235 research outputs found
Dilaton Stabilization in Brane Gas Cosmology
Brane Gas Cosmology is an M-theory motivated attempt to reconcile aspects of
the standard cosmology based on Einstein's theory of general relativity.
Dilaton gravity, when incorporating winding p-brane states, has verified the
Brandenberger--Vafa mechanism --a string-motivated conjecture which explains
why only three of the nine spatial dimensions predicted by string theory grow
large. Further investigation of this mechanism has argued for a hierarchy of
subspaces, and has shown the internal directions to be stable to initial
perturbations. These results, however, are dependent on a rolling dilaton, or
varying strength of Newton's gravitational constant. In these proceedings we
show that it is not possible to stabilize the dilaton and maintain the
stability of the internal directions within the standard Brane Gas Cosmology
setup.Comment: 6 pages, no figures. To appear in the Proceedings of MRST 2004, held
at Concordia University, Montreal, QC, 12-14 May 200
T and S dualities and The cosmological evolution of the dilaton and the scale factors
Cosmologically stabilizing radion along with the dilaton is one of the major
concerns of low energy string theory. One can hope that T and S dualities can
provide a plausible answer. In this work we study the impact of S and T duality
invariances on dilaton gravity. We have shown various instances where
physically interesting models arise as a result of imposing the mentioned
invariances. In particular S duality has a very privileged effect in that the
dilaton equations partially decouple from the evolution of the scale factors.
This makes it easy to understand the general rules for the stabilization of the
dilaton. We also show that certain T duality invariant actions become S duality
invariance compatible. That is they mimic S duality when extra dimensions
stabilize.Comment: Corrected a misleading interpretation of the S duality transformation
and a wrong comment on d=10. I thank A.Kaya for pointing this out to me in
time. So the new version is dealing with d=10 only. Added references and
corrected some typos. Minor re-editing. Omitted a section for elaboration in
a further study. Corrected further typo
Dynamical decompactification from brane gases in eleven-dimensional supergravity
Brane gas cosmology provides a dynamical decompactification mechanism that
could account for the number of spacetime dimensions we observe today. In this
work we discuss this scenario taking into account the full bosonic sector of
eleven-dimensional supergravity. We find new cosmological solutions that can
dynamically explain the existence of three large spatial dimensions
characterised by an universal asymptotic scaling behaviour and a large number
of initially unwrapped dimensions. This type of solutions enlarge the possible
initial conditions of the Universe in the Hagedorn phase and consequently can
potentially increase the probability of dynamical decompactification from
anisotropically wrapped backgrounds.Comment: 8 figures, JHEP3 styl
String windings in the early universe
We study string dynamics in the early universe. Our motivation is the
proposal of Brandenberger and Vafa, that string winding modes may play a key
role in decompactifying three spatial dimensions. We model the universe as a
homogeneous but anisotropic 9-torus filled with a gas of excited strings. We
adopt initial conditions which fix the dilaton and the volume of the torus, but
otherwise assume all states are equally likely. We study the evolution of the
system both analytically and numerically to determine the late-time behavior.
