fly away from each other.

Trouble is, when physicists calculated how much energy the cosmological constant would give empty space, the answer was 10 to the 120th power times more than needed to drive the expansion. (That’s 1 with 120 zeroes behind it.)

No one can say for certain why this should be so, but the fact that the cosmological constant is still the leading theory of the source of dark energy demonstrates just how mysterious the force remains.

In a paper published last June, Riess raised new questions about dark energy. He and his co-authors posit that the universe is expanding about 5 to 9 percent faster than previous estimates, based on data gathered by the European Space Agency’s Planck satellite between 2009 and 2013.

Speaking in his office, Riess notes that the Planck data have been disputed by Hopkins postdoctoral researcher Graeme Addison, in a recent paper with Bennett and five others at Hopkins and the University of British Columbia.

“At this moment in time I can’t say for sure whether there’s some more prosaic error, or whether we have started to collect one of the long-waited-for clues about the nature of dark energy and dark matter,” Riess says.

But if it turns out the runaway universe is expanding at the even-faster accelerated rate, he says, it’s possible “that dark energy is not the vanilla explanation,” which is the cosmological constant. “That would be exciting actually because it would be the kind of clue we’re hoping to get to understand it.”

Currently, Riess is honing the tools astrophysicists use to measure the acceleration. “This is the fun part,” he says, gesturing to his computer screen with two images of a supernova in galaxy NGC 5917, a companion to the Hooked Galaxy in Libra. He was trying to triangulate the star’s distance, using Hubble images made from two different points in Earth’s orbit.

“This is what I really like doing, is making measurements of the universe, very precise ones, and trying to interpret them in the cosmological framework to see what we can say about the universe,” he says.

The connection to black holes

Last year, two new ground-based U.S. observatories recorded the first-ever sighting of a gravity wave, the ripples in space predicted by Einstein that are triggered by the fall of a sparrow or the violent death of a star.

Twin Laser Interferometer Gravitational-Wave Observatory (LIGO) sites in Washington state and Louisiana recorded the faint blip of a wave, set in motion by the collision and merger of two black holes, ultra-dense objects with gravitational fields so strong that not even light can escape.

Shortly after LIGO’s findings were announced on February 11, Marc Kamionkowski hashed over them with his postdocs and students at the Cafe Azafrán, at the Space Telescope Science Institute, just across the street from Hopkins’ physics and astronomy building.

Rumors had been circulating for weeks about LIGO’s discovery, and the team had already met several times previously to brainstorm about possible follow-ups.

As he stood in line waiting to buy lunch, Adam Riess called over to his colleague’s table. “Marc, hey, could dark matter be big black holes?”

“Some of the discussions we were having had already been steering in that direction,” Kamionkowski says, but the fact that Riess was thinking along the same lines gave them new impetus. Kamionkowski and his students, led by postdoctoral fellow Simeon Bird, began to study the LIGO data and search the scientific literature.

Riess says it was an offhand comment, and at the time he didn’t expect anything to come of it. “And you know Marc and his postdocs just ran with it and I was extremely impressed,” he says. “And then they wrote me later and they said, well, you’re not going to believe this, but we’ve looked into this and it’s quite possible and we have written a paper on this and you’re welcome to be a co-author of it.”

Marc Kamionkowski photo

Every idea for dark matter is a nutty idea at this point.”Marc Kamionkowski

A couple of decades back, scientists searching for dark matter studied black holes created by collapsed stars, typically smaller than about 10 solar masses, and black holes at the center of galaxies, which have about a million solar masses. Neither was considered a good fit because there didn’t seem to be enough of them to make up the cosmos’ missing mass.

But LIGO detected the merger of two black holes with 36 and 29 solar masses, about the size predicted for a dark horse in the dark matter sweepstakes, “primordial” black holes, so-called because they’re thought to have formed in the infant universe.

The fact that LIGO spotted two primordial black holes locked in a binary death spiral so soon after it switched on last year suggests that the objects are pretty common. If so, Kamionkowski says, LIGO should detect several hundred or thousand similar mergers over the next decade.

Asked to make the best case he could think of for primordial black holes as the dark matter that physicists have been hunting for since the 1930s, Kamionkowski declined.

“It’s a nutty idea,” he says, then corrects himself. “It’s a very nutty idea. But on the other hand, every idea for dark matter is a nutty idea at this point. The reason why we wrote this paper is that this idea is not as nutty as you would think.” ■