491 research outputs found
CMB acoustic scale in the entropic-like accelerating universe
We consider generalizations of the entropic accelerating universe recently
proposed in Ref. [4,5] and show that their background equations can be made
equivalent to a model with a dark energy component with constant parameter of
state , where is related to the
coefficients of the new terms in the Friedman equations. After discussing all
the Friedman equations for an arbitrary , we show how to recover the
standard scalings for dust and radiation. The acoustic scale , related
to the peak positions in the pattern of the angular power spectrum of the
Cosmic Microwave Background anisotropies, is also computed and yields the
stringent bound . We then argue that future data might be able
to distinguish this model from pure CDM (corresponding to ).Comment: 6 pages, 2 figures. Accepted for publication in Physical Review
On boundary terms and conformal transformations in curved space-times
We intend to clarify the interplay between boundary terms and conformal
transformations in scalar-tensor theories of gravity. We first consider the
action for pure gravity in five dimensions and show that, on compactifing a la
Kaluza-Klein to four dimensions, one obtains the correct boundary terms in the
Jordan (or String) Frame form of the Brans-Dicke action. Further, we analyze
how the boundary terms change under the conformal transformations which lead to
the Pauli (or Einstein) frame and to the non-minimally coupled massless scalar
field. In particular, we study the behaviour of the total energy in
asymptotically flat space-times as it results from surface terms in the
Hamiltonian formalism.Comment: LaTeX 2e, 12 pages, no figure
Evolution of Large Scale Curvature Fluctuations During the Perturbative Decay of the Inflaton
We study the evolution of cosmological fluctuations during and after
inflation driven by a scalar field coupled to a perfect fluid through afriction
term. During the slow-roll regime for the scalar field, the perfect fluid is
also frozen and isocurvature perturbations are generated. After the end of
inflation, during the decay of the inflaton, we find that a change in the
observationally relevant large scale curvature fluctuations is possible.Comment: 9 pages, 2 figures; v2: version published in PR
Is a Dissipative Regime During Inflation in Agreement with Observations?
We study the spectral index of curvature perturbations for inflationary
models where the driving scalar field is coupled to a relativistic fluid
through a friction term . We find that only a very small friction term
- , with H being the Hubble parameter during inflation - is
allowed by observations, otherwise curvature fluctuations are generated with a
spectral index unacceptably red. These results are generic with respect
to the inflationary potential and known dependence of the friction term on the
scalar field and the energy density of the relativistic fluid. We compare our
findings with previous investigations.Comment: 4 pages, 2 tables, 1 figur
Noncommutative Inspired Reissner-Nordstr\"om Black Holes in Large Extra Dimensions
Recently, a new noncommutative geometry inspired solution of the coupled
Einstein-Maxwell field equations including black holes in 4-dimension is found.
In this paper, we generalize some aspects of this model to the
Reissner-Nordstr\"om (RN) like geometries with large extra dimensions. We
discuss Hawking radiation process based on noncommutative inspired solutions.
In this framework, existence of black hole remnant and possibility of its
detection in LHC are investigated.Comment: 24 pages, 12 figures, revised version to appear in Commun. Theor.
Phy
Discrete Symmetries and Localization in a Brane-world
Discrete symmetries are studied in warped space-times with one extra
dimension. In particular, we analyze the compatibility of five- and
four-dimensional charge conjugation, parity, time reversal and the orbifold
symmetry Z_2 with localization of fermions on the four-dimensional brane-world
and Lorentz invariance. We then show that, when a suitable topological scalar
field (the ``kink'') is included, fermion localization is a consequence of
(five-dimensional) CPT invariance.Comment: REVTeX, 8 pages, 1 EPS figure include
Pre - Inflationary Clues from String Theory ?
