483 research outputs found
Confining the Electroweak Model to a Brane
We introduce a simple scenario where, by starting with a five-dimensional
SU(3) gauge theory, we end up with several 4-D parallel branes with localized
fermions and gauge fields. Similar to the split fermion scenario, the
confinement of fermions is generated by the nontrivial topological solution of
a SU(3) scalar field. The 4-D fermions are found to be chiral, and to have
interesting properties coming from their 5-D group representation structure.
The gauge fields, on the other hand, are localized by loop corrections taking
place at the branes produced by the fermions. We show that these two confining
mechanisms can be put together to reproduce the basic structure of the
electroweak model for both leptons and quarks. A few important results are:
Gauge and Higgs fields are unified at the 5-D level; and new fields are
predicted: One left-handed neutrino with zero-hypercharge, and one massive
vector field coupling together the new neutrino with other left-handed leptons.
The hierarchy problem is also addressed.Comment: 9 pages, 8 figures; references added; version published in PR
The string swampland constraints require multi-field inflation
An important unsolved problem that affects practically all attempts to
connect string theory to cosmology and phenomenology is how to distinguish
effective field theories belonging to the string landscape from those that are
not consistent with a quantum theory of gravity at high energies (the "string
swampland"). It was recently proposed that potentials of the string landscape
must satisfy at least two conditions, the "swampland criteria", that severely
restrict the types of cosmological dynamics they can sustain. The first
criterion states that the (multi-field) effective field theory description is
only valid over a field displacement (in units where the Planck mass is 1), measured as a distance in the
target space geometry. A second, more recent, criterion asserts that, whenever
the potential is positive, its slope must be bounded from below, and
suggests . A recent analysis
concluded that these two conditions taken together practically rule out
slow-roll models of inflation. In this note we show that the two conditions
rule out inflationary backgrounds that follow geodesic trajectories in field
space, but not those following curved, non-geodesic, trajectories (which are
parametrized by a non-vanishing bending rate of the multi-field
trajectory). We derive a universal lower bound on (relative to the
Hubble parameter ) as a function of and the number of efolds
, assumed to be at least of order 60. If later studies confirm and
to be strictly , the bound implies strong turns with
. Slow-roll inflation in the landscape is not
ruled out, but it is strongly multi-field.Comment: v1: 15 pages; v2: 16 pages, references added, improved discussions,
version accepted for publication in JCA
Shapes and features of the primordial bispectrum
If time-dependent disruptions from slow-roll occur during inflation, the
correlation functions of the primordial curvature perturbation should have
scale-dependent features, a case which is marginally supported from the cosmic
microwave background (CMB) data. We offer a new approach to analyze the
appearance of such features in the primordial bispectrum that yields new
consistency relations and justifies the search of oscillating patterns
modulated by orthogonal and local templates. Under the assumption of sharp
features, we find that the cubic couplings of the curvature perturbation can be
expressed in terms of the bispectrum in two specific momentum configurations,
for example local and equilateral. This allows us to derive consistency
relations among different bispectrum shapes, which in principle could be tested
in future CMB surveys. Furthermore, based on the form of the consistency
relations, we construct new two-parameter templates for features that include
all the known shapes.Comment: (v1) 16 pages, 3 figures, 1 table; (v2) minor clarifications
including updated abstract, to appear in Journal of Cosmology and
Astroparticle Physic
Cumulative effects in inflation with ultra-light entropy modes
In multi-field inflation one or more non-adiabatic modes may become light,
potentially inducing large levels of isocurvature perturbations in the cosmic
microwave background. If in addition these light modes are coupled to the
adiabatic mode, they influence its evolution on super horizon scales. Here we
consider the case in which a non-adiabatic mode becomes approximately massless
("ultralight") while still coupled to the adiabatic mode, a typical situation
that arises with pseudo-Nambu-Goldstone bosons or moduli. This ultralight mode
freezes on super-horizon scales and acts as a constant source for the curvature
perturbation, making it grow linearly in time and effectively suppressing the
isocurvature component. We identify a Stuckelberg-like emergent shift symmetry
that underlies this behavior. As inflation lasts for many e-folds, the
integrated effect of this source enhances the power spectrum of the adiabatic
mode, while keeping the non-adiabatic spectrum approximately untouched. In this
case, towards the end of inflation all the fluctuations, adiabatic and
non-adiabatic, are dominated by a single degree of freedom.Comment: 27 pages, 1 figure; v2: improved discussions, version published in
JCA
Effective field theory of weakly coupled inflationary models
The application of Effective Field Theory (EFT) methods to inflation has
taken a central role in our current understanding of the very early universe.
The EFT perspective has been particularly useful in analyzing the
self-interactions determining the evolution of co-moving curvature
perturbations (Goldstone boson modes) and their influence on low-energy
observables. However, the standard EFT formalism, to lowest order in spacetime
differential operators, does not provide the most general parametrization of a
theory that remains weakly coupled throughout the entire low-energy regime.
Here we study the EFT formulation by including spacetime differential operators
implying a scale dependence of the Goldstone boson self-interactions and its
dispersion relation. These operators are shown to arise naturally from the
low-energy interaction of the Goldstone boson with heavy fields that have been
integrated out. We find that the EFT then stays weakly coupled all the way up
to the cutoff scale at which ultraviolet degrees of freedom become operative.
This opens up a regime of new physics where the dispersion relation is
dominated by a quadratic dependence on the momentum \omega ~ p^2. In addition,
provided that modes crossed the horizon within this energy range, the
prediction of inflationary observables - including non-Gaussian signatures -
are significantly affected by the new scales characterizing it.Comment: 36 pages, v2: references added, minor changes to match published
versio
- …