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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:
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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.
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 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).
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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.
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 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.
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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.
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 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.
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The 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.
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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.
KICP researchers are involved in three projects that will yield new SNe Ia data and improved constraints on the dark
energy. The Nearby Supernova Factory will carry out detailed spectroscopic studies of about 300 nearby supernovae
using a specially built integral-field spectrograph on the U. Hawaii 88-in telescope. This sample will increase the
dark energy precision of on-going high-redshift SN Ia surveys by a factor of two and will also enable detailed
studies of the systematic properties of the SN Ia population. The proposed SDSS II will be used in part to discover
and measure ~200 SN Ia in the intermediate redshift range z=0.1-0.3; as a dark energy probe, it will complement the
Factory and on-going surveys at higher redshift. The proposed Dark Energy Survey will measure roughly 1900 SNe Ia at
higher redshift (z=0.3-0.8) through repeat scanning of 40 square degrees of sky.
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