465 research outputs found
Comment on "Accelerating cosmological expansion from shear and bulk viscosity"
In a recent Letter [Phys. Rev. Lett. 114 091301 (2105)] the cause of the
acceleration of the present Universe has been identified with the shear
viscosity of an imperfect relativistic fluid even in the absence of any bulk
viscous contribution. The gist of this comment is that the shear viscosity, if
anything, can only lead to an accelerated expansion over sufficiently small
scales well inside the Hubble radius
Stochastic GW backgrounds and Ground based detectors
The interplay between different ground based detectors and stochastic
backgrounds of relic GW is described. A simultaneous detection of GW in the kHz
and in the MHz--GHz region can point towards a cosmological nature of the
signal. The sensitivity of a pair of VIRGO detectors to string cosmological
models is presented. The implications of microwave cavities for stochastic GW
backgrounds are discussed.Comment: 4 pages in Latex style, one figur
Cosmic backgrounds of relic gravitons and their absolute normalization
Provided the consistency relations are not violated, the recent Bicep2
observations pin down the absolute normalization, the spectral slope and the
maximal frequency of the cosmic graviton background produced during inflation.
The properly normalized spectra are hereby computed from the lowest frequencies
(of the order of the present Hubble rate) up to the highest frequency range in
the GHz region. Deviations from the conventional paradigm cannot be excluded
and are examined by allowing for different physical possibilities including, in
particular, a running of the tensor spectral index, an explicit breaking of the
consistency relations and a spike in the high-frequency tail of the spectrum
coming either from a post-inflationary phase dominated by a stiff fluid of from
the contribution of waterfall fields in a hybrid inflationary context. The
direct determinations of the tensor to scalar ratio at low frequencies, if
confirmed by the forthcoming observations, will also affect and constrain the
high-frequencies uncertainties. The limits on the cosmic graviton backgrounds
coming from wide-band interferometers (such as Ligo/Virgo, Lisa and Bbo/Decigo)
together with a more accurate scrutiny of the tensor B mode polarization at low
frequencies will set direct bounds on the post-inflationary evolution and on
other unconventional completions of the standard lore.Comment: 29 pages, 6 figures; to appear in Classical and Quantum Gravit
Magnetized birefringence and CMB polarization
The polarization plane of the cosmic microwave background radiation can be
rotated either in a magnetized plasma or in the presence of a quintessential
background with pseudoscalar coupling to electromagnetism. A unified treatment
of these two phenomena is presented for cold and warm electron-ion plasmas at
the pre-recombination epoch. The electron temperature is only relevant to the
relativistic correction of the cold plasma results. The spectrum of plasma
excitations is obtained from a generalized Appleton--Hartree equation,
describing simultaneously the high-frequency propagation of electromagnetic
waves in a magnetized plasma with a dynamical quintessence field. It is shown
that these two effects are comparable for the plausible range of parameters
allowed by present constraints. It is then argued that the generalized
expressions derived in the present study may be relevant for direct searches of
a possible rotation of the cosmic microwave background polarization.Comment: 9 pages; corrected typos, references adde
Stringy bounces and gradient instabilities
Bouncing solutions are obtained from a generally covariant action
characterized by a potential which is a nonlocal functional of the dilaton
field at two separated space-time points. Gradient instabilities are shown to
arise in this context but they are argued to be nongeneric. After performing a
gauge-invariant and frame-invariant derivation of the evolution equations of
the fluctuations, a heuristic criterion for the avoidance of pathological
instabilities is proposed and corroborated by a number of explicit examples
that turn out to be compatible with a quasi-flat spectrum of curvature
inhomogeneities for typical wavelengths larger than the Hubble radius.Comment: 25 pages; comments added and corrected typos; to appear in Phys. Rev.
