3,842 research outputs found
Cosmological milestones and energy conditions
Until recently, the physically relevant singularities occurring in FRW
cosmologies had traditionally been thought to be limited to the "big bang", and
possibly a "big crunch". However, over the last few years, the zoo of
cosmological singularities considered in the literature has become considerably
more extensive, with "big rips" and "sudden singularities" added to the mix, as
well as renewed interest in non-singular cosmological events such as "bounces"
and "turnarounds". In this talk, we present an extensive catalogue of such
cosmological milestones, both at the kinematical and dynamical level. First,
using generalized power series, purely kinematical definitions of these
cosmological events are provided in terms of the behaviour of the scale factor
a(t). The notion of a "scale-factor singularity" is defined, and its relation
to curvature singularities (polynomial and differential) is explored. Second,
dynamical information is extracted by using the Friedmann equations (without
assuming even the existence of any equation of state) to place constraints on
whether or not the classical energy conditions are satisfied at the
cosmological milestones. Since the classification is extremely general, and
modulo certain technical assumptions complete, the corresponding results are to
a high degree model-independent.Comment: 8 pages, 1 table, conference proceedings for NEB XII conference in
Nafplio, Greec
Bouncing Universes with Varying Constants
We investigate the behaviour of exact closed bouncing Friedmann universes in
theories with varying constants. We show that the simplest BSBM varying-alpha
theory leads to a bouncing universe. The value of alpha increases
monotonically, remaining approximately constant during most of each cycle, but
increasing significantly around each bounce. When dissipation is introduced we
show that in each new cycle the universe expands for longer and to a larger
size. We find a similar effect for closed bouncing universes in Brans-Dicke
theory, where also varies monotonically in time from cycle to cycle.
Similar behaviour occurs also in varying speed of light theories
Fully Anisotropic String Cosmologies, Maxwell Fields and Primordial Shear
We present a class of exact cosmological solutions of the low energy string
effective action in the presence of a homogeneous magnetic fields. We discuss
the physical properties of the obtained (fully anisotropic) cosmologies paying
particular attention to their vacuum limit and to the possible isotropization
mechanisms. We argue that quadratic curvature corrections are able to
isotropize fully anisotropic solutions whose scale factors describe accelerated
expansion. Moreover, the degree of isotropization grows with the duration of
the string phase. We follow the fate of the shear parameter in a decelerated
phase where, dilaton, magnetic fields and radiation fluid are simultaneously
present. In the absence of any magnetic field a long string phase immediately
followed by radiation is able to erase large anisotropies. Conversely, if a
short string phase is followed by a long dilaton dominated phase the
anisotropies can be present, in principle, also at later times. The presence of
magnetic seeds after the end of the string phase can induce further
anisotropies which can be studied within the formalism reported in this paper.Comment: 19 pages in Revtex style, 14 Encapsulated figure
Evidence of vorticity and shear at large angular scales in the WMAP data: a violation of cosmological isotropy?
Motivated by the large-scale asymmetry observed in the cosmic microwave
background sky, we consider a specific class of anisotropic cosmological models
-- Bianchi type VII_h -- and compare them to the WMAP first-year data on large
angular scales. Remarkably, we find evidence of a correlation which is ruled
out as a chance alignment at the 3sigma level. The best fit Bianchi model
corresponds to x=0.55, Omega_0=0.5, a rotation axis in the direction
(l,b)=(222degr,-62degr), shear (sigma/H)_0=2.4e-10 and a right--handed
vorticity (omega/H)_0=6.1e-10. Correcting for this component greatly reduces
the significance of the large-scale power asymmetry, resolves several anomalies
detected on large angular scales (ie. the low quadrupole amplitude and
quadrupole/octopole planarity and alignment), and can account for a
non--Gaussian "cold spot" on the sky. Despite the apparent inconsistency with
the best-fit parameters required in inflationary models to account for the
acoustic peaks, we consider the results sufficiently provocative to merit
further consideration.Comment: 4 pages, 3 figures; emulateapj.cls; ApJL accepted version plus fixed
error in vorticity calculation (sqrt(2) off in Table 1, abstract, and
conclusions); basic conclusions unchange
New Isotropic and Anisotropic Sudden Singularities
We show the existence of an infinite family of finite-time singularities in
isotropically expanding universes which obey the weak, strong, and dominant
energy conditions. We show what new type of energy condition is needed to
exclude them ab initio. We also determine the conditions under which
finite-time future singularities can arise in a wide class of anisotropic
cosmological models. New types of finite-time singularity are possible which
are characterised by divergences in the time-rate of change of the
anisotropic-pressure tensor. We investigate the conditions for the formation of
finite-time singularities in a Bianchi type universe with anisotropic
pressures and construct specific examples of anisotropic sudden singularities
in these universes.Comment: Typos corrected. Published versio
Chaplygin gas dominated anisotropic brane world cosmological models
We present exact solutions of the gravitational field equations in the
generalized Randall-Sundrum model for an anisotropic brane with Bianchi type I
geometry, with a generalized Chaplygin gas as matter source. The generalized
Chaplygin gas, which interpolates between a high density relativistic era and a
non-relativistic matter phase, is a popular dark energy candidate. For a
Bianchi type I space-time brane filled with a cosmological fluid obeying the
generalized Chaplygin equation of state the general solution of the
gravitational field equations can be expressed in an exact parametric form,
with the comoving volume taken as parameter. In the limiting cases of a stiff
cosmological fluid, with pressure equal to the energy density, and for a
pressureless fluid, the solution of the field equations can be expressed in an
exact analytical form. The evolution of the scalar field associated to the
Chaplygin fluid is also considered and the corresponding potential is obtained.
The behavior of the observationally important parameters like shear, anisotropy
and deceleration parameter is considered in detail.Comment: 13 pages, 6 figures, accepted for publication in PR
Spherical Curvature Inhomogeneities in String Cosmology
We study the evolution of non-linear spherically symmetric inhomogeneities in
string cosmology. Friedmann solutions of different spatial curvature are
matched to produce solutions which describe the evolution of non-linear density
and curvature inhomogeneities. The evolution of bound and unbound
inhomogeneities are studied. The problem of primordial black hole formation is
discussed in the string cosmological context and the pattern of evolution is
determined in the pre- and post-big-bang phases of evolution.Comment: 19 pages, Latex, 4 figure
Rotational inhomogeneities from pre-big bang?
The evolution of the rotational inhomogeneities is investigated in the
specific framework of four-dimensional pre-big bang models. While minimal
(dilaton-driven) scenarios do not lead to rotational fluctuations, in the case
of non-minimal (string-driven) models, fluid sources are present in the pre-big
bang phase. The rotational modes of the geometry, coupled to the divergenceless
part of the velocity field, can then be amplified depending upon the value of
the barotropic index of the perfect fluids. In the light of a possible
production of rotational inhomogeneities, solutions describing the coupled
evolution of the dilaton field and of the fluid sources are scrutinized in both
the string and Einstein frames. In semi-realistic scenarios, where the
curvature divergences are regularized by means of a non-local dilaton
potential, the rotational inhomogeneities are amplified during the pre-big bang
phase but they decay later on. Similar analyses can also be performed when a
contraction occurs directly in the string frame metric.Comment: 21 pages, corrected typos, references added; to appear in Class.
Quantum Gra
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