963 research outputs found
Initial Conditions for Imperfect Dark Matter
We discuss initial conditions for the recently proposed Imperfect Dark Matter
(Modified Dust). We show that they are adiabatic under fairly moderate
assumptions about the cosmological evolution of the Universe at the relevant
times.Comment: 16 pages; major revision, references added, conclusions unchange
Gravitational focusing of Imperfect Dark Matter
Motivated by the projectable Horava--Lifshitz model/mimetic matter scenario,
we consider a particular modification of standard gravity, which manifests as
an imperfect low pressure fluid. While practically indistinguishable from a
collection of non-relativistic weakly interacting particles on cosmological
scales, it leaves drastically different signatures in the Solar system. The
main effect stems from gravitational focusing of the flow of Imperfect Dark
Matter passing near the Sun. This entails strong amplification of Imperfect
Dark Matter energy density compared to its average value in the surrounding
halo. The enhancement is many orders of magnitude larger than in the case of
Cold Dark Matter, provoking deviations of the metric in the second order in the
Newtonian potential. Effects of gravitational focusing are prominent enough to
substantially affect the planetary dynamics. Using the existing bound on the
PPN parameter , we deduce a stringent constraint on the unique
constant of the model.Comment: 34 pages, 1 figure. Clarifications and references added. Matches
published versio
Constraining anisotropic models of the early Universe with WMAP9 data
We constrain several models of the early Universe that predict a statistical
anisotropy of the cosmic microwave background (CMB) sky. We make use of WMAP9
maps deconvolved with beam asymmetries. As compared to previous releases of
WMAP data, they do not exhibit the anomalously large quadrupole of statistical
anisotropy. This allows to strengthen the limits on the parameters of models
established earlier in the literature. In particular, the amplitude of the
special quadrupole is constrained as |g_*|<0.072 at 95% C.L. (-0.046<g_*<0.048
at 68% C.L.) independently of the preferred direction in the sky. The upper
limit is obtained on the total number of e-folds in anisotropic inflation with
the Maxwellian term nonminimally coupled to the inflaton, namely N_{tot}
<N_{CMB} +82 at 95% C.L. (+14 at 68% C.L.) for N_{CMB}=60. We also constrain
models of the (pseudo)conformal universe. The strongest constraint is obtained
for spectator scenarios involving a long stage of subhorizon evolution after
conformal rolling, which reads h^2 < 0.006 at 95% C.L., in terms of the
relevant parameter. The analogous constraint is much weaker in dynamical
models, e.g., Galilean genesis.Comment: 28 pages, 8 figures; references added, matches published versio
Caustic free completion of pressureless perfect fluid and k-essence
Both k-essence and the pressureless perfect fluid develop caustic
singularities at finite time. We further explore the connection between the two
and show that they belong to the same class of models, which admits the caustic
free completion by means of the canonical complex scalar field. Specifically,
the free massive/self-interacting complex scalar reproduces dynamics of
pressureless perfect fluid/shift-symmetric k-essence under certain initial
conditions in the limit of large mass/sharp self-interacting potential. We
elucidate a mechanism of resolving caustic singularities in the complete
picture. The collapse time is promoted to complex number. Hence, the
singularity is not developed in real time. The same conclusion holds for a
collection of collisionless particles modelled by means of the Schroedinger
equation, or ultra-light axions (generically, coherent oscillations of bosons
in the Bose--Einstein condensate state).Comment: 20 pages, 2 figures; clarifications and references added. Matches
published versio
New mechanism of producing superheavy Dark Matter
We study in detail the recently proposed mechanism of generating superheavy
Dark Matter with the mass larger than the Hubble rate at the end of inflation.
A real scalar field constituting Dark Matter linearly couples to the inflaton.
As a result of this interaction, the scalar gets displaced from its zero
expectation value. This offset feeds into the energy density of Dark Matter.
This mechanism is universal and can be implemented in a generic inflationary
scenario. Phenomenology of the model is comprised of Dark Matter decay into
inflatons, which in turn decay into Standard Model species triggering cascades
of high energy particles contributing to the cosmic ray flux. We evaluate the
lifetime of Dark Matter and obtain limits on the inflationary scenarios, where
this mechanism does not lead to the conflict with the Dark Matter stability
considerations/studies of cosmic ray propagation.Comment: 19 pages; discussion of Dark Matter production mechanism is expanded,
references added; matches journal versio
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
Affleck-Dine baryogenesis via mass splitting
We introduce a class of non-supersymmetric models explaining baryogenesis a
la Affleck-Dine, which use a decay of two superheavy scalar fields with close
masses. These scalars acquire non-zero expectation values during inflation
through linear couplings to a function of an inflaton. After the inflaton
decay, the model possesses approximate U(1)-invariance, explicitly broken by a
small mass splitting. This splitting leads to the baryogenesis in the early
Universe. Resulting baryon asymmetry is automatically small for the scalars
with the masses about the Grand Unification scale and larger. It is fully
determined by the inflaton dynamics and the Lagrangian parameters, i.e., is
independent of initial pre-inflationary conditions for the scalars. As a
consequence, baryon perturbations are purely adiabatic. We point out a possible
origin of the mass splitting: masses of scalars degenerate at some large energy
scale may acquire different loop corrections due to the interaction with the
inflaton. Compared to electroweak baryogenesis and conventional Affleck-Dine
scenarios, our mechanism generically leads to the proton decay suppressed by
the powers of the superheavy scalar masses, which makes this scenario
potentially testable.Comment: 6 pages; matches published versio
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
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