771 research outputs found
On string cosmology and the RG flow in 2d field theory
Time-dependent solutions of bosonic string theory resemble renormalisation
group trajectories in the space of 2d field theories: they often interpolate
between repulsive and attractive static solutions. It is shown that the
attractive static solutions are those whose spatial sections are minima of
|\bar c-25|, where \bar c is the `c-function'. The size of the domain of
attraction of such a solution may be a measure of the probability of the
corresponding string vacuum. Our discussion has also an implication for the RG
flow in theories coupled to dynamical 2d gravity: the flow from models with
c>25 to models with c<25 is forbidden.Comment: 16 pages, Latex, BUTP-94/7, Imperial/TP/93-94/29 (some footnotes and
references added.
Quantum Poisson-Lie T-duality and WZNW model
A pair of conformal sigma models related by Poisson-Lie T-duality is
constructed by starting with the O(2,2) Drinfeld double. The duality relates
the standard SL(2,R) WZNW model to a constrained sigma model defined on SL(2,R)
group space. The quantum equivalence of the models is established by using a
path integral argument.Comment: 19 pages, late
Elements of String Cosmology
Aspects of string cosmology for critical and non-critical strings are
discussed emphasizing the necessity to account for the dilaton dynamics for a
proper incorporation of ``large - small" duality. This drastically modifies the
intuition one has with Einstein's gravity. For example winding modes, even
though contribute to energy density, oppose expansion and if not annihilated
will stop the expansion. Moreover we find that the radiation dominated era of
the standard cosmology emerges quite naturally in string cosmology. Our
analysis of non-critical string cosmology provides a reinterpretation of the
(universal cover of the) recently studied two dimensional black hole solution
as a conformal realization of cosmological solutions found previously by
Mueller.Comment: 34 page
Invariances and Equations of Motion in Double Field Theory
We investigate the full set of equations of motion in double field theory and
discuss their O(D,D) symmetry and gauge transformation properties. We obtain a
Ricci-like tensor, its associated Bianchi identities, and relate our results to
those with a generalized metric formulation.Comment: 24 page
Exact four dimensional string solutions and Toda-like sigma models from `null-gauged' WZNW theories
We construct a new class of exact string solutions with a four dimensional
target space metric of signature () by gauging the independent left
and right nilpotent subgroups with `null' generators of WZNW models for rank 2
non-compact groups . The `null' property of the generators () implies the consistency of the gauging and the absence of
\a'-corrections to the semiclassical backgrounds obtained from the gauged
WZNW models. In the case of the maximally non-compact groups () the construction corresponds to gauging some of the
subgroups generated by the nilpotent `step' operators in the Gauss
decomposition. The rank 2 case is a particular example of a general
construction leading to conformal backgrounds with one time-like direction. The
conformal theories obtained by integrating out the gauge field can be
considered as sigma model analogs of Toda models (their classical equations of
motion are equivalent to Toda model equations). The procedure of `null gauging'
applies also to other non-compact groups.Comment: 33 p., harvmac, Imperial/TP/93-94/17 (minor changes: few explanations
added
A tachyonic extension of the stringy no-go theorem
We investigate the tachyon-dilaton-metric system to study the "graceful exit"
problem in string theoretic inflation, where tachyon plays the role of the
scalar field. From the phase space analysis, we find that the inflationary
phase does not smoothly connect to a Friedmann-Robertson-Walker (FRW) expanding
universe, thereby providing a simple tachyonic extension of the recently proved
stringy no-go theorem.Comment: TeX file (PHYZZX), 10 pages, change in the title, many changes in the
text (the version to appear in Phys. Rev. D
Finite sigma models and exact string solutions with Minkowski signature metric
We consider sigma models with a - dimensional Minkowski
signature target space metric having a covariantly constant null Killing
vector. These models are UV finite. The -dimensional target space metric
can be explicitly determined for a class of supersymmetric sigma models with
-dimensional `transverse' part of the target space being homogeneous
K\"ahler. The corresponding `transverse' sub-theory is an supersymmetric
sigma model with the exact \gb-function coinciding with its one-loop
expression. For example, the finite model has supersymmetric sigma
model as its `transverse' part. Moreover, there exists a non-trivial dilaton
field such that the Weyl invariance conditions are also satisfied, i.e. the
resulting models correspond to string vacua. Generic solutions are represented
in terms of the RG flow in `transverse' theory. We suggest a possible
application of the constructed Weyl invariant sigma models to quantisation of
gravity. They may be interpreted as `effective actions' of the quantum
dilaton gravity coupled to a (non-conformal) -dimensional `matter'
theory. The conformal factor of the metric and `dilaton' are
identified with the light cone coordinates of the - dimensional sigma
model.Comment: 24 pages, harvmac, Imperial/TP/92-93/
S-Duality and Exact Type IIB Superstring Backgrounds
A geometrical approach in the non-symmetric connection framework is employed
to examine the issue of higher order corrections to D=10 type IIB
superstring backgrounds with a covariantly constant null Killing isometry and
non-zero Ramond-Ramond field content. These describe generalized supersymmetric
string waves and were obtained recently by us through the S-duality
transformation of purely NS-NS plane wave backgrounds. We find that the
backgrounds are exact subject to the existence of certain field redefinitions
and provided certain restrictive conditions are satisfied.Comment: 21 Pages, LATEX fil
Vanishing of the conformal anomaly for strings in a gravitational wave
Using the non-symmetric-connection approach proposed by Osborn, we
demonstrate that, for a bosonic string in a specially chosen plane-fronted
gravitational wave and an axion background, the conformal anomaly vanishes at
the two-loop level. Under some conditions, the anomaly vanishes at all orders.Comment: Previously not available in hep-th. Published as Physics Letters B
313, 10 (1993). Plain TeX 6 pages. No figure
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