742 research outputs found
DSR as an explanation of cosmological structure
Deformed special relativity (DSR) is one of the possible realizations of a
varying speed of light (VSL). It deforms the usual quadratic dispersion
relations so that the speed of light becomes energy dependent, with preferred
frames avoided by postulating a non-linear representation of the Lorentz group.
The theory may be used to induce a varying speed of sound capable of generating
(near) scale-invariant density fluctuations, as discussed in a recent Letter.
We identify the non-linear representation of the Lorentz group that leads to
scale-invariance, finding a universal result. We also examine the higher order
field theory that could be set up to represent it
The four fixed points of scale invariant single field cosmological models
We introduce a new set of flow parameters to describe the time dependence of
the equation of state and the speed of sound in single field cosmological
models. A scale invariant power spectrum is produced if these flow parameters
satisfy specific dynamical equations. We analyze the flow of these parameters
and find four types of fixed points that encompass all known single field
models. Moreover, near each fixed point we uncover new models where the scale
invariance of the power spectrum relies on having simultaneously time varying
speed of sound and equation of state. We describe several distinctive new
models and discuss constraints from strong coupling and superluminality.Comment: 24 pages, 6 figure
Creating Statistically Anisotropic and Inhomogeneous Perturbations
In almost all structure formation models, primordial perturbations are
created within a homogeneous and isotropic universe, like the one we observe.
Because their ensemble averages inherit the symmetries of the spacetime in
which they are seeded, cosmological perturbations then happen to be
statistically isotropic and homogeneous. Certain anomalies in the cosmic
microwave background on the other hand suggest that perturbations do not
satisfy these statistical properties, thereby challenging perhaps our
understanding of structure formation. In this article we relax this tension. We
show that if the universe contains an appropriate triad of scalar fields with
spatially constant but non-zero gradients, it is possible to generate
statistically anisotropic and inhomogeneous primordial perturbations, even
though the energy momentum tensor of the triad itself is invariant under
translations and rotations.Comment: 20 pages, 1 figure. Uses RevTeX
A Dynamical Solution to the Problem of a Small Cosmological Constant and Late-time Cosmic Acceleration
Increasing evidence suggests that most of the energy density of the universe
consists of a dark energy component with negative pressure, a ``cosmological
constant" that causes the cosmic expansion to accelerate. In this paper, we
address the puzzle of why this component comes to dominate the universe only
recently rather than at some much earlier epoch. We present a class of theories
based on an evolving scalar field where the explanation is based entirely on
internal dynamical properties of the solutions. In the theories we consider,
the dynamics causes the scalar field to lock automatically into a negative
pressure state at the onset of matter-domination such that the present epoch is
the earliest possible time, consistent with nucleosynthesis restrictions, when
it can start to dominate.Comment: 5 pages, 3 figure
Tensor ghosts in the inflationary cosmology
Theories with curvature squared terms in the action are known to contain
ghost modes in general. However, if we regard curvature squared terms as
quantum corrections to the original theory, the emergence of ghosts may be
simply due to the perturbative truncation of a full non-perturbative theory. If
this is the case, there should be a way to live with ghosts. In this paper, we
take the Euclidean path integral approach, in which ghost degrees of freedom
can be, and are integrated out in the Euclideanized spacetime. We apply this
procedure to Einstein gravity with a Weyl curvature squared correction in the
inflationary background. We find that the amplitude of tensor perturbations is
modified by a term of O(alpha^2 H^2) where alpha^2 is a coupling constant in
front of the Weyl squared term and H is the Hubble parameter during inflation.Comment: 16 pages, no figure
Where does Cosmological Perturbation Theory Break Down?
We apply the effective field theory approach to the coupled metric-inflaton
system, in order to investigate the impact of higher dimension operators on the
spectrum of scalar and tensor perturbations in the short-wavelength regime. In
both cases, effective corrections at tree-level become important when the
Hubble parameter is of the order of the Planck mass, or when the physical wave
number of a cosmological perturbation mode approaches the square of the Planck
mass divided by the Hubble constant. Thus, the cut-off length below which
conventional cosmological perturbation theory does not apply is likely to be
much smaller than the Planck length. This has implications for the
observability of "trans-Planckian" effects in the spectrum of primordial
perturbations.Comment: 25 pages, uses FeynM
Quantum coherence and carriers mobility in organic semiconductors
We present a model of charge transport in organic molecular semiconductors
based on the effects of lattice fluctuations on the quantum coherence of the
electronic state of the charge carrier. Thermal intermolecular phonons and
librations tend to localize pure coherent states and to assist the motion of
less coherent ones. Decoherence is thus the primary mechanism by which
conduction occurs. It is driven by the coupling of the carrier to the molecular
lattice through polarization and transfer integral fluctuations as described by
the hamiltonian of Gosar and Choi. Localization effects in the quantum coherent
regime are modeled via the Anderson hamiltonian with correlated diagonal and
non-diagonal disorder leading to the determination of the carrier localization
length. This length defines the coherent extension of the ground state and
determines, in turn, the diffusion range in the incoherent regime and thus the
mobility. The transfer integral disorder of Troisi and Orlandi can also be
incorporated. This model, based on the idea of decoherence, allowed us to
predict the value and temperature dependence of the carrier mobility in
prototypical organic semiconductors that are in qualitative accord with
experiments
Preheating in Derivatively-Coupled Inflation Models
We study preheating in theories where the inflaton couples derivatively to
scalar and gauge fields. Such couplings may dominate in natural models of
inflation, in which the flatness of the inflaton potential is related to an
approximate shift symmetry of the inflaton. We compare our results with
previously studied models with non-derivative couplings. For sufficiently heavy
scalar matter, parametric resonance is ineffective in reheating the universe,
because the couplings of the inflaton to matter are very weak. If scalar matter
fields are light, derivative couplings lead to a mild long-wavelength
instability that drives matter fields to non-zero expectation values. In this
case however, long-wavelength fluctuations of the light scalar are produced
during inflation, leading to a host of cosmological problems. In contrast,
axion-like couplings of the inflaton to a gauge field do not lead to production
of long-wavelength fluctuations during inflation. However, again because of the
weakness of the couplings to the inflaton, parametric resonance is not
effective in producing gauge field quanta.Comment: 10 pages, 9 figure
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