375 research outputs found
Non-perturbative over-production of axion-like-particles (ALPs) via derivative interaction
Axion like particles (ALPs) are quite generic in many scenarios for physics
beyond the Standard Model, they are pseudoscalar Nambu-Goldstone bosons, and
appear once any global symmetry is broken spontaneously. The ALPs can
gain mass from various non-perturbative quantum effects, such as anomalies or
instantons. ALPs can couple to the matter sector incluidng a scalar condensate
such as inflaton or moduli field via derivative interactions, which are
suppressed by the axion {\it decay constant}, . Although weakly
interacting, the ALPs can be produced abundantly from the coherent oscillations
of a homogeneous condensate. In this paper we will study such a scenario where
the ALPs can be produced abundantly, and in some cases can even overclose the
Universe via odd and even dimensional operators, as long as , where denotes the initial amplitude of the coherent
oscillations of the scalar condensate, . We will briefly mention how such
dangerous overproduction would affect dark matter and dark radiation abundances
in the Universe.Comment: 17 pages, 18 figure
Primordial blackholes and gravitational waves for an inflection-point model of inflation
In this article we provide a new closed relationship between cosmic abundance
of primordial gravitational waves and primordial blackholes originated from
initial inflationary perturbations for inflection-point models of inflation
where inflation occurs below the Planck scale. The current Planck constraint on
tensor-to-scalar ratio, running of the spectral tilt, and from the abundance of
dark matter content in the universe, we can deduce a strict bound on the
current abundance of primordial blackholes to be within a range, .Comment: 7 pages, 3 figures, Revision accepted by Physics Letters
Nonlocal star as a blackhole mimicker
In the context of ghost-free, infinite derivative gravity, we will provide a
quantum mechanical framework in which we can describe astrophysical objects
devoid of curvature singularity and event horizon. In order to avoid ghosts and
singularity, the gravitational interaction has to be nonlocal, therefore, we
call these objects as nonlocal stars. Quantum mechanically a nonlocal star is a
self-gravitational bound system of many gravitons interacting nonlocally.
Outside the nonlocal star the spacetime is well described by the Schwarzschild
metric, while inside we have a non-vacuum spacetime metric which tends to be
conformally flat at the origin. Remarkably, in the most compact scenario the
radius of a nonlocal star is of the same order of the Buchdahl limit, therefore
slightly larger than the Schwarzschild radius, such that there can exist a
photosphere. These objects live longer than a Schwarzschild blackhole and they
are very good absorbers, due to the fact that the number of available states is
larger than that of a blackhole. As a result nonlocal stars, not only can be
excellent blackhole mimickers, but can also be considered as dark matter
candidates. In particular, nonlocal stars with masses below g can be
made stable compared to the age of the Universe.Comment: 12 pages. V2: references added, typos fixed. V3: accepted for
publication in PR
Reheating in supersymmetric high scale inflation
Motivated by Refs \cite{am1,am2}, we analyze how the inflaton decay reheats
the Universe within supersymmetry. In a non-supersymmetric case the inflaton
usually decays via preheating unless its couplings to other fields are very
small. Naively one would expect that supersymmetry enhances bosonic preheating
as it introduces new scalars such as squarks and sleptons. On the contrary, we
point out that preheating is unlikely within supersymmetry. The reason is that
flat directions in the scalar potential, classified by gauge invariant
combinations of slepton and squark fields, are generically displaced towards a
large vacuum expectation value (VEV) in the early Universe. They induce
supersymmetry preserving masses to the inflaton decay products through the
Standard Model Yukawa couplings, which kinematically blocks preheating for VEVs
GeV. The decay will become allowed only after the flat directions
start oscillating, and once the flat direction VEV is sufficiently redshifted.
For models with weak scale supersymmetry, this generically happens at a Hubble
expansion rate: , at which time the
inflaton decays in the perturbative regime. This is to our knowledge first
analysis where the inflaton decay to the Standard Model particles is treated
properly within supersymmetry. There are number of important consequences: no
overproduction of dangerous supersymmetric relics (particularly gravitinos), no
resonant excitation of superheavy dark matter, and no non-thermal leptogenesis
through non-perturbative creation of the right-handed (s)neutrinos. Finally
supersymmetric flat directions can even spoil hybrid inflation all together by
not allowing the auxiliary field become tachyonic.Comment: 13 revtex pages, 2 tables. Title changed, few clarifications added,
final version accepted for publication in Phys. Rev.
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