136 research outputs found
Cosmological Phase Transitions and Radius Stabilization in Higher Dimensions
Recently there has been considerable interest in field theories and string
theories with large extra spacetime dimensions. In this paper, we explore the
role of such extra dimensions for cosmology, focusing on cosmological phase
transitions in field theory and the Hagedorn transition and radius
stabilization in string theory. In each case, we find that significant
distinctions emerge from the usual case in which such large extra dimensions
are absent. For example, for temperatures larger than the scale of the
compactification radii, we show that the critical temperature above which
symmetry restoration occurs is reduced relative to the usual four-dimensional
case, and consequently cosmological phase transitions in extra dimensions are
delayed. Furthermore, we argue that if phase transitions do occur at
temperatures larger than the compactification scale, then they cannot be of
first-order type. Extending our analysis to string theories with large internal
dimensions, we focus on the Hagedorn transition and the new features that arise
due to the presence of large internal dimensions. We also consider the role of
thermal effects in establishing a potential for the radius of the compactified
dimension, and we use this to propose a thermal mechanism for generating and
stabilizing a large radius of compactification.Comment: 37 pages, LaTeX, 5 figure
Single-Brane Cosmological Solutions with a Stable Compact Extra Dimension
We consider 5-dimensional cosmological solutions of a single brane. The
correct cosmology on the brane, i.e., governed by the standard 4-dimensional
Friedmann equation, and stable compactification of the extra dimension is
guaranteed by the existence of a non-vanishing \hat{T}^5_5 which is
proportional to the 4-dimensional trace of the energy-momentum tensor. We show
that this component of the energy-momentum tensor arises from the backreaction
of the dilaton coupling to the brane. The same positive features are exhibited
in solutions found in the presence of non-vanishing cosmological constants both
on the brane (\Lambda_{br}) and in the bulk (\Lambda_B). Moreover, the
restoration of the Friedmann equation, with the correct sign, takes place for
both signs of so long as the sign of is opposite
in order to cancel the energy densities of the two cosmological
constants. We further extend our single-brane thin-wall solution to allow a
brane with finite thickness.Comment: 25 pages, Latex file, no figures, comments added, references updated,
final version to appear in Physical Review
On the Realization of Assisted Inflation
We consider conditions necessary for a successful implementation of so-called
assisted inflation. We generalize the applicability of assisted inflation
beyond exponential potentials as originally proposed to include standard
chaotic (\lambda \phi^4 or m^2 \phi^2) models as well. We also demonstrate that
in a purely 4-dimensional theory, unless the assisted sector is in fact
decoupled, the additional fields of the assisted sector actually impede
inflation. As a specific example of an assisted sector, we consider a
5-dimensional KK model for which the extra dimension may be somewhat or much
larger than the inverse Planck scale. In this case, the assisted sector (coming
from a KK compactification) eliminates the need for a fine-tuned quartic
coupling to drive chaotic inflation. This is a general result of models with
one or more "large" extra dimensions.Comment: 25 pages, LaTeX fil
Constraints on the Variations of the Fundamental Couplings
We reconsider several current bounds on the variation of the fine-structure
constant in models where all gauge and Yukawa couplings vary in an
interdependent manner, as would be expected in unified theories. In particular,
we re-examine the bounds established by the Oklo reactor from the resonant
neutron capture cross-section of 149Sm. By imposing variations in \Lambda_{QCD}
and the quark masses, as dictated by unified theories, the corresponding bound
on the variation of the fine-structure constant can be improved by about 2
orders of magnitude in such theories. In addition, we consider possible bounds
on variations due to their effect on long lived \alpha- and \beta-decay
isotopes, particularly 147Sm and 187Re. We obtain a strong constraint on \Delta
\alpha / \alpha, comparable to that of Oklo but extending to a higher redshift
corresponding to the age of the solar system, from the radioactive life-time of
187Re derived from meteoritic studies. We also analyze the astrophysical
consequences of perturbing the decay Q values on bound state \beta-decays
operating in the s-process.Comment: 25 pages, latex, 5 eps figure
A Calculable Toy Model of the Landscape
Motivated by recent discussions of the string-theory landscape, we propose
field-theoretic realizations of models with large numbers of vacua. These
models contain multiple U(1) gauge groups, and can be interpreted as
deconstructed versions of higher-dimensional gauge theory models with fluxes in
the compact space. We find that the vacuum structure of these models is very
rich, defined by parameter-space regions with different classes of stable vacua
