259 research outputs found
Screening of cosmological constant in non-local cosmology
We consider a model of non-local gravity with a large bare cosmological
constant, , and study its cosmological solutions. The model is
characterized by a function where
and is a real dimensionless parameter. In the
absence of matter, we find an expanding universe solution with
, that is, a universe with decelarated expansion without any fine-tuning
of the parameter. Thus the effect of the cosmological constant is effectively
shielded in this solution. It has been known that solutions in non-local
gravity often suffer from the existence of ghost modes. In the present case we
find the solution is ghost-free if . This is
quite a weak condition. We argue that the solution is stable against the
includion of matter fields. Thus our solution opens up new possibilities for
solution to the cosmological constant problem.Comment: 7 pages, 1 figure, LaTeX, V2:Some clarifications and references adde
Dynamical Compactification and Inflation in Einstein-Yang-Mills Theory with Higher Derivative Coupling
We study cosmology of the Einstein-Yang-Mills theory in ten dimensions with a
quartic term in the Yang-Mills field strength. We obtain analytically a class
of cosmological solutions in which the extra dimensions are static and the
scale factor of the four-dimensional Friedmann-Lemaitre-Robertson-Walker metric
is an exponential function of time. This means that the model can explain
inflation. Then we look for solutions that describe dynamical compactification
of the extra dimensions. The effective cosmological constant in the
four-dimensional universe is determined from the gravitational coupling,
ten-dimensional cosmological constant, gauge coupling and higher derivative
coupling. By numerical integration, the solution with is found to
behave as a matter-dominated universe which asymptotically approaches flat
space-time, while the solution with a non-vanishing approaches de
Sitter space-time in the asymptotic future.Comment: 30 pages, 7 figure
Tensor ghosts in the inflationary cosmology
Theories with curvature squared terms in the action are known to contain
ghost modes in general. However, if we regard curvature squared terms as
quantum corrections to the original theory, the emergence of ghosts may be
simply due to the perturbative truncation of a full non-perturbative theory. If
this is the case, there should be a way to live with ghosts. In this paper, we
take the Euclidean path integral approach, in which ghost degrees of freedom
can be, and are integrated out in the Euclideanized spacetime. We apply this
procedure to Einstein gravity with a Weyl curvature squared correction in the
inflationary background. We find that the amplitude of tensor perturbations is
modified by a term of O(alpha^2 H^2) where alpha^2 is a coupling constant in
front of the Weyl squared term and H is the Hubble parameter during inflation.Comment: 16 pages, no figure
Calculating the mass fraction of primordial black holes
We reinspect the calculation for the mass fraction of primordial black holes (PBHs) which are formed from primordial perturbations, finding that performing the calculation using the comoving curvature perturbation c in the standard way vastly overestimates the number of PBHs, by many orders of magnitude. This is because PBHs form shortly after horizon entry, meaning modes significantly larger than the PBH are unobservable and should not affect whether a PBH forms or not - this important effect is not taken into account by smoothing the distribution in the standard fashion. We discuss alternative methods and argue that the density contrast, Î, should be used instead as super-horizon modes are damped by a factor k2. We make a comparison between using a Press-Schechter approach and peaks theory, finding that the two are in close agreement in the region of interest. We also investigate the effect of varying the spectral index, and the running of the spectral index, on the abundance of primordial black holes
Spatial averaging and apparent acceleration in inhomogeneous spaces
As an alternative to dark energy that explains the observed acceleration of
the universe, it has been suggested that we may be at the center of an
inhomogeneous isotropic universe described by a Lemaitre-Tolman-Bondi (LTB)
solution of Einstein's field equations. To test this possibility, it is
necessary to solve the null geodesics. In this paper we first give a detailed
derivation of a fully analytical set of differential equations for the radial
null geodesics as functions of the redshift in LTB models. As an application we
use these equaions to show that a positive averaged acceleration obtained
in LTB models through spatial averaging can be incompatible with cosmological
observations. We provide examples of LTB models with positive which fail
to reproduce the observed luminosity distance . Since the apparent
cosmic acceleration is obtained from fitting the observed luminosity
distance to a FLRW model we conclude that in general a positive in LTB
models does not imply a positive .Comment: 16 pages, 12 figures. Explicit derivation of the fully analytical
null geodesic equations has been added. Published in GR
Discovery of Bright Variable X-ray Sources in NGC 1569 with Chandra
From the analysis of a ~100 ks Chandra observation of the dwarf starburst
galaxy NGC 1569, we have found that the X-ray point sources, CXOU
043048.1+645050 and CXOU 043048.6+645058, showed significant time variability.
