2,357 research outputs found
Phantom Energy Accretion onto Black Holes in Cyclic Universe
Black holes pose a serious problem in the cyclic or oscillating cosmology. It
is speculated that, in the cyclic universe with phantom turnarounds, black
holes will be torn apart by the phantom energy before turnaround before they
can create any problems. In this paper, using the mechanism of the phantom
accretion onto black holes, we find that black holes do not disappear before
the phantom turnaround. But the remanent black holes will not cause any
problems due to the Hawking evaporation.Comment: 8 pages, no figure; typographical errors are correcte
The Tolman Surface Brightness Test for the Reality of the Expansion. II. The Effect of the Point-Spread Function and Galaxy Ellipticity on the Derived Photometric Parameters
To complete the Tolman surface brightness test on the reality of the
expansion of the Universe, we need to measure accurately the surface brightness
profiles of the high-redshift galaxy sample. We, therefore, investigate the
effects of various sizes of point-spread-functions composed of telescope
diffraction, CCD pixel resolutions, and ground-based seeing on the measurements
of mean surface brightness. We have done the calculations using two synthetic
galaxies of effective radii of 0.70" and 0.25" with point-spread functions of
0.1, 0.3, and 0.9 arcseconds. We have also compared actual observations of
three high-redshift galaxies in the cluster Cl 1324 + 3011 (z = 0.76) made both
with the Keck telescopes in seeing of about 0.9" and with HST which has a PSF
that is approximately ten times smaller. The conclusion is that HST data can be
used as far into the galaxy image as a Petrosian metric radius of eta = 1.3
magnitudes, whereas the ground-based data will have systematic errors of up to
2.9 magnitudes in the mean surface brightness at eta values of less than 2.2
magnitudes. In the final section, we compare the differences in derived average
surface brightness for nearly circular galaxy images compared with highly
flattened images. The comparison is made by using the two reduction procedures
of (1) integrating the profile curves using circular apertures, and (2)
approximating an ``equivalent circular'' galaxy that is highly elongated by
using an ``effective'' radius of sqrt{ab}, where a and b are the semi-major and
semi-minor axis, respectively, of the best-fitting ellipse. The conclusion is
that the two methods of reduction give nearly identical results and that either
method can be used to analyze the low and high-redshift galaxy samples used in
the Tolman test.Comment: 15 pages, 9 figures; accepted for publication in Astronomical Journa
Relativistic gyratons in asymptotically AdS spacetime
We study the gravitational field of a spinning radiation beam-pulse (a
gyraton) in a D-dimensional asymptotically AdS spacetime. It is shown that the
Einstein equations for such a system reduce to a set of two linear equations in
a (D-2)-dimensional space. By solving these equations we obtain a metric which
is an exact solution of gravitational equations with the (negative)
cosmological constant. The explicit metrics for 4D and 5D gyratons in
asymptotically AdS spacetime are given and their properties are discussed.Comment: 10 page
The Tolman-Bondi--Vaidya Spacetime: matching timelike dust to null dust
The Tolman-Bondi and Vaidya solutions are two solutions to Einstein equations
which describe dust particles and null fluid, respectively. We show that it is
possible to match the two solutions in one single spacetime, the
Tolman-Bondi--Vaidya spacetime. The new spacetime is divided by a null surface
with Tolman-Bondi dust on one side and Vaidya fluid on the other side. The
differentiability of the spacetime is discussed. By constructing a specific
solution, we show that the metric across the null surface can be at least
and the stress-energy tensor is continuous.Comment: 5 pages, no figur
Are we seeing the beginnings of Inflation?
Phantom Cosmology provides an unique opportunity to "connect" the phantom
driven (low en- ergy meV scale) dark energy phase to the (high energy GUT
scale) inflationary era. This is possible because the energy density increases
in phantom cosmology. We present a concrete model where the energy density, but
not the scale factor, cycles through phases of standard radiation/matter domi-
nation followed by dark energy/inflationary phases, and the pattern repeating
itself. An interesting feature of the model is that once we include
interactions between the "phantom fluid" and ordinary matter, the Big rip
singularity is avoided with the phantom phase naturally giving way to a near
exponential inflationary expansion.Comment: 17 pages, 1 figur
Persistence of black holes through a cosmological bounce
We discuss whether black holes could persist in a universe which recollapses
and then bounces into a new expansion phase. Whether the bounce is of classical
or quantum gravitational origin, such cosmological models are of great current
interest. In particular, we investigate the mass range in which black holes
might survive a bounce and ways of differentiating observationally between
black holes formed just after and just before the last bounce. We also discuss
the consequences of the universe going through a sequence of dimensional
changes as it passes through a bounce.Comment: 8 pages, 1 figur
Energy dependence on fractional charge for strongly interacting subsystems
The energies of a pair of strongly-interacting subsystems with arbitrary
noninteger charges are examined from closed and open system perspectives. An
ensemble representation of the charge dependence is derived, valid at all
interaction strengths. Transforming from resonance-state ionicity to ensemble
charge dependence imposes physical constraints on the occupation numbers in the
strong-interaction limit. For open systems, the chemical potential is evaluated
using microscopic and thermodynamic models, leading to a novel correlation
between ground-state charge and an electronic temperature.Comment: 4 pages, 3 figs.; as accepted (Phys. Rev. Lett.
String Gyratons in Supergravity
We study solutions of the supergravity equations with the string-like sources
moving with the speed of light. An exact solution is obtained for the
gravitational field of a boosted ring string in any dimension greater than
three.Comment: 7 pages;v2 minor changes & references added, final in PR
Spatial Localization and Relativistic Transformation of Quantum Spins
The purity of a reduced state for spins that is pure in the rest frame will
most likely appear to degrade because spin and momentum become mixed when
viewed by a moving observer. We show that such a boost-induced decrease in spin
purity observed in a moving reference frame is intrinsically related to the
spatial localization properties of the wave package observed in the rest frame.
Furthermore, we prove that, for any localized pure state with separable spin
and momentum in the rest frame, its reduced density matrix for spins inevitably
appears to be mixed whenever viewed from a moving reference frame.Comment: 5 pages, 1 figur
Failure of the work-Hamiltonian connection for free energy calculations
Extensions of statistical mechanics are routinely being used to infer free
energies from the work performed over single-molecule nonequilibrium
trajectories. A key element of this approach is the ubiquitous expression
dW/dt=\partial H(x,t)/ \partial t which connects the microscopic work W
performed by a time-dependent force on the coordinate x with the corresponding
Hamiltonian H(x,t) at time t. Here we show that this connection, as pivotal as
it is, cannot be used to estimate free energy changes. We discuss the
implications of this result for single-molecule experiments and atomistic
molecular simulations and point out possible avenues to overcome these
limitations
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