separated by boundaries. This allows us to explicitly calculate physical
quantities such as the supersymmetry-breaking scale, the presence or absence of
R-symmetries, and probabilities of stable versus unstable vacua. Furthermore,
we find that this landscape picture evolves with energy, allowing vacua to
undergo phase transitions as they cross the boundaries between different
regions in the landscape. We also demonstrate that supergravity effects are
crucial in order to stabilize most of these vacua, and in order to allow the
possibility of cancelling the cosmological constant.Comment: 49 pages, LaTeX, 13 figures, references adde
Successful Targeting and Disruption of an Integrated Reporter Lentivirus Using the Engineered Homing Endonuclease Y2 I-AniI
Current antiviral therapy does not cure HIV-infected individuals because the virus establishes lifelong latent infection within long-lived memory T cells as integrated HIV proviral DNA. Here, we report a new therapeutic approach that aims to cure cells of latent HIV infection by rendering latent virus incapable of replication and pathogenesis via targeted cellular mutagenesis of essential viral genes. This is achieved by using a homing endonuclease to introduce DNA double-stranded breaks (dsb) within the integrated proviral DNA, which is followed by triggering of the cellular DNA damage response and error-prone repair. To evaluate this concept, we developed an in vitro culture model of viral latency, consisting of an integrated lentiviral vector with an easily evaluated reporter system to detect targeted mutagenesis events. Using this system, we demonstrate that homing endonucleases can efficiently and selectively target an integrated reporter lentivirus within the cellular genome, leading to mutation in the proviral DNA and loss of reporter gene expression. This new technology offers the possibility of selectively disabling integrated HIV provirus within latently infected cells
Explaining Institutional Change: Why Elected Politicians Implement Direct Democracy
In existing models of direct democratic institutions, the median voter benefits, but representative politicians are harmed since their policy choices can be overridden. This is a puzzle, since representative politicians were instrumental in creating these institutions. I build a model of direct democracy that explains why a representative might benefit from tying his or her own hands in this way. The key features are (1) that voters are uncertain about their representative's preferences; (2) that direct and representative elections are complementary ways for voters to control outcomes. The model shows that some politicians benefit from the introduction of direct democracy, since they are more likely to survive representative elections: direct democracy credibly prevents politicians from realising extreme outcomes. Historical evidence from the introduction of the initiative, referendum and recall in America broadly supports the theory, which also explains two empirical results that have puzzled scholars: legislators are trusted less, but reelected more, in US states with direct democracy. I conclude by discussing the potential for incomplete information and signaling models to improve our understanding of institutional change more generally
Physics Implications of Flat Directions in Free Fermionic Superstring Models I: Mass Spectrum and Couplings
From the "top-down" approach we investigate physics implications of the class
of D- and F- flat directions formed from non-Abelian singlets which are proven
flat to all orders in the nonrenormalizable superpotential, for a prototype
quasi-realistic free fermionic string model with the standard model gauge group
and three families (CHL5). These flat directions have at least an additional
U(1)' unbroken at the string scale. For each flat direction, the complete set
of effective mass terms and effective trilinear superpotential terms in the
observable sector are computed to all orders in the VEV's of the fields in the
flat direction. The "string selection-rules" disallow a large number of
couplings allowed by gauge invariance, resulting in a massless spectrum with a
large number of exotics, in most cases excluded by experiment, thus signifying
a generic flaw of these models. Nevertheless, the resulting trilinear couplings
of the massless spectrum possess a number of interesting features which we
analyse for two representative flat directions: for the fermion texture;
baryon- and lepton-number violating couplings; R-parity breaking; non-canonical
mu terms; and the possibility of electroweak and intermediate scale symmetry
breaking scenarios for U(1)'. The gauge coupling predictions are obtained in
the electroweak scale case. Fermion masses possess t-b and tau-mu universality,
with the string scale Yukawa couplings g and , respectively.
Fermion textures are present for certain flat directions, but only in the
down-quark sector. Baryon- and lepton- number violating couplings can trigger
proton-decay, oscillations, leptoquark interactions and R-parity
violation, leading to the absence of a stable LSP.Comment: 36 pages, 5 tables, 4 figures, RevTeX, minor change
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