Dark energy is a mysterious force that is accelerating the expansion of the universe. The expansion has slowed the clustering of dark matter, one of the universe's main building blocks. If we could measure the precise history of the Hubble expansion, and chart the development of mass structure, we could test theories of the physics of dark energy. Diffrent theories predict different scenarios.
The LSST will enable scientists to study the dark energy in four different and complementary ways:
- The telescope will image dark matter over cosmic time, via a "gravitational mirage." All the galaxies behind a clump of dark matter are deflected to a new place in the sky, causing their images to be distorted. This is effectively 3-D mass tomography of the universe.
- Galaxies clump in a non-random way, guided by the natural scale that was imprinted in the fireball of the Big Bang. This angular scale will be measured over cosmic time by LSST, yielding valuable information on the changing Hubble expansion.
- The numbers of huge clusters of dark matter are a diagnostic of the underlying cosmology. By charting the numbers of these (via their graviational mirage) over cosmic time, LSST will place another sensitive constraint on the physics of dark energy.
- Finally, a million supernovae will be monitored by LSST, giving yet another complemtary view of the history of the Hubble expansion.
Something Is Ripping The Universe Apart
Recently the composition of the universe has become even more puzzling: observations imply an acceleration of the universe's expansion over the past few billion years. In order to explain such an acceleration, we need "dark energy" with large negative pressure to generate a repulsive gravitational force. The evidence comes from studies of the total energy density of the universe and from supernova observations. Precision measurements of the cosmic microwave background have shown that the total energy density of the universe is very near the critical density needed to make the universe flat (i.e. the curvature of space-time, defined in General Relativity, goes to zero on large scales). Since energy is equivalent to mass (Special Relativity: E = mc2), this is usually expressed in terms of a critical mass density needed to make the universe flat. Ordinary matter such as stars, dust, and gas account for only 5% of the necessary mass density. Observations have shown that dark matter cannot account for more than ~25% of the critical mass density. Both the microwave background and supernova observations suggest that dark energy should make up ~70% of the critical energy density. When added to the mass-energy of matter, the total energy density is consistent with what is needed to make the universe flat.
Over the expansion history of our universe, densities have fallen by factors of trillions. Why is the dark energy density today within a factor of three of that of dark matter, whereas it evolves very differently with time? Moreover, the dark matter density is only a factor of five larger than that of ordinary matter. Understanding this may lead to advances in 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. Now, even the notion that the galaxies and stars comprise most of our universe has been abandoned. Emerging is a universe largely governed by dark matter and an even stranger dominance of a smoothly distributed and pervasive dark energy.
Dark Energy And The Fate Of The Universe
Cosmologists understand almost nothing about dark energy even though it appears to comprise about 70 percent of the universe. They are desperately seeking to uncover its fundamental properties: its strength, its permanence, and any variation with direction. The evolution of the universe is governed by the amount of dark matter and dark energy. The densities of dark matter and dark energy scale differently with cosmic scale as the universe expands. This evolution in cosmic scale is schematically shown in the figure below for several cosmologies. In a universe with a high density of dark matter, the Hubble expansion continues to decelerate due to the gravitation attraction of the dark matter filling the universe, ending in a big crunch. In a universe with a lower critical density of dark matter, the expansion coasts. In a universe with dark energy as well as dark matter, the initial deceleration is reversed at late times by the increasing dominance of the dark energy.
If the hypothetical dark energy continues to dominate the universe's energy balance, then the current expansion of space will continue to accelerate, exponentially. Structures which are not already gravitationally bound will ultimately fly apart. The Earth and the Milky Way would remain undisturbed while the rest of the universe appears to run away from us.
The nature of dark energy is currently a matter of speculation. Some believe that dark energy might be "vacuum energy", represented by the "cosmological constant" (Λ) in general relativity, a constant uniform density of dark energy throughout all of space that is independent of time or the universe's expansion. This notion was introduced by Einstein, and is consistent with our limited observations to date. Alternatively, dark energy might vary with cosmic time. Only new kinds of observations can settle the issue.
PROBING THE NATURE OF DARK ENERGY
What kind of universe do we live in? To test theories of dark energy we would like to measure the way the expansion of our universe 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. Dark energy affects two things: distances and the growth of mass structure. Measuring how dark matter structures and ratios of distances grow with cosmic time -- via LSST weak gravitational lensing observations -- will provide clues to the nature of dark energy. A key strength of the LSST is its ability to image huge volumes of the universe. Such a probe will be a natural part of the all-sky imaging survey. Billions of distant galaxies will have their shapes and colors measured. Sufficient color data will be obtained for an estimate of the distance to each galaxy. This will enable a unique probe of the physical nature of the dark energy that appears to fill the universe.
If, due to this dark energy, the expansion of the universe has been accelerating the development of mass structures via ordinary gravitational infall will be impeded. The time development of mass concentrations is sensitive to the physical nature of the dark energy itself: the so-called equation of state (pressure divided by energy density). In its deep wide-angle survey the LSST will be able to pin down the equation of state of dark energy to better than a few percent, at high confidence. Due to its wide coverage of the sky, LSST is uniquely capable of detecting any variation in the dark energy with direction. In turn, this will 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 measure cosmic shear 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.