45 research outputs found
Worldsheet instantons and coupling selection rules in heterotic orbifolds
We review recent results on string coupling selection rules for heterotic
orbifolds, derived using conformal field theory. Such rules are the first step
towards understanding the viability of the recently obtained compactifications
with potentially realistic particle spectra. They arise from the properties of
the worldsheet instantons that mediate the couplings, and include stringy
effects that would seem 'miraculous' to an effective field theory observer.Comment: 4 pages, talk presented at SQS'13, JINR, Dubna, Russia, 29 July - 03
August, 201
Wilson lines and Chern-Simons flux in explicit heterotic Calabi-Yau compactifications
We study to what extent Wilson lines in heterotic Calabi-Yau
compactifications lead to non-trivial H-flux via Chern-Simons terms. Wilson
lines are basic ingredients for Standard Model constructions but their induced
H-flux may affect the consistency of the leading order background geometry and
of the two-dimensional worldsheet theory. Moreover H-flux in heterotic
compactifications would play an important role for moduli stabilization and
could strongly constrain the supersymmetry breaking scale. We show how to
compute H-flux and the corresponding superpotential, given an explicit complete
intersection Calabi-Yau compactification and choice of Wilson lines. We do so
by classifying special Lagrangian submanifolds in the Calabi-Yau, understanding
how the Wilson lines project onto these submanifolds, and computing their
Chern-Simons invariants. We illustrate our procedure with the quintic
hypersurface as well as the split-bicubic, which can provide a potentially
realistic three generation model.Comment: 41 pages, 7 figures. v2: Minor corrections, published versio
Dark Energy with a Little Help from its Friends
We analyse theories that do not have a de Sitter vacuum and cannot lead to
slow-roll quintessence, but which nevertheless support a transient era of
accelerated cosmological expansion due to interactions between a scalar
and either a hidden sector thermal bath, which evolves as Dark Radiation, or an
extremely-light component of Dark Matter. We show that simple models can
explain the present-day Dark Energy of the Universe consistently with current
observations. This is possible both when 's potential has a hilltop form
and when it has a steep exponential run-away, as might naturally arise from
string theory. We also discuss a related theory of multi-field quintessence, in
which is coupled to a sector that sources a subdominant component of
Dark Energy, which overcomes many of the challenges of slow-roll quintessence.Comment: 30 pages + appendices (two columns), 9 figure
Gravity at the Tip of the Throat
We study the gravitational signatures that arise from compactifying Type IIB
supergravity on a compact space containing a Klebanov-Strassler warped throat.
After reviewing the dimensional reduction of the 10d graviton and explicitly
obtaining the equations of motion for the 4d tensor , vector
and scalar modes, we find the masses and wavefunctions of
the Kaluza-Klein tower of spin-2 states. We explore how the masses and
wavefunctions depend on the balance between the strength of the warping and the
size of the bulk, and how these relate to the range and strength of the
interactions which correct the Newtonian gravitational potential. By computing
the modified Newtonian potential for sources on a brane somewhere along the
throat, and applying consistency constraints on the Klebanov-Strassler
parameters, we obtain predictions for the phenomenological parameter space. In
the case of a fully warped throat, and depending on where the brane is along
the throat, these predictions are narrow in range and consistent with current
observational and experimental constraints. We also begin an exploration of
gravitational wave signatures of KK gravitons in warped throats, finding that
strong warping can bring the corresponding frequencies down to the windows of
current and proposed experiments.Comment: 49 pages, 13 figures, 5 table
De Sitter vacua -- when are `subleading corrections' really subleading?
We consider various string-loop, warping and curvature corrections that are
expected to appear in type IIB moduli stabilisation scenarios. It has recently
been argued, in the context of strongly-warped LVS de Sitter vacua, that it is
impossible to achieve parametric suppression in all of these corrections
simultaneously \cite{Junghans:2022exo}. We investigate corrections in the
context of the recently discovered weakly-warped LVS de Sitter vacua, which
represent a distinct branch of solutions in type IIB flux compactifications,
supported by small conifold flux numbers . Warping corrections
become less problematic in this regime, and some corrections even help to reach
the weakly-warped regime of parameter space. Other corrections continue to be
dangerous and would require numerical coefficients to be computed -- and found
to be small -- in order not to destroy the consistency of the weakly-warped LVS
de Sitter solution.Comment: 35 pages, 5 figures and 3 table
A guide to frames, 's, scales and corrections in string compactifications
This note is intended to serve as a reference for conventions used in the
literature on string compactifications, and how to move between them, collected
in a single and easy-to-find place, using type IIB as an illustrative example.
