19 research outputs found
On the Possibility of Anisotropic Curvature in Cosmology
In addition to shear and vorticity a homogeneous background may also exhibit
anisotropic curvature. Here a class of spacetimes is shown to exist where the
anisotropy is solely of the latter type, and the shear-free condition is
supported by a canonical, massless 2-form field. Such spacetimes possess a
preferred direction in the sky and at the same time a CMB which is isotropic at
the background level. A distortion of the luminosity distances is derived and
used to test the model against the CMB and supernovae (using the Union
catalog), and it is concluded that the latter exhibit a higher-than-expected
dependence on angular position. It is shown that future surveys could detect a
possible preferred direction by observing ~ 20 / (\Omega_{k0}^2) supernovae
over the whole sky.Comment: Extended SNe analysis and corrected some CMB results. Text also
extended and references added. 8 pages, 5 figure
Qualitative Analysis of Universes with Varying Alpha
Assuming a Friedmann universe which evolves with a power-law scale factor,
, we analyse the phase space of the system of equations that describes
a time-varying fine structure 'constant', , in the
Bekenstein-Sandvik-Barrow-Magueijo generalisation of general relativity. We
have classified all the possible behaviours of in ever-expanding
universes with different and find new exact solutions for . We
find the attractors points in the phase space for all . In general, will be a non-decreasing function of time that increases logarithmically in
time during a period when the expansion is dust dominated (), but
becomes constant when . This includes the case of negative-curvature
domination (). also tends rapidly to a constant when the
expansion scale factor increases exponentially. A general set of conditions is
established for to become asymptotically constant at late times in an
expanding universe.Comment: 26 pages, 6 figure
Ellipsoidal configurations in the de Sitter spacetime
The cosmological constant modifies certain properties of large
astrophysical rotating configurations with ellipsoidal geometries, provided the
objects are not too compact. Assuming an equilibrium configuration and so using
the tensor virial equation with we explore several equilibrium
properties of homogeneous rotating ellipsoids. One shows that the bifurcation
point, which in the oblate case distinguishes the Maclaurin ellipsoid from the
Jacobi ellipsoid, is sensitive to the cosmological constant. Adding to that,
the cosmological constant allows triaxial configurations of equilibrium
rotating the minor axis as solutions of the virial equations. The significance
of the result lies in the fact that minor axis rotation is indeed found in
nature. Being impossible for the oblate case, it is permissible for prolate
geometries, with zero and positive. For the triaxial case, however,
an equilibrium solution is found only for non-zero positive . Finally,
we solve the tensor virial equation for the angular velocity and display
special effects of the cosmological constant there.Comment: 15 pages, 11 figures, published in Class. Quant. Grav. References
adde
The present universe in the Einstein frame, metric-affine R+1/R gravity
We study the present, flat isotropic universe in 1/R-modified gravity. We use
the Palatini (metric-affine) variational principle and the Einstein
(metric-compatible connected) conformal frame. We show that the energy density
scaling deviates from the usual scaling for nonrelativistic matter, and the
largest deviation occurs in the present epoch. We find that the current
deceleration parameter derived from the apparent matter density parameter is
consistent with observations. There is also a small overlap between the
predicted and observed values for the redshift derivative of the deceleration
parameter. The predicted redshift of the deceleration-to-acceleration
transition agrees with that in the \Lambda-CDM model but it is larger than the
value estimated from SNIa observations.Comment: 11 pages; published versio
Ellipsoidal universe in the brane world
We study a scenario of the ellipsoidal universe in the brane world cosmology
with a cosmological constant in the bulk . From the five-dimensional Einstein
equations we derive the evolution equations for the eccentricity and the scale
factor of the universe, which are coupled to each other. It is found that if
the anisotropy of our universe is originated from a uniform magnetic field
inside the brane, the eccentricity decays faster in the bulk in comparison with
a four-dimensional ellipsoidal universe. We also investigate the ellipsoidal
universe in the brane-induced gravity and find the evolution equation for the
eccentricity which has a contribution determined by the four- and
five-dimensional Newton's constants. The role of the eccentricity is discussed
