1,036 research outputs found
Matter Bounce in Horava-Lifshitz Cosmology
Horava-Lifshitz gravity, a recent proposal for a UV-complete renormalizable
gravity theory, may lead to a bouncing cosmology. In this note we argue that
Horava-Lifshitz cosmology may yield a concrete realization of the matter bounce
scenario, and thus give rise to an alternative to inflation for producing a
scale-invariant spectrum of cosmological perturbations. In this scenario,
quantum vacuum fluctuations exit the Hubble radius in the pre-bounce phase and
the spectrum is transformed into a scale-invariant one on super-Hubble scales
before the bounce because the long wavelength modes undergo squeezing of their
wave-functions for a longer period of time than shorter wavelength modes. The
scale-invariance of the spectrum of curvature fluctuations is preserved during
and after the bounce. A distinctive prediction of this scenario is the
amplitude and shape of the bispectrum.Comment: 6 pages, 1 figure, a couple of minor wording change
Current Acceleration from Dilaton and Stringy Cold Dark Matter
We argue that string theory has all the ingredients to provide us with
candidates for the cold dark matter and explain the current acceleration of our
Universe. In any generic string compactification the dilaton plays an important
role as it couples to the Standard Model and other heavy non-relativistic
degrees of freedom such as the string winding modes and wrapped branes, we
collectively call them stringy cold dark matter. These couplings are
non-universal which results in an interesting dynamics for a rolling dilaton.
Initially, its potential can track radiation and matter while beginning to
dominate the dynamics recently, triggering a phase of acceleration. This
scenario can be realized as long as the dilaton also couples strongly to some
heavy modes. We furnish examples of such modes. We provide analytical and
numerical results and compare them with the current supernovae result. This
favors certain stringy candidates.Comment: 16 pages, 4 figures (colour
Cosmological Solution in M-theory on S^1/Z_2
We provide the first example of a cosmological solution of the Horava-Witten
supergravity. This solution is obtained by exchanging the role of time with the
radial coordinate of the transverse space to the five-brane soliton. On the
boundary this corresponds to rotating an instanton solution into a tunneling
process in a space with Lorentzian signature, leading to an expanding universe.
Due to the freedom to choose different non-trivial Yang-Mills backgrounds on
the boundaries, the two walls of the universe ( visible and hidden worlds)
expand differently. However at late times the anisotropy is washed away by
gravitational interactions.Comment: 10 pages, latex, no figur
Dilaton stabilization by massive fermion matter
The study started in a former work about the Dilaton mean field stabilization
thanks to the effective potential generated by the existence of massive
fermions, is here extended. Three loop corrections are evaluated in addition to
the previously calculated two loop terms. The results indicate that the Dilaton
vacuum field tend to be fixed at a high value close to the Planck scale, in
accordance with the need for predicting Einstein gravity from string theory.
The mass of the Dilaton is evaluated to be also a high value close to the
Planck mass, which implies the absence of Dilaton scalar signals in modern
cosmological observations. These properties arise when the fermion mass is
chosen to be either at a lower bound corresponding to the top quark mass, or
alternatively, at a very much higher value assumed to be in the grand
unification energy range. One of the three 3-loop terms is exactly evaluated in
terms of Master integrals. The other two graphs are however evaluated in their
leading logarithm correction in the perturbative expansion. The calculation of
the non leading logarithmic contribution and the inclusion of higher loops
terms could made more precise the numerical estimates of the vacuum field value
and masses, but seemingly are expected not to change the qualitative behavior
obtained. The validity of the here employed Yukawa model approximation is
argued for small value of the fermion masses with respect to the Planck one. A
correction to the two loop calculation done in the previous work is here
underlined.Comment: 18 pages, 5 figures, the study was extended and corrections on the
former calculations and redaction were done. The paper had been accepted for
publication in "Astrophysics and Space Science
Detection of transplanckian effects in the cosmic microwave background
Quantum gravity effects are expected to modify the primordial density
fluctuations produced during inflation and leave their imprint on the cosmic
microwave background observed today. We present a new analysis discussing
whether these effects are detectable, considering both currently available data
and simulated results from an optimal CMB experiment. We find that the WMAP
