109 research outputs found
NS fivebranes in type 0 string theory
The massless degrees of freedom of type 0 NS5-branes are derived. A
non-chiral, purely bosonic spectrum is found in both type 0A and 0B. This
non-chirality is confirmed by a one-loop computation in the bulk. Some puzzles
concerning type 0B S-duality are pointed out in this context. An interpretation
of the spectra in terms of ``type 0 little strings'' is proposed.Comment: LaTeX, 12 pages, no figures; v3: section on S-duality revised,
references added, to be published in JHE
Quantum evolution across singularities
Attempts to consider evolution across space-time singularities often lead to
quantum systems with time-dependent Hamiltonians developing an isolated
singularity as a function of time. Examples include matrix theory in certain
singular time-dependent backgounds and free quantum fields on the
two-dimensional compactified Milne universe. Due to the presence of the
singularities in the time dependence, the conventional quantum-mechanical
evolution is not well-defined for such systems. We propose a natural way,
mathematically analogous to renormalization in conventional quantum field
theory, to construct unitary quantum evolution across the singularity. We carry
out this procedure explicitly for free fields on the compactified Milne
universe and compare our results with the matching conditions considered in
earlier work (which were based on the covering Minkowski space).Comment: revised with an emphasis on local counterterm subtraction rather than
analyticity; version to be submitted for publicatio
Adiabaticity and emergence of classical space-time in time-dependent matrix theories
We discuss the low-curvature regime of time-dependent matrix theories
proposed to describe non-perturbative quantum gravity in asymptotically
plane-wave space-times. The emergence of near-classical space-time in this
limit turns out to be closely linked to the adiabaticity of the matrix theory
evolution. Supersymmetry restoration at low curvatures, which is crucial for
the usual space-time interpretation of matrix theories, becomes an obvious
feature of the adiabatic regime.Comment: 18 pages; v2: disclaimers and references added, version for
publicatio
On the Quantum Resolution of Cosmological Singularities using AdS/CFT
The AdS/CFT correspondence allows us to map a dynamical cosmology to a dual
quantum field theory living on the boundary of spacetime. Specifically, we
study a five-dimensional model cosmology in type IIB supergravity, where the
dual theory is an unstable deformation of supersymmetric SU(N) gauge
theory on \Rbar\times S^3. A one-loop computation shows that the coupling
governing the instability is asymptotically free, so quantum corrections cannot
turn the potential around. The big crunch singularity in the bulk occurs when a
boundary scalar field runs to infinity, in finite time. Consistent quantum
evolution requires that we impose boundary conditions at infinite scalar field,
i.e. a self-adjoint extension of the system. We find that quantum spreading of
the homogeneous mode of the boundary scalar leads to a natural UV cutoff in
particle production as the wavefunction for the homogeneous mode bounces back
from infinity. However a perturbative calculation indicates that despite this,
the logarithmic running of the boundary coupling governing the instability
generally leads to significant particle production across the bounce. This
prevents the wave packet of the homogeneous boundary scalar to return close to
its initial form. Translating back to the bulk theory, we conclude that a
quantum transition from a big crunch to a big bang is an improbable outcome of
cosmological evolution in this class of five-dimensional models.Comment: 91 pages, 24 figures; v2: minor reorganization of introduction,
clarifying comments throughout; 77 pages, 22 figures;v5: error corrected
which significantly changes conclusio
Strings in compact cosmological spaces
We confront the problem of giving a fundamental definition to perturbative
string theory in spacetimes with totally compact space (taken to be a torus for
simplicity, though the nature of the problem is very general) and non-compact
time. Due to backreaction induced by the presence of even a single string
quantum, the usual formulation of perturbative string theory in a fixed
classical background is infrared-divergent at all subleading orders in the
string coupling, and needs to be amended. The problem can be seen as a closed
string analogue of D0-brane recoil under an impact by closed strings (a
situation displaying extremely similar infrared divergences). Inspired by the
collective coordinate treatment of the D0-brane recoil, whereby the
translational modes of the D0-brane are introduced as explicit dynamical
variables in the path integral, we construct a similar formalism for the case
of string-induced gravitational backreaction, in which the spatially uniform
modes of the background fields on the compact space are quantized explicitly.
The formalism can equally well be seen as an ultraviolet completion of a
minisuperspace quantum cosmology with string degrees of freedom. We consider
the amplitudes for the universe to have two cross-sections with specified
spatial properties and string contents, and show (at the first non-trivial
order) that they are finite within our formalism.Comment: 37 pages, 4 figures; v2: cosmetic changes, published versio
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