311 research outputs found
Impact of other scalar fields on oscillons after hilltop inflation
Oscillons are spatially localized and relatively stable field fluctuations
which can form after inflation under suitable conditions. In order to reheat
the universe, the fields which dominate the energy density after inflation have
to couple to other degrees of freedom and finally produce the matter particles
present in the universe today. In this study, we use lattice simulations in 2+1
dimensions to investigate how such couplings can affect the formation and
stability of oscillons. We focus on models of hilltop inflation, where we have
recently shown that hill crossing oscillons generically form, and consider the
coupling to an additional scalar field which, depending on the value of the
coupling parameter, can get resonantly enhanced from the inhomogeneous inflaton
field. We find that three cases are realized: without a parametric resonance,
the additional scalar field has no effects on the oscillons. For a fast and
strong parametric resonance of the other scalar field, oscillons are strongly
suppressed. For a delayed parametric resonance, on the other hand, the
oscillons get imprinted on the other scalar field and their stability is even
enhanced compared to the single-field oscillons.Comment: 15 pages, 6 figures, version published in JCA
What can we learn from the stochastic gravitational wave background produced by oscillons?
The stochastic gravitational wave (GW) background provides a fascinating
window to the physics of the very early universe. Beyond the nearly
scale-invariant primordial GW spectrum produced during inflation, a spectrum
with a much richer structure is typically generated during the preheating phase
after inflation (or after some other phase transition at lower energies). This
raises the question of what one can learn from a future observation of the
stochastic gravitational wave background spectrum about the underlying physics
during preheating. Recently, it has been shown that during preheating
non-perturbative quasi-stable objects like oscillons can act as strong sources
for GW, leading to characteristic features such as distinct peaks in the
spectrum. In this paper, we study the GW production from oscillons using
semi-analytical techniques. In particular, we discuss how the GW spectrum is
affected by the parameters that characterise a given oscillon system, e.g. by
the background cosmology, the asymmetry of the oscillons and the evolution of
the number density of the oscillons. We compare our semi-analytic results with
numerical lattice simulations for a hilltop inflation model and a KKLT
scenario, which differ strongly in some of these characteristics, and find very
good agreement.Comment: 25 pages + Appendix, 16 figure
Hill crossing during preheating after hilltop inflation
In 'hilltop inflation', inflation takes place when the inflaton field slowly
rolls from close to a maximum of its potential (i.e. the 'hilltop') towards its
minimum. When the inflaton potential is associated with a phase transition,
possible topological defects produced during this phase transition, such as
domain walls, are efficiently diluted during inflation. It is typically assumed
that they also do not reform after inflation, i.e. that the inflaton field
stays on its side of the 'hill', finally performing damped oscillations around
the minimum of the potential. In this paper we study the linear and the
non-linear phases of preheating after hilltop inflation. We find that the
fluctuations of the inflaton field during the tachyonic oscillation phase grow
strong enough to allow the inflaton field to form regions in position space
where it crosses 'over the top of the hill' towards the 'wrong vacuum'. We
investigate the formation and behaviour of these overshooting regions using
lattice simulations: Rather than durable domain walls, these regions form
oscillon-like structures (i.e. localized bubbles that oscillate between the two
vacua) which should be included in a careful study of preheating in hilltop
inflation.Comment: 22 pages, 10 figures, v2 matches publication in JCAP. Animated movies
of our simulations are available online at
https://particlesandcosmology.unibas.ch/files/hilltop_preheating.htm
Gravitational waves from oscillons after inflation
We investigate the production of gravitational waves during preheating after
inflation in the common case of field potentials that are asymmetric around the
minimum. In particular, we study the impact of oscillons, comparatively long
lived and spatially localized regions where a scalar field (e.g. the inflaton)
oscillates with large amplitude. Contrary to a previous study, which considered
a symmetric potential, we find that oscillons in asymmetric potentials
associated with a phase transition can generate a pronounced peak in the
spectrum of gravitational waves, that largely exceeds the linear preheating
spectrum. We discuss the possible implications of this enhanced amplitude of
gravitational waves. For instance, for low scale inflation models, the
contribution from the oscillons can strongly enhance the observation prospects
at current and future gravitational wave detectors.Comment: 5 pages, 3 figures; v2: version published in PRL; v3: includes
Erratum (submitted to PRL
Hilltop inflation with preinflation from coupling to matter fields
We propose a class of models of supersymmetric hilltop inflation (also called