"Brane supersymmetry breaking" occurs in String Theory when the only
available combinations of D-branes and orientifolds are not mutually BPS and
yet do not introduce tree-level tachyon instabilities. It is characterized by
the emergence of a steep exponential potential, and thus by the absence of
maximally symmetric vacua. The corresponding low-energy supergravity admits
intriguing spatially-flat cosmological solutions where a scalar field is forced
to climb up toward the steep potential after an initial singularity, and
additional milder terms can inject an inflationary phase during the ensuing
descent. We show that, in the resulting power spectra of scalar perturbations,
an infrared suppression is typically followed by a pre-inflationary peak that
reflects the end of the climbing phase and can lie well apart from the
approximately scale invariant profile. A first look at WMAP9 raw data shows
that, while the chi^2 fits for the low-l CMB angular power spectrum are clearly
compatible with an almost scale invariant behavior, they display nonetheless an
eye-catching preference for this type of setting within a perturbative string
regime.Comment: 34 pages, LaTeX, 16 eps figures. Relative displacement in fig. 14 and
some typos corrected, references and acknowledgments updated. To appear in
JCA
Instability of reconstruction of the low CMB multipoles
We discuss the problem of the bias of the Internal Linear Combination (ILC)
CMB map and show that it is closely related to the coefficient of
cross-correlation K(l) of the true CMB and the foreground for each multipole l.
We present analysis of the cross-correlation for the WMAP ILC quadrupole and
octupole from the first (ILC(I)) and the third (ILC(III)) year data releases
and show that these correlations are about -0.52-0.6. Analysing 10^4 Monte
Carlo simulations of the random Gaussian CMB signals, we show that the
distribution function for the corresponding coefficient of the
cross-correlation has a polynomial shape P(K,l)\propto(1-K^2)^(l-1). We show
that the most probable value of the cross-correlation coefficient of the ILC
and foreground quadrupole has two extrema at K ~= +/-0.58$. Thus, the ILC(III)
quadrupole represents the most probable value of the coefficient K. We analyze
the problem of debiasing of the ILC CMB and pointed out that reconstruction of
the bias seems to be very problematic due to statistical uncertainties. In
addition, instability of the debiasing illuminates itself for the quadrupole
and octupole components through the flip-effect, when the even (l+m) modes can
be reconstructed with significant error. This error manifests itself as
opposite, in respect to the true sign of even low multipole modes, and leads to
significant changes of the coefficient of cross-correlation with the
foreground. We show that the CMB realizations, whose the sign of quadrupole
(2,0) component is negative (and the same, as for all the foregrounds), the
corresponding probability to get the positive sign after implementation of the
ILC method is about 40%.Comment: 11 pages, 5 figure
Early Dark Energy Cosmologies
We propose a novel parameterization of the dark energy density. It is
particularly well suited to describe a non-negligible contribution of dark
energy at early times and contains only three parameters, which are all
physically meaningful: the fractional dark energy density today, the equation
of state today and the fractional dark energy density at early times. As we
parameterize Omega_d(a) directly instead of the equation of state, we can give
analytic expressions for the Hubble parameter, the conformal horizon today and
at last scattering, the sound horizon at last scattering, the acoustic scale as
well as the luminosity distance. For an equation of state today w_0 < -1, our
model crosses the cosmological constant boundary. We perform numerical studies
to constrain the parameters of our model by using Cosmic Microwave Background,
Large Scale Structure and Supernovae Ia data. At 95% confidence, we find that
the fractional dark energy density at early times Omega_early < 0.06. This
bound tightens considerably to Omega_early < 0.04 when the latest Boomerang
data is included. We find that both the gold sample of Riess et. al. and the
SNLS data by Astier et. al. when combined with CMB and LSS data mildly prefer
w_0 < -1, but are well compatible with a cosmological constant.Comment: 6 pages, 3 figures; references added, matches published versio
Dominant Multipoles in WMAP5 Mosaic Data Correlation Maps
The method of correlation mapping on the full sphere is used to study the
properties of the ILC map, as well as the dust and synchrotron background
components. An anomalous correlation of the components with the ILC map in the
main plane and in the poles of the ecliptic and equatorial coordinate systems
was discovered. Apart from the bias, a dominant quadrupole contribution in the
power spectrum of the mosaic correlation maps was found in the pixel
correlation histogram. Various causes of the anomalous signal are discussed.Comment: 10 pages,11 figure
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