236 research outputs found
Cosmic Dynamics in the Chameleon Cosmology
We study in this paper chameleon cosmology applied to
Friedmann-Robertson-Walker space, which gives rise to the equation of state
(EoS) parameter larger than -1 in the past and less than -1 today, satisfying
current observations. We also study cosmological constraints on the model using
the time evolution of the cosmological redshift of distant sources which
directly probes the expansion history of the universe. Due to the evolution of
the universe's expansion rate, the model independent Cosmological Redshift
Drift (CRD)test is expected to experience a small, systematic drift as a
function of time. The model is supported by the observational data obtained
from the test.Comment: 16 pages, 9 figure
Construction of nonsingular cosmological solutions in string theories
We study nonsingular cosmological scenarios in a general -dimensional
string effective action of the dilaton-modulus-axion system in the presence of
the matter source. In the standard dilatonic Brans-Dicke parameter
() with radiation, we analytically obtain singularity-free bouncing
solutions where the universe starts out in a state with a finite curvature and
evolves toward the weakly coupled regime. We apply this analytic method to the
string-gas cosmology including the massive state in addition to the leading
massless state (radiation), with and without the axion. We numerically find
bouncing solutions which asymptotically approach an almost radiation-dominant
universe with a decreasing curvature irrespective of the presence of the axion,
implying that inclusion of the matter source is crucial for the existence of
such solutions for . In the theories with , it is
possible to obtain complete regular bouncing solutions with a finite dilaton
and curvature in both past and future asymptotics for the general dimension,
. We also discuss the case where dilatonic higher-order corrections are
involved to the tree-level effective action and demonstrate that the presence
of axion/modulus fields and the matter source does not significantly affect the
dynamics of the dilaton-driven inflation and the subsequent graceful exit.Comment: 27 pages, 6 eps figure
Gravity from Quantum Information
It is suggested that the Einstein equation can be derived from Landauer's
principle applied to an information erasing process at a local Rindler horizon
and Jacobson's idea linking the Einstein equation with thermodynamics. When
matter crosses the horizon, the information of the matter disappears and the
horizon entanglement entropy increases to compensate the entropy reduction. The
Einstein equation describes an information-energy relation during this process,
which implies that entropic gravity is related to the quantum entanglement of
the vacuum and has a quantum information theoretic origin.Comment: 7 pages, revtex4-1, 2 figures, recent supporting results adde
The accelerating universe and a limiting curvature proposal
We consider the hypothesis of a limiting minimal curvature in gravity as a
way to construct a class of theories exhibiting late-time cosmic acceleration.
Guided by the minimal curvature conjecture (MCC) we are naturally lead to a set
of scalar tensor theories in which the scalar is non-minimally coupled both to
gravity and to the matter Lagrangian. The model is compared to the Lambda Cold
Dark Matter concordance model and to the observational data using the gold
SNeIa sample of Riess et. al. (2004). An excellent fit to the data is achieved.
We present a toy model designed to demonstrate that such a new, possibly
fundamental, principle may be responsible for the recent period of cosmological
acceleration. Observational constraints remain to be imposed on these models.Comment: 22 pages, 7 figures; revised version to appear in JCAP; references
adde
Effective Field Theory Approach to String Gas Cosmology
We derive the 4D low energy effective field theory for a closed string gas on
a time dependent FRW background. We examine the solutions and find that
although the Brandenberger-Vafa mechanism at late times no longer leads to
radion stabilization, the radion rolls slowly enough that the scenario is still
of interest. In particular, we find a simple example of the string inspired
dark matter recently proposed by Gubser and Peebles.Comment: 19 pages, 2 figures, comments adde
Does brane cosmology have realistic principles?
The maximal symmetry, or Perfect Cosmological Principle(PCP), that prevents
AdS type spaces from degenerating into anti-inflationary collapse is argued to
be unphysical. For example, the simple requirement that brane-bulk models
should be the result of having evolved from even more energetic string
phenomena picks out a preferred time direction.
We question whether quantum cosmological reasoning can be applied in any
meaningful way to obtain, what are essentially, classical constructs . An
alternative scheme is to more readily accept the PCP and allow the branes to
also become eternal. A perpetually expanding and contracting brane model could
be driven by the presence of charged black holes in the AdS bulk, that
effectively violates the weak-energy condition as singularities are approached.
This can be contrasted with the so-called Ekpyrotic universe which also closely
accepts the PCP. This being broken only by occasional collisions between
branes, that can then simulate a big bang cosmology.Comment: extended version and title chang
Isotropization in Brane Gas Cosmology
Brane Gas Cosmology (BGC) is an approach to unifying string theory and
cosmology in which matter is described by a gas of strings and branes in a
dilaton gravity background. The Universe is assumed to start out with all
spatial dimensions compact and small. It has previously been shown that in this
context, in the approximation of neglecting inhomogeneities and anisotropies,
there is a dynamical mechanism which allows only three spatial dimensions to
become large. However, previous studies do not lead to any conclusions
concerning the isotropy or anisotropy of these three large spatial dimensions.
Here, we generalize the equations of BGC to the anisotropic case, and find that
isotropization is a natural consequence of the dynamics.Comment: 11 pages, 4 eps figures, references adde
On the Origin of Entropic Gravity and Inertia
It was recently suggested that quantum field theory is not fundamental but
emerges from the loss of phase space information about matter crossing causal
horizons. Possible connections between this formalism and Verlinde's entropic
gravity and Jacobson's thermodynamic gravity are proposed.
The holographic screen in Verlinde's formalism can be identified as local
Rindler horizons and its entropy as that of the bulk fields beyond the
horizons.
This naturally resolves some issues on entropic gravity.
The quantum fluctuation of the fields is the origin of the thermodynamic
nature of entropic gravity.
It is also suggested that inertia is related to dragging
Rindler horizons.Comment: 9 pages, revtex4-1, 3 figures, accepted for publication in
Foundations of Physic
String windings in the early universe
We study string dynamics in the early universe. Our motivation is the
proposal of Brandenberger and Vafa, that string winding modes may play a key
role in decompactifying three spatial dimensions. We model the universe as a
homogeneous but anisotropic 9-torus filled with a gas of excited strings. We
adopt initial conditions which fix the dilaton and the volume of the torus, but
otherwise assume all states are equally likely. We study the evolution of the
system both analytically and numerically to determine the late-time behavior.
We find that, although dynamical evolution can indeed lead to three large
spatial dimensions, such an outcome is not statistically favored.Comment: 26 pages, LaTeX, 4 eps figure
Reconstructing Single Field Inflationary Actions From CMBR Data
This paper describes a general program for deriving the action of single
field inflation models with nonstandard kinetic energy terms using CMBR power
spectrum data. This method assumes that an action depends on a set of
undetermined functions, each of which is a function of either the inflaton wave
function or its time derivative. The scalar, tensor and non-gaussianity of the
curvature perturbation spectrum are used to derive a set of reconstruction
equations whose solution set can specify up to three of the undetermined
functions. The method is then used to find the undetermined functions in
various types of actions assuming power law type scalar and tensor spectra. In
actions that contain only two unknown functions, the third reconstruction
equation implies a consistency relation between the non-gaussianty, sound speed
and slow roll parameters. In particular we focus on reconstructing a
generalized DBI action with an unknown potential and warp factor. We find that
for realistic scalar and tensor spectra, the reconstructed warp factor and
potential are very similar to the theoretically derived result. Furthermore,
physical consistency of the reconstructed warp factor and potential imposes
strict constraints on the scalar and tensor spectral indices.Comment: 33 pages, 3 figures: v3 - References adde
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