979 research outputs found
The Imprint of Gravitational Waves on the Cosmic Microwave Background
Long-wavelength gravitational waves can induce significant temperature
anisotropy in the cosmic microwave background. Distinguishing this from
anisotropy induced by energy density fluctuations is critical for testing
inflationary cosmology and theories of large-scale structure formation. We
describe full radiative transport calculations of the two contributions and
show that they differ dramatically at angular scales below a few degrees. We
show how anisotropy experiments probing large- and small-angular scales can
combine to distinguish the imprint due to gravitational waves.Comment: 11 pages, Penn Preprint-UPR-
Damping of Tensor Modes in Cosmology
An analytic formula is given for the traceless transverse part of the
anisotropic stress tensor due to free streaming neutrinos, and used to derive
an integro-differential equation for the propagation of cosmological
gravitational waves. The solution shows that anisotropic stress reduces the
squared amplitude by 35.6 % for wavelengths that enter the horizon during the
radiation-dominated phase, independent of any cosmological parameters. This
decreases the tensor temperature and polarization correlation functions for
these wavelengths by the same amount. The effect is less for wavelengths that
enter the horizon at later times. At the longest wavelengths the decrease in
the tensor correlation functions due to neutrino free streaming ranges from
10.7% for to 9.0% for . An Appendix gives a
general proof that tensor as well as scalar modes satisfy a conservation law
for perturbations outside the horizon, even when the anisotropic stress tensor
is not negligible.Comment: 14 pages. The original version of this paper has been expanded to
deal with perturbations of any wavelength. While for wavelengths short enough
to enter the horizon during radiation dominance, temperature and polarization
correlations are damped by 35.6%, at the longest wavelengths the damping is
from 9.0% to 11%. An added Appendix gives a general proof that tensor as well
as scalar modes satisfy a conservation law outside the horizon, even during
neutrino decoupling. Some references are also adde
Detection Limits for Super-Hubble Suppression of Causal Fluctuations
We investigate to what extent future microwave background experiments might
be able to detect a suppression of fluctuation power on large scales in flat
and open universe models. Such suppression would arise if fluctuations are
generated by causal processes, and a measurement of a small suppression scale
would be problematic for inflation models, but consistent with many defect
models. More speculatively, a measurement of a suppression scale of the order
of the present Hubble radius could provide independent evidence for a
fine-tuned inflation model leading to a low-density universe. We find that,
depending on the primordial power spectrum, a suppression scale modestly larger
than the visible Horizon can be detected, but that the detectability drops very
rapidly with increasing scale. For models with two periods of inflation, there
is essentially no possibility of detecting a causal suppression scale.Comment: 8 pages, 4 figures, revtex, In Press Physical Review D 200
Relativistic bound states in Yukawa model
The bound state solutions of two fermions interacting by a scalar exchange
are obtained in the framework of the explicitly covariant light-front dynamics.
The stability with respect to cutoff of the J= and J=
states is studied. The solutions for J= are found to be stable for
coupling constants below the critical value
and unstable above it. The asymptotic behavior of the
wave functions is found to follow a law. The coefficient
and the critical coupling constant are calculated from an
eigenvalue equation. The binding energies for the J= solutions
diverge logarithmically with the cutoff for any value of the coupling constant.
