921 research outputs found
Comments on Exclusive Electroproduction of Transversely Polarized Vector Mesons
We discuss the electroproduction of light vector mesons from transversely
polarized photons. Here QCD factorization cannot be applied as shown explicitly
in a leading order calculation of corresponding Feynman diagrams. It is
emphasized that present infrared singular contributions cannot be regularized
through phenomenological meson distribution amplitudes with suppressed endpoint
configurations. We point out that infrared divergencies arise also from
integrals over skewed parton distributions of the nucleons.
In a phenomenological analysis of transverse vector meson production model
dependent regularizations have to be applied. If this procedure preserves the
analytic structure suggested by a leading order calculation of Feynman
diagrams, one obtains contributions from nucleon parton distributions and their
derivatives. In particular polarized gluons enter only through their
derivative
Off-Forward Parton Distributions
Recently, there have been some interesting developments involving off-forward
parton distributions of the nucleon, deeply virtual Compton scattering, and
hard diffractive vector-meson production. These developments are triggered by
the realization that the off-forward distributions contain information about
the internal spin structure of the nucleon and that diffractive
electroproduction of vector mesons depends on these unconventional
distributions. This paper gives a brief overview of the recent developments
The Imprint of Gravitational Waves in Models Dominated by a Dynamical Cosmic Scalar Field
An alternative to the standard cold dark matter model has been recently
proposed in which a significant fraction of the energy density of the universe
is due to a dynamical scalar field () whose effective equation-of-state
differs from that of matter, radiation or cosmological constant (). In
this paper, we determine how the Q-component modifies the primordial inflation
gravitational wave (tensor metric) contribution to the cosmic microwave
background anisotropy and, thereby, one of the key tests of inflation.Comment: 15 pages, 14 figures, revtex, submitted to Phys. Rev.
Lorentz Invariance and the Cosmological Constant
Non-trivial solutions in string field theory may lead to the spontaneous
breaking of Lorentz invariance and to new tensor-matter interactions. It is
argued that requiring the contribution of the vacuum expectation values of
Lorentz tensors to account for the vacuum energy up to the level that
implies the new interactions range is . These conjectured violations of the Lorentz symmetry are
consistent with the most stringent experimental limits.Comment: 13 pages, plain Latex. This essay was selected for an honorable
mention in the 1997 Gravity Research Foundation essay competio
Graviton Production in Elliptical and Hyperbolic Universes
The problem of cosmological graviton creation for homogeneous and isotropic
universes with elliptical (\vae =+1) and hyperbolical (\vae =-1) geometries
is addressed. The gravitational wave equation is established for a
self-gravitating fluid satisfying the barotropic equation of state , which is the source of the Einstein's equations plus a cosmological
-term. The time dependent part of this equation is exactly solved in
terms of hypergeometric functions for any value of and spatial
curvature \vae. An expression representing an adiabatic vacuum state is then
obtained in terms of associated Legendre functions whenever , where n is an integer. This includes most
cases of physical interest such as . The mechanism of
graviton creation is reviewed and the Bogoliubov coefficients related to
transitions between arbitrary cosmic eras are also explicitly evaluated.Comment: 25 pages, uses REVTE
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-
Conformal Invariance and Cosmic Background Radiation
The spectrum and statistics of the cosmic microwave background radiation
(CMBR) are investigated under the hypothesis that scale invariance of the
primordial density fluctuations should be promoted to full conformal
invariance. As in the theory of critical phenomena, this hypothesis leads in
general to deviations from naive scaling. The spectral index of the two-point
function of density fluctuations is given in terms of the quantum trace anomaly
and is greater than one, leading to less power at large distance scales than a
strict Harrison-Zel'dovich spectrum. Conformal invariance also implies
non-gaussian statistics for the higher point correlations and in particular, it
completely determines the large angular dependence of the three-point
correlations of the CMBR.Comment: 4 pages, Revtex file, uuencoded with one figur
Recovering the Inflationary Potential
A procedure is developed for the recovery of the inflationary potential over
the interval that affects astrophysical scales (\approx 1\Mpc - 10^4\Mpc).
The amplitudes of the scalar and tensor metric perturbations and their
power-spectrum indices, which can in principle be inferred from large-angle CBR
anisotropy experiments and other cosmological data, determine the value of the
inflationary potential and its first two derivatives. From these, the
inflationary potential can be reconstructed in a Taylor series and the
consistency of the inflationary hypothesis tested. A number of examples are
presented, and the effect of observational uncertainties is discussed.Comment: 13 pages LaTeX, 6 Figs. available on request, FNAL-Pub-93/182-
Can the Gravitational Wave Background from Inflation be Detected Locally?
The Cosmic Background Explorer (COBE) detection of microwave background
anisotropies may contain a component due to gravitational waves generated by
inflation. It is shown that the gravitational waves from inflation might be
seen using `beam-in-space' detectors, but not the Laser Interferometer Gravity
Wave Observatory (LIGO). The central conclusion, dependent only on weak
assumptions regarding the physics of inflation, is a surprising one. The larger
the component of the COBE signal due to gravitational waves, the {\em smaller}
the expected local gravitational wave signal.Comment: 8 pages, standard LaTeX (no figures), SUSSEX-AST 93/7-
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