Anomalous Magnetohydrodynamics
Anomalous symmetries induce currents which can be parallel rather than
orthogonal to the hypermagnetic field. Building on the analogy with charged
liquids at high magnetic Reynolds numbers, the persistence of anomalous
currents is scrutinized for parametrically large conductivities when the plasma
approximation is accurate. Different examples in globally neutral systems
suggest that the magnetic configurations minimizing the energy density with the
constraint that the helicity be conserved coincide, in the perfectly conducting
limit, with the ones obtainable in ideal magnetohydrodynamics where the
anomalous currents are neglected. It is argued that this is the rationale for
the ability of extending to anomalous magnetohydrodynamics the hydromagnetic
solutions characterized by finite gyrotropy. The generally covariant aspects of
the problem are addressed with particular attention to conformally flat
geometries which are potentially relevant for the description of the
electroweak plasma prior to the phase transition.Comment: 25 pages, no figure
Primordial magnetic fields
Large scale magnetic fields represent a triple point where cosmology,
high-energy physics and astrophysics meet for different but related purposes.
After reviewing the implications of large scale magnetic fields in these
different areas, the role of primordial magnetic fields is discussed in various
physical processes occurring prior to the decoupling epoch with particular
attention to the big bang nucleosynthesis (BBN) epoch and to the electroweak
(EW) epoch. The generation of matter--antimatter isocurvature fluctuations,
induced by hypermagnetic fields, is analyzed in light of a possible increase of
extra-relativistic species at BBN. It is argued that stochastic GW backgrounds
can be generated by hypermagnetic fields at the LISA frequency. The problem of
the origin of large scale magnetic fields is also scrutinized.Comment: 41 pages in Latex style, 5 figure
Fluctuations of inflationary magnetogenesis
This analysis aims at exploring what can be said about the growth rate of
magnetized inhomogeneities under two concurrent hypotheses: a phase of quasi-de
Sitter dynamics driven by a single inflaton field and the simultaneous presence
of a spectator field coupled to gravity and to the gauge sector. Instead of
invoking ad hoc correlations between the various components, the system of
scalar inhomogeneities is diagonalized in terms of two gauge-invariant
quasi-normal modes whose weighted sum gives the curvature perturbations on
comoving orthogonal hypersurfaces. The predominance of the conventional
adiabatic scalar mode implies that the growth rate of magnetized
inhomogeneities must not exceed 2.2 in Hubble units if the conventional
inflationary phase is to last about 70 efolds and for a range of slow roll
parameters between 0.1 and 0.001. Longer and shorter durations of the quasi-de
Sitter stage lead, respectively, either to tighter or to looser bounds which
are anyway more constraining than the standard backreaction demands imposed on
the gauge sector. Since a critical growth rate of order 2 leads to a quasi-flat
magnetic energy spectrum, the upper bounds on the growth rate imply a lower
bound on the magnetic spectral index. The advantages of the uniform curvature
gauge are emphasized and specifically exploited throughout the treatment of the
multicomponent system characterizing this class of problems.Comment: 37 pages, 4 figure
Faraday scaling and the Bicep2 observations
As repeatedly speculated in the past, the linear polarization of the Cosmic
Microwave Background can be rotated via the Faraday effect. An economic
explanation of the recent Bicep2 observations, not relying on long-wavelength
tensor modes of the geometry, would stipulate that the detected B mode comes
exclusively from a Faraday rotated E mode polarization. We show hereunder that
this interpretation is ruled out by the existing upper limits on the B mode
polarization obtained by independent experiments at observational frequencies
much lower than the operating frequency of the Bicep2 experiment. We then
derive the fraction of the observed B mode polarization ascribable to the
Faraday effect and suggest a dedicated experimental strategy for its detection.Comment: 8 pages, no figure
Viscous modes, isocurvature perturbations and CMB initial conditions
When the predecoupling plasma is thermodynamically reversible its
fluctuations are classified in terms of the adiabatic and entropic modes. A
different category of physical solutions, so far unexplored, arises when the
inhomogeneities of the viscosity coefficients induce computable curvature
perturbations. The viscous modes are explicitly illustrated and compared with
the conventional isocurvature solutions.Comment: 9 pages, no figures; corrected typos; to appear in Physical Review as
a Rapid Communicatio
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