67,967 research outputs found
Baryon Oscillations and Consistency Tests for Photometrically-Determined Redshifts of Very Faint Galaxies
Weak lensing surveys that can potentially place strong constraints on dark
energy parameters can only do so if the source redshift means and error
distributions are very well known. We investigate prospects for controlling
errors in these quantities by exploiting their influence on the power spectra
of the galaxies. Although, from the galaxy power spectra alone, sufficiently
precise and simultaneous determination of redshift biases and variances is not
possible, a strong consistency test is. Given the redshift error rms, galaxy
power spectra can be used to determine the mean redshift of a group of galaxies
to subpercent accuracy. Although galaxy power spectra cannot be used to
determine the redshift error rms, they can be used to determine this rms
divided by the Hubble parameter, a quantity that may be even more valuable for
interpretation of cosmic shear data than the rms itself. We also show that
galaxy power spectra, due to the baryonic acoustic oscillations, can
potentially lead to constraints on dark energy that are competitive with those
due to the cosmic shear power spectra from the same survey.Comment: 8 pages, 6 figures, submitted to Ap
Entanglement creation between two causally-disconnected objects
We study the full entanglement dynamics of two uniformly accelerated
Unruh-DeWitt detectors with no direct interaction in between but each coupled
to a common quantum field and moving back-to-back in the field vacuum. For two
detectors initially prepared in a separable state our exact results show that
quantum entanglement between the detectors can be created by the quantum field
under some specific circumstances, though each detector never enters the
other's light cone in this setup. In the weak coupling limit, this entanglement
creation can occur only if the initial moment is placed early enough and the
proper acceleration of the detectors is not too large or too small compared to
the natural frequency of the detectors. Once entanglement is created it lasts
only a finite duration, and always disappears at late times. Prior result by
Reznik derived using the time-dependent perturbation theory with extended
integration domain is shown to be a limiting case of our exact solutions at
some specific moment. In the strong coupling and high acceleration regime,
vacuum fluctuations experienced by each detector locally always dominate over
the cross correlations between the detectors, so entanglement between the
detectors will never be generated.Comment: 16 pages, 8 figures; added Ref.[7] and related discussion
Noise kernel for a quantum field in Schwarzschild spacetime under the Gaussian approximation
A method is given to compute an approximation to the noise kernel, defined as
the symmetrized connected 2-point function of the stress tensor, for the
conformally invariant scalar field in any spacetime conformal to an
ultra-static spacetime for the case in which the field is in a thermal state at
an arbitrary temperature. The most useful applications of the method are flat
space where the approximation is exact and Schwarzschild spacetime where the
approximation is better than it is in most other spacetimes. The two points are
assumed to be separated in a timelike or spacelike direction. The method
involves the use of a Gaussian approximation which is of the same type as that
used by Page to compute an approximate form of the stress tensor for this field
in Schwarzschild spacetime. All components of the noise kernel have been
computed exactly for hot flat space and one component is explicitly displayed.
Several components have also been computed for Schwarzschild spacetime and
again one component is explicitly displayed.Comment: 34 pages, no figures. Substantial revisions in Secs. I, IV, and V;
minor revisions elsewhere; new results include computation of the exact noise
kernel for hot flat space and an approximate computation of the noise kernel
for a thermal state at an arbitrary temperature in Schwarzschild spacetime
when the points are split in the time directio
Variational Monte Carlo study of gapless spin liquid in the spin- XXZ antiferromagnetic model on the kagome lattice
By using the variational Monte Carlo technique, we study the spin- XXZ
antiferromagnetic model (with easy-plane anisotropy) on the kagome lattice. A
class of Gutzwiller projected fermionic states with a spin Jastrow factor is
considered to describe either spin liquids (with or symmetry) or
magnetically ordered phases (with or ). We
find that the magnetic states are not stable in the thermodynamic limit.
