280 research outputs found
Some Late-time Asymptotics of General Scalar-Tensor Cosmologies
We study the asymptotic behaviour of isotropic and homogeneous universes in
general scalar-tensor gravity theories containing a p=-rho vacuum fluid stress
and other sub-dominant matter stresses. It is shown that in order for there to
be approach to a de Sitter spacetime at large 4-volumes the coupling function,
omega(phi), which defines the scalar-tensor theory, must diverge faster than
|phi_infty-phi|^(-1+epsilon) for all epsilon>0 as phi rightarrow phi_infty 0
for large values of the time. Thus, for a given theory, specified by
omega(phi), there must exist some phi_infty in (0,infty) such that omega ->
infty and omega' / omega^(2+epsilon) -> 0 as phi -> 0 phi_infty in order for
cosmological solutions of the theory to approach de Sitter expansion at late
times. We also classify the possible asymptotic time variations of the
gravitation `constant' G(t) at late times in scalar-tensor theories. We show
that (unlike in general relativity) the problem of a profusion of ``Boltzmann
brains'' at late cosmological times can be avoided in scalar-tensor theories,
including Brans-Dicke theory, in which phi -> infty and omega ~ o(\phi^(1/2))
at asymptotically late times.Comment: 14 page
Source Matching in the SDSS and RASS: Which Galaxies are Really X-ray Sources?
The current view of galaxy formation holds that all massive galaxies harbor a
massive black hole at their center, but that these black holes are not always
in an actively accreting phase. X-ray emission is often used to identify
accreting sources, but for galaxies that are not harboring quasars
(low-luminosity active galaxies), the X-ray flux may be weak, or obscured by
dust. To aid in the understanding of weakly accreting black holes in the local
universe, a large sample of galaxies with X-ray detections is needed. We
cross-match the ROSAT All Sky Survey (RASS) with galaxies from the Sloan
Digital Sky Survey Data Release 4 (SDSS DR4) to create such a sample. Because
of the high SDSS source density and large RASS positional errors, the
cross-matched catalog is highly contaminated by random associations. We
investigate the overlap of these surveys and provide a statistical test of the
validity of RASS-SDSS galaxy cross-matches. SDSS quasars provide a test of our
cross-match validation scheme, as they have a very high fraction of true RASS
matches. We find that the number of true matches between the SDSS main galaxy
sample and the RASS is highly dependent on the optical spectral classification
of the galaxy; essentially no star-forming galaxies are detected, while more
than 0.6% of narrow-line Seyferts are detected in the RASS. Also, galaxies with
ambiguous optical classification have a surprisingly high RASS detection
fraction. This allows us to further constrain the SEDs of low-luminosity active
galaxies. Our technique is quite general, and can be applied to any
cross-matching between surveys with well-understood positional errors.Comment: 10 pages, 10 figures, submitted to The Astronomical Journal on 19
June 200
Anisotropy in the dielectric spectrum of hydration water and its relation to water dynamics
Proteins, molecules, and macromolecular assemblies in water are surrounded by
a nanometer-sized hydration layer with properties very different from bulk
water. Here, we use classical molecular dynamics simulations to study the
dielectric response of hydration water next to hydrophobic and hydrophilic
planar surfaces. We find the interfacial dielectricabsorption of water to be
strongly anisotropic: compared to bulk water, which shows a broad
dielectricabsorption maximum around 15 GHz in the imaginary part of the
dielectric function, the absorption for electric fields parallel to the
surface is of similar strength and shows a slight redshift, while for
perpendicular electric fields it is strongly attenuated and blueshifted. This
anisotropy is generic for hydrophobic and hydrophilic surfaces. From our
spatially resolved dielectric functions and a modified Maxwell-Garnett theory
that accounts for anisotropic hydration layers around spherical particles, the
dielectricabsorption of solutions of organic molecules and micelles is derived
to exhibit the experimentally known attenuation in combination with a
redshift. These two features are traced back to the subtle interplay of
interfacial depolarization effects and the dielectricanisotropy in the
hydration layer. By a detailed analysis of the individual water molecule
dynamics the perpendicular blueshift is shown not to be linked to accelerated
water reorientation, but rather to dielectric boundary effects. Carefully
conducted angularly resolved experiments at planar aqueous interfaces will be
able to resolve this dielectricanisotropy and thus to confirm the subtle
connection between spectralabsorption features and the molecular water
dynamics in hydration layers
New Experimental Limits on Macroscopic Forces Below 100 Microns
Results of an experimental search for new macroscopic forces with Yukawa
range between 5 and 500 microns are presented. The experiment uses 1 kHz
mechanical oscillators as test masses with a stiff conducting shield between
them to suppress backgrounds. No signal is observed above the instrumental
thermal noise after 22 hours of integration time. These results provide the
strongest limits to date between 10 and 100 microns, improve on previous limits
by as much as three orders of magnitude, and rule out half of the remaining
parameter space for predictions of string-inspired models with low-energy
supersymmetry breaking. New forces of four times gravitational strength or
greater are excluded at the 95% confidence level for interaction ranges between
200 and 500 microns.Comment: 25 Pages, 7 Figures: Minor Correction
Bouncing Universes with Varying Constants
We investigate the behaviour of exact closed bouncing Friedmann universes in
theories with varying constants. We show that the simplest BSBM varying-alpha
theory leads to a bouncing universe. The value of alpha increases
monotonically, remaining approximately constant during most of each cycle, but
increasing significantly around each bounce. When dissipation is introduced we
show that in each new cycle the universe expands for longer and to a larger
size. We find a similar effect for closed bouncing universes in Brans-Dicke
theory, where also varies monotonically in time from cycle to cycle.
