3,479 research outputs found
Scalar field dark energy perturbations and the Integrated Sachs Wolfe effect
Dark energy perturbation affects the growth of matter perturbations even in
scenarios with noninteracting dark energy. We investigate the Integrated Sachs
Wolfe (ISW) effect in various canonical scalar field models with perturbed dark
energy. We do this analysis for models belonging to the thawing and freezing
classes. We show that between these classes there is no clear difference for
the ISW effect. We show that on taking perturbations into account, the
contribution due to different models is closer to each other and to the
cosmological constant model as compared to the case of a smooth dark energy.
Therefore considering dark energy to be homogeneous gives an overestimate in
distinction between different models. However there are significant difference
between contribution to the angular power spectrum due to different models.Comment: 4 pages, 3 postscript figures. references added, changes in text,
conclusions remain the sam
Vacuum Fluctuations of Energy Density can lead to the observed Cosmological Constant
The energy density associated with Planck length is while the energy density associated with the Hubble length is
where . The observed value of the dark
energy density is quite different from {\it either} of these and is close to
the geometric mean of the two: .
It is argued that classical gravity is actually a probe of the vacuum {\it
fluctuations} of energy density, rather than the energy density itself. While
the globally defined ground state, being an eigenstate of Hamiltonian, will not
have any fluctuations, the ground state energy in the finite region of space
bounded by the cosmic horizon will exhibit fluctuations . When used as a source of gravity, this should
lead to a spacetime with a horizon size . This bootstrapping condition
leads naturally to an effective dark energy density which is precisely the observed value. The model
requires, either (i) a stochastic fluctuations of vacuum energy which is
correlated over about a Hubble time or (ii) a semi- anthropic interpretation.
The implications are discussed.Comment: r pages; revtex; comments welcom
Scalar Field Dark Energy Perturbations and their Scale Dependence
We estimate the amplitude of perturbation in dark energy at different length
scales for a quintessence model with an exponential potential. It is shown that
on length scales much smaller than hubble radius, perturbation in dark energy
is negligible in comparison to that in in dark matter. However, on scales
comparable to the hubble radius () the
perturbation in dark energy in general cannot be neglected. As compared to the
CDM model, large scale matter power spectrum is suppressed in a
generic quintessence dark energy model. We show that on scales , this suppression is primarily due to different background
evolution compared to CDM model. However, on much larger scales
perturbation in dark energy can effect matter power spectrum significantly.
Hence this analysis can act as a discriminator between CDM model and
other generic dark energy models with .Comment: 12 pages, 13 figures, added new section, accepted for publication in
Phys. Rev.
Dark energy transition between quintessence and phantom regimes - an equation of state analysis
The dark energy transition between quintessence () and phantom ()
regimes (the crossing of the cosmological constant boundary) is studied using
the dark energy equation of state. Models characterized by this type of
transition are explicitly constructed and their equation of state is found to
be {\em implicitly} defined. The behavior of the more general models with the
implicitly defined equation of state, obtained by the generalization of the
explicitly constructed models, is studied to gain insight into the necessary
conditions for the occurrence of the transition, as well as to investigate the
mechanism behind the transition. It is found that the parameters of the
generalized models need to satisfy special conditions for the transition to
happen and that the mechanism behind the transition is the cancellation of the
contribution of the cosmological constant boundary. The aspects of the behavior
of the generalized models which are not related to the transition are briefly
discussed and the role of the implicitly defined dark energy equation of state
in the description of the dark energy evolution is emphasized.Comment: v1: 9 pages, 6 figures. v2: references added. v3: minor changes.
Version accepted for publication in Phys. Rev.
