46 research outputs found
Modified gravity with negative and positive powers of the curvature: unification of the inflation and of the cosmic acceleration
The modified gravity, which eliminates the need for dark energy and which
seems to be stable, is considered. The terms with positive powers of the
curvature support the inflationary epoch while the terms with negative powers
of the curvature serve as effective dark energy, supporting current cosmic
acceleration. The equivalent scalar-tensor gravity may be compatible with the
simplest solar system experiments.Comment: 23 pages, 3 figures, discussion is extended, references added,
version to appear in PR
Statistical distribution of quantum entanglement for a random bipartite state
We compute analytically the statistics of the Renyi and von Neumann entropies
(standard measures of entanglement), for a random pure state in a large
bipartite quantum system. The full probability distribution is computed by
first mapping the problem to a random matrix model and then using a Coulomb gas
method. We identify three different regimes in the entropy distribution, which
correspond to two phase transitions in the associated Coulomb gas. The two
critical points correspond to sudden changes in the shape of the Coulomb charge
density: the appearance of an integrable singularity at the origin for the
first critical point, and the detachement of the rightmost charge (largest
eigenvalue) from the sea of the other charges at the second critical point.
Analytical results are verified by Monte Carlo numerical simulations. A short
account of some of these results appeared recently in Phys. Rev. Lett. {\bf
104}, 110501 (2010).Comment: 7 figure
Exact Minimum Eigenvalue Distribution of an Entangled Random Pure State
A recent conjecture regarding the average of the minimum eigenvalue of the
reduced density matrix of a random complex state is proved. In fact, the full
distribution of the minimum eigenvalue is derived exactly for both the cases of
a random real and a random complex state. Our results are relevant to the
entanglement properties of eigenvectors of the orthogonal and unitary ensembles
of random matrix theory and quantum chaotic systems. They also provide a rare
exactly solvable case for the distribution of the minimum of a set of N {\em
strongly correlated} random variables for all values of N (and not just for
large N).Comment: 13 pages, 2 figures included; typos corrected; to appear in J. Stat.
Phy
Primordial black holes in braneworld cosmologies: Accretion after formation
We recently studied the formation and evaporation of primordial black holes
in a simple braneworld cosmology, namely Randall-Sundrum Type II. Here we study
the effect of accretion from the cosmological background onto the black holes
after formation. While it is generally believed that in the standard cosmology
such accretion is of negligible importance, we find that during the high-energy
regime of braneworld cosmology accretion can be the dominant effect and lead to
a mass increase of potentially orders of magnitude. However, unfortunately the
growth is exponentially sensitive to the accretion efficiency, which cannot be
determined accurately. Since accretion becomes unimportant once the high-energy
regime is over, it does not affect any constraints expressed at the time of
black hole evaporation, but it can change the interpretation of those
constraints in terms of early Universe formation rates.Comment: 6 pages RevTeX4 file. Extension to discussion of thermal balance and
grey-body factor
Spin polarised nuclear matter and its application to neutron stars
An equation of state(EOS) of nuclear matter with explicit inclusion of a
spin-isospin dependent force is constructed from a finite range, momentum and
density dependent effective interaction. This EOS is found to be in good
agreement with those obtained from more sophisticated models for unpolarised
nuclear matter. Introducing spin degrees of freedom, it is found that at
density about 2.5 times the density of normal nuclear matter the neutron matter
undergoes a ferromagnetic transition. The maximum mass and the radius of the
neutron star agree favourably with the observations. Since finding quark matter
rather than spin polarised nuclear matter at the core of neutron stars is more
probable, the proposed EOS is also applied to the study of hybrid stars. It is
found using the bag model picture that one can in principle describe both the
mass and size as well as the surface magnetic field of hybrid stars
satisfactorily.Comment: 26 pages, 11 figures available on reques
Primordial black holes in braneworld cosmologies: astrophysical constraints
In two recent papers we explored the modifications to primordial black hole
physics when one moves to the simplest braneworld model, Randall--Sundrum type
II. Both the evaporation law and the cosmological evolution of the population
can be modified, and additionally accretion of energy from the background can
be dominant over evaporation at high energies. In this paper we present a
detailed study of how this impacts upon various astrophysical constraints,
analyzing constraints from the present density, from the present high-energy
photon background radiation, from distortion of the microwave background
spectrum, and from processes affecting light element abundances both during and
after nucleosynthesis. Typically, the constraints on the formation rate of
primordial black holes weaken as compared to the standard cosmology if black
hole accretion is unimportant at high energies, but can be strengthened in the
case of efficient accretion.Comment: 17 pages RevTeX4 file with three figures incorporated; final paper in
series astro-ph/0205149 and astro-ph/0208299. Minor changes to match version
accepted by Physical Review
1D Frustrated Ferromagnetic Model with Added Dzyaloshinskii-Moriya Interaction
The one-dimensional (1D) isotropic frustrated ferromagnetic spin-1/2 model is
considered. Classical and quantum effects of adding a Dzyaloshinskii-Moriya
(DM) interaction on the ground state of the system is studied using the
analytical cluster method and numerical Lanczos technique. Cluster method
results, show that the classical ground state magnetic phase diagram consists
of only one single phase: "chiral". The quantum corrections are determined by
means of the Lanczos method and a rich quantum phase diagram including the
gapless Luttinger liquid, the gapped chiral and dimer orders is obtained.
