275 research outputs found
Late time cosmic acceleration from natural infrared cutoff?
In this paper, inspired by the ultraviolet deformation of the
Friedmann-Lema\^{\i}tre-Robertson-Walker geometry in loop quantum cosmology, we
formulate an infrared-modified cosmological model. We obtain the associated
deformed Friedmann and Raychaudhuri equations and we show that the late time
cosmic acceleration can be addressed by the infrared corrections. As a
particular example, we applied the setup to the case of matter dominated
universe. This model has the same number of parameters as CDM, but a
dynamical dark energy generates in the matter dominated era at the late time.
According to our model, as the universe expands, the energy density of the cold
dark matter dilutes and when the Hubble parameter approaches to its minimum,
the infrared effects dominate such that the effective equation of state
parameter smoothly changes from to .
Interestingly and nontrivially, the unstable de Sitter phase with is corresponding to and the universe crosses
the phantom divide from the quintessence phase with and
to the phantom phase with and which shows that the model is observationally viable. The
results show that the universe finally ends up in a big rip singularity for a
finite time proportional to the inverse of the minimum of the Hubble parameter.
Moreover, we consider the dynamical stability of the model and we show that the
universe starts from the matter dominated era at the past attractor with
and ends up in a future attractor at the big rip with
.Comment: 11 pages, 2 figures, accepted for publication in PL
Deviation from the Standard Uncertainty Principle and the Dark Energy Problem
Quantum fluctuations of a real massless scalar field are studied in the
context of the Generalized Uncertainty Principle (GUP). The dynamical finite
vacuum energy is found in spatially flat Friedmann-Robertson- Walker (FRW)
spacetime which can be identified as dark energy to explain late time cosmic
speed-up. The results show that a tiny deviation from the standard uncertainty
principle is necessary on cosmological ground. By using the observational data
we have constraint the GUP parameter even more stronger than ever.Comment: 9 pages, no figures, to appear in Gen. Rel. Gra
Spin-2 dark matter from inflation
The seed of dark matter can be generated from light spectator fields during
inflation through a similar mechanism that the seed of observed large scale
structures are produced from the inflaton field. The accumulated energy density
of the corresponding excited modes, which is subdominant during inflation,
dominates energy density of the universe later around the time of matter and
radiation equality and plays the role of dark matter. For spin-2 spectator
fields, Higuchi bound may seem to prevent excitation of such light modes since
deviation of the inflationary background from the exact de Sitter spacetime is
very small. However, sizable interactions with the inflaton field breaks (part
of) isometries of the de Sitter space in the inflationary background and
relaxes the Higuchi bound. Looking for this possibility in the context of
effective field theory of inflation, we suggest a dark matter model consisting
of spin-2 particles that produce during inflation.Comment: 23 pages, 2 figure
Migraine in children and adolescents
ObjectiveHeadache is a frequent symptom in children and adolescents. Migraine is one of the most common types of primary headache disorders in children that at times can be extremely disabling. Many clinical features of migraine in childrendiffer from that starting in adulthood. This review discusses the epidemiology,clinical features, management, and prognosis of migraine headache in childrenand adolescents
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