11,640 research outputs found
Constraining the braneworld with gravitational wave observations
Some braneworld models may have observable consequences that, if detected,
would validate a requisite element of string theory. In the infinite
Randall-Sundrum model (RS2), the AdS radius of curvature, L, of the extra
dimension supports a single bound state of the massless graviton on the brane,
thereby reproducing Newtonian gravity in the weak-field limit. However, using
the AdS/CFT correspondence, it has been suggested that one possible consequence
of RS2 is an enormous increase in Hawking radiation emitted by black holes. We
utilize this possibility to derive two novel methods for constraining L via
gravitational wave measurements. We show that the EMRI event rate detected by
LISA can constrain L at the ~1 micron level for optimal cases, while the
observation of a single galactic black hole binary with LISA results in an
optimal constraint of L <= 5 microns.Comment: 4 pages, replaced with version published in Phys. Rev. Lett
Improvements in Birch's theorem on forms in many variables
We show that a non-singular integral form of degree d is soluble
non-trivially over the integers if and only if it is soluble non-trivially over
the reals and the p-adic numbers, provided that the form has at least
(d-\sqrt{d}/2)2^d variables. This improves on a longstanding result of Birch.Comment: 36 page
Is the Universe More Transparent to Gamma Rays Than Previously Thought?
The MAGIC collaboration has recently reported the detection of the strong
gamma-ray blazar 3C279 during a 1-2 day flare. They have used their spectral
observations to draw conclusions regarding upper limits on the opacity of the
Universe to high energy gamma-rays and, by implication, upper limits on the
extragalactic mid-infrared background radiation. In this paper we examine the
effect of gamma-ray absorption by the extragalactic infrared radiation on
intrinsic spectra for this blazar and compare our results with the
observational data on 3C279. We find agreement with our previous results,
contrary to the recent assertion of the MAGIC group that the Universe is more
transparent to \gray s than our calculations indicate. Our analysis indicates
that in the energy range between ~80 and ~500 GeV, 3C279 has a best-fit
intrinsic spectrum with a spectral index ~1.78 using our fast evolution model
and ~2.19 using our baseline model. However, we also find that spectral indices
in the range of 0.0 to 3.0 are almost as equally acceptable as the best fit
spectral indices. Assuming the same intrinsic spectral index for this flare as
for the 1991 flare from 3C279 observed by EGRET, viz., 2.02, which lies between
our best fit indices, we estimate that the MAGIC flare was ~3 times brighter
than the EGRET flare observed 15 years earlier.Comment: version accepted for publication in ApJ Letter
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