39 research outputs found
Semi analytic approach to understanding the distribution of neutral hydrogen in the universe
Analytic derivations of the correlation function and the column density
distribution for neutral hydrogen in the IGM are presented, assuming that the
non-linear baryonic mass density distribution in the IGM is lognormal. This
ansatz was used earlier by Bi & Davidsen (1997) to perform 1D simulations of
lines-of-sight and analyse the properties of absorption systems. Our approach
is completely analytic, which allows us to explore a wide region of the
parameter space for our model. The analytic results have been compared with
observations, whenever possible. Two kinds of correlation functions are
defined: along the line-of-sight (LOS) and across the transverse direction. We
find that the effects on the LOS correlation due to change in cosmology and the
slope of the equation of state of the IGM, \gamma are of the same order, which
means that we cannot constrain both the parameters simultaneously. However, it
is possible to constrain \gamma and its evolution using the observed LOS
correlation function at different epochs, provided one knows the background
cosmology. We suggest that the constraints on the evolution of \gamma obtained
using the LOS correlation can be used as an independent tool to probe the
reionisation history of the universe. From the transverse correlation function,
we find that the excess probability, over random, of finding two neutral
hydrogen overdense regions separated by an angle \theta, is always less than 1
per cent for redshifts greater than 2. Our models also reproduce the observed
column density distribution for neutral hydrogen and the shape of the
distribution depends on \gamma. Our calculations suggest that one can rule out
\gamma > 1.6 for z \simeq 2.31 using the column density distribution. However,
one cannot rule higher values of \gamma at higher redshifts.Comment: 16 pages, 8 figures. Accepted for publication in MNRAS. Revised
following referee's comment
Reply to [arXiv:1105.5653]: "Comment on 'Quasinormal modes in Schwarzschild-de Sitter spacetime: A simple derivation of the level spacing of the frequencies'"
We explain why the analysis in our paper [Phys. Rev. D 69, 064033 (2004),
arXiv:gr-qc/0311064 ] is relevant and correct.Comment: 2 page
Theoretical and observational constraints on the H i intensity power spectrum
Mapping of the neutral hydrogen (H i) 21-cm intensity fluctuations across redshifts promises a novel and powerful probe of cosmology. The neutral hydrogen gas mass density and bias parameter are key astrophysical inputs to the H i intensity fluctuation power spectrum. We compile the latest theoretical and observational constraints on and at various redshifts in the post-reionization universe. Constraints are incorporated from galaxy surveys, H i intensity mapping experiments, damped Lyman α system observations, theoretical prescriptions for assigning H i to dark matter haloes and the results of numerical simulations. Using a minimum variance interpolation scheme, we obtain the predicted uncertainties on the H i intensity fluctuation power spectrum across redshifts 0-3.5 for three different confidence scenarios. We provide a convenient tabular form for the interpolated values of , and the H i power spectrum amplitude and their uncertainties. We discuss the consequences for the measurement of the power spectrum by current and future intensity mapping experiment
A theoretician's analysis of the supernova data and the limitations in determining the nature of dark energy
Current cosmological observations show a strong signature of the existence of
a dark energy component with negative pressure. The most obvious candidate for
this dark energy is the cosmological constant (with the equation of state
w_X=p/\rho=-1), which, however, raises several theoretical difficulties. This
has led to models for dark energy component which evolves with time. We discuss
certain questions related to the determination of the nature of dark energy
component from observations of high redshift supernova. The main results of our
analysis are: (i) Even if the precise value of w_X is known from observations,
it is not possible to determine the nature of the unknown dark energy source
using only kinematical and geometrical measurements. We have given explicit
examples to show that different types of sources can give rise to a given w_X.
(ii) Although the full data set of supernova observations (which are currently
available) strongly rule out models without dark energy, the high (z>0.25) and
low (z<0.25) redshift data sets, individually, admit decelerating models with
zero dark energy. Any possible evolution in the absolute magnitude of the
supernovae, if detected, might allow the decelerating models to be consistent
with the data. (iii) We have introduced two parameters, which can be obtained
entirely from theory, to study the sensitivity of the luminosity distance on
w_X. Using these two parameters, we have argued that although one can determine
the present value of w_X accurately from the data, one cannot constrain the
evolution of w_X.Comment: Revised versio
Dark Energy and Gravity
I review the problem of dark energy focusing on the cosmological constant as
the candidate and discuss its implications for the nature of gravity. Part 1
briefly overviews the currently popular `concordance cosmology' and summarises
the evidence for dark energy. It also provides the observational and
theoretical arguments in favour of the cosmological constant as the candidate
and emphasises why no other approach really solves the conceptual problems
usually attributed to the cosmological constant. Part 2 describes some of the
approaches to understand the nature of the cosmological constant and attempts
to extract the key ingredients which must be present in any viable solution. I
argue that (i)the cosmological constant problem cannot be satisfactorily solved
until gravitational action is made invariant under the shift of the matter
lagrangian by a constant and (ii) this cannot happen if the metric is the
dynamical variable. Hence the cosmological constant problem essentially has to
do with our (mis)understanding of the nature of gravity. Part 3 discusses an
alternative perspective on gravity in which the action is explicitly invariant
under the above transformation. Extremizing this action leads to an equation
determining the background geometry which gives Einstein's theory at the lowest
order with Lanczos-Lovelock type corrections. (Condensed abstract).Comment: Invited Review for a special Gen.Rel.Grav. issue on Dark Energy,
edited by G.F.R.Ellis, R.Maartens and H.Nicolai; revtex; 22 pages; 2 figure
Concept of temperature in multi-horizon spacetimes: Analysis of Schwarzschild-De Sitter metric
In case of spacetimes with single horizon, there exist several
well-established procedures for relating the surface gravity of the horizon to
a thermodynamic temperature. Such procedures, however, cannot be extended in a
straightforward manner when a spacetime has multiple horizons. In particular,
it is not clear whether there exists a notion of global temperature
characterizing the multi-horizon spacetimes. We examine the conditions under
which a global temperature can exist for a spacetime with two horizons using
the example of Schwarzschild-De Sitter (SDS) spacetime. We systematically
extend different procedures (like the expectation value of stress tensor,
response of particle detectors, periodicity in the Euclidean time etc.) for
identifying a temperature in the case of spacetimes with single horizon to the
SDS spacetime. This analysis is facilitated by using a global coordinate chart
which covers the entire SDS manifold. We find that all the procedures lead to a
consistent picture characterized by the following features: (a) In general, SDS
spacetime behaves like a non-equilibrium system characterized by two
temperatures. (b) It is not possible to associate a global temperature with SDS
spacetime except when the ratio of the two surface gravities is rational (c)
Even when the ratio of the two surface gravities is rational, the thermal
nature depends on the coordinate chart used. There exists a global coordinate
chart in which there is global equilibrium temperature while there exist other
charts in which SDS behaves as though it has two different temperatures. The
coordinate dependence of the thermal nature is reminiscent of the flat
spacetime in Minkowski and Rindler coordinate charts. The implications are
discussed.Comment: 12 page