In 1990, the aptly-named space shuttle Discovery carried something revolutionary into earth’s orbit—a telescope that could see into the far reaches of space and take pictures of distant galaxies never before been seen by the human eye.
Then in 1998, astrophysicists Saul Perlmutter, Brian Schmidt, and Adam G. Riess, and their respective research teams, working at three different institutions, began using these revolutionary images to examine the luminosity of supernovae in the far reaches of the universe. Applying complicated algorithms to images of distant stars, they calculated the rate at which the universe had expanded during the time that the starlight took to travel to earth.
But let’s backtrack for a moment. There are two Hubbles important to this story: the telescope, and its namesake, Edwin Hubble. His astronomical observations in 1929 led to the theory of universal expansion in the first place. However, until 1998, physicists believed that this expansion was slowing down. Perlmutter, Schmidt, and Riess set out to determine once and for all which was occurring. What they found instead is that an undetected force in our universe is pushing galaxies apart at an accelerating rate, blowing the lid off of existing theories concerning the amount of energy contained in space.
In the 13 years since their discovery, the scientific community has been grappling with the implications of this new force, which by current calculations must make up some 70 percent of the universe in order to exert its observed effects. This discovery, in tandem with ongoing research studying the composition of dark matter, has presented a new picture of the universe in which everything we understand as matter, everything we can taste, touch, smell, hear, and see; every planet, star, asteroid, and speck of dust in the farthest reaches of space, comprises less than 5 percent of the universe. Which raises the question, what, then, is the other 95 percent?
A NEW MATTER
Brown University Physics professor Ian Dell’Antonio has some answers. In an interview with The Independent, he shed light on the dramatic universal questions raised by astrophysics in the last decade and a half. Dell’Antonio’s research focuses primarily on the study of dark matter, using telescopes to examine a phenomenon known as gravitational lensing—how light is bent around large masses in space.
Objects of immense mass, such as stars and galaxies, exert a gravitational force large enough to bend the fabric of space. The amount of distortion is proportional to the amount of mass, Dell’Antonio explained. “You can weigh the mass that’s in front of [a source of light] by measuring how much the light is distorted.” By measuring the distortion of light from distant stars, astrophysicists can estimate the mass of far-flung galaxies.
They work at the problem from another angle as well, using galaxies close to Earth (whose weight can be measured concretely through means other than gravitational lensing) to approximate a hypothetical weight for distant galaxies. Because the mass of a galaxy should be proportional to the amount of light distortion, physicists can predict how much the light from each source should be distorted.
Thus, they work at the problem from two angles: calculating the actual mass of the galaxy based on the distortion of light, and calculating how much the light should be distorted based on intelligent estimates of galaxy mass. Here’s where it gets tricky: in reality, these figures don’t match. The predicted galaxy mass is based on the typical amount of matter in neighboring galaxies that is observable to earth’s instruments, but it isn’t at all close to the mass of the galaxy calculated from the distortion of light. According to Dell’Antonio, the disparity between the estimated distortion and the perceived distortion is “not small… there’s typically ten to twenty times more mass than you can account for by the galaxies.” That unexplained mass is what scientists refer to as dark matter.
Dark matter, as the name suggests, does not emit light or any other form of electromagnetic radiation and has not yet been detected by any human instruments. We know it exists, but we do not yet know what it is. In fact, it is most easily defined by what scientists have ruled out. It is not a charged particle (if it were charged it would emit detectable electromagnetic radiation), or a particle that has strong interactions with normal matter (if it did, these interactions would have occurred during the pressure cooker conditions of the Big Bang and altered the elemental composition of the universe in measurable ways).We are certain that it has mass because of its gravitational effects on light, and that it composes some 25 percent of the known universe based on these gravitational effects. However, these broad constraints leave astrophysicists with a plethora of candidate particles vying for the lead role on the cosmological stage. Understanding which particle constitutes dark matter would allow physicists and cosmologists to fill in a huge gap in our understanding of the composition of the universe, and might even lead to the discovery of previously unknown particle interactions that would expand our picture of the intangible forces at work in nature. The current conundrum is this: researchers do not know what they do not know about dark matter. On the one hand, our growing understanding of its unique properties could open doors into unexplored realms of particle physics; or history could end up relegating it to an interesting but tangential sidebar in an astrophysics textbook. It is still too early to tell.
Current research in astrophysics at Brown focuses on the construction of dark matter detectors that would be able to measure the rare collisions between dark matter and real matter. Others around the world focus on the creation of dark matter in Large Hadron Colliders such as the one at CERN, the underground complex in Switzerland leading the field in particle physics. The hope is that by studying the decay properties and sub-atomic interactions of candidate particles, scientists will be able to rule out (or decide on) whether those particles fit the criteria for dark matter. Thus far, none of the many possible candidate particles have been proven to exist in the mass and quantities required to constitute nearly a quarter of the universe.
