78 research outputs found
Hidden Negative Energies in Strongly Accelerated Universes
We point out that theories of cosmological acceleration which have equation
of state, w, such that 1+w is small but positive may still secretly violate the
null energy condition. This violation implies the existence of observers for
whom the background has infinitely negative energy densities, despite the fact
that the perturbations are free of ghosts and gradient instabilities.Comment: 5 pages, 1 figure. v2 reflects version accepted for publication in
PRD. Changes: additional discussion of gauge-dependence in perturbed
cosmologie
Dark Matter via Many Copies of the Standard Model
We propose a cosmological scenario based on the assumption that the Standard
Model possesses a large number of copies. It is demonstrated that baryons in
the hidden copies of the standard model can naturally account for the dark
matter. The right abundance of the hidden-sector baryons and the correct
spectrum of density perturbations are simultaneously generated during modulated
reheating. We show that for the natural values of inflaton coupling constants,
dictated by unitarity, the dark-matter abundance is predicted to be
proportional to the ratio of observed cosmological parameters: the square of
the amplitude of cosmological perturbations and the baryon-to-photon number
ratio.Comment: 11 pages, 2 figure
B-Inflation
We propose a novel model of inflation based on a large class of covariant
effective actions containing only the second derivatives of a scalar field. The
initial conditions leading to the inflationary solutions in this model are
rather generic. The graceful exit from the inflationary regime is natural once
the first order derivative terms are included.Comment: 21 pages,10 figures, references added, Fig.1 slightly corrected,
discussion about stability is extende
Looking beyond the horizon
In this short talk we review our results from the paper hep-th/0604075 with
the main stress on the issues of causality and acoustic metric in eikonal
approximation. In addition we correct the formula for the redshift of sound
signals from our original work. We show that, if there exists a special kind of
Born-Infeld type scalar field, then one can send information from inside a
black hole. This information is encoded in perturbations of the field
propagating in non-trivial scalar field backgrounds, which serve as a "new
ether". Although the action of the theory is manifestly Lorentz-invariant the
non-trivial solutions break this symmetry spontaneously, allowing, the
superluminal propagation of perturbations with respect to this "new ether". We
found the stationary solution for background, which describes the accretion of
the scalar field onto a black hole. Examining the propagation of small
perturbations around this solution we show that the signals emitted inside the
Schwarzschild horizon can reach an observer located outside the black hole.
Thus the accreting field forms a hydrodynamical analog of a black hole whose
horizon is inside of the gravitational black hole drawing in the scalar field.Comment: To appear in the proceedings of 11th Marcel Grossmann Meeting on
Recent Developments in Theoretical and Experimental General Relativity,
Gravitation, and Relativistic Field Theories, Berlin, Germany, 23-29 Jul 200
When Matter Matters
We study a recently proposed scenario for the early universe: Subluminal
Galilean Genesis. We prove that without any other matter present in the
spatially flat Friedmann universe, the perturbations of the Galileon scalar
field propagate with a speed at most equal to the speed of light. This proof
applies to all cosmological solutions -- to the whole phase space.
However, in a more realistic situation, when one includes any matter which is
not directly coupled to the Galileon, there always exists a region of phase
space where these perturbations propagate superluminally, indeed with
arbitrarily high speed. We illustrate our analytic proof with numerical
computations. We discuss the implications of this result for the possible UV
completion of the model.Comment: 16 pages, 3 figures. v2 reflects version accepted for publication in
JCAP. Changes include a reorganisation of section order, a new figure 1 and
additional reference
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