Structures in the Universe
The Large-Scale structures we see in the Universe today --- clusters of galaxies, voids, and filaments --- evolved from small fluctuations in the density of the Universe that likely formed in the first fraction of a second after the Big Bang. By studying these structures observationally and, in parallel, constructing theoretical models for how they form and grow, researchers at the Kavli Institute for Cosmological Physics and their colleagues are piecing together a coherent story for the formation of large-scale structure. While not all elements of this story are fully in place, many of them have been identified: we know that gravity is the engine that drives the growth of structure, we have good evidence that for most of the history of the Universe gravity acted primarily on the (as-yet-undetected but inferred) dark matter to grow structure, and we have a candidate theory' primordial inflation --- for the origin of the initial fluctuations.

With such a paradigm for structure formation in place, detailed studies of these large-scale structures become powerful probes of cosmology, including studies of the dark matter and of the more recently discovered dark energy. In 1998, there was a striking discovery: observations of Type Ia supernovae (SNe Ia) indicated that the expansion of the Universe is speeding up, not slowing down. This acceleration is caused either by a mysterious new component of the Universe --- dark energy --- or by a breakdown of Einstein's theory of gravity (General Relativity) at large scales. In either case, the implications for our understanding of the fundamental laws of nature and for cosmology would be profound.

These results have been verified by independent experimental techniques such as observations of fluctuations in the anisotropy of the cosmic microwave background radiation (CMB) by ground-based experiments, such as DASI, and most recently by the WMAP satellite, along with data from the Sloan Digital Sky Survey (SDSS) and other galaxy surveys on the large scale distribution of galaxies. One of the major research objectives of the Kavli Institute for Cosmological Physics is to use observations and simulations of structures in the Universe to measure the expansion history of the Universe and thereby probe the nature of the dark energy. KICP research in this area includes four complementary approaches listed below. The first three use surveys that reveal the distribution, abundance, and evolution of structures in the Universe, in order to explore the underlying cosmology, especially the dark energy:

Strong and weak gravitational lensing
Picture: Strong and weak gravitational lensing
A dense mass concentration in the Universe, e.g., a galaxy or cluster of galaxies, can act like a lens, gravitationally bending light from objects behind it. In strong lensing, this effect is so pronounced that two or more images of the same background object can be seen. The Sloan Digital Sky Survey is discovering new gravitationally lensed quasars and is allowing refinements in the understanding of how the frequency of lensed quasars constrains the dark energy density of the Universe.

A lensed quasar, 0903+5028, discovered by the SDSS: two images of the same quasar are seen; the lower right image is superposed on the foreground lensing galaxy. D. Johnston, et. al, Astronomical Journal, 126, 2281 (2003).

Such strongly lensed systems are relatively rare; more common is weak lensing, in which light bending by the distribution of dark matter in the Universe causes slight distortions in the images of distant galaxies. The pattern of such distortions in the SDSS is being used to probe how the distribution of luminous galaxies is related to that of the underlying dark matter and also to constrain the cosmic density of dark matter. In the future, these techniques will be applied in the deeper, proposed Dark Energy Survey (DES) to probe the nature of the dark energy; the DES will measure shapes for approximately 300 million galaxies in a survey covering 5000 square degrees in 4 optical passbands using a new wide-field camera to be built for the Blanco 4-meter telescope at Cerro Telolo Interamerican Observatory.

Galaxy clustering
Picture: Galaxy clustering
The large size of the SDSS is enabling researchers to study in fine detail how galaxies with different properties (e.g., color, luminosity) cluster in the Universe. These results are providing new insights into how galaxies are distributed in dark matter halos and are also constraining the dark matter density and other cosmological parameters. In the future, studies of galaxy clustering as a function of redshift in the DES will provide a new probe of the dark energy.

KICP theorists are developing models of galaxy clustering to advance our understanding of physical processes that determine how galaxies are distributed in space. A key question is how many galaxies are populating different overdense regions. For example, how many galaxies would be typically found in a cluster of a given mass? KICP researchers are trying to answer this question using high-resolution supercomputer simulations of structure formation in the Universe [more].

These figures show a simulation of a galaxy cluster that includes both dynamics of dark matter and gas. Figure 1 shows distribution of dark matter within and around the cluster, while Figure 2 shows the distribution of stars. The clumps of stars here would be identified as galaxies in observations.

The Sunyaev-Zel'dovich Effect (SZE)
Picture: The Sunyaev-Zeldovich Effect (SZE)
A complementary approach to studying the dark energy is to capitalize on a remarkable property of the Sunyaev-Zel'dovich effect (SZE). The gas between galaxies in a cluster contains extremely energetic electrons. When photons from the cosmic microwave background radiation encounter these electrons, they can gain energy when scattering from the electrons, via a process called the inverse-Compton effect. This effect can be observed by measuring the intensity of CMB photons at different wavelengths in the direction of a cluster.

The Sunyaev-Zel'dovich effect (SZE) toward the z = 0.54 cluster of galaxies CL 0016+16. The SZ effect shown by the contours is a depression in the intensity of the long-wavelength CMB radiation due to inverse Compton scattering of the CMB photons to higher energies. The false color image is the X-ray emission from the hot cluster gas. As a spectral distortion of the CMB, the intensity of the SZE signal is independent of the redshift of the cluster and only dependent on the cluster properties. In this way, SZE surveys offer an ideal probe of the high redshift universe.

he SZ Array (SZA) is an interferometric array of eight 3.5m telescopes which operate at 26 - 36 GHz and 85-115 GHz. The initial one-year survey will be conducted at 30 GHz and cover 12 square degrees. It will be coordinated with optical and X-ray surveys. It will provide tight constraints on the matter and dark energy densities of the universe, but not on the dark energy equation of state. In order to determine the equation of state, a larger area must be surveyed and the cluster gas properties must be better understood. The SZA will be used at 90 GHz and with the six 10.4 m telescopes of the OVRO mm-wave array to provide detailed images for pursuing cluster gas evolution studies. These studies are closely connected to ongoing numerical simulations which are designed to make detailed models of the properties of the gas in clusters.

The South Pole Telescope (SPT) will greatly increase the size of the sample. The planned SPT is an 8m off-axis telescope equipped with novel new 1000 element bolometric array. It will be able to cover 4000 square degrees in a single Austral winter. In principle, such should provide a tight constraint on the equation of state of the Dark Energy. These SZE surveys will directly measure the abundance of clusters at high redshifts. Also, they will constrain non-Gaussianity in the initial conditions providing a new constraint on inflation.
The proposed DES will provide photometric redshift estimates for the majority of the clusters detected by the SPT, a necessary ingredient for measuring the cluster abundance.

Observations of Type Ia Supernovae
Picture: Observations of Type Ia Supernovae
In 1998, two groups used measurements of Type Ia supernovae to demonstrate that the expansion of the Universe is accelerating, ushering in the current fascination and preoccupation with dark energy. Type Ia supernovae are extremely useful cosmological probes because they are nearly standard candles --- a class of objects with roughly the same intrinsic luminosity when they reach their brightest point --- and can therefore be used to measure cosmological distances with high accuracy. By measuring distances and therefore lookback times along with redshifts (and therefore the cosmological scale factor), one can trace the expansion history of the Universe and thereby probe the properties of the dark energy. Supernovae of Type Ia are uniquely suited to this task, because their distances can be measured to high accuracy over cosmologically great distances.