25,179 research outputs found
Cosmologies with variable parameters and dynamical cosmon: implications on the cosmic coincidence problem
Dynamical dark energy (DE) has been proposed to explain various aspects of
the cosmological constant (CC) problem(s). For example, it is very difficult to
accept that a strictly constant Lambda-term constitutes the ultimate
explanation for the DE in our Universe. It is also hard to acquiesce in the
idea that we accidentally happen to live in an epoch where the CC contributes
an energy density value right in the ballpark of the rapidly diluting matter
density. It should perhaps be more plausible to conceive that the vacuum
energy, is actually a dynamical quantity as the Universe itself. More
generally, we could even entertain the possibility that the total DE is in fact
a mixture of vacuum energy and other dynamical components (e.g. fields, higher
order terms in the effective action etc) which can be represented collectively
by an effective entity X (dubbed the ``cosmon''). The ``cosmon'', therefore,
acts as a dynamical DE component different from the vacuum energy. While it can
actually behave phantom-like by itself, the overall DE fluid may effectively
appear as standard quintessence, or even mimic at present an almost exact CC
behavior. Thanks to the versatility of such cosmic fluid we can show that a
composite DE system of this sort (``LXCDM'') may have a key to resolving the
mysterious coincidence problem.Comment: LaTeX, 13 pages, 5 figure
Head-On Collision of Neutron Stars As A Thought Experiment
The head-on collision of identical neutron stars from rest at infinity
requires a numerical simulation in full general relativity for a complete
solution. Undaunted, we provide a relativistic, analytic argument to suggest
that during the collision, sufficient thermal pressure is always generated to
support the hot remnant in quasi-static stable equilibrium against collapse
prior to slow cooling via neutrino emission. Our conclusion is independent of
the total mass of the progenitors and holds even if the remnant greatly exceeds
the maximum mass of a cold neutron star.Comment: to appear in Physical Review D (revtex, 3 figs, 5 pgs
Phased-In Tax Cuts and Economic Activity
Phased-in tax reductions are a common feature of tax legislation. This paper uses a dynamic general equilibrium model to quantify the effects of delaying tax cuts. According to the analysis of the model, the phased-in tax cuts of the 2001 tax law substantially reduced employment, output, and investment during the phase-in period. In contrast, the immediate tax cuts of the 2003 tax law provided significant incentives for immediate production and investment. The paper argues that the rules and accounting procedures used by Congress for formulating tax policy have a significant impact in shaping the details of tax policy and led to the phase-ins, sunsets, and temporary tax changes in both the 2001 and 2003 tax laws.Fiscal Policy, Tax Policy
Photoassociation adiabatic passage of ultracold Rb atoms to form ultracold Rb_2 molecules
We theoretically explore photoassociation by Adiabatic Passage of two
colliding cold ^{85}Rb atoms in an atomic trap to form an ultracold Rb_2
molecule. We consider the incoherent thermal nature of the scattering process
in a trap and show that coherent manipulations of the atomic ensemble, such as
adiabatic passage, are feasible if performed within the coherence time window
dictated by the temperature, which is relatively long for cold atoms. We show
that a sequence of ~2*10^7 pulses of moderate intensities, each lasting ~750
ns, can photoassociate a large fraction of the atomic ensemble at temperature
of 100 microkelvin and density of 10^{11} atoms/cm^3. Use of multiple pulse
sequences makes it possible to populate the ground vibrational state. Employing
spontaneous decay from a selected excited state, one can accumulate the
molecules in a narrow distribution of vibrational states in the ground
electronic potential. Alternatively, by removing the created molecules from the
beam path between pulse sets, one can create a low-density ensemble of
molecules in their ground ro-vibrational state.Comment: RevTex, 23 pages, 9 figure
Dark energy: a quantum fossil from the inflationary Universe?
The discovery of dark energy (DE) as the physical cause for the accelerated
expansion of the Universe is the most remarkable experimental finding of modern
cosmology. However, it leads to insurmountable theoretical difficulties from
the point of view of fundamental physics. Inflation, on the other hand,
constitutes another crucial ingredient, which seems necessary to solve other
cosmological conundrums and provides the primeval quantum seeds for structure
formation. One may wonder if there is any deep relationship between these two
paradigms. In this work, we suggest that the existence of the DE in the present
Universe could be linked to the quantum field theoretical mechanism that may
have triggered primordial inflation in the early Universe. This mechanism,
based on quantum conformal symmetry, induces a logarithmic,
asymptotically-free, running of the gravitational coupling. If this evolution
persists in the present Universe, and if matter is conserved, the general
covariance of Einstein's equations demands the existence of dynamical DE in the
form of a running cosmological term whose variation follows a power law of the
redshift.Comment: LaTeX, 14 pages, extended discussion. References added. Accepted in
J. Phys. A: Mathematical and Theoretica
Is There a Relationship between the Density of Primordial Black Holes in a Galaxy and the Rate of Cosmological Gamma-Ray Bursts?
The rate of accretion of matter from a solar-type star onto a primordial
black hole (PBH) that passes through it is calculated. The probability that a
PBH is captured into an orbit around a star in a galaxy is found. The mean
lifetime of the PBH in such an orbit and the rate of orbital captures of PBHs
in the galaxy are calculated. It is shown that this rate does not depend on the
mass of the PBH. This mechanism cannot make an appreciable contribution to the
rate of observed gamma-ray bursts. The density of PBHs in the galaxy can reach
a critical value - the density of the mass of dark matter in the galaxy.Comment: 7 page
Binary Black-Hole Mergers in Magnetized Disks: Simulations in Full General Relativity
We present results from the first fully general relativistic,
magnetohydrodynamic (GRMHD) simulations of an equal-mass black hole binary
(BHBH) in a magnetized, circumbinary accretion disk. We simulate both the pre
and post-decoupling phases of a BHBH-disk system and both "cooling" and
"no-cooling" gas flows. Prior to decoupling, the competition between the binary
tidal torques and the effective viscous torques due to MHD turbulence depletes
the disk interior to the binary orbit. However, it also induces a two-stream
accretion flow and mildly relativistic polar outflows from the BHs. Following
decoupling, but before gas fills the low-density "hollow" surrounding the
remnant, the accretion rate is reduced, while there is a prompt electromagnetic
(EM) luminosity enhancement following merger due to shock heating and accretion
onto the spinning BH remnant. This investigation, though preliminary, previews
more detailed GRMHD simulations we plan to perform in anticipation of future,
simultaneous detections of gravitational and EM radiation from a merging
BHBH-disk system.Comment: 5 pages, 5 figure
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