Similar behaviour occurs also in varying speed of light theories
A Low Matter Density Decaying Vacuum Cosmology from Complex Metric
A low matter density decaying vacuum cosmology is proposed on the assumption
that the universe's radius is a complex quantity \hat{R} if it is regarded as
having a zero energy-momentum tensor. But we find that when the radius is real,
it contains matter. Using the Einstein-Hilbert action principle, the physical
scale factor R(t) =|\hat{R}| is obtained as equal to (R_0^{2} + t^{2})^{1/2}
with R_0 representing the finite radius of the universe at t=0. The resulting
physical picture is roughly a theoretical justification of the old Ozer-Taha
model. The new model is devoid of all cosmological problems. In particular, it
confirms the bounds on H_p, the present value of the Hubble parameter: 0.85 <
H_p t_p < 1.91 and faces no age problem. We argue that the total energy density
consists of parts corresponding to relativistic/non-relativistic matter, a
positive vacuum energy and a form of matter with equation of state p_K = -(1/3)
rho_K (textures or generally K-matter), and the following predictions are made
for the present nonrelativistic era: Omega_{M,n.rel.} \approx 2/3,
Omega_{V,n.rel.} \approx 1/3, Omega_ <<1, Omega_K \approx 1, where a parameter
corresponding to K-matter is taken to be unity. It is shown that the spacetime
with complex metric has signature changing properties. Using quantum
cosmological considerations, it is shown that the wave function is peaked about
the classical contour of evolution and the minimum radius R_0 of the
nonsingular model is predicted as comparable with the Planck length.
PACS No(s); 98.80 Hw, 04.20, 04.60Comment: 21 pages, no figure
Extensive air showers with TeV-scale quantum gravity
One of the possible consequences of the existence of extra degrees of freedom
beyond the electroweak scale is the increase of neutrino-nucleon cross sections
() beyond Standard Model predictions. At ultra-high energies
this may allow the existence of neutrino-initiated extensive air showers. In
this paper, we examine the most relevant observables of such showers. Our
analysis indicates that the future Pierre Auger Observatory could be
potentially powerful in probing models with large compact dimensions.Comment: 7 pages revtex, 5 eps fig
Evolution of the Scale Factor with a Variable Cosmological Term
Evolution of the scale factor a(t) in Friedmann models (those with zero
pressure and a constant cosmological term Lambda) is well understood, and
elegantly summarized in the review of Felten and Isaacman [Rev. Mod. Phys. 58,
689 (1986)]. Developments in particle physics and inflationary theory, however,
increasingly indicate that Lambda ought to be treated as a dynamical quantity.
We revisit the evolution of the scale factor with a variable Lambda-term, and
also generalize the treatment to include nonzero pressure. New solutions are
obtained and evaluated using a variety of observational criteria. Existing
arguments for the inevitability of a big bang (ie., an initial state with a=0)
are substantially weakened, and can be evaded in some cases with Lambda_0 (the
present value of Lambda) well below current experimental limits.Comment: 29 pages, 12 figures (not included), LaTeX, uses Phys Rev D style
files (revtex.cls, revtex.sty, aps.sty, aps10.sty, prabib.sty). To appear in
Phys Rev
Spin-2 spectrum of defect theories
We study spin-2 excitations in the background of the recently-discovered
type-IIB solutions of D'Hoker et al. These are holographically-dual to defect
conformal field theories, and they are also of interest in the context of the
Karch-Randall proposal for a string-theory embedding of localized gravity. We
first generalize an argument by Csaki et al to show that for any solution with
four-dimensional anti-de Sitter, Poincare or de Sitter invariance the spin-2
excitations obey the massless scalar wave equation in ten dimensions. For the
interface solutions at hand this reduces to a Laplace-Beltrami equation on a
Riemann surface with disk topology, and in the simplest case of the
supersymmetric Janus solution it further reduces to an ordinary differential
equation known as Heun's equation. We solve this equation numerically, and
exhibit the spectrum as a function of the dilaton-jump parameter .
In the limit of large a nearly-flat linear-dilaton dimension grows
large, and the Janus geometry becomes effectively five-dimensional. We also
discuss the difficulties of localizing four-dimensional gravity in the more
general backgrounds with NS5-brane or D5-brane charge, which will be analyzed
in detail in a companion paper.Comment: 41 pages, 6 figure
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