Probing the Dark Side of the Universe

One of the remarkable discoveries in astrophysics has been the recognition that the material we see and are familiar with, which makes up the earth, the sun, the stars, and everyday objects, such as a table, is only a small fraction of all of the matter in the universe. The rest is dark matter, possibly a new form of elementary particle that does not emit or absorb light, and can only be detected from its gravitational effects.

In the last decade, astronomical observations of several kinds, particularly of distant supernovae and the cosmic microwave background, also indicate the existence of what is known as dark energy, a uniform background field that is accelerating the expansion of the universe. The presence of dark energy suggests a fundamental gap in our current understanding of the basic forces of nature.

According to the standard cosmological model, dark matter comprises about 22 percent of the universe, while dark energy makes up 74 percent. “There is a few percent of residual dirt left over,” says Scott Tremaine, Richard Black Professor in the School of Natural Sciences, “and that is us and the stars, the galaxies, and everything we know.”

With the discovery of dark energy ten years ago, a better understanding of the properties of dark matter, and a more precise accounting of the composition of the universe, the two fields of astrophysics (the physics of the very large) and particle physics (the physics of the very small) are each providing some of the most important new experimental data and theoretical concepts for the other. Re­search at the Institute for Advanced Study has played a significant role in this development. The late Institute Professor John Bahcall, through his research on solar neutrinos, was a pioneer in demonstrating the importance of astrophysical phenomena for understanding fundamental physics.

Current knowledge of the fundamental forces of physics is based on two ­well-­established theories: the Standard Model of particle physics, which gives an impressively accurate description of elementary particles and their interactions, but ignores gravity and only accounts for about ­one-­sixth of the matter in the universe; and Einstein’s theory of general relativity, which describes the observed gravitational behavior of large objects in the universe, such as galaxies and clusters of galaxies, but has yet to be reconciled with quantum principles.

These three images created by Member Douglas Rudd show the various matter components in a simulation encompassing a volume 86 Megaparsec on a side (for reference, the distance between the Milky Way and its nearest neighbor is 0.75 Mpc). The three components are dark matter (blue), gas (green), and stars (orange). The stars form in galaxies which lie at the intersection of filaments as seen in the dark matter and gas profiles. The dark matter, as the dominant matter component in the universe, drives the formation of those filaments and structures and dark energy “pushes back,” effectively stopping the growth of that structure and therefore galaxies.

Institute Faculty and Members, who have contributed many of the theoretical foundations of the Standard Model and its possible modifications, have been at the forefront of trying to resolve the apparent incompatibility of general relativity and quantum theory that has been a central paradox of theoretical physics for several decades.

Today, the Institute is home to what is one of the world’s leading groups of particle physicists—Professors Stephen Adler, Nima ­Arkani-­Hamed, Juan Maldacena, Nathan Seiberg, and Edward Witten. They and other physicists who have worked at the Institute are responsible for many of the radically new ideas about the ultimate structure of matter and the nature of space and time that will be tested at the Large Hadron Collider (LHC), a particle accelerator expected to begin operating this year at the European Center for Nuclear Research (CERN) near Geneva.

At the same time, astrophysics research at the Institute, led by Tremaine and Professor Peter Goldreich, has contributed to many of the most significant advances in astronomy and astrophysics, from the formation of stars and planets to the discovery of black holes to the distribution and properties of dark matter.

The discovery of dark energy a decade ago showed that empty space is filled with a mysterious energy that is not diluted as the universe expands. While Einstein initially proposed a cosmological constant that could explain the dark energy (he prematurely discounted it as his biggest blunder when Edwin Hubble discovered the expansion of the universe in the 1920s), it is the amount of cosmologically observed dark energy that is difficult to reconcile with our current understanding of physics. Quantum fluctuations of the vacuum are thought to be a source of the dark energy field but most quantum field theories predict that it should be 10120 times larger than it is, meaning that the rate of acceleration would be so fast that galaxies and stars would not have a chance to form.

“Dark energy is really an embarrassment for particle physics,” says Edward Witten, Charles Simonyi Professor in the School of Natural Sciences, who spoke about dark energy at the Space Telescope Science Institute in May. “Its discovery has greatly changed how we think about the laws of nature. The nature of the change depends crucially on whether dark energy is a ‘cosmological constant.’ For me, the discovery of cosmic acceleration/dark energy was the most dramatic finding in physics since perhaps the discovery of the J/psi particle [a cornerstone of the Standard Model] in 1974.”