During this observation, the X-ray flux of CXOU 043048.1+645050 increased by 10
times in only 2 x 10^4 s. Since the spectrum in its bright phase was fitted
with a disk blackbody model with kT_in ~0.43 keV and the bolometric luminosity
is L_bol ~10^38 ergs s^-1, this source is an X-ray binary with a stellar mass
black-hole. Since the spectrum in its faint phase was also fitted with a disk
blackbody model, the time variability can be explained by a change of the
accretion rate onto the black hole. The other variable source, CXOU
043048.6+645058, had a flat spectrum with a photon index of ~1.6. This source
may be an X-ray binary with an X-ray luminosity of several x 10^37 ergs s^-1.
In addition, three other weak sources showed possible time variability. Taking
all of the variability into account may suggest an abundant population of
compact X-ray sources in NGC 1569.Comment: 15 pages including 4 Postscript figures; accepted for publication in
ApJ
Self-force Regularization in the Schwarzschild Spacetime
We discuss the gravitational self-force on a particle in a black hole
space-time. For a point particle, the full (bare) self-force diverges. The
metric perturbation induced by a particle can be divided into two parts, the
direct part (or the S part) and the tail part (or the R part), in the harmonic
gauge, and the regularized self-force is derived from the R part which is
regular and satisfies the source-free perturbed Einstein equations. But this
formulation is abstract, so when we apply to black hole-particle systems, there
are many problems to be overcome in order to derive a concrete self-force.
These problems are roughly divided into two parts. They are the problem of
regularizing the divergent self-force, i.e., ``subtraction problem'' and the
problem of the singularity in gauge transformation, i.e., ``gauge problem''. In
this paper, we discuss these problems in the Schwarzschild background and
report some recent progress.Comment: 34 pages, 2 figures, submitted to CQG, special volume for Radiation
Reaction (CAPRA7
A New delta N Formalism for Multi-Component Inflation
The delta N formula that relates the final curvature perturbation on comoving
slices to the inflaton perturbation on flat slices after horizon crossing is a
powerful and intuitive tool to compute the curvature perturbation spectrum from
inflation. However, it is customarily assumed further that the conventional
slow-roll condition is satisfied, and satisfied by all components, during
horizon crossing. In this paper, we develop a new delta N formalism for
multi-component inflation that can be applied in the most general situations.
This allows us to generalize the idea of general slow-roll inflation to the
multi-component case, in particular only applying the general slow-roll
condition to the relevant component. We compute the power spectrum of the
curvature perturbation in multi-component general slow-roll inflation, and find
that under quite general conditions it is invertible.Comment: 24 pages, no figur
Classical and quantum radiation from a moving charge in an expanding universe
We investigate photon emission from a moving particle in an expanding
universe. This process is analogous to the radiation from an accelerated charge
in the classical electromagnetic theory. Using the framework of quantum field
theory in curved spacetime, we demonstrate that the Wentzel-Kramers-Brillouin
(WKB) approximation leads to the Larmor formula for the rate of the radiation
energy from a moving charge in an expanding universe. Using exactly solvable
models in a radiation-dominated universe and in a Milne universe, we examine
the validity of the WKB formula. It is shown that the quantum effect suppresses
the radiation energy in comparison with the WKB formula.Comment: 16 pages, JCAP in pres
Vacuum destabilization from Kaluza-Klein modes in an inflating brane
We discuss the effects from the Kaluza-Klein modes in the brane world
scenario when an interaction between bulk and brane fields is included. We
focus on the bulk inflaton model, where a bulk field drives inflation in
an almost bulk bounded by an inflating brane. We couple to a
brane scalar field representing matter on the brane. The bulk field
is assumed to have a light mode, whose mass depends on the expectation
value of . To estimate the effects from the KK modes, we compute the
1-loop effective potential V_\eff(\phi). With no tuning of the parameters of
the model, the vacuum becomes (meta)stable -- V_\eff(\phi) develops a true
vacuum at a nonzero . In the true vacuum, the light mode of
becomes heavy, degenerates with the KK modes and decays. We comment on some
implications for the bulk inflaton model. Also, we clarify some aspects of the
renormalization procedure in the thin wall approximation, and show that the
fluctuations in the bulk and on the brane are closely related.Comment: 15 pages, 2 eps figures. Notation improved, references adde
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