2,019 research outputs found
Sudden variations in the speed of sound during inflation: features in the power spectrum and bispectrum
We employ the formalism of the effective field theory of inflation to study
the effects of a sudden change in the speed of sound of the inflationary
perturbations. Such an event generates a feature with high frequency
oscillations both in the two- and in the three-point functions of the curvature
fluctuations. We study, at first order in the magnitude of the change of the
speed of sound, the dependence of the power spectrum and of the bispectrum on
the duration of the change. In the limit of a very short duration, the
oscillations in the power spectrum persist up to very large momenta and the
amplitude of the feature in the bispectrum diverges while its location moves to
increasing momenta.Comment: 7 pp, 2 figure
Adding helicity to inflationary magnetogenesis
The most studied mechanism of inflationary magnetogenesis relies on the
time-dependence of the coefficient of the gauge kinetic term
. Unfortunately, only extremely finely tuned versions
of the model can consistently generate the cosmological magnetic fields
required by observations. We propose a generalization of this model, where also
the pseudoscalar invariant is multiplied by a
time dependent function. The new parity violating term allows more freedom in
tuning the amplitude of the field at the end of inflation. Moreover, it leads
to a helical magnetic field that is amplified at large scales by
magnetohydrodynamical processes during the radiation dominated epoch. As a
consequence, our model can satisfy the observational lower bounds on fields in
the intergalactic medium, while providing a seed for the galactic dynamo, if
inflation occurs at an energy scale ranging from to GeV. Such
energy scale is well below that suggested by the recent BICEP2 result, if the
latter is due to primordial tensor modes. However, the gauge field is a source
of tensors during inflation and generates a spectrum of gravitational waves
that can give a sizable tensor to scalar ratio even if
inflation occurs at low energies. This system therefore evades the Lyth bound.
For smaller values of , lower values of the inflationary energy scale are
required. The model predicts fully helical cosmological magnetic fields and a
chiral spectrum of primordial gravitational waves.Comment: 17 pages, 4 figures. Minor changes to match the version accepted for
publication in JCA
Preheating of massive fermions after inflation: analytical results
Non-perturbative production of fermions after chaotic inflation has been the
object of several studies in the very recent past. However, the results in the
most interesting case of production of massive fermions in an expanding
Universe were so far known only numerically. We provide very simple and
readable analytical formulae, both for the spectra of the created fermions and
for their total energy density. Their derivation is closely related to the one
adopted for bosons and exploits the fact that the production occurs during very
short intervals of nonadiabatical change of the fermionic frequency. Our
formulae show the presence of resonance bands if the expansion of the Universe
is neglected, and their disappearance when the latter is included. As in the
bosonic case, this last effect is due to the stochastic character that the
expansion gives to the whole process. Backreaction is considered in the last
part of the work. All our analytical results are in excellent agreement with
the previous numerical ones in the regime of validity of the latter. However, a
more accurate scaling for the energy density of the produced fermions is here
found.Comment: Final version, 31 pages, 9 figure
- …