3,661 research outputs found
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
Galilean invariance and the consistency relation for the nonlinear squeezed bispectrum of large scale structure
We discuss the constraints imposed on the nonlinear evolution of the Large
Scale Structure (LSS) of the universe by galilean invariance, the symmetry
relevant on subhorizon scales. Using Ward identities associated to the
invariance, we derive fully nonlinear consistency relations between statistical
correlators of the density and velocity perturbations, such as the power
spectrum and the bispectrum. These relations are valid up to O (f_{NL}^2)
corrections. We then show that most of the semi-analytic methods proposed so
far to resum the perturbative expansion of the LSS dynamics fail to fulfill the
constraints imposed by galilean invariance, and are therefore susceptible to
non-physical infrared effects. Finally, we identify and discuss a
nonperturbative semi-analytical scheme which is manifestly galilean invariant
at any order of its expansion.Comment: 29 pages, 7 figure
Galilean invariant resummation schemes of cosmological perturbations
Many of the methods proposed so far to go beyond Standard Perturbation Theory
break invariance under time-dependent boosts (denoted here as extended Galilean
Invariance, or GI). This gives rise to spurious large scale effects which spoil
the small scale predictions of these approximation schemes. By using
consistency relations we derive fully non-perturbative constraints that GI
imposes on correlation functions. We then introduce a method to quantify the
amount of GI breaking of a given scheme, and to correct it by properly tailored
counterterms. Finally, we formulate resummation schemes which are manifestly
GI, discuss their general features, and implement them in the so called
Time-Flow, or TRG, equations.Comment: 21 pages, 5 figure
A review of Axion Inflation in the era of Planck
Because the inflationary mechanism is extremely sensitive to UV-physics, the
construction of theoretically robust models of inflation provides a unique
window on Planck-scale physics. We review efforts to use an axion with a shift
symmetry to ensure a prolonged slow-roll background evolution. The symmetry
dictates which operators are allowed, and these in turn determine the
observational predictions of this class of models, which include observable
gravitational waves (potentially chiral), oscillations in all primordial
correlators, specific deviations from scale invariance and Gaussianity and
primordial black holes. We discuss the constraints on this class of models in
light of the recent Planck results and comment on future perspectives. The
shift symmetry is very useful in models of large-field inflation, which
typically have monomial potentials, but it cannot explain why two or more terms
in the potential are fine-tuned against each other, as needed for typical
models of small-field inflation. Therefore some additional symmetries or
fine-tuning will be needed if forthcoming experiments will constrain the
tensor-to-scalar ratio to be r < 0.01.Comment: 24 pages, 3 figures. Invited review to appear in Class. Quantum Gra
On the Construction of Quintessential Inflation Models
Attention has been recently drawn towards models in which inflation and
quintessence schemes are unified. In such `quintessential inflation' models, a
unique scalar field is required to play both the role of the inflaton and of
the late-time dynamical cosmological constant. We address the issue of the
initial conditions for quintessence in this context and find that, in the two
explicit examples provided, inflation can uniquely fix them to be in the
allowed range for a present day tracking.Comment: LaTex; 14 pages, 1 figure. V2: improved version, some points
clarifie
Trajectories with suppressed tensor-to-scalar ratio in Aligned Natural Inflation
In Aligned Natural Inflation, an alignment between different potential terms
produces an inflaton excursion greater than the axion scales in the potential.
We show that, starting from a general potential of two axions with two aligned
potential terms, the effective theory for the resulting light direction is
characterized by four parameters: an effective potential scale, an effective
axion constant, and two extra parameters (related to ratios of the axion scales
and the potential scales in the field theory). For all choices of these
extra parameters, the model can support inflation along valleys (in the
field space) that end in minima of the potential. This leads to a
phenomenology similar to that of single field Natural Inflation. For a
significant range of the extra two parameters, the model possesses also higher
altitude inflationary trajectories passing through saddle points of the
field potential, and disconnected from any minimum. These plateaus end when
the heavier direction becomes unstable, and therefore all of inflation takes
place close to the saddle point, where - due to the higher altitude - the
potential is flatter (smaller parameter). As a consequence, a
tensor-to-scalar ratio can be
easily achieved in the allowed region, well within the latest
CMB contours
Gravitational leptogenesis in Natural Inflation
We compute the gravitational leptogenesis generated from the parity-violating
gravitational waves sourced by an abelian gauge field coupled to a
pseudo-scalar inflation. We show that, once the CMB bound on the
tensor-to-scalar ratio is enforced, the lepton asymmetry produced by this
mechanism during inflation is too small to account for the observed baryon
asymmetry of the universe, irrespectively of the inflaton potential, the
strength of its coupling to the gauge field, and the details of reheating.Comment: 18 pages, 2 figures; published versio
Ward identities and consistency relations for the large scale structure with multiple species
We present fully nonlinear consistency relations for the squeezed bispectrum
of Large Scale Structure. These relations hold when the matter component of the
Universe is composed of one or more species, and generalize those obtained in
Peloso, Pietroni '13 and Kehagias, Riotto '13 in the single species case. The
multi-species relations apply to the standard dark matter + baryons scenario,
as well as to the case in which some of the fields are auxiliary quantities
describing a particular population, such as dark matter halos or a specific
galaxy class. If a large scale velocity bias exists between the different
populations new terms appear in the consistency relations with respect to the
single species case. As an illustration, we discuss two physical cases in which
such a velocity bias can exist: (1) a new long range scalar force in the dark
matter sector (resulting in a violation of the equivalence principle in the
dark matter-baryon system), and (2) the distribution of dark matter halos
relative to that of the underlying dark matter field.Comment: Published versio
Stability analysis of chromo-natural inflation and possible evasion of Lyth's bound
We perform the complete stability study of the model of chromo-natural
inflation (Adshead and Wyman '12), where, due to its coupling to a SU(2)
vector, a pseudo-scalar inflaton chi slowly rolls on a steep potential. As a
typical example, one can consider an axion with a sub-Planckian decay constant
f. The phenomenology of the model was recently studied (Dimastrogiovanni,
Fasiello, and Tolley '12) in the m_g >> H limit, where m_g is the mass of the
fluctuations of the vector field, and H the Hubble rate. We show that the
inflationary solution is stable for m_g > 2 H, while it otherwise experiences a
strong instability due to scalar perturbations in the sub-horizon regime. The
tensor perturbations are instead standard, and the vector ones remain
perturbatively small. Depending on the parameters, this model can give a
gravity wave signal that can be detected in ongoing or forthcoming CMB
experiments. This detection can occur even if, during inflation, the inflaton
spans an interval of size Delta chi = O (f) which is some orders of magnitude
below the Planck scale, evading a well known bound that holds for a free
inflaton (Lyth '97).Comment: 15 pages, 4 figures. Revised study of tensor mode
L^p-summability of Riesz means for the sublaplacian on complex spheres
In this paper we study the L^p-convergence of the Riesz means for the
sublaplacian on the sphere S^{2n-1} in the complex n-dimensional space C^n. We
show that the Riesz means of order delta of a function f converge to f in
L^p(S^{2n-1}) when delta>delta(p):=(2n-1)|1\2-1\p|. The index delta(p) improves
the one found by Alexopoulos and Lohoue', 2n|1\2-1\p|, and it coincides with
the one found by Mauceri and, with different methods, by Mueller in the case of
sublaplacian on the Heisenberg group.Comment: Rapporto interno Politecnico di Torino, Novembre 200
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