17,792 research outputs found
A note on the Gauge Symmetries of Unimodular Gravity
The symmetries of Unimodular Gravity are clarified somewhat.Comment: 4 pages, v2: acknowledgments correcte
Quantum Corrections to Unimodular Gravity
The problem of the cosmological constant appears in a new light in Unimodular
Gravity. In particular, the zero momentum piece of the potential (that is, the
constant piece independent of the matter fields) does not automatically produce
a cosmological constant proportional to it. The aim of this paper is to give
some details on a calculation showing that quantum corrections do not
renormalize the classical value of this observable.Comment: 34 page
Big brake singularity is accommodated as an exotic quintessence field
We describe a big brake singularity in terms of a modified Chaplygin gas
equation of state p=(\ga_{m}-1)\rho+\al\ga_{m}\rho^{-n}, accommodate this
late-time event as an exotic quintessence model obtained from an
energy-momentum tensor, and focus on the cosmological behavior of the exotic
field, its kinetic energy and the potential energy. At the background level,
the exotic field does not blow up whereas its kinetic energy and potential both
grow without limit near the future singularity. We evaluate the classical
stability of this background solution by examining the scalar perturbations of
the metric along with the inclusion of entropy perturbation in the perturbed
pressure. Within the Newtonian gauge, the gravitational field approaches a
constant near the singularity plus additional regular terms. When the perturbed
exotic field is associated with \al>0 the perturbed pressure and contrast
density both diverge, whereas the perturbed exotic field and the divergence of
the exotic field's velocity go to zero exponentially. When the perturbed exotic
field is associated with \al<0 the contrast density always blows up, but the
perturbed pressure can remain bounded. In addition, the perturbed exotic field
and the divergence of the exotic field's velocity vanish near the big brake
singularity. We also briefly look at the behavior of the intrinsic entropy
perturbation near the singular event.Comment: 11 pages, no figures. Accepted for its publication in PR
Interacting dark sector with transversal interaction
We investigate the interacting dark sector composed of dark matter, dark
energy, and dark radiation for a spatially flat Friedmann-Robertson-Walker
(FRW) background by introducing a three-dimensional internal space spanned by
the interaction vector and solve the source equation for a linear
transversal interaction. Then, we explore a realistic model with dark matter
coupled to a scalar field plus a decoupled radiation term, analyze the amount
of dark energy in the radiation era and find that our model is consistent with
the recent measurements of cosmic microwave background anisotropy coming from
Planck along with the future constraints achievable by CMBPol experiment.Comment: To appear in the proceedings of "CosmoSur II - Gravitation and
Cosmology in the Southern Cone" (Valparaiso, Chile, 27-31 May 2013
Dark radiation and dark matter coupled to holographic Ricci dark energy
We investigate a universe filled with interacting dark matter, holographic
dark energy, and dark radiation for the spatially flat
Friedmann-Robertson-Walker (FRW) spacetime. We use a linear interaction to
reconstruct all the component energy densities in terms of the scale factor by
directly solving the balance's equations along with the source equation. We
apply the method to the observational Hubble data for constraining
the cosmic parameters, contrast with the Union 2 sample of supernovae, and
analyze the amount of dark energy in the radiation era. It turns out that our
model exhibits an excess of dark energy in the recombination era whereas the
stringent bound at big-bang
nucleosynthesis is fulfilled. We find that the interaction provides a physical
mechanism for alleviating the triple cosmic coincidence and this leads to
.Comment: 8 pages, 5 figures, 2 tables. Accepted for its publication in The
European Physical Journal C (2013).
