1,302 research outputs found
Exact metric around a wiggly cosmic string
The exact metric around a wiggly cosmic string is found by modifying the
energy momentum-tensor of a straight infinitely thin cosmic string to include
an electric current along the symmetry axis.Comment: 5 page
Cosmic string formation by flux trapping
We study the formation of cosmic strings by confining a stochastic magnetic
field into flux tubes in a numerical simulation. We use overdamped evolution in
a potential that is minimized when the flux through each face in the simulation
lattice is a multiple of the fundamental flux quantum. When the typical number
of flux quanta through a correlation-length-sized region is initially about 1,
we find a string network similar to that generated by the Kibble-Zurek
mechanism. With larger initial flux, the loop distribution and the Brownian
shape of the infinite strings remain unchanged, but the fraction of length in
infinite strings is increased. A 2D slice of the network exhibits bundles of
strings pointing in the same direction, as in earlier 2D simulations. We find,
however, that strings belonging to the same bundle do not stay together in 3D
for much longer than the correlation length. As the initial flux per
correlation length is decreased, there is a point at which infinite strings
disappear, as in the Hagedorn transition.Comment: 16 pages and 9 figures. (Minor changes and new references added
Weak-Field Gravity of Revolving Circular Cosmic Strings
A weak-field solution of Einstein's equations is constructed. It is generated
by a circular cosmic string revolving in its plane about the centre of the
circle. (The revolution is introduced to prevent the string from collapsing.)
This solution exhibits a conical singularity, and the corresponding deficit
angle is the same as for a straight string of the same linear energy density,
irrespective of the angular velocity of the string.Comment: 13 pages, LaTe
Spinning cosmic strings: a general class of solutions
In this work, we give a general class of solutions of the spinning cosmic
string in Einstein's theory of gravity. After treating same problem in Einstein
Cartan (EC) theory of gravity, the exact solution satisfying both exterior and
interior space-times representing a spin fluid moving along the symmetry axis
is presented in the EC theory. The existence of closed timelike curves in this
spacetime are also examined
Prediction and explanation in the multiverse
Probabilities in the multiverse can be calculated by assuming that we are
typical representatives in a given reference class. But is this class well
defined? What should be included in the ensemble in which we are supposed to be
typical? There is a widespread belief that this question is inherently vague,
and that there are various possible choices for the types of reference objects
which should be counted in. Here we argue that the ``ideal'' reference class
(for the purpose of making predictions) can be defined unambiguously in a
rather precise way, as the set of all observers with identical information
content. When the observers in a given class perform an experiment, the class
branches into subclasses who learn different information from the outcome of
that experiment. The probabilities for the different outcomes are defined as
the relative numbers of observers in each subclass. For practical purposes,
wider reference classes can be used, where we trace over all information which
is uncorrelated to the outcome of the experiment, or whose correlation with it
is beyond our current understanding. We argue that, once we have gathered all
practically available evidence, the optimal strategy for making predictions is
to consider ourselves typical in any reference class we belong to, unless we
have evidence to the contrary. In the latter case, the class must be
correspondingly narrowed.Comment: Minor clarifications adde
Comment on "Formation of primordial black holes by cosmic strings"
We show that in a pioneering paper by Polnarev and Zembowicz, some
conclusions concerning the characteristics of the Turok-strings are generally
not correct. In addition we show that the probability of string collapse given
there, is off by a large prefactor (~1000).Comment: 5 pages, LaTeX and 1 figure, postscript. To appear in PR
Quantum effects in gravitational wave signals from cuspy superstrings
We study the gravitational emission, in Superstring Theory, from fundamental
strings exhibiting cusps. The classical computation of the gravitational
radiation signal from cuspy strings features strong bursts in the special null
directions associated to the cusps. We perform a quantum computation of the
gravitational radiation signal from a cuspy string, as measured in a
gravitational wave detector using matched filtering and located in the special
null direction associated to the cusp. We study the quantum statistics
(expectation value and variance) of the measured filtered signal and find that
it is very sharply peaked around the classical prediction. Ultimately, this
result follows from the fact that the detector is a low-pass filter which is
blind to the violent high-frequency quantum fluctuations of both the string
worldsheet, and the incoming gravitational field.Comment: 16 pages, no figur
Black Holes from Nucleating Strings
We evaluate the probability that a loop of string that has spontaneously
nucleated during inflation will form a black hole upon collapse, after the end
of inflation. We then use the observational bounds on the density of primordial
black holes to put constraints on the parameters of the model. Other
constraints from the distortions of the microwave background and emission of
gravitational radiation by the loops are considered. Also, observational
constraints on domain wall nucleation and monopole pair production during
inflation are briefly discussed.Comment: 27 pages, tutp-92-
Surplus Angle and Sign-flipped Coulomb Force in Projectable Horava-Lifshitz Gravity
We obtain the static spherically symmetric vacuum solutions of
Horava-Lifshitz gravity theory, imposing the detailed balance condition only in
the UV limit. We find the solutions in two different coordinate systems, the
Painlev\'e-Gullstrand coordinates and the Poincare coordinates, to examine the
consequences of imposing the projectability condition. The solutions in two
coordinate systems are distinct due to the non-relativistic nature of the HL
gravity. In the Painleve-Gullstrand coordinates complying with the
projectability condition, the solution involves an additional integration
constant which yields surplus angle and implies attractive Coulomb force
between same charges.Comment: 13 page
Cosmic string scaling in flat space
We investigate the evolution of infinite strings as a part of a complete
cosmic string network in flat space. We perform a simulation of the network
which uses functional forms for the string position and thus is exact to the
limits of computer arithmetic. Our results confirm that the wiggles on the
strings obey a scaling law described by universal power spectrum. The average
distance between long strings also scales accurately with the time. These
results suggest that small-scale structure will also scale in expanding
universe, even in the absence of gravitational damping.Comment: 13 pages,7 figure
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