1,566 research outputs found
Quantum effects in the Alcubierre warp drive spacetime
The expectation value of the stress-energy tensor of a free conformally
invariant scalar field is computed in a two-dimensional reduction of the
Alcubierre ``warp drive'' spacetime. The stress-energy is found to diverge if
the apparent velocity of the spaceship exceeds the speed of light. If such
behavior occurs in four dimensions, then it appears implausible that ``warp
drive'' behavior in a spacetime could be engineered, even by an arbitrarily
advanced civilization.Comment: 9 pages, ReVTe
Quantum fields and "Big Rip" expansion singularities
The effects of quantized conformally invariant massless fields on the
evolution of cosmological models containing a ``Big Rip'' future expansion
singularity are examined. Quantized scalar, spinor, and vector fields are found
to strengthen the accelerating expansion of such models as they approach the
expansion singularity.Comment: 7 pages; REVTeX
An Analytic Model with Critical Behavior in Black Hole Formation
A simple analytic model is presented which exhibits a critical behavior in
black hole formation, namely, collapse of a thin shell coupled with outgoing
null fluid. It is seen that the critical behavior is caused by the
gravitational nonlinearity near the event horizon. We calculate the value of
the critical exponent analytically and find that it is very dependent on the
coupling constants of the system.Comment: 21pp., ReVTeX, 7 figures (postscript, compressed and uuencoded),
TIT/HEP-266/COSMO-4
Renormalization of the charged scalar field in curved space
The DeWitt-Schwinger proper time point-splitting procedure is applied to a
massive complex scalar field with arbitrary curvature coupling interacting with
a classical electromagnetic field in a general curved spacetime. The scalar
field current is found to have a linear divergence. The presence of the
external background gauge field is found to modify the stress-energy tensor
results of Christensen for the neutral scalar field by adding terms of the form
to the logarithmic counterterms. These results are shown to be
expected from an analysis of the degree of divergence of scalar quantum
electrodynamics.Comment: 24 pages REVTe
Astrophysical Bounds on Global Strings
Global topological defects produce nonzero stress-energy throughout
spacetime, and as a result can have observable gravitational influence on
surrounding matter. Gravitational effects of global strings are used to place
bounds on their cosmic abundance. The minimum separation between global strings
is estimated by considering the defects' contribution to the cosmological
energy density. More rigorous constraints on the abundance of global strings
are constructed by examining the tidal forces such defects will have on
observable astrophysical systems. The small number of observed tidally
disrupted systems indicates there can be very few of these objects in the
observable universe.Comment: 14 pages, REVTe
Semiclassical Methods for Hawking Radiation from a Vaidya Black Hole
We derive the general form of Hawking temperature for Vaidya black hole in
the tunneling pictures. This kind of black hole is regarded as the description
of a more realistic one since it's time dependent decreasing mass due to the
evaporation process. Clearly, the temperature would be time dependent as our
findings. We use the semiclassical methods, namely radial null geodesic and
complex paths methods. Both methods are found to give the same results. Then,
we discuss the possible form of corresponding entropy.Comment: REVTeX 4, 11 pages, no figures, accepted for publication in IJMPA;
v2: eq.5 is correcte
Gamma-Ray Bursts and Quantum Cosmic Censorship
Gamma-ray bursts are believed to result from the coalescence of binary
neutron stars. However, the standard proposals for conversion of the
gravitational energy to thermal energy have difficulties. We show that if the
merger of the two neutron stars results in a naked singularity, instead of a
black hole, the ensuing quantum particle creation can provide the requisite
thermal energy in a straightforward way. The back-reaction of the created
particles can avoid the formation of the naked singularity predicted by the
classical theory. Hence cosmic censorship holds in the quantum theory, even if
it were to be violated in classical general relativity.Comment: Latex File, 6 pages. This essay received the third award from the
Gravity Research Foundation for the year 199
Semiclassical Stability of the Extreme Reissner-Nordstrom Black Hole
The stress-energy tensor of a free quantized scalar field is calculated in
the extreme Reissner-Nordstr\"{o}m black hole spacetime in the zero temperature
vacuum state. The stress-energy appears to be regular on the event horizon,
contrary to the suggestion provided by two-dimensional calculations. An
analytic calculation on the event horizon for a thermal state shows that if the
temperature is nonzero then the stress-energy diverges strongly there.Comment: 10 pages, REVTeX, 4 figures in separate uuencoded compressed fil
Algebraic approach to time-delay data analysis for LISA
Cancellation of laser frequency noise in interferometers is crucial for
attaining the requisite sensitivity of the triangular 3-spacecraft LISA
configuration. Raw laser noise is several orders of magnitude above the other
noises and thus it is essential to bring it down to the level of other noises
such as shot, acceleration, etc. Since it is impossible to maintain equal
distances between spacecrafts, laser noise cancellation must be achieved by
appropriately combining the six beams with appropriate time-delays. It has been
shown in several recent papers that such combinations are possible. In this
paper, we present a rigorous and systematic formalism based on algebraic
geometrical methods involving computational commutative algebra, which
generates in principle {\it all} the data combinations cancelling the laser
frequency noise. The relevant data combinations form the first module of
syzygies, as it is called in the literature of algebraic geometry. The module
is over a polynomial ring in three variables, the three variables corresponding
to the three time-delays around the LISA triangle. Specifically, we list
several sets of generators for the module whose linear combinations with
polynomial coefficients generate the entire module. We find that this formalism
can also be extended in a straight forward way to cancel Doppler shifts due to
optical bench motions. The two modules are infact isomorphic.
We use our formalism to obtain the transfer functions for the six beams and
for the generators. We specifically investigate monochromatic gravitational
wave sources in the LISA band and carry out the maximisiation over linear
combinations of the generators of the signal-to-noise ratios with the frequency
and source direction angles as parameters.Comment: 27 Pages, 6 figure
Massive spinor fields in flat spacetimes with non-trivial topology
The vacuum expectation value of the stress-energy tensor is calculated for
spin massive fields in several multiply connected flat spacetimes.
We examine the physical effects of topology on manifolds such as , , , the Mobius strip and the Klein bottle.
We find that the spinor vacuum stress tensor has the opposite sign to, and
twice the magnitude of, the scalar tensor in orientable manifolds. Extending
the above considerations to the case of Misner spacetime, we calculate the
vacuum expectation value of spinor stress-energy tensor in this space and
discuss its implications for the chronology protection conjecture.Comment: 18 pages, Some of the equations in section VI as well as
typographical errors corrected, 5 figures, Revtex
- …