(Wilkinson Microwave Anisotropy Probe) data show no evidence for the particular
signature considered in this work but give an upper bound on the parameters of
the model. However, a hypothetical experiment shows that with proper data, the
trans-Planckian effects should be detectable through alternate sampling
methods. This fuzzy conclusion is a result of the nature of the oscillations,
since they give rise to a likelihood hypersurface riddled with local maxima. A
simple Bayesian analysis shows no significant evidence for the simulated data
to prefer a trans-Planckian model. Conventional Markov chain Monte Carlo (MCMC)
methods are not suitable for exploring this complicated landscape, but
alternative methods are required to solve the problem. This, however, requires
extremely high-precision data.Comment: 9 pages, 22 figure
String Gas Cosmology and Structure Formation
It has recently been shown that a Hagedorn phase of string gas cosmology may
provide a causal mechanism for generating a nearly scale-invariant spectrum of
scalar metric fluctuations, without the need for an intervening period of de
Sitter expansion. A distinctive signature of this structure formation scenario
would be a slight blue tilt of the spectrum of gravitational waves. In this
paper we give more details of the computations leading to these results.Comment: 12 pages, 3 figure
Unconventional Cosmology
I review two cosmological paradigms which are alternative to the current
inflationary scenario. The first alternative is the "matter bounce", a
non-singular bouncing cosmology with a matter-dominated phase of contraction.
The second is an "emergent" scenario, which can be implemented in the context
of "string gas cosmology". I will compare these scenarios with the inflationary
one and demonstrate that all three lead to an approximately scale-invariant
spectrum of cosmological perturbations.Comment: 45 pages, 10 figures; invited lectures at the 6th Aegean Summer
School "Quantum Gravity and Quantum Cosmology", Chora, Naxos, Greece, Sept.
12 - 17 2012, to be publ. in the proceedings; these lecture notes form an
updated version of arXiv:1003.1745 and arXiv:1103.227
Trans-Planckian Physics and the Spectrum of Fluctuations in a Bouncing Universe
In this paper, we calculate the spectrum of scalar field fluctuations in a
bouncing, asymptotically flat Universe, and investigate the dependence of the
result on changes in the physics on length scales shorter than the Planck
length which are introduced via modifications of the dispersion relation. In
this model, there are no ambiguities concerning the choice of the initial
vacuum state. We study an example in which the final spectrum of fluctuations
depends sensitively on the modifications of the dispersion relation without
needing to invoke complex frequencies. Changes in the amplitude and in the
spectral index are possible, in addition to modulations of the spectrum. This
strengthens the conclusions of previous work in which the spectrum of
cosmological perturbations in expanding inflationary cosmologies was studied,
and it was found that, for dispersion relations for which the evolution is not
adiabatic, the spectrum changes from the standard prediction of
scale-invariance.Comment: 10 pages, 6 figures, RevTeX4. Analytical determination of the
spectrum, corrected some typos, conclusions unchange
The Coherent State Representation of Quantum Fluctuations in the Early Universe
Using the squeezed state formalism the coherent state representation of
quantum fluctuations in an expanding universe is derived. It is shown that this
provides a useful alternative to the Wigner function as a phase space
representation of quantum fluctuations. The quantum to classical transition of
fluctuations is naturally implemented by decohering the density matrix in this
representation. The entropy of the decohered vacua is derived. It is shown that
the decoherence process breaks the physical equivalence between vacua that
differ by a coordinate dependent phase generated by a surface term in the
Lagrangian. In particular, scale invariant power spectra are only obtained for
a special choice of surface term.Comment: 25 pages in revtex 3. This version is completely revised with
corrections and significant new calculation
DBI with Primordial Magnetic Field in the Sky
In this paper, we study the generation of a large scale magnetic field with
amplitude of order G in an inflationary model which has been introduced in
hep-th/0310221. This inflationary model based on existence of a speed limit for
inflaton field. Generating a mass for inflaton at scale above the ,
breaks the conformal triviality of the Maxwell equation and causes to originate
a magnetic field during the inflation. The amplitude strongly depends on the
details of reheating stage and also depends on the e-foldings parameter N. We
find the amplitude of the primordial magnetic field at decoupling time in this
inflationary background using late time behavior of the theory.Comment: 12 pages, no figure, typos correcte
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