28 research outputs found
Non-equilibrium Thermodynamics of Spacetime
It has previously been shown that the Einstein equation can be derived from
the requirement that the Clausius relation dS = dQ/T hold for all local
acceleration horizons through each spacetime point, where dS is one quarter the
horizon area change in Planck units, and dQ and T are the energy flux across
the horizon and Unruh temperature seen by an accelerating observer just inside
the horizon. Here we show that a curvature correction to the entropy that is
polynomial in the Ricci scalar requires a non-equilibrium treatment. The
corresponding field equation is derived from the entropy balance relation dS
=dQ/T+dS_i, where dS_i is a bulk viscosity entropy production term that we
determine by imposing energy-momentum conservation. Entropy production can also
be included in pure Einstein theory by allowing for shear viscosity of the
horizon.Comment: 4 pages. Dedicated to Rafael Sorkin on the occasion of his 60th
birthda
Gravitational Waves from Sub-lunar Mass Primordial Black Hole Binaries - A New Probe of Extradimensions
In many braneworld models, gravity is largely modified at the electro-weak
scale ~ 1TeV. In such models, primordial black holes (PBHs) with lunar mass M ~
10^{-7}M_sun might have been produced when the temperature of the universe was
at ~ 1TeV. If a significant fraction of the dark halo of our galaxy consists of
these lunar mass PBHs, a huge number of BH binaries will exist in our
neighborhood. Third generation detectors such as EURO can detect gravitational
waves from these binaries, and can also determine their chirp mass. With a new
detector designed to be sensitive at high frequency bands greater than 1 kHz,
the existence of extradimensions could be confirmed.Comment: 4 pages, 1 figure, typos correcte
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
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
Braneworld black holes in cosmology and astrophysics
The braneworld description of our universe entails a large extra dimension
and a fundamental scale of gravity that might be lower by several orders of
magnitude compared to the Planck scale. An interesting consequence of the
braneworld scenario is in the nature of spherically symmetric vacuum solutions
to the brane gravitational field equations which could represent black holes
with properties quite distinct compared to ordinary black holes in
4-dimensions. We discuss certain key features of some braneworld black hole
geometries. Such black holes are likely to have diverse cosmological and
astrophysical ramifications. The cosmological evolution of primordial
braneworld black holes is described highlighting their longevity due to
modified evaporation and effective accretion of radiation during the early
braneworld high energy era. Observational abundance of various evaporation
products of the black holes at different eras impose constraints on their
initial mass fraction. Surviving primordial black holes could be candidates of
dark matter present in galactic haloes. We discuss gravitational lensing by
braneworld black holes. Observables related to the relativistic images of
strong field gravitational lensing could in principle be used to distinguish
between different braneworld black hole metrics in future observations.Comment: Latex, 35 pages, Review article published in Int. J. Mod. Phys. D
(uses stylefile ws-ijmpd.cls); typos corrected; references adde
Interaction of a TeV Scale Black Hole with the Quark-Gluon Plasma at LHC
If the fundamental Planck scale is near a TeV, then parton collisions with
high enough center-of-mass energy should produce black holes. The production
rate for such black holes has been extensively studied for the case of a
proton-proton collision at \sqrt s = 14 TeV and for a lead-lead collision at
\sqrt s = 5.5 TeV at LHC. As the parton energy density is much higher at
lead-lead collisions than in pp collisions at LHC, one natural question is
whether the produced black holes will be able to absorb the partons formed in
the lead-lead collisions and eventually `eat' the quark-gluon plasma formed at
LHC. In this paper, we make a quantitative analysis of this possibility and
find that since the energy density of partons formed in lead-lead collisions at
LHC is about 500 GeV/fm^3, the rate of absorption for one of these black holes
is much smaller than the rate of evaporation. Hence, we argue that black holes
formed in such collisions will decay very quickly, and will not absorb very
many nearby partons. More precisely, we show that for the black hole mass to
increase via parton absorption at the LHC the typical energy density of quarks
and gluons should be of the order of 10^{10} GeV/fm^3. As LHC will not be able
to produce such a high energy density partonic system, the black hole will not
be able to absorb a sufficient number of nearby partons before it decays. The
typical life time of the black hole formed at LHC is found to be a small
fraction of a fm/c.Comment: 7 pages latex (double column), 3 eps figure
Randall-Sundrum black holes and strange stars
It has recently been suggested that the existence of bare strange stars is
incompatible with low scale gravity scenarios. It has been claimed that in such
models, high energy neutrinos incident on the surface of a bare strange star
would lead to catastrophic black hole growth. We point out that for the flat
large extra dimensional case, the parts of parameter space which give rise to
such growth are ruled out by other methods. We then go on to show in detail how
black holes evolve in the the Randall-Sundrum two brane scenario where the
extra dimensions are curved. We find that catastrophic black hole growth does
not occur in this situation either. We also present some general expressions
for the growth of five dimensional black holes in dense media.Comment: 16 pages, more numerics has lead to different path to same
conclusion. Accepted in PR
Validity of Generalized Second Law of Thermodynamics in the Logamediate and Intermediate scenarios of the Universe
In this work, we have investigated the validity of the generalized second law
of thermodynamics in logamediate and intermediate scenarios of the universe
bounded by the Hubble, apparent, particle and event horizons using and without
using first law of thermodynamics. We have observed that the GSL is valid for
Hubble, apparent, particle and event horizons of the universe in the
logamediate scenario of the universe using first law and without using first
law. Similarly the GSL is valid for all horizons in the intermediate scenario
of the universe using first law. Also in the intermediate scenario of the
universe, the GSL is valid for Hubble, apparent and particle horizons but it
breaks down whenever we consider the universe enveloped by the event horizon
Black Hole Production at LHC: String Balls and Black Holes from pp and Lead-lead Collisions
If the fundamental planck scale is near a TeV, then parton collisions with
high enough center-of-mass energy should produce black holes. The production
rate for such black holes at LHC has been extensively studied for the case of a
proton-proton collision. In this paper, we extend this analysis to a lead-lead
collision at LHC. We find that the cross section for small black holes which
may in principle be produced in such a collision is either enhanced or
suppressed, depending upon the black hole mass. For example, for black holes
with a mass around 3 TeV we find that the differential black hole production
cross section, d\sigma/dM, in a typical lead-lead collision is up to 90 times
larger than that for black holes produced in a typical proton-proton collision.
We also discuss the cross-sections for `string ball' production in these
collisions. For string balls of mass about 1 (2) TeV, we find that the
differential production cross section in a typical lead-lead collision may be
enhanced by a factor up to 3300 (850) times that of a proton-proton collision
at LHC.Comment: Added some discussion, final version to appear in Phys. Rev. D (rapid
communications