1,607 research outputs found
COBE Constraints on a Compact Toroidal Low-density Universe
In this paper, the cosmic microwave background (CMB) anisotropy in a
multiply-connected compact flat 3-torus model with the cosmological constant is
investigated. Using the COBE-DMR 4-year data, a full Bayesian analysis revealed
that the constraint on the topology of the flat 3-torus model with
low-matter-density is less stringent. As in compact hyperbolic models, the
large-angle temperature fluctuations can be produced as the gravitational
potential decays at the -dominant epoch well after the last
scattering. The maximum allowed number of images of the cell (fundamental
domain) within the observable region at present is approximately 49 for
and whereas for and
.Comment: 13 pages using RevTeX, 5 eps files, typos correcte
Cosmological Parameter Estimation from CMB Experiments
I review the general aspects of cosmological parameter estimation from
observations of the cosmic microwave background (CMB) temperature anisotropies
in the framework of inflationary adiabatic models. The most recent CMB datasets
are starting to give good constraints on the relevant parameters of
inflationary adiabatic models. They point toward a model consistent with the
basic predictions of inflation: a nearly flat universe, with a nearly scale
invariant spectrum of primordial fluctuation.Comment: 9 pages, 2 figures, invited talk, to appear in ``Cosmology and
Particle Physics'', Proc. of the CAPP 2000 Conference, Verbier, Switzerland,
July 2000, eds. J. Garcia-Bellido, R. Durrer and M. Shaposhnikov (AIP, 2001
Cosmology and Time
Time has always played a crucial role in cosmology. I review some of the
aspects of the present cosmological model which are more directly related to
time, such as: the definition of a cosmic time; the existence of typical
timescales and epochs in an expanding universe; the problem of the initial
singularity and the origin of time; the cosmological arrow of time.Comment: Invited talk given at the Time Machine Factory conference, October
14-19, 2012, Turin, Italy. To appear in the proceedings published by EPJ
(2013
Mapping the galactic gravitational potential with peculiar acceleration
It has been suggested recently that the change in cosmological redshift (the
Sandage test of expansion) could be observed in the next generation of large
telescopes and ultra-stable spectrographs. In a recent paper we estimated the
change of peculiar velocity, i.e. the peculiar acceleration, in nearby galaxies
and clusters and shown it to be of the same order of magnitude as the typical
cosmological signal. Mapping the acceleration field allows for a reconstruction
of the galactic gravitational potential without assuming virialization. In this
paper we focus on the peculiar acceleration in our own Galaxy, modeled as a
Kuzmin disc and a dark matter spherical halo. We estimate the peculiar
acceleration for all known Galactic globular clusters and find some cases with
an expected velocity shift in excess of 20 cm/sec for observations fifteen
years apart, well above the typical cosmological acceleration. We then compare
the predicted signal for a MOND (modified Newtonian dynamics) model in which
the spherical dark matter halo is absent. We find that the signal pattern is
qualitatively different, showing that the peculiar acceleration field could be
employed to test competing theories of gravity. However the difference seems
too small to be detectable in the near future.Comment: 11 pages, 10 figures, 3 tables, minor changes, accepted for
publication by MNRA
The time evolution of cosmological redshift as a test of dark energy
The variation of the expansion rate of the Universe with time produces an
evolution in the cosmological redshift of distant sources (for example quasar
Lyman- absorption lines), that might be directly observed by future
ultra stable, high-resolution spectrographs (such as CODEX) coupled to
extremely large telescopes (such as European Southern Observatory's Extremely
Large Telescope, ELT). This would open a new window to explore the physical
mechanism responsible for the current acceleration of the Universe. We
investigate the evolution of cosmological redshift from a variety of dark
energy models, and compare it with simulated data. We perform a Fisher matrix
analysis and discuss the prospects for constraining the parameters of these
models and for discriminating among competing candidates. We find that, because
of parameter degeneracies, and of the inherent technical difficulties involved
in this kind of observations, the uncertainties on parameter reconstruction can
be rather large unless strong external priors are assumed. However, the method
could be a valuable complementary cosmological tool, and give important
insights on the dynamics of dark energy, not obtainable using other probes.Comment: 9 pages, 2 figures. Matching published versio
Forecasting isocurvature models with CMB lensing information: axion and curvaton scenarios
Some inflationary models predict the existence of isocurvature primordial
fluctuations, in addition to the well known adiabatic perturbation. Such mixed
models are not yet ruled out by available data sets. In this paper we explore
the possibility of obtaining better constraints on the isocurva- ture
contribution from future astronomical data. We consider the axion and curvaton
inflationary scenarios, and use Planck satellite experimental specifications
together with SDSS galaxy survey to forecast for the best parameter error
estimation by means of the Fisher information matrix formal- ism. In
particular, we consider how CMB lensing information can improve this forecast.
We found substantial improvements for all the considered cosmological
parameters. In the case of isocurvature amplitude this improvement is strongly
model dependent, varying between less than 1% and above 20% around its fiducial
value. Furthermore, CMB lensing enables the degeneracy break between the
isocurvature amplitude and correlation phase in one of the models. In this
sense, CMB lensing information will be crucial in the analysis of future data.Comment: Accepted for publication in PR
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