“The frustrating thing for me, as an astrophysicist,” Dell’Antonio said, “is that through gravitational lensing it’s easy to tell how much of the dark matter there is. But you learn almost nothing about what it is.” However, there is hope that the many cosmological mysteries of dark matter may be revealed over time, as scientists across the world—whether in underground complexes exploding streams of particles or in aboveground observatories poring over images of the farthest reaches of space—ahead in the darkness.
What Perlmutter, Schmidt, and Riess set out to measure in 1998 was how much the rate of expansion of the universe had slowed since the Big Bang. Prior to their discovery, physicists universally believed that the mass of the universe due to dark and normal matter would cause the expansion started by the Big Bang to become slower and slower over time. The matter within the universe, they thought, must exert inwardly-directed gravitational effects on itself in opposition to its outward momentum, acting against the tendency toward expansion. Imagine their surprise, then, when they discovered that the rate of expansion hadn’t decreased at all. It had, in fact, been accelerating.
The scientists earned the Nobel Prize in Physics in 2011 for the discovery that radically altered the understanding of energy distribution within the universe. Their research led to the revelation that in order to counter the gravitational attraction of masses within the universe, there had to be an energy source pushing these masses outward. In keeping with tradition, scientists dubbed this unknown source “dark energy.”
The moniker is fitting, because dark energy acts somewhat like normal energy’s evil twin. Here’s why: in a relativistic sense, energy has many mathematical properties, one of which is pressure, a physical quality that determines the gravitational attraction between substances. By convention, the pressure of normal energy and matter is positive. In order to overcome the attractive forces between masses, the pressure of dark energy must be opposite tothat of normal matter and energy. Dell’Antonio explained that astrophysicists “think dark energy is something that has enough negative pressure that [it] pushes the universe further and further apart, but we know of no physical thing that has negative pressure, and that’s why dark energy is fundamentally a very strange thing.” Essentially, it is energy that acts contrary to anything physicists have seen before.
However, the concept of dark energy was not wholly without precedent. In his early theories of relativity, Einstein included a factor he designed called the “cosmological constant,” a generic force that pushed the masses of the universe outward and away from one another at exactly the same strength that gravity pulled them inwards. He envisioned a cosmos that neither contracted nor expanded. However, such a perfectly balanced equilibrium seemed impossible, and Einstein abandoned the term when Hubble’s 1929 observations showed that the universe was expanding. Historical reports have Einstein calling the creation of the cosmological constant his “biggest mistake” in creating a model of the universe. But Einstein may have been too quick to dismiss the concept. Now decades-old theories regarding the cosmological constant are being applied to the study of dark energy because, as Dell’Antonio said, “it has exactly the right property: it causes space to want to accelerate.”
By playing with Einstein’s equations relating the curvature of the universe to its contents, theoretical physicists have re-imagined the cosmological constant as something called “vacuum energy,” a type of energy associated with (and evenly distributed throughout) empty space. Thus, as the size of the universe increases, the amount of empty space increases, upping the amount of vacuum energy in the universe—and pushing it outward faster and faster. It is not yet understood exactly how and why dark energy is distributed, and, as with dark matter, we do not yet know how to place it meaningfully in our existing picture of the cosmos. Will the galaxies of our universe will gradually drift farther and farther apart to become lone islands of light in the vast black gulf of space? It’s possible. And if dark energy is evenly distributed throughout the universe, how then are we constantly surrounded by it yet have never before detected it? The decade old discovery has raised more questions than it has answered, but the potential for this vast source of energy seems as limitless as the source itself.
The future of dark energy research is both macroscopic and incredibly myopic. Physicists are studying the farthest reaches of space to gather as many data points as possible about rates of expansion throughout time. They are analyzing their data for subtle trends that might lend further information to the effects wrought by dark energy on our changing cosmos.
But why should we care about these unseen forces and particles hovering in the black inter-galactic voids that we now know are not as empty as they seem? If you’re not convinced of the exciting potential of dark matter and dark energy research, it’s worth making a trip up to Dell’Antonio’s charmingly cluttered corner office in the Barus and Holley building at Brown University. He puts it best when he says, “You open a window and you see, ‘Oh, there’s a bigger universe out there’….expanding our view of the possibilities of where things go—that’s the biggest excitement. It may well be that the discovery of dark matter will lead us to a new force of nature, or will lead us to something really profoundly new about how the universe works. But fundamentally, for now, being able to strive to piece together how the universe works and how we fit into that working of the universe... I think of it as an intellectual challenge.”
ASHTON STRAIT B‘13 is a cosmological mystery.