328 research outputs found
Statistical anisotropy of CMB as a probe of conformal rolling scenario
Search for the statistical anisotropy in the CMB data is a powerful tool for
constraining models of the early Universe. In this paper we focus on the
recently proposed cosmological scenario with conformal rolling. We consider two
sub-scenarios, one of which involves a long intermediate stage between
conformal rolling and conventional hot epoch. Primordial scalar perturbations
generated within these sub-scenarios have different direction-dependent power
spectra, both characterized by a single parameter h^2. We search for the
signatures of this anisotropy in the seven-year WMAP data using quadratic
maximum likelihood method, first applied for similar purposes by Hanson and
Lewis. We confirm the large quadrupole anisotropy detected in V and W bands,
which has been argued to originate from systematic effects rather than from
cosmology. We construct an estimator for the parameter h^2. In the case of the
sub-scenario with the intermediate stage we set an upper limit h^2 < 0.045 at
the 95% confidence level. The constraint on h^2 is much weaker in the case of
another sub-scenario, where the intermediate stage is absent.Comment: 27 pages, 4 figures. Stronger constraint in case of sub-scenario A
obtained. Version accepted for publication in JCA
Revisiting constraints on (pseudo)conformal Universe with Planck data
We revisit constraints on the (pseudo)conformal Universe from the
non-observation of statistical anisotropy in the Planck data. The quadratic
maximal likelihood estimator is applied to the Planck temperature maps at
frequencies 143 GHz and 217 GHz as well as their cross-correlation. The
strongest constraint is obtained in the scenario of the (pseudo)conformal
Universe with a long intermediate evolution after conformal symmetry breaking.
In terms of the relevant parameter (coupling constant), the limit is h^2
<0.0013 at 95% C.L. (using the cross-estimator). The analogous limit is much
weaker in the scenario without the intermediate stage (h^2 \ln
\frac{H_0}{\Lambda}<0.52) allowing the coupling constant to be of order one. In
the latter case, the non-Gaussianity in the 4-point function appears to be a
more promising signature.Comment: 13 pages, 2 figures. Appendix with detailed computation of the Fisher
matrix adde
Effect of intermediate Minkowskian evolution on CMB bispectrum
We consider a non-inflationary early Universe scenario in which relevant
scalar perturbations get frozen out at some point, but then are defrosted and
follow a long nearly Minkowskian evolution before the hot era. This
intermediate stage leaves specific imprint on the CMB 3-point function, largely
independent of details of microscopic physics. In particular, the CMB
bispectrum undergoes oscillations in the multipole l space with roughly
constant amplitude. The latter is in contrast to the oscillatory bispectrum
enhanced in the flattened triangle limit, as predicted by inflation with
non-Bunch--Davies vacuum. Given this and other peculiar features of the
bispectrum, stringent constraints imposed by the Planck data may not apply. The
CMB 3-point function is suppressed by the inverse duration squared of the
Minkowskian evolution, but can be of observable size for relatively short
intermediate Minkowskian stage.Comment: 27 pages, 5 figures; references added, representation of the material
improved; matches journal versio
Living with ghosts in Horava-Lifshitz gravity
We consider the branch of the projectable Horava-Lifshitz model which
exhibits ghost instabilities in the low energy limit. It turns out that, due to
the Lorentz violating structure of the model and to the presence of a finite
strong coupling scale, the vacuum decay rate into photons is tiny in a wide
range of phenomenologically acceptable parameters. The strong coupling scale,
understood as a cutoff on ghosts' spatial momenta, can be raised up to TeV. At lower momenta, the projectable Horava-Lifshitz gravity is
equivalent to General Relativity supplemented by a fluid with a small positive
sound speed squared () , that could
be a promising candidate for the Dark Matter. Despite these advantages, the
unavoidable presence of the strong coupling obscures the implementation of the
original Horava's proposal on quantum gravity. Apart from the Horava-Lifshitz
model, conclusions of the present work hold also for the mimetic matter
scenario, where the analogue of the projectability condition is achieved by a
non-invertible conformal transformation of the metric.Comment: 33 pages, 1 figure. The proof of an equivalence between the IR limit
of the projectable Horava-Lifshitz gravity and the mimetic matter scenario is
given in Appendix A. Version accepted for publication in JHE
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