163 research outputs found
Generation of seed perturbations from Quantum Cosmology
The origin of seed perturbations in the Universe is studied within the
framework of a specific minisuperspace model. It is shown that the `creation'
of the Universe as a result of a quantum transition from a flat empty spacetime
would lead to a flat FLRW (Friedmann Lema\^\i tre Robertson-Walker) Universe
with weak inhomogeneous perturbations at large wavelengths. The power spectrum
of these perturbations is found to be scale invariant at horizon crossing
(i.e., the Harrison-Zeldovich spectrum). It is also recognised that the seed
perturbations generated in our model would be generically of the isocurvature
kind.Comment: Plain Tex 24 Page
Suppressing CMB low multipoles with ISW effect
Recent results of Planck data reveal that the power in the low multipoles of
the CMB angular power spectrum, approximately up to , is significantly
lower than the theoretically predicted in the best fit CDM model. In
this paper we investigate the possibility of invoking the Integrated
Sachs-Wolfe (ISW) effect to explain this power deficit at low multipoles. The
ISW effect that originates from the late time expansion history of the universe
is rich in possibilities given the limited understanding of the origin of dark
energy (DE). It is a common understanding that the ISW effect adds to the power
at the low multipoles of the CMB angular power spectrum. In this paper we carry
out an analytic study to show that there are some expansion histories in which
the ISW effect, instead of adding power, provides negative contribution to the
power at low multipoles. Guided by the analytic study, we present examples of
the features required in the late time expansion history of the universe that
could explain the power deficiency through the ISW effect. We also show that an
ISW origin of power deficiency is consistent, at present, with other
cosmological observations that probe the expansion history such as distance
modulus, matter power spectrum and the evolution of cluster number count. We
also show that the ISW effect may be distinguished from power deficit
originating from features in the PPS using the measurements of the CMB
polarization spectrum at low multipoles expected from Planck. We conclude that
the power at low multipoles of the CMB anisotropy could well be closely linked
to Dark Energy puzzle in cosmology and this observation could be actually
pointing to richer phenomenology of DE beyond the cosmological constant
. (abbreviated)Comment: 20 pages, 7 figure
SCoPE: An efficient method of Cosmological Parameter Estimation
Markov Chain Monte Carlo (MCMC) sampler is widely used for cosmological
parameter estimation from CMB and other data. However, due to the intrinsic
serial nature of the MCMC sampler, convergence is often very slow. Here we
present a fast and independently written Monte Carlo method for cosmological
parameter estimation named as Slick Cosmological Parameter Estimator (SCoPE),
that employs delayed rejection to increase the acceptance rate of a chain, and
pre-fetching that helps an individual chain to run on parallel CPUs. An
inter-chain covariance update is also incorporated to prevent clustering of the
chains allowing faster and better mixing of the chains. We use an adaptive
method for covariance calculation to calculate and update the covariance
automatically as the chains progress. Our analysis shows that the acceptance
probability of each step in SCoPE is more than and the convergence of
the chains are faster. Using SCoPE, we carry out some cosmological parameter
estimations with different cosmological models using WMAP-9 and Planck results.
One of the current research interests in cosmology is quantifying the nature of
dark energy. We analyze the cosmological parameters from two illustrative
commonly used parameterisations of dark energy models. We also asses primordial
helium fraction in the universe can be constrained by the present CMB data from
WMAP-9 and Planck. The results from our MCMC analysis on the one hand helps us
to understand the workability of the SCoPE better, on the other hand it
provides a completely independent estimation of cosmological parameters from
WMAP-9 and Planck data.Comment: 22 pages, 10 figures, 2 table
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