THE UNSEEN UNIVERSE:
DARK MATTER AND DARK ENERGY
The vast areas between the stars and galaxies appear empty and dark, but this impression is misleading. We now know that 96 percent of the universe is dominated by unknown and unseen forms of mass and energy: dark matter and dark energy. Two fundamental goals of cosmology are to determine the composition of the energy and matter in the universe. It appears that the simplest expectation, a universe made of ordinary matter (so-called "baryons" — the familiar neutrons and protons) is wrong. Instead, we appear to live in a universe which challenges our understanding of physics.
Modern cosmology has been built on two pillars of radiation: the residue from the big bang and the distribution and spectra of stars and galaxies. Yet it has long been known that mass, not luminosity, is the key to the structure of the universe. This is because gravity plays a central role in the formation of structure. Over cosmic time, "over-dense" regions become still denser. Tiny ripples of density existing 300,000 years after the big bang have grown into the complexity of mass structure — galaxies to clusters to super-clusters of galaxies — we see in today's universe some 14 billion years later. On the largest of scales, the overall expansion history of the universe is governed by its mass-energy (Einstein taught us that mass and energy are related). Since mass could not be seen directly, astronomers have until recently used the luminosity of the trace amounts of ordinary matter in existence as a proxy for total mass in cosmological studies. Yet this ordinary matter, the baryons we are made of, cannot be the chief component of most of the mass in the universe.
A huge amount of dark matter — roughly ten times as much mass as there is in all the stars and gas and dust — controls the early evolution of structure in the universe. The dark matter is thought to be some very different kind of particle created during the hot big bang that interacts only weakly with the familiar particles of "normal" matter. In the earliest moments of the universe, corresponding to temperatures and energies far higher than any attainable or even imaginable on Earth, a legacy in the form of dark-matter particles was created. This legacy is detectable today in its cumulative gravitational effects on large-scale structures in the universe.
OVER THE LAST FEW years the composition of the universe has become even more puzzling, as the observed luminosity of Type Ia supernovae at high red shift, and other observations, appear to imply an acceleration of the universe's expansion in recent times. In order to explain such an acceleration, we need dark energy with large negative pressure to generate a repulsive gravitational force. Even more puzzling is the fine-tuning of parameters which seems to be required to explain why the dark-energy density today is about the same as that of dark matter, whereas it evolves very differently with time. Moreover, this density is only a factor of ten larger than that of baryons and neutrinos. This may imply, as the Ptolemaic epicycles did, that we are lacking a sufficiently deep understanding of fundamental physics. It is possible that what we call dark matter and dark energy arise from some unknown aspect of gravity. Thus, the highest energies and the universe on the largest scales are connected. Today the worlds of particle physics and cosmology are coming together in a transformed world view: Copernicus dethroned Earth from a central position in the cosmos, and Edwin Hubble demoted our galaxy from any significant location in space. Now, even the notion that the galaxies and stars comprise most of our universe is being abandoned. Emerging is a universe largely governed by dark matter and, we are beginning to think, by an even stranger dominance of a smoothly-distributed and pervasive dark energy.
HOW DO WE STUDY dark matter if we cannot see it? These mountains of mass will bend light rays from background galaxies like a lens and create "cosmic mirages." This tool was made possible by the discovery of a population of distant galaxies so numerous on the sky that they could be used as sources for the statistical reconstruction of an image of the foreground dark matter that served as the lens. From their warping of the visible matter behind them, we now can see the dark matter clumps, map them and chart their development over cosmic time. To see the dark matter, we have to "invert" the cosmic mirage it produces. We have to look deep enough and wide enough into the background universe so that there are thousands of galaxies projected near every foreground lens. Exploring the full range of mass structures will require a new facility which will image billions of distant galaxies.
How much dark matter in our universe resides outside clusters of galaxies? If most of the mass in clusters is in a smooth distribution extending out millions of light years, perhaps most of the dark matter of the universe is distributed more broadly than clusters. To make an analogy closer to home, clusters are the Mount Everests of the universe. But most of the mass in Earth's mountains resides in the more numerous foothills. Though we are currently limited to fields of view less than a degree across, telescopes are nevertheless being used to probe this universal dark-matter distribution.
