30 research outputs found
Formation of Galaxy Clusters
In this review, we describe our current understanding of cluster formation:
from the general picture of collapse from initial density fluctuations in an
expanding Universe to detailed simulations of cluster formation including the
effects of galaxy formation. We outline both the areas in which highly accurate
predictions of theoretical models can be obtained and areas where predictions
are uncertain due to uncertain physics of galaxy formation and feedback. The
former includes the description of the structural properties of the dark matter
halos hosting cluster, their mass function and clustering properties. Their
study provides a foundation for cosmological applications of clusters and for
testing the fundamental assumptions of the standard model of structure
formation. The latter includes the description of the total gas and stellar
fractions, the thermodynamical and non-thermal processes in the intracluster
plasma. Their study serves as a testing ground for galaxy formation models and
plasma physics. In this context, we identify a suitable radial range where the
observed thermal properties of the intra-cluster plasma exhibit the most
regular behavior and thus can be used to define robust observational proxies
for the total cluster mass. We put particular emphasis on examining assumptions
and limitations of the widely used self-similar model of clusters. Finally, we
discuss the formation of clusters in non-standard cosmological models, such as
non-Gaussian models for the initial density field and models with modified
gravity, along with prospects for testing these alternative scenarios with
large cluster surveys in the near future.Comment: 66 pages, 17 figures, review to be published in 2012 Annual Reviews
of Astronomy & Astrophysic
The Reionization of the Universe by the First Stars and Quasars
The first light from stars and quasars ended the ``dark ages'' of the
universe and led to the reionization of hydrogen by redshift 7. Current
observations are at the threshold of probing this epoch. The study of
high-redshift sources is likely to attract major attention in observational and
theoretical cosmology over the next decade.Comment: 60 pages, including 21 figures; to be published in the 2001 Volume of
Annual Reviews of Astronomy and Astrophysics; A more extensive review, for
Physics Reports, is also available, with a different astro-ph number, or at
http://www.cita.utoronto.ca/~barkana/review.htm
Probing the Universe with Weak Lensing
Gravitational lenses can provide crucial information on the geometry of the
Universe, on the cosmological scenario of formation of its structures as well
as on the history of its components with look-back time. In this review, I
focus on the most recent results obtained during the last five years from the
analysis of the weak lensing regime. The interest of weak lensing as a probe of
dark matter and the for study of the coupling between light and mass on scales
of clusters of galaxies, large scale structures and galaxies is discussed
first. Then I present the impact of weak lensing for the study of distant
galaxies and of the population of lensed sources as function of redshift.
Finally, I discuss the potential interest of weak lensing to constrain the
cosmological parameters, either from pure geometrical effects observed in
peculiar lenses, or from the coupling of weak lensing with the CMB.Comment: To appear Annual Review of Astronomy and Astrophysiscs Vol. 37. Latex
and psfig.sty. Version without figure, 54 pages, 73Kb. Complete version
including 13 figures (60 pages) available on ftp.iap.fr anonymous account in
/pub/from_users/mellier/AnnualReview ; file ARAAmellier.ps.gz 1.6 M
Bose-Einstein condensation of photons in an optical microcavity
Bose-Einstein condensation, the macroscopic ground state accumulation of
particles with integer spin (bosons) at low temperature and high density, has
been observed in several physical systems, including cold atomic gases and
solid state physics quasiparticles. However, the most omnipresent Bose gas,
blackbody radiation (radiation in thermal equilibrium with the cavity walls)
does not show this phase transition, because the chemical potential of photons
vanishes and, when the temperature is reduced, photons disappear in the cavity
walls. Theoretical works have considered photon number conserving
thermalization processes, a prerequisite for Bose-Einstein condensation, using
Compton scattering with a gas of thermal electrons, or using photon-photon
scattering in a nonlinear resonator configuration. In a recent experiment, we
have observed number conserving thermalization of a two-dimensional photon gas
in a dye-filled optical microcavity, acting as a 'white-wall' box for photons.
Here we report on the observation of a Bose-Einstein condensation of photons in
a dye-filled optical microcavity. The cavity mirrors provide both a confining
potential and a non-vanishing effective photon mass, making the system formally
equivalent to a two-dimensional gas of trapped, massive bosons. By multiple
scattering off the dye molecules, the photons thermalize to the temperature of
the dye solution (room temperature). Upon increasing the photon density we
observe the following signatures for a BEC of photons: Bose-Einstein
distributed photon energies with a massively populated ground state mode on top
of a broad thermal wing, the phase transition occurring both at the expected
value and exhibiting the predicted cavity geometry dependence, and the ground
state mode emerging even for a spatially displaced pump spot