We find that, although dynamical evolution can indeed lead to three large
spatial dimensions, such an outcome is not statistically favored.Comment: 26 pages, LaTeX, 4 eps figure
Structural basis for the RING catalyzed synthesis of K63 linked ubiquitin chains
This work was supported by grants from Cancer Research UK (C434/A13067), the Wellcome Trust (098391/Z/12/Z) and Biotechnology and Biological Sciences Research Council (BB/J016004/1).The RING E3 ligase catalysed formation of lysine 63 linked ubiquitin chains by the Ube2V2–Ubc13 E2 complex is required for many important biological processes. Here we report the structure of the RING domain dimer of rat RNF4 in complex with a human Ubc13~Ub conjugate and Ube2V2. The structure has captured Ube2V2 bound to the acceptor (priming) ubiquitin with Lys63 in a position that could lead to attack on the linkage between the donor (second) ubiquitin and Ubc13 that is held in the active “folded back” conformation by the RING domain of RNF4. The interfaces identified in the structure were verified by in vitro ubiquitination assays of site directed mutants. This represents the first view of the synthesis of Lys63 linked ubiquitin chains in which both substrate ubiquitin and ubiquitin-loaded E2 are juxtaposed to allow E3 ligase mediated catalysis.PostprintPeer reviewe
String Gas Cosmology and Structure Formation
It has recently been shown that a Hagedorn phase of string gas cosmology may
provide a causal mechanism for generating a nearly scale-invariant spectrum of
scalar metric fluctuations, without the need for an intervening period of de
Sitter expansion. A distinctive signature of this structure formation scenario
would be a slight blue tilt of the spectrum of gravitational waves. In this
paper we give more details of the computations leading to these results.Comment: 12 pages, 3 figure
A millisecond pulsar in a stellar triple system
Gravitationally bound three-body systems have been studied for hundreds of
years and are common in our Galaxy. They show complex orbital interactions,
which can constrain the compositions, masses, and interior structures of the
bodies and test theories of gravity, if sufficiently precise measurements are
available. A triple system containing a radio pulsar could provide such
measurements, but the only previously known such system, B1620-26 (with a
millisecond pulsar, a white dwarf, and a planetary-mass object in an orbit of
several decades), shows only weak interactions. Here we report precision timing
and multi-wavelength observations of PSR J0337+1715, a millisecond pulsar in a
hierarchical triple system with two other stars. Strong gravitational
interactions are apparent and provide the masses of the pulsar (1.4378(13)
Msun, where Msun is the solar mass and the parentheses contain the uncertainty
in the final decimal places) and the two white dwarf companions (0.19751(15)
Msun and 0.4101(3) Msun), as well as the inclinations of the orbits (both
approximately 39.2 degrees). The unexpectedly coplanar and nearly circular
orbits indicate a complex and exotic evolutionary past that differs from those
of known stellar systems. The gravitational field of the outer white dwarf
strongly accelerates the inner binary containing the neutron star, and the
system will thus provide an ideal laboratory in which to test the strong
equivalence principle of general relativity.Comment: 17 pages, 3 figures, 1 table. Published online by Nature on 5 Jan
2014. Extremely minor differences with published version may exis
String Gas Cosmology
We present a critical review and summary of String Gas Cosmology. We include
a pedagogical derivation of the effective action starting from string theory,
emphasizing the necessary approximations that must be invoked. Working in the
effective theory, we demonstrate that at late-times it is not possible to
stabilize the extra dimensions by a gas of massive string winding modes. We
then consider additional string gases that contain so-called enhanced symmetry
states. These string gases are very heavy initially, but drive the moduli to
locations that minimize the energy and pressure of the gas. We consider both
classical and quantum gas dynamics, where in the former the validity of the
theory is questionable and some fine-tuning is required, but in the latter we
find a consistent and promising stabilization mechanism that is valid at
late-times. In addition, we find that string gases provide a framework to
explore dark matter, presenting alternatives to CDM as recently
considered by Gubser and Peebles. We also discuss quantum trapping with string
gases as a method for including dynamics on the string landscape.Comment: 55 pages, 1 figure, minor corrections, version to appear in Reviews
of Modern Physic
Moduli Stabilization in Brane Gas Cosmology with Superpotentials
In the context of brane gas cosmology in superstring theory, we show why it
is impossible to simultaneously stabilize the dilaton and the radion with a
general gas of strings (including massless modes) and D-branes. Although this
requires invoking a different mechanism to stabilize these moduli fields, we
find that the brane gas can still play a crucial role in the early universe in
assisting moduli stabilization. We show that a modest energy density of
specific types of brane gas can solve the overshoot problem that typically
afflicts potentials arising from gaugino condensation.Comment: minor changes to match the journal versio
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