At the faint magnitudes reached by large telescopes, the sky is studded with tens of billions of faint distant blue galaxies. In recent years astronomers have become adept at mapping the dark matter associated with known galaxy clusters using these background galaxies as a cosmic wallpaper for weak gravitational lensing analyses.
This is an image of dark matter in a 2 degree by 2 degree field of the sky from the Deep Lens Survey. Many mass clusters may be seen in projection. With color redshift information on the background galaxies, three-dimensional maps can be constructed. The LSST is needed to make these tomographic mass maps over a cosmologically significant area.
With multi-wavelength deep imaging of the faint blue galaxies, we can construct photometric redshifts for them and go beyond a simple foreground/background paradigm. Photometric redshifts enable tomographic analysis of slices of the projected sky in redshift bins. By obtaining weak lensing maps for sources at a variety of redshifts, we can obtain a three-dimensional mass map of the universe back to half its current age. Only the Large Synoptic Survey Telescope with its combination of huge field and large light grasp would enable such a survey in our lifetimes.
Structures as large as 500 million light-years are known to exist. The Large Synoptic Survey Telescope will for the first time map the evolution of these mass structures over cosmic time. This will directly test theories of the evolution of our universe and the nature of dark matter and dark energy.
32-Mpc simulation of large-scale structure in a standard cold-dark matter Universe. Courtesy R. Cen, Princeton University.
To undertake tomographic gravitational lens reconstruction of dark matter images at high redshift and large look-back times requires superb imaging of distant background galaxies. At 29th magnitude per square arcsecond surface brightness, there is a distant blue galaxy every several arcseconds on the sky. One requires good angular resolution over a 10 square degree field of view, coupled with the light gathering power of an 8-meter class mirror.
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.
