6,472 research outputs found
Parametric Resonance in the Early Universe - A Fitting Analysis
Particle production via parametric resonance in the early Universe, is a
nonperturbative, non-linear and out-of-equilibrium phenomenon. Although it is a
well studied topic, whenever a new scenario exhibits parametric resonance, a
full re-analysis is normally required. To avoid this tedious task, many works
present often only a simplified linear treatment of the problem. In order to
surpass this circumstance in the future, we provide a fitting analysis of
parametric resonance through all its relevant stages: initial linear growth,
non-linear evolution, and relaxation towards equilibrium. Using lattice
simulations in an expanding grid in 3 + 1 dimensions, we parametrize the
dynamics outcome scanning over the relevant ingredients: role of the
oscillatory field, particle coupling strength, initial conditions, and
background expansion rate. We emphasize the inaccuracy of the linear
calculation of the decay time of the oscillatory field, and propose a more
appropriate definition of this scale based on the subsequent non-linear
dynamics. We provide simple fits to the relevant time scales and particle
energy fractions at each stage. Our fits can be applied to post-inflationary
preheating scenarios, where the oscillatory field is the inflaton, or to
spectator-field scenarios, where the oscillatory field can be e.g. a curvaton,
or the Standard Model Higgs.Comment: Extended discussion about the late-time dynamics of the system in
quadratic models. Minor changes in numerical fits with respect first version.
It matches version published in JCAP (30 pages + Appendices + Bibliography,
13 figures
A new gravitational wave background from the Big Bang
The reheating of the universe after hybrid inflation proceeds through the
nucleation and subsequent collision of large concentrations of energy density
in the form of bubble-like structures moving at relativistic speeds. This
generates a significant fraction of energy in the form of a stochastic
background of gravitational waves, whose time evolution is determined by the
successive stages of reheating: First, tachyonic preheating makes the amplitude
of gravity waves grow exponentially fast. Second, bubble collisions add a new
burst of gravitational radiation. Third, turbulent motions finally sets the end
of gravitational waves production. From then on, these waves propagate
unimpeded to us. We find that the fraction of energy density today in these
primordial gravitational waves could be significant for GUT scale models of
inflation, although well beyond the frequency range sensitivity of
gravitational wave observatories like LIGO, LISA or BBO. However, low-scale
models could still produce a detectable signal at frequencies accessible to BBO
or DECIGO. For comparison, we have also computed the analogous background from
some chaotic inflation models and obtained similar results to those of other
groups. The discovery of such a background would open a new observational
window into the very early universe, where the details of the process of
reheating could be explored. Thus, it could also serve as a new experimental
tool for testing the Inflationary Paradigm.Comment: 20 pages, 8 figures, to appear in the Proceedings of JGRG17, Nagoya
(Japan), 3-7 December 200
Inconsistency of an inflationary sector coupled only to Einstein gravity
From a model-building perspective, the inflationary sector might very well
have no direct couplings to other species, apart from inevitable gravitational
interactions. Within the context of General Relativity, a thermal universe can
still emerge after inflation if: some radiation sector is excited towards
the end of inflation, and the post-inflationary equation of state becomes
sufficiently stiff , with a
threshold depending on the inflationary scale and the initial
radiation-to-inflaton energy ratio . Furthermore, a stiff period in
the expansion history enhances significantly the inflationary gravitational
wave (GW) background, making this signal (potentially) observable by aLIGO,
LISA and other experiments. The very same enhancement leads however to an
inconsistency of the scenario: the energy of the GWs becomes too large compared
to the rest of the radiation sector, violating standard BBN and CMB bounds on
GW backgrounds. Except for very special scenarios where the initial radiation
sector comprises hundreds of fields with couplings tuned to specific values,
our result applies independently of , and . This suggests
that in order to reheat the universe, the inflationary sector should be coupled
directly to other particle species. Alternatively the inflationary sector could
be implemented in modified gravity theories.Comment: Comments added to match published version in JCAP, 22 pages (+
appendix + references), 4 figure
The Standard Model Higgs as the origin of the hot Big Bang
If the Standard Model (SM) Higgs is weakly coupled to the inflationary
sector, the Higgs is expected to be universally in the form of a condensate
towards the end of inflation. The Higgs decays rapidly after inflation - via
non-perturbative effects - into an out-of-equilibrium distribution of SM
species, which thermalize soon afterwards. If the post-inflationary equation of
state of the universe is stiff, , the SM species eventually
dominate the total energy budget. This provides a natural origin for the
relativistic thermal plasma of SM species, required for the onset of the `hot
Big Bang' era. The viability of this scenario requires the inflationary Hubble
scale to be lower than the instability scale for Higgs vacuum decay, the
Higgs not to generate too large curvature perturbations at cosmological scales,
and the SM dominance to occur before Big Bang Nucleosynthesis. We show that
successful reheating into the SM can only be obtained in the presence of a
non-minimal coupling to gravity , with a reheating temperature
of .Comment: 6 pages, 2 figures, minor changes with new figures to match published
version in PL
Ability of LIGO and LISA to probe the equation of state of the early Universe
The expansion history of the Universe between the end of inflation and the
onset of radiation-domination (RD) is currently unknown. If the equation of
state during this period is stiffer than that of radiation, , the
gravitational wave (GW) background from inflation acquires a blue-tilt
at
frequencies corresponding to modes re-entering the horizon
during the stiff-domination (SD), where is the frequency today of
the horizon scale at the SD-to-RD transition. We characterized in detail the
transfer function of the GW energy density spectrum, considering both 'instant'
and smooth modelings of the SD-to-RD transition. The shape of the spectrum is
controlled by , , and (the Hubble scale of
inflation). We determined the parameter space compatible with a detection of
this signal by LIGO and LISA, including possible changes in the number of
relativistic degrees of freedom, and the presence of a tensor tilt. Consistency
with upper bounds on stochastic GW backgrounds, however, rules out a
significant fraction of the observable parameter space. We find that this
renders the signal unobservable by Advanced LIGO, in all cases. The GW
background remains detectable by LISA, though only in a small island of
parameter space, corresponding to scenarios with an equation of state in the
range and a high inflationary scale , but low reheating temperature (equivalently, ). Implications for early
Universe scenarios resting upon an SD epoch are briefly discussed.Comment: Matching published version in JCAP, 32 pages, 8 figure
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