In the coming years, astrophysical observations will play a complementary role to experiments conducted in high energy physics laboratories, most particularly at the LHC. The synergy of collider experiments and astrophysical observations may tell us what dark matter and dark energy ­are—­potentially creating a revolution in our understanding of both particle physics and the universe.

“Particle physicists have turned more and more to astrophysics as a way of getting new data,” says Tremaine. “Now, perhaps, in the next few years, the shoe will be on the other foot. With the LHC, particle physicists may, in turn, be able to discover things that will help the cosmologists understand what the universe is made of.”

Current research by Members in the School of Natural Sciences includes investigating the formation of dark matter substructure in our galaxy through analytical methods as well as numerical simulations; using ­large-­scale computer simulations to study the growth and evolution of clusters of galaxies, and exploring their use as a probe of dark energy; studying the relationship between galaxies and dark matter from galaxy clustering to learn about cosmology and galaxy formation and evolution; and using weak gravitational lensing to answer a variety of astrophysical questions that will provide clues about the nature of dark matter and dark energy.

“Because the Institute has an exceptionally strong group of both particle physicists and astrophysicists, we are in an ideal position to understand any unexpected connections between particle physics and the cosmology as they emerge from either the LHC or new astronomical observations,” says Tremaine.

From a physicist’s point of view, “dark” matter or energy is something that does not emit or absorb any light, and thus does not interact with electromagnetic radiation in the same way that a table or the stars do. Since almost all astronomical observations are based on collecting light through telescopes, dark matter is extremely difficult to detect and characterize. Although the existence of dark matter was suggested in the 1930s, only in the last couple of decades have scientists made substantial progress in understanding its properties. We now know that every galaxy is surrounded by a halo of dark matter that can be detected indirectly by observing its gravitational effects. Still, there is no direct evidence about its actual nature.

“In galaxies, only a small fraction of the normal matter can give off light. The question is how can you use this tiny portion of bright things to probe the dark side­—the dark matter and dark energy—and also, on the other hand, understand the formation and evolution of the bright side of these galaxies,” says Zheng Zheng, a Member in the School of Natural Sciences. Zheng’s current research involves using the halo occupation distribution—the relation between galaxies and dark matter at the level of individual dark matter halos—to learn about cosmology and galaxy formation and evolution.

Most cosmologists believe that the dark matter represents some new form of elementary particle that was formed in large amounts during the Big Bang, and has survived until the present day. “There is a question of whether the evidence for dark matter is really evidence for some new form of matter, or evidence that the law of gravity is breaking down in some way,” says Tremaine. “There are people who argue that what we are seeing is not this invisible form of matter for which there is no other evidence, but a fundamental change in the laws of gravity on astronomical scales. I think the preponderance of evidence favors the interpretation that it is dark matter. But I have a lot of respect for several of the people who have been arguing for modifications to the laws of gravity. My guess is that dark matter is going to win out.”

The leading candidate that might explain the fundamental makeup of dark matter is a hypothetical particle called the weakly interacting massive particle (WIMP) whose existence may be confirmed with NASA’s recently launched ­Gamma-­ray Large Area Space Telescope (GLAST).

While gamma rays originate from a multitude of ­high-­energy, astronomical sources, such as black holes and exploding stars, current theory suggests they can also come from WIMPs. Predicted by supersymmetry, a theory that extends the Standard Model, WIMPs could annihilate when they interact or undergo free decay. In either case, if gamma rays were among the secondary particles, these might be detected by GLAST.

Elaborate computer simulations have provided clues about the nature of dark matter and how clusters and superclusters of galaxies evolved out of a slight lumpiness in the early universe. Over time, gravity has amplified the lumpiness (see cover illustration).

Member Michael Kuhlen has worked on modeling the lumpiness of dark matter, employing the highest resolution numerical simulations available, including a recently completed one billion particle simulation that resolves dark matter structure in the very inner, dense regions of a galaxy like our own (see illustration, right). “We start very early on and create an artificial computer representation of the dark matter distribution, and we allow it to evolve forward in time by solving just the equations that govern how dark matter clumps under the influence of gravity,” says Kuhlen. “Then we can look at the distribution of dark matter today after a simulation has progressed for 13 billion years on the computer.”

