137 research outputs found
Newtonian limit of the singular f(R) gravity in the Palatini formalism
Recently D. Vollick [Phys. Rev. D68, 063510 (2003)] has shown that the
inclusion of the 1/R curvature terms in the gravitational action and the use of
the Palatini formalism offer an alternative explanation for cosmological
acceleration. In this work we show not only that this model of Vollick does not
have a good Newtonian limit, but also that any f(R) theory with a pole of order
n in R=0 and its second derivative respect to R evaluated at Ro is not zero,
where Ro is the scalar curvature of background, does not have a good Newtonian
limit.Comment: 9 page
Supergravity Inflation on the Brane
We study N=1 Supergravity inflation in the context of the braneworld
scenario. Particular attention is paid to the problem of the onset of inflation
at sub-Planckian field values and the ensued inflationary observables. We find
that the so-called -problem encountered in supergravity inspired
inflationary models can be solved in the context of the braneworld scenario,
for some range of the parameters involved. Furthermore, we obtain an upper
bound on the scale of the fifth dimension, M_5 \lsim 10^{-3} M_P, in case the
inflationary potential is quadratic in the inflaton field, . If the
inflationary potential is cubic in , consistency with observational data
requires that .Comment: 6 pages, 1 figure, to appear in Phys. Rev.
The CMB power spectrum at l=30-200 from QMASK
We measure the cosmic microwave background (CMB) power spectrum on angular
scales l~30-200 (1-6 degrees) from the QMASK map, which combines the data from
the QMAP and Saskatoon experiments. Since the accuracy of recent measurements
leftward of the first acoustic peak is limited by sample-variance, the large
area of the QMASK map (648 square degrees) allows us to place among the
sharpest constraints to date in this range, in good agreement with BOOMERanG
and (on the largest scales) COBE/DMR. By band-pass-filtering the QMAP and
Saskatoon maps, we are able to spatially compare them scale-by-scale to check
for beam- and pointing-related systematic errors.Comment: Revised to match accepted PRD version. Substantially expanded. Window
functions, map and covariance matrix at
http://www.hep.upenn.edu/~xuyz/qmask.htm
Acceleration of the universe, vacuum metamorphosis, and the large-time asymptotic form of the heat kernel
We investigate the possibility that the late acceleration observed in the
rate of expansion of the universe is due to vacuum quantum effects arising in
curved spacetime. The theoretical basis of the vacuum cold dark matter (VCDM),
or vacuum metamorphosis, cosmological model of Parker and Raval is revisited
and improved. We show, by means of a manifestly nonperturbative approach, how
the infrared behavior of the propagator (related to the large-time asymptotic
form of the heat kernel) of a free scalar field in curved spacetime causes the
vacuum expectation value of its energy-momentum tensor to exhibit a resonance
effect when the scalar curvature R of the spacetime reaches a particular value
related to the mass of the field. we show that the back reaction caused by this
resonance drives the universe through a transition to an accelerating expansion
phase, very much in the same way as originally proposed by Parker and Raval.
Our analysis includes higher derivatives that were neglected in the earlier
analysis, and takes into account the possible runaway solutions that can follow
from these higher-derivative terms. We find that the runaway solutions do not
occur if the universe was described by the usual classical FRW solution prior
to the growth of vacuum energy-density and negative pressure (i.e., vacuum
metamorphosis) that causes the transition to an accelerating expansion of the
universe in this theory.Comment: 33 pages, 3 figures. Submitted to Physical Review D15 (Dec 23, 2003).
v2: 1 reference added. No other change
Gravitational Lensing and f(R) theories in the Palatini approach
We investigate gravitational lensing in the Palatini approach to the f(R)
extended theories of gravity. Starting from an exact solution of the f(R) field
equations, which corresponds to the Schwarzschild-de Sitter metric and, on the
basis of recent studies on this metric, we focus on some lensing observables,
in order to evaluate the effects of the non linearity of the gravity
Lagrangian. We give estimates for some astrophysical events, and show that
these effects are tiny for galactic lenses, but become interesting for
extragalactic ones.Comment: 7 Pages, RevTex, 1 eps figure; references added; revised to match the
version accepted for publication in General Relativity and Gravitatio
Probing Kaluza-Klein Dark Matter with Neutrino Telescopes
In models in which all of the Standard Model fields live in extra universal
dimensions, the lightest Kaluza-Klein (KK) particle can be stable. Calculations
of the one-loop radiative corrections to the masses of the KK modes suggest
that the identity of the lightest KK particle (LKP) is mostly the first KK
excitation of the hypercharge gauge boson. This LKP is a viable dark matter
candidate with an ideal present-day relic abundance if its mass is moderately
large, between 600 to 1200 GeV. Such weakly interacting dark matter particles
are expected to become gravitationally trapped in large bodies, such as the
Sun, and annihilate into neutrinos or other particles that decay into
neutrinos. We calculate the annihilation rate, neutrino flux and the resulting
event rate in present and future neutrino telescopes. The relatively large mass
implies that the neutrino energy spectrum is expected to be well above the
energy threshold of AMANDA and IceCube. We find that the event rate in IceCube
is between a few to tens of events per year.Comment: 13 pages, 3 figures, LaTeX; typos fixed, version to appear in PR
Can inflationary models of cosmic perturbations evade the secondary oscillation test?
We consider the consequences of an observed Cosmic Microwave Background (CMB)
temperature anisotropy spectrum containing no secondary oscillations. While
such a spectrum is generally considered to be a robust signature of active
structure formation, we show that such a spectrum {\em can} be produced by
(very unusual) inflationary models or other passive evolution models. However,
we show that for all these passive models the characteristic oscillations would
show up in other observable spectra. Our work shows that when CMB polarization
and matter power spectra are taken into account secondary oscillations are
indeed a signature of even these very exotic passive models. We construct a
measure of the observability of secondary oscillations in a given experiment,
and show that even with foregrounds both the MAP and \pk satellites should be
able to distinguish between models with and without oscillations. Thus we
conclude that inflationary and other passive models can {\em not} evade the
secondary oscillation test.Comment: Final version accepted for publication in PRD. Minor improvements
have been made to the discussion and new data has been included. The
conclusions are unchagne
Phantom Field with O(N) Symmetry in Exponential Potential
In this paper, we study the phase space of phantom model with O(\emph{N})
symmetry in exponential potential. Different from the model without O(\emph{N})
symmetry, the introduction of the symmetry leads to a lower bound on the
equation of state for the existence of stable phantom dominated attractor
phase. The reconstruction relation between the potential of O(\textit{N})
phantom system and red shift has been derived.Comment: 5 pages, 3 figures, replaced with the version to appear on Phys. Rev.
Primordial Nucleosynthesis as a Test of the Friedmann Equation in the Early Universe
In the standard hot big bang model, the expansion of the early universe is
given by the Friedmann equation with an energy density dominated by
relativistic particles. Since in a variety of models this equation is altered,
we introduce modifications in the Friedmann equation and show that we can
constrain them using big bang nucleosynthesis data. When there is no
neutrino/antineutrino asymmetry these modifications are tightly bounded but in
presence of an asymmetry the bounds become much looser. As an illustration, we
apply our results to a model where the second and third families couple to
gravity differently than the first family (non-universal gravity).Comment: 6 figures. Revised version. Matches with the accepted one for
publication in PR
- âŠ