494 research outputs found
dS/CFT at uniform energy density and a de Sitter "bluewall"
We describe a class of spacetimes that are asymptotically de Sitter in the
Poincare slicing. Assuming that a dS/CFT correspondence exists, we argue that
these are gravity duals to a CFT on a circle leading to uniform energy-momentum
density, and are equivalent to an analytic continuation of the Euclidean AdS
black brane. These are solutions with a complex parameter which then gives a
real energy-momentum density. We also discuss a related solution with the
parameter continued to a real number, which we refer to as a de Sitter
"bluewall". This spacetime has two asymptotic de Sitter universes and Cauchy
horizons cloaking timelike singularities. We argue that the Cauchy horizons
give rise to a blue-shift instability.Comment: Latex, 13pgs, 2 figs. v2: 14pgs, published version, some rephrasing
of language in terms of Euclidean CFT on a circle, more elaborate discussion
on blueshif
\u3cem\u3edS/CFT\u3c/em\u3e at uniform energy density and a de Sitter “bluewall”
We describe a class of spacetimes that are asymptotically de Sitter in the Poincare slicing. Assuming that a dS/CFT correspondence exists, we argue that these are gravity duals to a CFT on a circle leading to uniform energy-momentum density, and are equivalent to an analytic continuation of the Euclidean AdS black brane. These are solutions with a complex parameter which then gives a real energy-momentum density. We also discuss a related solution with the parameter continued to a real number, which we refer to as a de Sitter “bluewall”. This spacetime has two asymptotic de Sitter universes and Cauchy horizons cloaking timelike singularities. We argue that the Cauchy horizons give rise to a blue-shift instability
What is the Simplest Linear Ramp?
We discuss conditions under which a deterministic sequence of real numbers,
interpreted as the set of eigenvalues of a Hamiltonian, can exhibit features
usually associated to random matrix spectra. A key diagnostic is the spectral
form factor (SFF) -- a linear ramp in the SFF is often viewed as a signature of
random matrix behavior. Based on various explicit examples, we observe
conditions for linear and power law ramps to arise in deterministic spectra. We
note that a very simple spectrum with a linear ramp is .
Despite the presence of ramps, these sequences do exhibit conventional
level repulsion, demonstrating that the lore about their concurrence needs
refinement. However, when a small noise correction is added to the spectrum,
they lead to clear level repulsion as well as the (linear) ramp. We note some
remarkable features of logarithmic spectra, apart from their linear ramps: they
are closely related to normal modes of black hole stretched horizons, and their
partition function with argument is the Riemann zeta function
. An immediate consequence is that the spectral form factor is simply
. Our observation that log spectra have a linear ramp, is
closely related to the Lindel\"of hypothesis on the growth of the zeta
function. With elementary numerics, we check that the slope of a best fit line
through on a log-log plot is indeed , to the fourth decimal.
We also note that truncating the Riemann zeta function sum at a finite integer
causes the would-be-eternal ramp to end on a plateau.Comment: 16 pages, many plots, v2: minor corrections, reference
Fuzzballs and Random Matrices
Black holes are believed to have the fast scrambling properties of random
matrices. If the fuzzball proposal is to be a viable model for quantum black
holes, it should reproduce this expectation. This is considered challenging,
because it is natural for the modes on a fuzzball microstate to follow Poisson
statistics. In a previous paper, we noted a potential loophole here, thanks to
the modes depending not just on the -quantum number, but also on the
-quantum numbers of the compact dimensions. For a free scalar field ,
by imposing a Dirichlet boundary condition at the stretched horizon,
we showed that this -dependence leads to a linear ramp in the Spectral Form
Factor (SFF). Despite this, the status of level repulsion remained mysterious.
