|
Clusters of galaxies --
group page
Clusters of galaxies are the
largest
bound structures in the Universe. They typically contain a few thousand
galaxies, but most of the mass is in the form of dark matter, and hot plasma at a temperature of 107-108 K. The cluster
abundance probes cosmological parameters and the evolution of the
universe while
studies of the physical processes within
the cluster itself show how they formed and how they continue to evolve.
At KIPAC clusters are studied
across a broad range of wavelengths including optical, radio,
mm-sub/mm, X-rays and gamma rays. At optical wavelengths, in addition
to light from the
cluster galaxies themselves, background galaxies are visible that
are lensed by
the
enormous quantity of matter. The hot plasma (the
intracluster medium or ICM) can
studied in
X-rays using both
imaging and spectroscopic techniques. The
plasma also
scatters
light from the Cosmic Microwave Background (CMB) causing a distortion
to the CMB called the
Sunyaev-Zeldovich (SZ) effect. Observations of
the SZ
effect by KIPAC members allow measurements of cluster peculiar
velocities -- the
velocity of cluster over and above the Hubble flow.
|
 An observation of Abell 2218 is shown that combines galaxies with strong lensing (inset), SZ (white contours), and the X-ray image (red/orange plot). |
 This figure shows a weak lensing map (contours) superposed over an optical image of a galaxy cluster near the strongly gravitationally lensed quasar RXJ0911 |
Gravitational Lensing -- group page
Gravitational lensing is a powerful tool for studying the mass distributions of galaxies and clusters, observing the faintest and most distant objects, and inferring the properties of the Universe's expanding geometry. We are involved in studying all but the very weakest of gravitational lensing effects, from multiply-imaged quasars to weak lensing by galaxy clusters to the microlensing of stars by compact halo objects. We are also playing a leading role in the optical strong gravitational lens surveys to be performed in the not-too-distant future by SNAP and LSST, observatories that will enable cosmological and astrophysical gravitational lensing experiments of unprecedented precision. return to top |
| Cosmic Microwave Background Radiation -- group page The spatial distribution of polarization is the third important property of the Cosmic Background Radiation (after the spectrum and the spatial distribution of anisotropies). Although the signal is tiny (1-10% of the temperature fluctuations) the scientific rewards are large. The spatial pattern of CMB polarization can be decomposed in a component with curl (the B-mode) and a gradient-only component (E-mode). A measurement of B-mode polarization can help to distinguish between inflationary models. Stanford is involved in several efforts to measure CMB polarization, including the QUaD experiment, a 62-element bolometric array receiver that will be operational in late 2004, and the Planck satellite, which will launch in 2007. return to top |
 The QUaD telescope being installed at the South Pole in early 2004. |
 The Crab Nebula in X-Rays .
The Crab Nebula contains one of the most famous pulsars.
Credit: Chandra X-ray Observatory, NASA |
Neutron
stars Neutron stars are one of the end products of stellar
evolution and are formed during the supernova explosion of massive
stars. They are seen in many incarnations, such as as radio
pulsars, accreting X-ray pulsars, isolated cooling neutron stars, and
magnetars, and the observations ofneutron stars cover the entire
electromagnetic spectrum, from radio through
X-rays and gamma-rays, possibly in the future, gravitational
waves. Researchers at KIPAC study neutron stars to better understand
the behavior of matter and radiation in the strong gravity, high
densities, and large magnetic fields characteristic of neutron star
environments.
return to top |
|
Optical
Surveys
Large-scale surveys of the
optical sky can be used for a wide variety
of astronomical studies, from finding and tracking near-earth
asteroids to understanding the nature of dark matter and dark
energy. Members of KIPAC are involved in analysis of current
existing survey data including those from the Sloan Digital Sky Survey (SDSS), the
Hubble Space Telescope Deep Fields (e.g., GOODS and UDF), the Deep Lens Survey (DLS), and others.
KIPAC is also involved in hardware development and scientific/mission
planning of future observatories that will probe deeper into the
universe: the Large Synoptic Survey Telescope (LSST) and the Supernova
Acceleration Probe (SNAP).
return to top |
 |
 KIPAC members are part of the CDMS experiment, which is searching for direct interactions between baryons and matter |
Dark
Matter
Dynamical estimates of the masses of galaxies and clusters show that the total mass greatly exceeds the observed luminous mass. More recently, the density of baryons has been measured independently, and is too small by a factor of 5 to explain the observed gravitational mass in the universe. The most recent accurate measurements from the anisotropies in CMB and the distances to Type Ia supernovae have converged on a consistent picture -- the "concordance model" -- with 4.4% baryons, 22% dark matter and 73% dark energy. The nature of both dark matter and dark energy, which make up approximately 95% of the energy budget of the universe, is completely unknown. To explain dark matter, a new
particle is needed that is weakly
interacting
and stable over the age of the universe. Various models of particle
physics beyond the standard
model
provide candidates, and it is the job of theorists and
experimentalists to understand how they might be
detected, and then to build
experiments to attempt detection. One method is to directly
measure the expected, but rar,e interactions between dark matter and
baryons. The CDMS project,
part of which is based at Stanford, is
attempting this measurement. It could also be possible to indirectly
measure radiation from rare dark matter annihilation events in our
galactic halo
using the GLAST
satellite, which will launch in 2007 and in which
Stanford is playing a major role.
return to top |
Contact us
Kavli Institute for Particle Astrophysics and Cosmology
2575 Sand Hill Rd, MS 29
Menlo Park, CA 94025
650.926.2846
kipac@slac.stanford.edu