261 research outputs found
Singularities and qualitative study in LQC
We will perform a detailed analysis of singularities in Einstein Cosmology and in LQC (Loop Quantum Cosmology). We will obtain explicit analytical expressions for the energy density and the Hubble constant for a given set of possible Equations of State. We will also consider the case when the background is driven by a single scalar field, obtaining analytical expressions for the corresponding potential. And, in a given particular case, we will perform a qualitative study of the orbits in the associated phase space of the scalar. eld
Reheating via gravitational particle production in simple models of quintessence or CDM inflation
We have tested some simple CDM (the same test is also valid for
quintessence) inflation models, imposing that they match with the recent
observational data provided by the BICEP and Planck's team and leading to a
reheating temperature, which is obtained via gravitational particle production
after inflation, supporting the nucleosynthesis success.Comment: Proceedings of the workshop "Cosmology and the Quantum Vacuum IV"
Segovia, Spain. Accepted for publication in the special Issue "Cosmology and
the Quantum Vacuum" of Galaxie
Note on the reheating temperature in Starobinsky-type potentials
The relation between the reheating temperature, the number of e-folds and the
spectral index is shown for the Starobinsky model and some of its descendants
through a very detailed calculation of these three quantities. The conclusion
is that for viable temperatures between MeV and GeV the
corresponding values of the spectral index enter perfectly in its
C.L., which shows the viability of this kind of models.Comment: Version accepted for publication in GER
The spectrum of Gravitational Waves, their overproduction in quintessential inflation and its influence in the reheating temperature
One of the most important issues in an inflationary theory as standard or
quintessential inflation is the mechanism to reheat the universe after the end
of the inflationary period in order to match with the Hot Big Bang universe. In
quintessential inflation two mechanisms are frequently used, namely the
reheating via gravitational particle production which is, as we will see, very
efficient when the phase transition from the end of inflation to a kinetic
regime (all the energy of the inflaton field is kinetic) is very abrupt, and
the so-called instant preheating which is used for a very smooth phase
transition because in that case the gravitational particle production is very
inefficient.
In the present work, a detailed study of these mechanisms is done, obtaining
bounds for the reheating temperature and the range of the parameters involved
in each reheating mechanism in order that the Gravitational Waves (GWs)
produced at the beginning of kination do not disturb the Big Bang
Nucleosynthesis (BBN) success.Comment: 51 pages, 4 figures, 1 table, new material and references added.
Version accepted for publication in Univers
Quintessential inflation at low reheating temperatures
We have tested some simple quintessential inflation models, imposing that
they match with the recent observational data provided by the BICEP and
Planck's team and leading to a reheating temperature, which is obtained via
gravitational particle production after inflation, supporting the
nucleosynthesis success. Moreover, for the models coming from supergravity one
needs to demand low temperatures in order to avoid problems such as the
gravitino overproduction or the gravitational production of moduli fields,
which are obtained only when the reheating temperature is due to the production
of massless particles with a coupling constant very close to its conformal
value.Comment: 21 pages, 10 figures. Version accepted for publication in EPJ
Cosmological solutions in spatially curved universes with adiabatic particle production
We perform a qualitative and thermodynamic study of two models when one takes
into account adiabatic particle production. In the first one, there is a
constant particle production rate, which leads to solutions depicting the
current cosmic acceleration but without inflation. The other one has solutions
that unify the early and late time acceleration. These solutions converge
asymptotically to the thermal equilibrium.Comment: 13 pages, 10 figures. Version accepted for publication in CQ
Reheating constraints in quintessential inflation
We study the consequences of reheating in quintessential inflation. From
simple inflationary quintessential models introduced in cite{hap, hap2}, we
show that when the reheating is due to the production of heavy massive
particles conformally coupled with gravity, a viable model which matches with
the current observational data cite{Ade, Planck, bicep2} is only possible for
reheating temperatures that range between GeV and GeV. On the other
hand, when the universe reheats via the production of massless particles, the
viability of the model is only possible when those particles are nearly
conformally coupled with gravity, leading to a reheating temperature between
MeV and GeV.Comment: Version accepted for publication in PR
The matter-ekpyrotic bounce scenario in Loop Quantum Cosmology
We will perform a detailed study of the matter-ekpyrotic bouncing scenario in
Loop Quantum Cosmology using the methods of the dynamical systems theory. We
will show that when the background is driven by a single scalar field, at very
late times, in the contracting phase, all orbits depict a matter dominated
Universe, which evolves to an ekpyrotic phase. After the bounce the Universe
enters in the expanding phase, where the orbits leave the ekpyrotic regime
going to a kination (also named deflationary) regime. Moreover, this scenario
supports the production of heavy massive particles conformally coupled with
gravity, which reheats the universe at temperatures compatible with the
nucleosynthesis bounds and also the production of massless particles
non-conformally coupled with gravity leading to very high reheating
temperatures but ensuring the nucleosynthesis success. Dealing with
cosmological perturbations, these background dynamics produce a nearly scale
invariant power spectrum for the modes that leave the Hubble radius, in the
contracting phase, when the Universe is quasi-matter dominated, whose spectral
index and corresponding running is compatible with the recent experimental data
obtained by PLANCK's team.Comment: 39 pages, 19 figures. Version accepted for publication in JCA
Qualitative study in Loop Quantum Cosmology
This work contains a detailed qualitative analysis, in General Relativity and
in Loop Quantum Cosmology, of the dynamics in the associated phase space of a
scalar field minimally coupled with gravity, whose potential mimics the
dynamics of a perfect fluid with a linear Equation of State (EoS). Dealing with
the orbits (solutions) of the system, we will see that there are analytic ones,
which lead to the same dynamics as the perfect fluid, and our goal is to check
their stability, depending on the value of the EoS parameter, i.e., to show
whether the other orbits converge or diverge to these analytic solutions at
early and late times.Comment: 12 pages, 7 figures. Version accepted for publication in CQ
Cosmological perturbations in a class of fully covariant modified theories: Application to models with the same background as standard LQC
Bouncing cosmologies are obtained by adding to the Einstein-Hilbert action a
term of the form , with a scalar depending on the
Hubble parameter only, not on its derivatives, and which is here shown to arise
from the divergence of the unitary time-like eigenvector of the stress tensor.
At background level, the dynamical equations for a given -theory are
calculated, showing that the simplest bouncing cosmology resulting leads to
exactly the same equations as those for holonomy corrected Loop Quantum
Cosmology (LQC). When dealing with perturbations, the equation for tensor ones
is the same as in General Relativity (GR); for scalar perturbations, when one
uses the -theory which leads to the same background as the standard version
of holonomy corrected LQC, one obtains similar equations (although a bit more
elaborated) as those coming from LQC in the so-called deformed algebra
approach.Comment: 18 pages and one figure. Version accepted for publication in EPJ
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