By calculating a wide range of dark matter particle masses and interaction probabilities based on particle physics calculations, Kuhlen and his collaborators have predicted that GLAST might very well detect the signatures of annihilations in dark matter clumps. “We want to know how we could possibly detect it because so far we only know about dark matter through indirect measurements, by detecting the influence of its gravity,” says Kuhlen.

Gravitational lensing is another way to discern evidence of dark matter in images of large clusters of galaxies. Strange optical effects are formed when the light from a very distant, bright source is warped by the gravitational pull of matter in the cluster. The intensity of the distortion indicates the strength of the cluster’s overall gravitational field and total mass.

Member Rachel Mandelbaum has been working on using gravitational lensing to measure the properties of the distribution of dark matter and dark energy. “My main area of research is weak gravitational lensing, the very small perturbations in the shapes of distant source galaxies due to massive foreground galaxies and clusters,” says Mandelbaum. “There are many useful applications of weak lensing due to the fact that it is sensitive to the full matter density projected along the line of sight, regardless of whether that matter is luminous—visible through a telescope—or not—the mysterious dark matter.” In one project, Mandelbaum and her collaborators have been comparing weak lensing by and gravitational clustering of galaxies in an effort to constrain and possibly exclude theories of modified gravity, which cast doubt on the existence of dark energy and the theory of general relativity.

“With dark matter, there are all sorts of approaches by which we might reasonably expect to learn a lot more,” says Goldreich. “Dark matter clusters gravitationally, so we more or less know where it is. We think that it is composed of a non-relativistic particle that moves in the gravitational fields of galaxies about as fast as the stars do.”

“Dark energy doesn’t appear to cluster. Our best guess is that it is a pervasive ether that bathes the universe. At the moment we don’t have any clever idea of how to investigate its properties other than to accurately measure the history of the expansion of the universe and the growth of structure. Trying to sense dark energy is like trying to sense the air in a room. You can feel the air when you wave your hand, but we lack a comparably sensitive detector for dark energy.”

Since the late 1920s, astronomers have known that the universe is expanding. Physicists thought that the combined gravitational pull of galaxies would gradually slow cosmic expansion, but in 1998 two independent research teams discovered what has become known as dark energy. By monitoring the light from distant supernovas to measure how the expansion rate of the universe had changed over time, the researchers found that while the universe’s expansion had been slowing down for its first few billion years, more recently it had started speeding up. Current theory is that when the universe was young and dense, the gravitational attraction of matter held sway. As the universe continued to expand and thin out, the repulsive effects of dark energy became dominant.

“The abundance of galaxy clusters is very sensitive to the amount of dark matter and the amount of dark energy,” says Member Douglas Rudd (see illustration, page 1). “The more dark matter you have, the more matter you have, the more objects you form. The more dark energy you have, the more formation is suppressed. So there is a competition between dark matter and dark energy.”

Rudd uses numerical simulations to test the relationship between what is known as the Sunyaev-Zel’dovich effect and the mass of galaxy clusters (see illustration, page 4). The SZ effect, which is the imprint left on the cosmic microwave background when light travels through the hot gas in a cluster’s core, allows researchers to locate galaxy clusters over cosmic history. By measuring the number of clusters at many different redshifts, or times, in the universe, researchers can map out how dark energy starts to affect the growth of structure.

“We can predict the number of dark matter halos very well from theories and from simulations that have been done over the last several decades,” says Rudd. “What is not clear is how to go from observations of the SZ effect to mass very accurately, which is necessary if you want to get information about dark energy.”
Although scientists since Einstein had recognized that some form of dark energy was a theoretical possibility, it was regarded as nothing more than some exotic possibility for which there was no independent evidence. Over the last decade, it was gradually realized that if you added a component of dark energy to more sophisticated models of the properties of the universe—its expansion history, the clustering of galaxies, the clustering of the background radiation—suddenly everything fit much better, and over the past decade, that fit has continued to improve.

The hope is that the LHC will provide new data that will eliminate a lot of possibilities that have been discussed as no longer consistent with the data, while providing enough clues for particle physicists to move forward and gain a better understanding of basic laws that drive the universe. That increased understanding would then lead to definite predictions about the properties of the universe that would give more insight to cosmologists. “It is far from certain that the LHC or other projects will enable us to achieve the goal of understanding the nature of dark matter within the next few years,” says Tremaine, “but the prize will be really wonderful if it comes to pass.”