Hypothesis of path integral duality: Applications to QED
We use the modified propagator for quantum field based on a ``principle of
path integral duality" proposed earlier in a paper by Padmanabhan to
investigate several results in QED. This procedure modifies the Feynman
propagator by the introduction of a fundamental length scale. We use this
modified propagator for the Dirac particles to evaluate the first order
radiative corrections in QED. We find that the extra factor of the modified
propagator acts like a regulator at the Planck scales thereby removing the
divergences that otherwise appear in the conventional radiative correction
calculations of QED. We find that:(i) all the three renormalisation factors
, , and pick up finite corrections and (ii) the modified
propagator breaks the gauge invariance at a very small level of
. The implications of this result to generation of the
primordial seed magnetic fields are discussed.Comment: 15 pages, LaTeX2e (uses ijmpd.sty); To appear in IJMP-D; References
adde
The hypothesis of path integral duality II: corrections to quantum field theoretic results
In the path integral expression for a Feynman propagator of a spinless
particle of mass , the path integral amplitude for a path of proper length
connecting events and in a spacetime
described by the metric tensor is . In a recent paper, assuming the path integral amplitude to be
invariant under the duality transformation ,
Padmanabhan has evaluated the modified Feynman propagator in an arbitrary
curved spacetime. He finds that the essential feature of this `principle of
path integral duality' is that the Euclidean proper distance
between two infinitesimally separated spacetime events is replaced by . In other words, under the duality principle the spacetime
behaves as though it has a `zero-point length' , a feature that is
expected to arise in a quantum theory of gravity. In the Schwinger's proper
time description of the Feynman propagator, the weightage factor for a path
with a proper time is . Invoking Padmanabhan's `principle of
path integral duality' corresponds to modifying the weightage factor
to . In this paper, we use this modified
weightage factor in Schwinger's proper time formalism to evaluate the quantum
gravitational corrections to some of the standard quantum field theoretic
results in flat and curved spacetimes. We find that the extra factor
acts as a regulator at the Planck scale thereby `removing' the
divergences that otherwise appear in the theory. Finally, we discuss the wider
implications of our analysis.Comment: 26 pages, Revte
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
511 KeV Photons From Color Superconducting Dark Matter
We discuss the possibility that the recent detection of 511 keV gamma rays
from the galactic bulge, as observed by INTEGRAL, can be naturally explained by
the supermassive very dense droplets (strangelets) of dark matter. These
droplets are assumed to be made of ordinary light quarks (or antiquarks)
condensed in non-hadronic color superconducting phase. The droplets can carry
electrons (or positrons) in the bulk or/and on the surface. The e^+e^-
annihilation events take place due to the collisions of electrons from the
visible matter with positrons from dark matter droplets which may result in the
bright 511 KeV gamma-ray line from the bulge of the Galaxy.Comment: Final version to appear in PRL. Added: estimation of the width, 3Ke
Thermal Bremsstrahlung Radiation in a Two-Temperature Plasma
In the normal one-temperature plasma the motion of ions is usually neglected
when calculating the Bremsstrahlung radiation of the plasma. Here we calculate
the Bremsstrahlung radiation of a two-temperature plasma by taking into account
of the motion of ions. Our results show that the total radiation power is
always lower if the motion of ions is considered. We also apply the
two-temperature Bremsstrahlung radiation mechanism for an analytical
Advection-Dominated Accretion Flow (ADAF) model; we find the two-temperature
correction to the total Bremsstrahlung radiation for ADAF is negligible.Comment: 5 pages, 4 figures, accepted for publication in CHJAA. Some
discussions and references adde
Constraints on the evolution of the relationship between H i mass and halo mass in the last 12 Gyr
The neutral hydrogen (H I) content of dark matter haloes forms an intermediate state in the baryon cycle that connects the hot shock-heated gas and cold star-forming gas in haloes. Measurement of the relationship between H I mass and halo mass therefore puts important constraints on galaxy formation models. We combine radio observations of H I in emission at low redshift (z ∼ 0) with optical/UV observations of H I in absorption at high redshift (1 < z < 4) to derive constraints on the evolution of the H I-mass–halo-mass (HIHM) relation from redshift z = 4 to 0. We find that one can model the HIHM relation similar to the stellar-mass–halo-mass (SHM) relation at z ∼ 0. At z = 0, haloes with mass 1011.7 M⊙ have the highest H I mass fraction (∼1 per cent), which is about four times smaller than their stellar-mass fraction. We model the evolution of the HIHM relation in a manner similar to that of the SHM relation. Combining this parametrization with a redshift- and mass-dependent modified Navarro–Frenk–White profile for the H I density within a halo, we draw constraints on the evolution of the HIHM relation from the observed H I column density, incidence rate and clustering bias at high redshift. We compare these findings with results from hydrodynamical simulations and other approaches in the literature and find the models to be consistent with each other at the 68 per cent confidence level
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