In this letter, motivated by the profile functions of BPS fuzzballs, we
consider a generic profile instead of at the
stretched horizon. For various notions of genericity (eg. when the Fourier
coefficients of are suitably Gaussian distributed), we find
that the -dependence of the spectrum exhibits striking evidence of level
repulsion, along with the linear ramp. We also find that varying the profile
leads to natural interpolations between Poisson and Wigner-Dyson(WD)-like
spectra. The linear ramp in our previous work can be understood as arising via
an extreme version of level repulsion in such a limiting spectrum. We also
explain how the stretched horizon/fuzzball is different in these aspects from
simply putting a cut-off in flat space or AdS (ie., without a horizon).Comment: v2: minor corrections, improvements in wordin
Time Dependent Cosmologies and Their Duals
We construct a family of solutions in IIB supergravity theory. These are time
dependent or depend on a light-like coordinate and can be thought of as
deformations of AdS_5 x S^5. Several of the solutions have singularities. The
light-like solutions preserve 8 supersymmetries. We argue that these solutions
are dual to the N=4 gauge theory in a 3+1 dimensional spacetime with a metric
and a gauge coupling that is varying with time or the light-like direction
respectively. This identification allows us to map the question of singularity
resolution to the dual gauge theory.Comment: 13 pages REVTeX and AMSLaTeX. v2: corrected typos and made some
clarifications; reference added; v3: more clarifications, references adde
Gauge theory duals of cosmological backgrounds and their energy momentum tensors
We revisit type IIB supergravity backgrounds with null and spacelike singularities with natural gauge theory duals proposed in [S.R. Das, J. Michelson, K. Narayan, and S.P. Trivedi, Phys. Rev. D 74, 026002 (2006)] and [S.R. Das, J. Michelson, K. Narayan, and S.P. Trivedi, Phys. Rev. D 75, 026002 (2007)]. We show that for these backgrounds there are always choices of the boundaries of these deformed AdS5×S5 space-times, such that the dual gauge theories live on flat metrics and have space-time dependent couplings. We present a new time dependent solution of this kind where the effective string coupling is always bounded and vanishes at a spacelike singularity in the bulk, and the space-time becomes AdS5×S5 at early and late times. The holographic energy momentum tensor calculated with a choice of flat boundary is shown to vanish for null backgrounds and to be generically nonzero for time dependent backgrounds
Distinct, ecotype-specific genome and proteome signatures in the marine cyanobacteria Prochlorococcus
The marine cyanobacterium Prochlorococcus marinus, having multiple ecotypes of distinct genotypic/ phenotypic traits and being the first documented example of genome shrinkage in free-living organisms, offers an ideal system for studying niche-driven molecular micro-diversity in closely related microbes. The present study,through an extensive comparative analysis of various genomic/proteomic features of 6 high light (HL) and 6 low light (LL) adapted strains, makes an attempt to identify molecular determinants associated with their vertical niche partitioning. Pronounced strand-specific asymmetry in synonymous codon usage is observed exclusively in LL strains. Distinct dinucleotide abundance profiles are exhibited by 2 LL strains with larger genomes and G+C-content ≈ 50% (group LLa), 4 LL strains having reduced genomes and G+C-content ≈ 35-37% (group LLb), and 6 HL strains. Taking into account the emergence of LLa, LLb and HL strains (based on 16S rRNA phylogeny), a gradual increase in average aromaticity, pI values and beta- & coil-forming propensities and a decrease in mean hydrophobicity, instability indices and helix-forming propensities of core proteins are observed. Greater variations in orthologous gene repertoire are found between LLa and LLb strains, while higher number of positively selected genes exist between LL and HL strains. Strains of different Prochlorococcus groups are characterized by distinct compositional, physicochemical and structural traits that are not mere remnants of a continuous genetic drift, but are potential outcomes of a grand scheme of niche-oriented stepwise diversification, that might have driven them chronologically towards greater stability/fidelity and invoked upon them a special ability to inhabit diverse oceanic
environments
Molecular signature of hypersaline adaptation: insights from genome and proteome composition of halophilic prokaryotes
A comparative genomic and proteomic study of halophilic and non-halophilic prokaryotes identifies specific genomic and proteomic features typical of halophilic species that are independent from genomic GC-content and taxonomic position
Large scale synthesis of 2D graphene oxide by mechanical milling of 3D carbon nanoparticles in air
Graphene oxide (GO) is one of the important functional materials. Large-scale
synthesis of it is very challenging. Following a simple cost-effective route,
large-scale GO was produced by mechanical (ball) milling, in air, of carbon
nanoparticles (CNPs) present in carbon soot in the present study. The thickness
of the GO layer was seen to decrease with an increase in milling time. Ball
milling provided the required energy to acquire the in-plane graphitic order in
the CNPs reducing the disorders in it. As the surface area of the layered
structure became more and more with the increase in milling time, more and more
oxygen of air got attached to the carbon in graphene leading to the formation
of GO. An increase in the time of the ball mill up to 5 hours leads to a
significant increase in the content of GO. Thus ball milling can be useful to
produce large-scale two-dimensional GO for a short time.Comment: 23 pages, 10 figure
Cosmologies with Null Singularities and their Gauge Theory Duals
We investigate backgrounds of Type IIB string theory with null singularities
and their duals proposed in hep-th/0602107. The dual theory is a deformed N=4
Yang-Mills theory in 3+1 dimensions with couplings dependent on a light-like
direction. We concentrate on backgrounds which become AdS_5 x S^5 at early and
late times and where the string coupling is bounded, vanishing at the
singularity. Our main conclusion is that in these cases the dual gauge theory
is nonsingular. We show this by arguing that there exists a complete set of
gauge invariant observables in the dual gauge theory whose correlation
functions are nonsingular at all times. The two-point correlator for some
operators calculated in the gauge theory does not agree with the result from
the bulk supergravity solution. However, the bulk calculation is invalid near
the singularity where corrections to the supergravity approximation become
important. We also obtain pp-waves which are suitable Penrose limits of this
general class of solutions, and construct the Matrix Membrane theory which
describes these pp-wave backgrounds.Comment: 43 pages REVTeX and AMSLaTeX. v2: references adde
- …