in explaining the quadrupole problem of the cosmic microwave background.Comment: 15 pages, 1 figure, Version 3, references added, contents expande
Constraining f(R) gravity in the Palatini formalism
Although several models of theories of gravity within the Palatini
approach have been studied already, the interest was concentrated on those that
have an effect on the late-time evolution of the universe, by the inclusion for
example of terms inversely proportional to the scalar curvature in the
gravitational action. However, additional positive powers of the curvature also
provide interesting early-time phenomenology, like inflation, and the presence
of such terms in the action is equally, if not more, probable. In the present
paper models with both additional positive and negative powers of the scalar
curvature are studied. Their effect on the evolution of the universe is
investigated for all cosmological eras, and various constraints are put on the
extra terms in the actions. Additionally, we examine the extent to which the
new terms in positive powers affect the late-time evolution of the universe and
the related observables, which also determines our ability to probe their
presence in the gravitational action.Comment: reference update and minor changes to match published versio
Cosmology with exponential potentials
We examine in the context of general relativity the dynamics of a spatially
flat Robertson-Walker universe filled with a classical minimally coupled scalar
field \phi of exponential potential ~ e^{-\mu\phi} plus pressureless baryonic
matter. This system is reduced to a first-order ordinary differential equation,
providing direct evidence on the acceleration/deceleration properties of the
system. As a consequence, for positive potentials, passage into acceleration
not at late times is generically a feature of the system, even when the
late-times attractors are decelerating. Furthermore, the structure formation
bound, together with the constraints on the present values of \Omega_{m},
w_{\phi} provide, independently of initial conditions and other parameters,
necessary conditions on \mu. Special solutions are found to possess intervals
of acceleration. For the almost cosmological constant case w_{\phi} ~ -1, as
well as, for the generic late-times evolution, the general relation
\Omega_{\phi}(w_{\phi}) is obtained.Comment: RevTex4, 9 pages, 2 figures, References adde
Comparing two approaches to Hawking radiation of Schwarzschild-de Sitter black holes
We study two different ways to analyze the Hawking evaporation of a
Schwarzschild-de Sitter black hole. The first one uses the standard approach of
surface gravity evaluated at the possible horizons. The second method derives
its results via the Generalized Uncertainty Principle (GUP) which offers a yet
different method to look at the problem. In the case of a Schwarzschild black
hole it is known that this methods affirms the existence of a black hole
remnant (minimal mass ) of the order of Planck mass
and a corresponding maximal temperature also of the order of
. The standard dispersion relation is, in the GUP
formulation, deformed in the vicinity of Planck length which is
the smallest value the horizon can take. We generalize the uncertainty
principle to Schwarzschild-de Sitter spacetime with the cosmological constant
and find a dual relation which, compared to
and , affirms the existence of a maximal mass
of the order , minimum
temperature . As compared to the standard
approach we find a deformed dispersion relation close to
and in addition at the maximally possible horizon approximately at
. agrees with the standard results at
(or equivalently at ).Comment: new references adde
Modified-Source Gravity and Cosmological Structure Formation
One way to account for the acceleration of the universe is to modify general
relativity, rather than introducing dark energy. Typically, such modifications
introduce new degrees of freedom. It is interesting to consider models with no
new degrees of freedom, but with a modified dependence on the conventional
energy-momentum tensor; the Palatini formulation of theories is one
example. Such theories offer an interesting testing ground for investigations
of cosmological modified gravity. In this paper we study the evolution of
structure in these ``modified-source gravity'' theories. In the linear regime,
density perturbations exhibit scale dependent runaway growth at late times and,
in particular, a mode of a given wavenumber goes nonlinear at a higher redshift
than in the standard CDM model. We discuss the implications of this
behavior and why there are reasons to expect that the growth will be cut off in
the nonlinear regime. Assuming that this holds in a full nonlinear analysis, we
briefly describe how upcoming measurements may probe the differences between
the modified theory and the standard CDM model.Comment: 22 pages, 6 figures, uses iopart styl