4,481 research outputs found
String spectra near some null cosmological singularities
We construct cosmological spacetimes with null Kasner-like singularities as
purely gravitational solutions with no other background fields turned on. These
can be recast as anisotropic plane-wave spacetimes by coordinate
transformations. We analyse string quantization to find the spectrum of string
modes in these backgrounds. The classical string modes can be solved for
exactly in these time-dependent backgrounds, which enables a detailed study of
the near singularity string spectrum, (time-dependent) oscillator masses and
wavefunctions. We find that for low lying string modes(finite oscillation
number), the classical near-singularity string mode functions are non-divergent
for various families of singularities. Furthermore, for any infinitesimal
regularization of the vicinity of the singularity, we find a tower of string
modes of ultra-high oscillation number which propagate essentially freely in
the background. The resulting picture suggests that string interactions are
non-negligible near the singularity.Comment: Latex, 30pgs; v2. minor clarifications, references adde
Development of a Large-Area Aerogel Cherenkov Counter Onboard BESS
This paper describes the development of a threshold type aerogel Cherenkov
counter with a large sensitive area of 0.6 m to be carried onboard the BESS
rigidity spectrometer to detect cosmic-ray antiprotons. The design incorporates
a large diffusion box containing 46 finemesh photomultipliers, with special
attention being paid to achieving good performance under a magnetic field and
providing sufficient endurance while minimizing material usage. The refractive
index of the aerogel was chosen to be 1.03. By utilizing the muons and protons
accumulated during the cosmic-ray measurements at sea level, a rejection factor
of 10 was obtained against muons with , while keeping 97%
efficiency for protons below the threshold.Comment: 13 pages, LaTex, 9 eps figures included, submitted to NIM
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
Null cosmological singularities and free strings
We continue exploring free strings in the background of null Kasner-like
cosmological singularities, following arXiv:0904.4532 [hep-th]. We study the
free string Schrodinger wavefunctional along the lines of arXiv:0807.1517
[hep-th]. We find the wavefunctional to be nonsingular in the vicinity of
singularities whose Kasner exponents satisfy certain relations. We compare this
with the description in other variables. We then study certain regulated
versions of these singularities where the singular region is replaced by a
substringy but nonsingular region and study the string spectra in these
backgrounds. The string modes can again be solved for exactly, giving some
insight into how string oscillator states get excited near the singularity.Comment: Latex, 24pgs, v3. PRD version, various clarifications on AdS/CFT
connections added, references adde
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
Intersecting black branes in strong gravitational waves
We consider intersecting black branes with strong gravitational waves
propagating along their worldvolume in the context of supergravity theories.
Both near-horizon and space-filling gravitational wave modes are included in
our ansatz. The equations of motion (originally, partial differential
equations) are shown to reduce to ordinary differential equations, which
include a Toda-like system. For special arrangements of intersecting black
branes, the Toda-like system becomes integrable, permitting a more thorough
analysis of the gravitational equations of motion.Comment: 17 pages; v2: cosmetic improvements, published versio
[O III] and X-ray Properties of a Complete Sample of Hard X-ray Selected AGNs in the Local Universe
We study the correlation between the [O III] and X-ray
luminosities of local Active Galactic Nuclei (AGNs), using a complete, hard
X-ray ( keV) selected sample in the Swift/BAT 9-month catalog. From our
optical spectroscopic observations at the South African Astronomical
Observatory and the literature, a catalog of [O III] line flux
for all 103 AGNs at Galactic latitudes of is complied.
Significant correlations with intrinsic X-ray luminosity () are
found both for observed () and extinction-corrected () luminosities, separately for X-ray unabsorbed and absorbed
AGNs. We obtain the regression form of and from the whole sample. The absorbed AGNs with low
(0.5\%) scattering fractions in soft X-rays show on average smaller and ratios than the
other absorbed AGNs, while those in edge-on host galaxies do not. These results
suggest that a significant fraction of this population are buried in tori with
small opening angles. By using these vs.
correlations, the X-ray luminosity function of local AGNs (including Compton
thick AGNs) in a standard population synthesis model gives much better
agreement with the [O III] luminosity function derived from the
Sloan Digital Sky Survey than previously reported. This confirms that hard
X-ray observations are a very powerful tool to find AGNs with high
completeness.Comment: 14 pages including 11 figures and 3 tables, accepted for publication
in ApJ. In this manuscript, the observed 14-195 keV luminosities in Table 1
have been corrected to be exactly the same as in the original Swift/BAT
9-month catalog. Accordingly, Figures 2(a) and 3(a) and a part of Tables 2
and 3 have been updated. The changes from the previous version are small and
do not affect the tex
Gauge Theories with Time Dependent Couplings and their Cosmological Duals
We consider the N=4 SYM theory in flat 3+1 dimensional spacetime with a time
dependent coupling constant which vanishes at , like . In an
analogous quantum mechanics toy model we find that the response is singular.
The energy diverges at , for a generic state. In addition, if the
phase of the wave function has a wildly oscillating behavior, which does not
allow it to be continued past . A similar effect would make the gauge
theory singular as well, though nontrivial effects of renormalization could
tame this singularity and allow a smooth continuation beyond . The gravity
dual in some cases is known to be a time dependent cosmology which exhibits a
space-like singularity at . Our results, if applicable in the gauge theory
for the case of the vanishing coupling, imply that the singularity is a genuine
sickness and does not admit a meaningful continuation. When the coupling
remains non-zero and becomes small at , the curvature in the bulk becomes
of order the string scale. The gauge theory now admits a time evolution beyond
this point. In this case, a finite amount of energy is produced which possibly
thermalizes and leads to a black hole in the bulk.Comment: 45 pages, 1 figure; v2. minor modifications; v3: PRD version,
conclusions in the field theory case significantly revised to include
possible renormalization effects, quantum mechanics toy model unchanged,
abstract and introduction revised, additional subsection 4.1 adde
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