60 research outputs found
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
CMBR Weak Lensing and HI 21-cm Cross-correlation Angular Power Spectrum
Weak gravitational lensing of the CMBR manifests as a secondary anisotropy in
the temperature maps. The effect, quantified through the shear and convergence
fields imprint the underlying large scale structure (LSS), geometry and
evolution history of the Universe. It is hence perceived to be an important
observational probe of cosmology. De-lensing the CMBR temperature maps is also
crucial for detecting the gravitational wave generated B-modes. Future
observations of redshifted 21-cm radiation from the cosmological neutral
hydrogen (HI) distribution hold the potential of probing the LSS over a large
redshift range. We have investigated the correlation between post-reionization
HI signal and weak lensing convergence field. Assuming that the HI follows the
dark matter distribution, the cross-correlation angular power spectrum at a
multipole \ell is found to be proportional to the cold dark matter power
spectrum evaluated at \ell/r, where r denotes the comoving distance to the
redshift where the HI is located. The amplitude of the ross-correlation depends
on quantities specific to the HI distribution, growth of perturbations and also
the underlying cosmological model. In an ideal ituation, we found that a
statistically significant detection of the cross-correlation signal is
possible. If detected, the cross-correlation signal hold the possibility of a
joint estimation of cosmological parameters and also test various CMBR
de-lensing estimators.Comment: 14 pages, 4 figures, publishe
Dynamics of Logamediate and Intermediate Scenarios in the Dark Energy Filled Universe
We have considered a model of two component mixture i.e., mixture of
Chaplygin gas and barotropic fluid with tachyonic field. In the case, when they
have no interaction then both of them retain their own properties. Let us
consider an energy flow between barotropic and tachyonic fluids. In both the
cases we find the exact solutions for the tachyonic field and the tachyonic
potential and show that the tachyonic potential follows the asymptotic
behavior. We have considered an interaction between these two fluids by
introducing a coupling term. Finally, we have considered a model of three
component mixture i.e., mixture of tachyonic field, Chaplygin gas and
barotropic fluid with or without interaction. The coupling functions decays
with time indicating a strong energy flow at the initial period and weak stable
interaction at later stage. To keep the observational support of recent
acceleration we have considered two particular forms (i) Logamediate Scenario
and (ii) Intermediate Scenario, of evolution of the Universe. We have examined
the natures of the recent developed statefinder parameters and slow-roll
parameters in both scenarios with and without interactions in whole evolution
of the universe.Comment: 28 pages, 20 figure
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
A tachyonic scalar field with mutually interacting components
We investigate the tachyonic cosmological potential in two
different cases of the quasi-exponential expansion of universe and discuss
various forms of interaction between the two components---matter and the
cosmological constant--- of the tachyonic scalar field, which leads to the
viable solutions of their respective energy densities. The distinction among
the interaction forms is shown to appear in the diagnostic. Further,
the role of the high- and low-redshift observations of the Hubble parameter is
discussed to determine the proportionality constants and hence the correct form
of matter--cosmological constant interaction.Comment: 14 page
A new view of k-essence
K-essence models, relying on scalar fields with non-canonical kinetic terms,
have been proposed as an alternative to quintessence in explaining the observed
acceleration of the Universe. We consider the use of field redefinitions to
cast k-essence in a more familiar form. While k-essence models cannot in
general be rewritten in the form of quintessence models, we show that in
certain dynamical regimes an equivalence can be made, which in particular can
shed light on the tracking behaviour of k-essence. In several cases, k-essence
cannot be observationally distinguished from quintessence using the homogeneous
evolution, though there may be small effects on the perturbation spectrum. We
make a detailed analysis of two k-essence models from the literature and
comment on the nature of the fine tuning arising in the models.Comment: 7 pages RevTeX4 file with four figures incorporate
Newtonian Collapse of Scalar Field Dark Matter
In this letter, we develop a Newtonian approach to the collapse of galaxy
fluctuations of scalar field dark matter under initial conditions inferred from
simple assumptions. The full relativistic system, the so called
Einstein-Klein-Gordon, is reduced to the Schr\"odinger-Newton one in the weak
field limit. The scaling symmetries of the SN equations are exploited to track
the non-linear collapse of single scalar matter fluctuations. The results can
be applied to both real and complex scalar fields.Comment: 4 pages RevTex4 file, 4 eps figure
Electromagnetic String Fluid in Rolling Tachyon
We study Born-Infeld type effective action for unstable D3-brane system
including a tachyon and an Abelian gauge field, and find the rolling tachyon
with constant electric and magnetic fields as the most general homogeneous
solution. Tachyonic vacua are characterized by magnitudes of the electric and
magnetic fields and the angle between them. Analysis of small fluctuations in
this background shows that the obtained configuration may be interpreted as a
fluid consisting of string-like objects carrying electric and magnetic fields.
They are stretched along one direction and the rolling tachyon move in a
perpendicular direction to the strings. Direction of the propagating waves
coincides with that of strings with velocity equal to electric field.Comment: LaTeX, 18 pages, 1 figure, minor correction
The unification of inflation and late-time acceleration in the frame of -essence
By using the formulation of the reconstruction, we explicitly construct
models of -essence, which unify the inflation in the early universe and the
late accelerating expansion of the present universe by a single scalar field.
Due to the higher derivative terms, the solution describing the unification can
be stable in the space of solutions, which makes the restriction for the
initial condition relaxed. The higher derivative terms also eliminate tachyon.
Therefore we can construct a model describing the time development, which
cannot be realized by a usual inflaton or quintessence models of the canonical
scalar field due to the instability or the existence of tachyon. We also
propose a mechanism of the reheating by the quantum effects coming from the
variation of the energy density of the scalar field.Comment: LaTeX, 13 pages, 10 figure
Simulation techniques for cosmological simulations
Modern cosmological observations allow us to study in great detail the
evolution and history of the large scale structure hierarchy. The fundamental
problem of accurate constraints on the cosmological parameters, within a given
cosmological model, requires precise modelling of the observed structure. In
this paper we briefly review the current most effective techniques of large
scale structure simulations, emphasising both their advantages and
shortcomings. Starting with basics of the direct N-body simulations appropriate
to modelling cold dark matter evolution, we then discuss the direct-sum
technique GRAPE, particle-mesh (PM) and hybrid methods, combining the PM and
the tree algorithms. Simulations of baryonic matter in the Universe often use
hydrodynamic codes based on both particle methods that discretise mass, and
grid-based methods. We briefly describe Eulerian grid methods, and also some
variants of Lagrangian smoothed particle hydrodynamics (SPH) methods.Comment: 42 pages, 16 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 12; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
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