http://link.springer.com/article/10.1140/epjc/s10052-013-2352-7 arXiv admin
note: substantial text overlap with arXiv:1210.550
Unimodular Gravity and General Relativity UV divergent contributions to the scattering of massive scalar particles
We work out the one-loop and order UV divergent
contributions, coming from Unimodular Gravity and General Relativity, to the S
matrix element of the scattering process
in a theory with mass . We show that both Unimodular
Gravity and General Relativity give rise to the same UV divergent contributions
in Dimension Regularization. This seems to be at odds with the known result
that in a multiplicative MS dimensional regularization scheme the General
Relativity corrections, in the de Donder gauge, to the beta function
of the coupling do not vanish, whereas the
Unimodular Gravity corrections, in a certain gauge, do vanish. Actually, we
show that the UV divergent contributions to the 1PI Feynman diagrams which give
rise to those non-vanishing corrections to do not contribute
to the UV divergent behaviour of the S matrix element of and this shows that any physical consequence
--such existence of asymptotic freedom due to gravitational interactions--
drawn from the value of is not physically meaningful.Comment: 13 pages, 4 figure
Nonbaryonic dark matter and scalar field coupled with a transversal interaction plus decoupled radiation
We analyze a universe filled with interacting dark matter, a scalar field
accommodated as dark radiation along with dark energy plus a decoupled
radiation term within the framework of spatially flat
Friedmann-Robertson-Walker (FRW) spacetime. We work in a three-dimensional
internal space spanned by the interaction vector and use a transversal
interaction for solving the source equation in order to find all
the interacting component energy densities. We asymptotically reconstruct the
scalar field and potential from an early radiation era to the late dominate
dark energy one, passing through an intermediate epoch dominated by dark
matter. We apply the method to the updated observational Hubble data
for constraining the cosmic parameters, contrast with the Union 2 sample of
supernovae, and analyze the amount of dark energy in the radiation era. It
turns out that our model fulfills the severe bound of at level, is consistent with the recent
analysis that includes cosmic microwave background anisotropy measurements from
the Atacama Cosmology Telescope and the South Pole Telescope along with the
future constraints achievable by Planck and CMBPol experiments, and satisfies
the stringent bound at
level in the big-bang nucleosynthesis epoch.Comment: 12 pages plus 5 pages of figures. In order to see the figures go to
the journal website
(http://link.springer.com/article/10.1140%2Fepjc%2Fs10052-013-2497-4). arXiv
admin note: substantial text overlap with arXiv:1303.3356, arXiv:1210.550
Dark matter and Ricci-like holographic dark energy coupled through a quadratic interaction
We examine a spatially flat Friedmann-Robertson-Walker (FRW) universe filled
with interacting dark matter and a modified holographic Ricci dark energy
(MHRDE). The interaction term is selected as a significant rational function of
the total energy density and its first derivative homogeneous of degree. We
show that the effective one-fluid obeys the equation of state of a relaxed
Chaplygin gas, then the universe turns to be dominated by pressureless dark
matter at early times and undergoes an accelerated expansion in the far future
driven by a strong negative pressure. Performing a -statistical
analysis with the observational Hubble data and the Union2 compilation of SNe
Ia, we place some constraints on cosmological parameters analyzing the
feasibleness of the modified holographic Ricci ansatz. It turned that MHRDE
gets the accelerated expansion faster than the CDM model. Finally, a
new model with a component that does not exchange energy with the interacting
dark sector is presented for studying bounds on the dark energy at early times.Comment: 7 pages, 3 figures, conference. To appear in the proceedings of the
CosmoSul conference, held in Rio de Janeiro, Brazil, 01-05 august of 201
Dark matter, dark energy, and dark radiation coupled with a transversal interaction
We investigate a cosmological scenario with three interacting components that
includes dark matter, dark energy, and radiation in the spatially flat
Friedmann-Robertson-Walker universe. We introduce a 3-dimensional internal
space, the interaction vector satisfying the
constraint plane , the barotropic index vector
\boldmath {\gamma}=(\ga_x,\ga_m,\ga_r) and select a transversal interaction
vector in a sense that \mathbf{Q_t}\cdot \boldmath
{\gamma}. We exactly solve the source equation for a linear
, that depends on the total energy density and its derivatives up
to third order, and find all the component energy densities. We obtain a large
set of interactions for which the source equation admits a power law solution
and show its asymptotic stability by constructing the Lyapunov function. We
apply the method to the observational Hubble data for constraining
the cosmic parameters, and analyze the amount of dark energy in the radiation
era for the above linear . It turns to be that our model fulfills
the severe bound of and is consistent with the
future constraints achievable by Planck and CMBPol experiments.Comment: 9 pages, 4 figures. Accepted for publication in Physical Review
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