725 research outputs found
Conformal Symmetries of FRW Accelerating Cosmologies
We show that any accelerating Friedmann-Robertson-Walker (FRW) cosmology with
equation of state w < -1/3 (and therefore not only a de Sitter stage with w
=-1) exhibits three-dimensional conformal symmetry on future constant-time
hypersurfaces. We also offer an alternative derivation of this result in terms
of conformal Killing vectors and show that long wavelength comoving curvature
perturbations of the perturbed FRW metric are just conformal Killing motions of
the FRW background. We then extend theb boundary conformal symmetry to the bulk
for accelerating cosmologies. Our findings indicate that one can easily
generate perturbations of scalar fields which are not only scale invariant, but
also fully conformally invariant on super-Hubble scales. Measuring a
scale-invariant power spectrum for the cosmological perturbation does not
automatically imply that the universe went through a de Sitter stage.Comment: 25 page
A Stochastic Approach to Thermal Fluctuations during a First Order Electroweak Phase Transition
We investigate the role played by subcritical bubbles at the onset of the
electroweak phase transition. Treating the configuration modelling the thermal
fluctuations around the homogeneous zero configuration of the Higgs field as a
stochastic variable, we describe its dynamics by a phenomenological Langevin
equation. This approach allows to properly take into account both the effects
of the thermal bath on the system: a systematic dyssipative force, which tends
to erase out any initial subcritical configuration, and a random stochastic
force responsible for the fluctuations. We show that the contribution to the
variance \lgh\phi^2(t)\rg_V in a given volume from any initial
subcritical configuration is quickly damped away and that, in the limit of long
times, \lgh\phi^2(t)\rg_V approaches its equilibrium value provided by the
stochastic force and independent from the viscosity coefficient, as predicted
by the fluctuation-dissipation theorem. In agreement with some recent claims,
we conclude that thermal fluctuations do not affect the nucleation of critical
bubbles at the onset of the electroweak phase transition making electroweak
baryogenesis scenarios still a viable possibility to explain the primordial
baryon asymmetry in the Universe.Comment: Two figures: fig1.metafile and fig2.metafile. Just print them as
usual file.p
Testing the Origin of Cosmological Magnetic Fields through the Large-Scale Structure Consistency Relations
We study the symmetries of the post-recombination cosmological
magnetohydrodynamical equations which describe the evolution of dark matter,
baryons and magnetic fields in a self-consistent way. This is done both at the
level of fluid equations and of Vlasov-Poisson-Maxwell equations in phase
space. We discuss some consistency relations for the soft limit of the (n +
1)-correlator functions involving magnetic fields and matter overdensities. In
particular, we stress that any violation of such consistency relations at
equal-time would point towards an inflationary origin of the magnetic field.Comment: 23 page
On the Starobinsky Model of Inflation from Supergravity
We discuss how the higher-derivative Starobinsky model of inflation
originates from N=1 supergravity. It is known that, in the old-minimal
supergravity description written by employing a chiral compensator in the
superconformal framework, the Starobinsky model is equivalent to a no-scale
model with F-term potential. We show that the Starobinsky model can also be
originated within the so-called new-minimal supergravity, where a linear
compensator superfield is employed. In this formulation, the Starobinsky model
is equivalent to standard supergravity coupled to a massive vector multiplet
whose lowest scalar component plays the role of the inflaton and the vacuum
energy is provided by a D-term potential. We also point out that higher-order
corrections to the supergravity Lagrangian represent a threat to the
Starobinsky model as they can destroy the flatness of the inflaton potential in
its scalar field equivalent description.Comment: 17 pages, 2 figures, published versio
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