Moreover, next nearest neighbors will be entangled by increasing DM interaction
and for open chains, end-spins are entangled which shows the long distance
entanglement (LDE) feature that can be controlled by DM interaction.Comment: 8 pages, 9 figure
Brans-Dicke Theory and primordial black holes in Early Matter-Dominated Era
We show that primordial black holes can be formed in the matter-dominated era
with gravity described by the Brans-Dicke theory. Considering an early
matter-dominated era between inflation and reheating, we found that the
primordial black holes formed during that era evaporate at a quicker than those
of early radiation-dominated era. Thus, in comparison with latter case, less
number of primordial black holes could exist today. Again the constraints on
primordial black hole formation tend towards the larger value than their
radiation-dominated era counterparts indicating a significant enhancement in
the formation of primordial black holes during the matter-dominaed era.Comment: 9 page
Cosmology from Rolling Massive Scalar Field on the anti-D3 Brane of de Sitter Vacua
We investigate a string-inspired scenario associated with a rolling massive
scalar field on D-branes and discuss its cosmological implications. In
particular, we discuss cosmological evolution of the massive scalar field on
the ant-D3 brane of KKLT vacua. Unlike the case of tachyon field, because of
the warp factor of the anti-D3 brane, it is possible to obtain the required
level of amplitude of density perturbations. We study the spectra of scalar and
tensor perturbations generated during the rolling scalar inflation and show
that our scenario satisfies the observational constraint coming from the Cosmic
Microwave Background anisotropies and other observational data. We also
implement the negative cosmological constant arising from the stabilization of
the modulus fields in the KKLT vacua and find that this leads to a successful
reheating in which the energy density of the scalar field effectively scales as
a pressureless dust. The present dark energy can be also explained in our
scenario provided that the potential energy of the massive rolling scalar does
not exactly cancel with the amplitude of the negative cosmological constant at
the potential minimum.Comment: RevTex4, 15 pages, 5 eps figures, minor clarifications and few
references added, final version to appear in PR
Constraints on the mass spectrum of primordial black holes and braneworld parameters from the high-energy diffuse photon background
We investigate the spectral shape of a high-energy diffuse photon emitted by
evaporating primordial black holes (PBHs) in the Randall-Sundrum type II (RS2)
braneworld. In their braneworld scenario, the nature of small PBHs is
drastically modified from the ordinary four-dimensional case for the following
two reasons. (i) dropping Hawking temperature, which equivalently lengthens the
lifetime of the individual PBH due to the change of space-time topology and
(ii) the effective increase of the total amount of PBHs caused by accretion
during the earliest part of the radiation-dominated epoch, the brane
high-energy phase. From studies of the expected spectral shape and its
dependence on braneworld parameters, we obtain two qualitatively distinctive
possibilities of constraints on the braneworld PBHs from the observations of
diffuse high-energy photon background. If the efficiency of accretion in the
high-energy phase exceeds a critical value, the existence of the extra
dimension gives a more stringent upper bound on the abundance of PBHs than the
4D case and a small length scale for the extra dimension is favored. On the
contrary, in the case below the critical accretion efficiency, we find that the
constraint on the PBH abundance can be relaxed by a few orders of magnitude in
exchange for the existence of the large extra dimension; its size may be even
bounded in the region above 10^{19} times 4D Planck length scale provided the
rest mass energy density of the PBHs relative to energy density of radiation is
actually larger than 10^{-27} (4D upper bound) at their formation time. The
above analytical studies are also confirmed numerically, and an allowed region
for braneworld parameters and PBH abundance is clearly obtained.Comment: 16 pages, 8 figures, REVTeX4; version published in PR