For a wide range of cutoff, the states with different angular momentum
projections are weakly split.Comment: 22 pages, 13 figures, .tar.gz fil
Detecting relic gravitational waves in the CMB: A statistical bias
Analyzing the imprint of relic gravitational waves (RGWs) on the cosmic
microwave background (CMB) power spectra provides a way to determine the signal
of RGWs. In this Letter, we discuss a statistical bias, which could exist in
the data analysis and has the tendency to overlook the RGWs. We also explain
why this bias exists, and how to avoid it.Comment: 4 pages, 1 figur
Simulating Cosmic Microwave Background maps in multi-connected spaces
This article describes the computation of cosmic microwave background
anisotropies in a universe with multi-connected spatial sections and focuses on
the implementation of the topology in standard CMB computer codes. The key
ingredient is the computation of the eigenmodes of the Laplacian with boundary
conditions compatible with multi-connected space topology. The correlators of
the coefficients of the decomposition of the temperature fluctuation in
spherical harmonics are computed and examples are given for spatially flat
spaces and one family of spherical spaces, namely the lens spaces. Under the
hypothesis of Gaussian initial conditions, these correlators encode all the
topological information of the CMB and suffice to simulate CMB maps.Comment: 33 pages, 55 figures, submitted to PRD. Higher resolution figures
available on deman
Triggering Threshold Spacecraft Charging with Changes in Electron Emission from Materials
Modest changes in spacecraft charging conditions can lead to abrupt changes in the spacecraft equilibrium, from small positive potentials to large negative potentials relative to the space plasma; this phenomenon is referred to as threshold charging. It is well known that temporal changes of the space plasma environment (electron plasma temperature or density) can cause threshold charging. Threshold charging can also result from by temporal changes in the juxtaposition of the spacecraft to the environment, including spacecraft orbit, orientation, and geometry. This study focuses on the effects of possible changes in electron emission properties of representative spacecraft materials. It is found that for electron-induced emission, the possible threshold scenarios are very rich, since this type of electron emission can cause either positive or negative charging. Alternately, modification of photon- or ion-induced electron emission is found to induce threshold charging only in certain favorable cases. Changes of emission properties discussed include modifications due to: contamination, degradation and roughening of surfaces and layered materials; biasing and charge accumulation; bandstructure occupation and density of states caused by heat, optical or particle radiation; optical reflectivity and absorptivity; and inaccuracies and errors in measurements and parameterization of materials properties. An established method is used here to quantitatively gauge the relative extent to which these various changes in electron emission alter a spacecraft’s charging behavior and possibly lead to threshold charging. The absolute charging behavior of a hypothetical flat, two-dimensional satellite panel of a single material (either polycrystalline conductor Au or the polymeric polyimide Kapton™ H) is modeled as it undergoes modification and concomitant changes in spacecraft charging in three representative geosynchronous orbit environments, from full sunlight to full shade (eclipse) are considered
Energy Flow in Interjet Radiation
We study the distribution of transverse energy, Q_Omega, radiated into an
arbitrary interjet angular region, Omega, in high-p_T two-jet events. Using an
approximation that emphasizes radiation directly from the partons that undergo
the hard scattering, we find a distribution that can be extrapolated smoothly
to Q_Omega=Lambda_QCD, where it vanishes. This method, which we apply
numerically in a valence quark approximation, provides a class of predictions
on transverse energy radiated between jets, as a function of jet energy and
rapidity, and of the choice of the region Omega in which the energy is
measured. We discuss the relation of our approximation to the radiation from
unobserved partons of intermediate energy, whose importance was identified by
Dasgupta and Salam.Comment: 26 pages, 8 eps figures. Revised to include a discussion of
non-global logarithm
Bosonic Quartic Couplings at LHC
We analyze the potential of the CERN Large Hadron Collider (LHC) to study
anomalous quartic vector-boson interactions Z Z gamma gamma, Z Z Z gamma, W+ W-
gamma gamma, and W+ W- Z gamma through the weak boson fusion processes q q -> q
q gamma gamma and q q -> q q gamma Z(-> l+ l-) with l = electron or muon. After
a careful study of the backgrounds and how to extract them from the data, we
show that the process p p -> j j gamma l+ l- is potentially the most sensitive
to deviations from the Standard Model, improving the sensitivity to anomalous
couplings by up to a factor 10^4 (10^2) with respect to the present direct
(indirect) limits.Comment: 18 pages, 2 figures, revised versio
On the degree of scale invariance of inflationary perturbations
Many, if not most, inflationary models predict the power-law index of the
spectrum of density perturbations is close to one, though not precisely equal
to one, |n-1| \sim O(0.1), implying that the spectrum of density perturbations
is nearly, but not exactly, scale invariant. Some models allow n to be
significantly less than one (n \sim 0.7); a spectral index significantly
greater than one is more difficult to achieve. We show that n \approx 1 is a
consequence of the slow-roll conditions for inflation and ``naturalness,'' and
thus is a generic prediction of inflation. We discuss what is required to
deviate significantly from scale invariance, and then show, by explicit
construction, the existence of smooth potentials that satisfy all the
conditions for successful inflation and give as large as 2.Comment: 7 pages, 2 figures, submitted to Phys. Rev.
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