2,782 research outputs found
Anthropic Bound on Dark Radiation and its Implications for Reheating
We derive an anthropic bound on the extra neutrino species, , based on the observation that a positive suppresses
the growth of matter fluctuations due to the prolonged radiation dominated era,
which reduces the fraction of matter that collapses into galaxies, hence, the
number of observers. We vary and the positive cosmological
constant while fixing the other cosmological parameters. We then show that the
probability of finding ourselves in a universe satisfying the current bound is
of order a few percents for a flat prior distribution. If
is found to be close to the current upper bound or the value suggested by the
tension, the anthropic explanation is not very unlikely. On the other
hand, if the upper bound on is significantly improved by
future observations, such simple anthropic consideration does not explain the
small value of . We also study simple models where dark
radiation consists of relativistic particles produced by heavy scalar decays,
and show that the prior probability distribution sensitively depends on the
number of the particle species.Comment: 14 pages, 4 figures; V2: Added references; V3: Published version,
Added the anthropic bound on the number of neutrino flavor
Isocurvature Perturbations of Dark Energy and Dark Matter from the Swampland Conjecture
We point out that the recently proposed Swampland conjecture on the potential
gradient can lead to isocurvature perturbations of dark energy, if the
quintessence field acquires large quantum fluctuations during high-scale
inflation preferred by the conjecture. Also, if the quintessence field is
coupled to a dark sector that contains dark matter, isocurvature perturbation
of dark matter is similarly induced. Both isocurvature perturbations can be
suppressed if the quintessence potential allows a tracker solution in the early
Universe. We find that a vector field of mass meV is an
excellent dark matter candidate in this context, not only because the right
abundance is known to be produced by quantum fluctuations during high-scale
inflation without running afoul of isocurvature bounds, but also because its
coupling to the quintessence does not spoil the flatness of the potential.Comment: 14 pages, 1 figure; (v2) minor changes, Fig. 1 adde
Diphoton excess from hidden U(1) gauge symmetry with large kinetic mixing
We show that the 750 GeV diphoton excess can be explained by introducing
vector-like quarks and hidden fermions charged under a hidden U(1) gauge
symmetry, which has a relatively large coupling constant as well as a
significant kinetic mixing with U(1). With the large kinetic mixing, the
standard model gauge couplings unify around GeV, suggesting the grand
unified theory without too rapid proton decay. Our scenario predicts events
with a photon and missing transverse momentum, and its cross section is related
to that for the diphoton excess through the kinetic mixing. We also discuss
other possible collider signatures and cosmology, including various ways to
evade constraints on exotic stable charged particles. In some cases where the
750 GeV diphoton excess is due to diaxion decays, our scenario also predicts
triphoton and tetraphoton signals.Comment: 19 pages, 2 figures; figures update
Adiabatic suppression of the axion abundance and isocurvature due to coupling to hidden monopoles
The string theory predicts many light fields called moduli and axions, which
cause a cosmological problem due to the overproduction of their coherent
oscillation after inflation. One of the prominent solutions is an adiabatic
suppression mechanism, which, however, is non-trivial to achieve in the case of
axions because it necessitates a large effective mass term which decreases as a
function of time. The purpose of this paper is twofold. First, we provide an
analytic method to calculate the cosmological abundance of coherent oscillation
in a general situation under the adiabatic suppression mechanism. Secondly, we
apply our method to some concrete examples, including the one where a string
axion acquires a large effective mass due to the Witten effect in the presence
of hidden monopoles.Comment: 30 pages, 3 figure
Suppressing the QCD Axion Abundance by Hidden Monopoles
We study the Witten effect of hidden monopoles on the QCD axion dynamics, and
show that its abundance as well as isocurvature perturbations can be
significantly suppressed if there is a sufficient amount of hidden monopoles.
When the hidden monopoles make up a significant fraction of dark matter, the
Witten effect suppresses the abundance of axion with the decay constant smaller
than GeV. The cosmological domain wall problem of the QCD axion can
also be avoided, relaxing the upper bound on the decay constant when the
Peccei-Quinn symmetry is spontaneously broken after inflation.Comment: 6 pages, 1 figur
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