Our new observational mass maps cover a limited range of scales. There are big and small clusters of mass, and there are what appear to be filaments of mass — enough mass to add up to about a third of the density that, in the absence of dark energy, would be required to ultimately slow the expansion of the universe to zero. Through these first probes, we have glimpsed a complex universe of mass: dilute filaments of mass coexisting with piles of dark matter centered on clusters of galaxies. This complex dark-matter structure took billions of years to grow. Its growth rate is predictable for a given model of the expanding universe. Probes of other features of the universe — from primeval deuterium to tiny fluctuations in the heat left over from the big bang to supernovae at large distances — suggest that some form of dark energy, when combined with the gravity of the dark matter, creates a flat cosmic geometry in which parallel light rays remain parallel. To determine what this dark energy is and how we can probe its physics, we look at its influence on the expansion rate of the universe. Dark energy acts against gravity, tending to accelerate the expansion.
To test theories of dark energy we would like to measure the way some volume which is co-moving with the Hubble flow changes with cosmic time. For a universe with a given mass density, the time history of the expansion encodes information on the amount and nature of dark energy. Measuring this change in co-moving volume by taking "snapshots" of mass clusters at different cosmic times would provide clues to the nature of dark energy. In turn, this would tell us something about physics at the earliest moments of our universe, setting the course for its future evolution.
The world of quantum gravity at a fraction of a second after the big bang, when the universe was so hot and dense that even protons and neutrons were broken up into a hot soup of quarks, connects to the world as we now see it — a vast expanding cosmos extending out 14 billion light-years. Dark energy and dark matter are relics of the first moments when unfamiliar physics of quantum gravity ruled. A route to understanding dark matter and probing the nature of dark energy is to count the number of mass clusters over the last half of the age of the universe — at a time when dark energy apparently had its greatest influence. LSST does this in several independent ways. These probes of the nature of dark energy by LSST are complimentary to those of space missions measuring the cosmic microwave background and very distant supernovae. Indeed, since we understand so little about dark energy, it is prudent to pursue all these lines of investigation.
MASS IN THREE DIMENSIONS
The faintest galaxies have a range of colors, each one's color depending on its type and its distance from us. The most distant galaxies have their spectra shifted to longer, redder wavelengths by the Hubble expansion, and their light has taken up to ten billion years to travel to us. Using the colors of the galaxies, it is possible to gauge the distance to the background galaxies. Mirages also rely on distance. This is the clue that unlocks the universe of mass in three dimensions; the more distant the source, the more warped its image. If there is a foreground mass, the mirage effect on the background galaxies is stronger for more distant galaxies.
By measuring both the warp and the distances to the background galaxies, it is possible to reconstruct the mass map and also to place the mass at its correct distance. This enables the exploration of mass in the universe, independent of light, since only the light from the background galaxies is used. By exploring mass in the universe in three dimensions we are also exploring mass at various cosmic ages. This is because mass seen at great distance is mass seen at a much earlier time. So we can chart the evolution of dark matter structure with cosmic time.
Surveying the numbers of cosmic mass clusters in our universe will ultimately lead to precision tests of theories of dark energy. To fully open this novel window of the three-dimensional universe of mass history, we need a new telescope and camera very unlike what we have now. We need LSST. Advances in technology have equipped us to mine the distant galaxies for data — in industrial quantity. LSST's wide-angle gravitational lens survey will generate millions of gigabytes of data and intriguing opportunities for unique understanding of the development of cosmic structure. Our challenge is twofold. These galaxies are faint, and we need to capture images of billions of them. LSST's combination of large light-collecting capability and unprecedented field of view will for the first time open this unique window on the physics of our universe. LSST will provide a wide and deep view of the universe, allowing us to conduct full 3-D mass tomography to chart not only dark matter, but the presence and influence of dark energy.
Do we trust our current view of the universe? Combining these results with other cosmic probes will lead to multiple tests of the foundations of our model for the universe. What will our concept of the universe be when those answers are in? Perhaps the most interesting outcome will be the unexpected; a clash between different precision measurements might prove to be a hint of a grander structure, possibly in higher dimensions. LSST provides that opportunity.