238 research outputs found
Representations of SU(1,1) in Non-commutative Space Generated by the Heisenberg Algebra
SU(1,1) is considered as the automorphism group of the Heisenberg algebra H.
The basis in the Hilbert space K of functions on H on which the irreducible
representations of the group are realized is explicitly constructed. The
addition theorems are derived.Comment: Latex, 8 page
Binary Cosmic Strings
The properties of cosmic strings have been investigated in detail for their
implications in early-universe cosmology. Although many variations of the basic
structure have been discovered, with implications for both the microscopic and
macroscopic properties of cosmic strings, the cylindrical symmetry of the
short-distance structure of the string is generally unaffected. In this paper
we describe some mechanisms leading to an asymmetric structure of the string
core, giving the defects a quasi-two-dimensional character. We also begin to
investigate the consequences of this internal structure for the microscopic and
macroscopic physics.Comment: 19 pages; uses harvmac (not included
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
Fractional Super Lie Algebras and Groups
n^{th} root of a Lie algebra and its dual (that is fractional supergroup)
based on the permutation group invariant forms are formulated in the Hopf
algebra formalism. Detailed discussion of -graided algebras is
done.Comment: 13 pages, detailed discussion of -graided is adde
Relic gravitational waves: latest revisions and preparations for new data
The forthcoming release of data from the Planck mission, and possibly from
the next round of Wilkinson Microwave Anisotropy Probe (WMAP) observations,
make it necessary to revise the evaluations of relic gravitational waves in the
existing data and, at the same time, to refine the assumptions and data
analysis techniques in preparation for the arrival of new data. We reconsider
with the help of the commonly used CosmoMC numerical package the previously
found indications of relic gravitational waves in the 7-year (WMAP7) data. The
CosmoMC approach reduces the confidence of these indications from approximately
2 level to approximately 1 level, but the indications do not
disappear altogether. We critically analyze the assumptions that are currently
used in the Cosmic Microwave Background (CMB) data analyzes and outline the
strategy that should help avoid the oversight of relic gravitational waves in
the future CMB data. In particular, it is important to keep away from the
unwarranted assumptions about density perturbations. The prospects of confident
detection of relic gravitational waves by the Planck satellite have worsened,
but they are still good. It appears that more effort will be required in order
to mitigate the foreground contamination.Comment: 11 pages, 4 figures, 3 tables; v.3: improvements, published versio
Dynamics of domain walls intersecting black holes
Previous studies concerning the interaction of branes and black holes
suggested that a small black hole intersecting a brane may escape via a
mechanism of reconnection. Here we consider this problem by studying the
interaction of a small black hole and a domain wall composed of a scalar field
and simulate the evolution of this system when the black hole acquires an
initial recoil velocity. We test and confirm previous results, however, unlike
the cases previously studied, in the more general set-up considered here, we
are able to follow the evolution of the system also during the separation, and
completely illustrate how the escape of the black hole takes place.Comment: 4 pages, 6 figure
Spherical functions on the de Sitter group
Matrix elements and spherical functions of irreducible representations of the
de Sitter group are studied on the various homogeneous spaces of this group. It
is shown that a universal covering of the de Sitter group gives rise to
quaternion Euler angles. An explicit form of Casimir and Laplace-Beltrami
operators on the homogeneous spaces is given. Different expressions of the
matrix elements and spherical functions are given in terms of multiple
hypergeometric functions both for finite-dimensional and unitary
representations of the principal series of the de Sitter group.Comment: 40 page
Why is Spacetime Lorentzian?
We expand on the idea that spacetime signature should be treated as a
dynamical degree of freedom in quantum field theory. It has been argued that
the probability distribution for signature, induced by massless free fields, is
peaked at the Lorentzian value uniquely in D=4 dimensions. This argument is
reviewed, and certain consistency constraints on the generalized signature
(i.e. the tangent space metric
\eta_{ab}(x)=\mbox{diag}[e^{i\theta(x)},1,1,1]) are derived. It is shown that
only one dynamical "Wick angle" can be introduced in the
generalized signature, and the magnitude of fluctuations away from Lorentzian
signature is estimated to be of order
, where is the Planck length, and is the length scale of
the Universe. For massless fields, the case of D=2 dimensions and the case of
supersymmetry are degenerate, in the sense that no signature is preferred. Mass
effects lift this degeneracy, and we show that a dynamical origin of Lorentzian
signature is also possible for (broken) supersymmetry theories in D=6
dimensions, in addition to the more general non-supersymmetric case in D=4
dimensions.Comment: 26 pages, plain LaTeX, NBI-HE-93-3
Imprints of cosmic strings on the cosmological gravitational wave background
The equation which governs the temporal evolution of a gravitational wave
(GW) in curved space-time can be treated as the Schrodinger equation for a
particle moving in the presence of an effective potential. When GWs propagate
in an expanding Universe with constant effective potential, there is a critical
value (k_c) of the comoving wave-number which discriminates the metric
perturbations into oscillating (k > k_c) and non-oscillating (k < k_c) modes.
As a consequence, if the non-oscillatory modes are outside the horizon they do
not freeze out. The effective potential is reduced to a non-vanishing constant
in a cosmological model which is driven by a two-component fluid, consisting of
radiation (dominant) and cosmic strings (sub-dominant). It is known that the
cosmological evolution gradually results in the scaling of a cosmic-string
network and, therefore, after some time (\Dl \ta) the Universe becomes
radiation-dominated. The evolution of the non-oscillatory GW modes during \Dl
\ta (while they were outside the horizon), results in the distortion of the GW
power spectrum from what it is anticipated in a pure radiation-model, at
present-time frequencies in the range 10^{-16} Hz < f < 10^5 Hz.Comment: 8 pages, 1 figure, accepted for publication to Physical Review
Evading the cosmological domain wall problem
Discrete symmetries are commonplace in field theoretical models but pose a
severe problem for cosmology since they lead to the formation of domain walls
during spontaneous symmetry breaking in the early universe. However if one of
the vacuua is favoured over the others, either energetically, or because of
initial conditions, it will eventually come to dominate the universe. Using
numerical methods, we study the evolution of the domain wall network for a
variety of field configurations in two and three dimensions and quantify the
rate at which the walls disappear. Good agreement is found with a recent
analytic estimate of the termination of the scaling regime of the wall network.Comment: 17 pages (revtex), including 9 figures (epsf); Revised to include
test of numerical approximation used; No change in results or conclusions;
accepted for publication in Phys Rev D. PostScript available at
ftp://ftp.physics.ox.ac.uk/pub/local/users/sarkar/Domainwalls.ps.g
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