"new inflation") where the initial conditions of the inflaton close to the
hilltop are generated through "matter field preinflation". This is achieved via
a coupling term between the inflaton and matter fields (i.e. Standard Model
fields or a right-handed neutrino). The same coupling also opens up a decay
channel for the inflaton into Standard Model fields, which allows efficient
reheating of the universe. We discuss the multifield dynamics of the inflaton
and matter fields during inflation using the delta N formalism and show under
which conditions the model effectively reduces to single-field hilltop
inflation during the last 60 e-folds. We also study perturbative reheating
through the matter-inflaton coupling for a specific example where the matter
field is identified with a right-handed (s)neutrino, and demonstrate that in
this case the model can generate the observed baryon asymmetry through
nonthermal leptogenesis.Comment: 26 pages, 10 figures, v2: reference added to match publication in
JCA
Detecting the Stochastic Gravitational Wave Background from Primordial Black Hole Formation
Primordial Black Holes (PBH) from peaks in the curvature power spectrum could
constitute today an important fraction of the Dark Matter in the Universe. At
horizon reentry, during the radiation era, order one fluctuations collapse
gravitationally to form black holes and, at the same time, generate a
stochastic background of gravitational waves coming from second order
anisotropic stresses in matter. We study the amplitude and shape of this
background for several phenomenological models of the curvature power spectrum
that can be embedded in waterfall hybrid inflation, axion, domain wall, and
boosts of PBH formation at the QCD transition. For a broad peak or a nearly
scale invariant spectrum, this stochastic background is generically enhanced by
about one order of magnitude, compared to a sharp feature. As a result,
stellar-mass PBH from Gaussian fluctuations with a wide mass distribution are
already in strong tension with the limits from Pulsar Timing Arrays, if they
constitute a non negligible fraction of the Dark Matter. But this result is
mitigated by the uncertainties on the curvature threshold leading to PBH
formation. LISA will have the sensitivity to detect or rule out light PBH down
to . Upcoming runs of LIGO/Virgo and future interferometers
such as the Einstein Telescope will increase the frequency lever arm to
constrain PBH from the QCD transition. Ultimately, the future SKA Pulsar Timing
Arrays could probe the existence of even a single stellar-mass PBH in our
Observable Universe.Comment: 20 pages, 5 figures, comments welcom
False vacuum energy dominated inflation with large and the importance of
We investigate to which extent and under which circumstances false vacuum
energy () dominated slow-roll inflation is compatible with a large
tensor-to-scalar ratio , as indicated by the recent BICEP2
measurement. With we refer to a constant contribution to the inflaton
potential, present before a phase transition takes place and absent in the true
vacuum of the theory, like e.g. in hybrid inflation. Based on model-independent
considerations, we derive an upper bound on the possible amount of
domination and highlight the importance of higher-order runnings of the scalar
spectral index (beyond ) in order to realise scenarios of
dominated inflation. We study the conditions for domination explicitly
with an inflaton potential reconstruction around the inflaton field value 50
e-folds before the end of inflation, taking into account the present
observational data. To this end, we provide the up-to-date parameter
constraints within CDM + + + using the
cosmological parameter estimation code Monte Python together with the Boltzmann
code CLASS.Comment: 16 pages, 5 figures; v2: matches publication in JCA
Oscillons from String Moduli
A generic feature of string compactifications is the presence of many scalar
fields, called moduli. Moduli are usually displaced from their
post-inflationary minimum during inflation. Their relaxation to the minimum
could lead to the production of oscillons: localised, long-lived, non-linear
excitations of the scalar fields. Here we discuss under which conditions
oscillons can be produced in string cosmology and illustrate their production
and potential phenomenology with two explicit examples: the case of an
initially displaced volume modulus in the KKLT scenario and the case of a
displaced blow-up Kaehler modulus in the Large Volume Scenario (LVS). One, in
principle, observable consequence of oscillon dynamics is the production of
gravitational waves which, contrary to those produced from preheating after
high scale inflation, could have lower frequencies, closer to the currently
observable range. We also show that, for the considered parameter ranges,
oscillating fibre and volume moduli do not develop any significant
non-perturbative dynamics. Furthermore, we find that the vacua in the LVS and
the KKLT scenario are stable against local overshootings of the field into the
decompatification region, which provides an additional check on the longevity
of these metastable configurations.Comment: 32 pages + appendix, 23 figures, for videos of the simulations see
https://particlesandcosmology.unibas.ch/downloads/oscillons-from-string-moduli-movies.htm
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