692 research outputs found
Gravitons and Lightcone Fluctuations
Gravitons in a squeezed vacuum state, the natural result of quantum creation
in the early universe or by black holes, will introduce metric fluctuations.
These metric fluctuations will introduce fluctuations of the lightcone. It is
shown that when the various two-point functions of a quantized field are
averaged over the metric fluctuations, the lightcone singularity disappears for
distinct points. The metric averaged functions remain singular in the limit of
coincident points. The metric averaged retarded Green's function for a massless
field becomes a Gaussian which is nonzero both inside and outside of the
classical lightcone. This implies some photons propagate faster than the
classical light speed, whereas others propagate slower. The possible effects of
metric fluctuations upon one-loop quantum processes are discussed and
illustrated by the calculation of the one-loop electron self-energy.Comment: 18pp, LATEX, TUTP-94-1
Recapitulation of the EEF1A2 D252H neurodevelopmental disorder-causing missense mutation in mice reveals a toxic gain of function
Surface Terms as Counterterms in the AdS/CFT Correspondence
We examine the recently proposed technique of adding boundary counterterms to
the gravitational action for spacetimes which are locally asymptotic to anti-de
Sitter. In particular, we explicitly identify higher order counterterms, which
allow us to consider spacetimes of dimensions d<=7. As the counterterms
eliminate the need of ``background subtraction'' in calculating the action, we
apply this technique to study examples where the appropriate background was
ambiguous or unknown: topological black holes, Taub-NUT-AdS and Taub-Bolt-AdS.
We also identify certain cases where the covariant counterterms fail to render
the action finite, and we comment on the dual field theory interpretation of
this result. In some examples, the case of vanishing cosmological constant may
be recovered in a limit, which allows us to check results and resolve
ambiguities in certain asymptotically flat spacetime computations in the
literature.Comment: Revtex, 18 pages. References updated and few typo's fixed. Final
versio
The Sensitivity of Ligo to a Stochastic Background, and its Dependance on the Detector Orientations
We analyze the sensitivity of a network of interferometer gravitational-wave
detectors to the gravitational-wave stochastic background, and derive the
dependence of this sensitivity on the orientations of the detector arms. We
build on and extend the recent work of Christensen, but our conclusion for the
optimal choice of orientations of a pair of detectors differs from his. For a
pair of detectors (such as LIGO) that subtends an angle at the center of the
earth of \,\alt 70^\circ, we find that the optimal configuration is for each
detector to have its arms make an angle of (modulo ) with
the arc of the great circle that joins them. For detectors that are farther
separated, each detector should instead have one arm aligned with this arc. We
also describe in detail the optimal data-analysis algorithm for searching for
the stochastic background with a detector network, which is implicit in earlier
work of Michelson. The LIGO pair of detectors will be separated by . The minimum detectable stochastic energy-density for these
detectors with their currently planned orientations is greater than
what it would be if the orientations were optimal.Comment: 56 pages, 10 figures, Caltech preprint GRP-347, submitted to Phys Rev
D, uses revtex macro
Gravitational Radiation From Cosmological Turbulence
An injection of energy into the early Universe on a given characteristic
length scale will result in turbulent motions of the primordial plasma. We
calculate the stochastic background of gravitational radiation arising from a
period of cosmological turbulence, using a simple model of isotropic
Kolmogoroff turbulence produced in a cosmological phase transition. We also
derive the gravitational radiation generated by magnetic fields arising from a
dynamo operating during the period of turbulence. The resulting gravitational
radiation background has a maximum amplitude comparable to the radiation
background from the collision of bubbles in a first-order phase transition, but
at a lower frequency, while the radiation from the induced magnetic fields is
always subdominant to that from the turbulence itself. We briefly discuss the
detectability of such a signal.Comment: 20 pages. Corrections for an errant factor of 2 in all the gravity
wave characteristic amplitudes. Accepted for publication in Phys. Rev.
Toward an internally consistent astronomical distance scale
Accurate astronomical distance determination is crucial for all fields in
astrophysics, from Galactic to cosmological scales. Despite, or perhaps because
of, significant efforts to determine accurate distances, using a wide range of
methods, tracers, and techniques, an internally consistent astronomical
distance framework has not yet been established. We review current efforts to
homogenize the Local Group's distance framework, with particular emphasis on
the potential of RR Lyrae stars as distance indicators, and attempt to extend
this in an internally consistent manner to cosmological distances. Calibration
based on Type Ia supernovae and distance determinations based on gravitational
lensing represent particularly promising approaches. We provide a positive
outlook to improvements to the status quo expected from future surveys,
missions, and facilities. Astronomical distance determination has clearly
reached maturity and near-consistency.Comment: Review article, 59 pages (4 figures); Space Science Reviews, in press
(chapter 8 of a special collection resulting from the May 2016 ISSI-BJ
workshop on Astronomical Distance Determination in the Space Age
Dark Energy and Gravity
I review the problem of dark energy focusing on the cosmological constant as
the candidate and discuss its implications for the nature of gravity. Part 1
briefly overviews the currently popular `concordance cosmology' and summarises
the evidence for dark energy. It also provides the observational and
theoretical arguments in favour of the cosmological constant as the candidate
and emphasises why no other approach really solves the conceptual problems
usually attributed to the cosmological constant. Part 2 describes some of the
approaches to understand the nature of the cosmological constant and attempts
to extract the key ingredients which must be present in any viable solution. I
argue that (i)the cosmological constant problem cannot be satisfactorily solved
until gravitational action is made invariant under the shift of the matter
lagrangian by a constant and (ii) this cannot happen if the metric is the
dynamical variable. Hence the cosmological constant problem essentially has to
do with our (mis)understanding of the nature of gravity. Part 3 discusses an
alternative perspective on gravity in which the action is explicitly invariant
under the above transformation. Extremizing this action leads to an equation
determining the background geometry which gives Einstein's theory at the lowest
order with Lanczos-Lovelock type corrections. (Condensed abstract).Comment: Invited Review for a special Gen.Rel.Grav. issue on Dark Energy,
edited by G.F.R.Ellis, R.Maartens and H.Nicolai; revtex; 22 pages; 2 figure
Hantavirus and Arenavirus Antibodies in Persons with Occupational Rodent Exposure, North America
Risk for infection was low among those who handled neotomine or sigmodontine rodents on the job
Detector Description and Performance for the First Coincidence Observations between LIGO and GEO
For 17 days in August and September 2002, the LIGO and GEO interferometer
gravitational wave detectors were operated in coincidence to produce their
first data for scientific analysis. Although the detectors were still far from
their design sensitivity levels, the data can be used to place better upper
limits on the flux of gravitational waves incident on the earth than previous
direct measurements. This paper describes the instruments and the data in some
detail, as a companion to analysis papers based on the first data.Comment: 41 pages, 9 figures 17 Sept 03: author list amended, minor editorial
change
- …