We hope it may be useful to beginners in the field and busy experts. E.g.
string constructions proposed to address the moduli stabilisation problem are
generically in regions of parameter space at the boundaries of control, so that
consistent use of 's and frame conventions can be pivotal when computing
their potentially dangerous corrections.Comment: 17 page
Runaway quintessence, out of the swampland
We propose a simple, well-motivated and robust alternative to a metastable de
Sitter vacuum in string theory, consistent with current observations of dark
energy and naturally satisfying conjectured swampland constraints. Inflation
ends in a supersymmetric Minkowski minimum, with a flat direction that is
protected by non-renormalisation theorems. At some scale non-perturbative
effects kick in, inducing a runaway scalar potential. The tail of this runaway
potential cannot sustain a late-time dominating, slow-roll quintessence.
However, the potential always contains a dS maximum. If the modulus starts
close to the hilltop, it remains frozen there by Hubble friction for much of
the cosmological history, at first sourcing a tiny classical vacuum energy and
then constituting a rolling quintessence, with observable consequences. So long
as the modulus is localised away from the Standard Model in the extra
dimensions, there are no observable fifth forces nor is there time-variation of
fundamental constants, and the modulus mass is protected from radiative
corrections. We revisit concrete string models based on heterotic orbifold
compactifications, and show that their de Sitter extrema satisfy the recently
refined dS Swampland Conjecture.Comment: 12 pages, 5 figures. v2: minor modifications, references and comments
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Modular invariance, misalignment and finiteness in non-supersymmetric strings
In this article we show that finite perturbative corrections in
non-supersymmetric strings can be understood via an interplay between modular
invariance and misaligned supersymmetry. While modular invariance is known to
be crucial in closed-string models, its presence and role for open strings is
more subtle. Nevertheless, we argue that it leads to cancellations in physical
quantities such as the one-loop cosmological constant and prevents them from
diverging. In particular, we show that if the sector-averaged number of states
does not grow exponentially, as predicted by misaligned supersymmetry, all
exponential divergences in the one-loop cosmological constant cancel out as
well. To account for the absence of power-law divergences, instead, we need to
resort to the modular structure of the partition function. We finally comment
on the presence of misaligned supersymmetry in the known 10-dimensional
tachyon-free non-supersymmetric string theories.Comment: 36 pages + appendices, 4 figure
String cosmology: from the early universe to today
We review applications of string theory to cosmology, from primordial times to the present-day accelerated expansion. Starting with a brief overview of cosmology and string compactifications, we discuss in detail moduli stabilisation, inflation in string theory, the impact of string theory on post-inflationary dynamics (reheating, moduli domination, kination), dark energy (the cosmological constant from a string landscape and models of quintessence) and various alternative scenarios (string/brane gases, the pre big-bang scenario, rolling tachyons, ekpyrotic/cyclic cosmologies, bubbles of nothing, S-brane and holographic cosmologies). The state of the art in string constructions is described in each topic and, where relevant, connections to swampland conjectures are made. The possibilities for novel particles and excitations (axions, moduli, cosmic strings, branes, solitons, oscillons and boson stars) are emphasised. Implications for the physics of the CMB, gravitational waves, dark matter and dark radiation are discussed along with potential observational signatures
A new de Sitter solution with a weakly warped deformed conifold
We revisit moduli stabilisation for type IIB flux compactifications that
include a warped throat region corresponding to a warped deformed conifold,
with an anti-D3-brane sitting at its tip. The warping induces a coupling
between the conifold's deformation modulus and the bulk volume modulus in the
K\"ahler potential. Previous works have studied the scalar potential assuming a
strong warping such that this coupling term dominates, and found that the
anti-D3-brane uplift may destabilise the conifold modulus and/or volume
modulus, unless flux numbers within the throat are large, which makes tadpole
cancellation a challenge. We explore the regime of parameter space
corresponding to a weakly-but-still warped throat, such that the coupling
between the conifold and volume moduli is subdominant. We thus discover a new
metastable de Sitter solution within the four-dimensional effective field
theory. We discuss the position of this de Sitter vacuum in the string theory
landscape and swampland.Comment: 36 pages, 7 figures and 4 tables; references added and minor typos
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