Moreover, there is no energy gain to break the gauge symmetry from to
within the spin-liquid states, as previously found in the Heisenberg
model. The best variational wave function is therefore the Dirac state,
supplemented by the spin Jastrow factor. Furthermore, a vanishing spin
gap is obtained at the variational level, in the whole regime from the to
the Heisenberg model.Comment: 7 pages, 7 figure
On the non-Gaussianity from Recombination
The non-linear effects operating at the recombination epoch generate a
non-Gaussian signal in the CMB anisotropies. Such a contribution is relevant
because it represents a major part of the second-order radiation transfer
function which must be determined in order to have a complete control of both
the primordial and non-primordial part of non-Gaussianity in the CMB
anisotropies. We provide an estimate of the level of non-Gaussianity in the CMB
arising from the recombination epoch which shows up mainly in the equilateral
configuration. We find that it causes a contamination to the possible
measurement of the equilateral primordial bispectrum shifting the minimum
detectable value of the non-Gaussian parameter f^equil_NL by Delta f^equil_NL=
O(10) for an experiment like Planck.Comment: LaTeX file; 11 pages. v2: Typos corrected; references added; comments
about the effective non-linearity parameter added in Sec. IV; comments added
in the conclusions of Sec. IV. v3: References added; some clarifications
added as footnotes 4 and 6, and in Sec. 3. Matches version accepted for
publication in JCA
Entanglement, recoherence and information flow in an accelerated detector - quantum field system: Implications for black hole information issue
We study an exactly solvable model where an uniformly accelerated detector is
linearly coupled to a massless scalar field initially in the Minkowski vacuum.
Using the exact correlation functions we show that as soon as the coupling is
switched on one can see information flowing from the detector to the field and
propagating with the radiation into null infinity. By expressing the reduced
density matrix of the detector in terms of the two-point functions, we
calculate the purity function in the detector and study the evolution of
quantum entanglement between the detector and the field. Only in the ultraweak
coupling regime could some degree of recoherence in the detector appear at late
times, but never in full restoration. We explicitly show that under the most
general conditions the detector never recovers its quantum coherence and the
entanglement between the detector and the field remains large at late times. To
the extent this model can be used as an analog to the system of a black hole
interacting with a quantum field, our result seems to suggest in the prevalent
non-Markovian regime, assuming unitarity for the combined system, that black
hole information is not lost but transferred to the quantum field degrees of
freedom. Our combined system will evolve into a highly entangled state between
a remnant of large area (in Bekenstein's black hole atom analog) without any
information of its initial state, and the quantum field, now imbued with
complex information content not-so-easily retrievable by a local observer.Comment: 16 pages, 12 figures; minor change
Metric fluctuations of an evaporating black hole from back reaction of stress tensor fluctuations
This paper delineates the first steps in a systematic quantitative study of
the spacetime fluctuations induced by quantum fields in an evaporating black
hole under the stochastic gravity program. The central object of interest is
the noise kernel, which is the symmetrized two-point quantum correlation
function of the stress tensor operator. As a concrete example we apply it to
the study of the spherically-symmetric sector of metric perturbations around an
evaporating black hole background geometry. For macroscopic black holes we find
that those fluctuations grow and eventually become important when considering
sufficiently long periods of time (of the order of the evaporation time), but
well before the Planckian regime is reached. In addition, the assumption of a
simple correlation between the fluctuations of the energy flux crossing the
horizon and far from it, which was made in earlier work on
spherically-symmetric induced fluctuations, is carefully scrutinized and found
to be invalid. Our analysis suggests the existence of an infinite amplitude for
the fluctuations when trying to localize the horizon as a three-dimensional
hypersurface, as in the classical case, and, as a consequence, a more accurate
picture of the horizon as possessing a finite effective width due to quantum
fluctuations. This is supported by a systematic analysis of the noise kernel in
curved spacetime smeared with different functions under different conditions,
the details are collected in the appendices. This case study shows a pathway
for probing quantum metric fluctuations near the horizon and understanding
their physical meaning.Comment: 21 pages, REVTe
Variational Monte Carlo study of chiral spin liquid in the extended Heisenberg model on the Kagome lattice
We investigate the extended Heisenberg model on the Kagome lattice by using
Gutzwiller projected fermionic states and the variational Monte Carlo
technique. In particular, when both second- and third-neighbor super-exchanges
are considered, we find that a gapped spin liquid described by non-trivial
magnetic fluxes and long-range chiral-chiral correlations is energetically
favored compared to the gapless U(1) Dirac state. Furthermore, the topological
Chern number, obtained by integrating the Berry curvature, and the degeneracy
of the ground state, by constructing linearly independent states, lead us to
identify this flux state as the chiral spin liquid with fractionalized
Chern number.Comment: 9 pages, 7 figure
- …