4,531 research outputs found
Distinguishing modified gravity models
Modified gravity models with screening in local environments appear in three
different guises: chameleon, K-mouflage and Vainshtein mechanisms. We propose
to look for differences between these classes of models by considering
cosmological observations at low redshift. In particular, we analyse the
redshift dependence of the fine structure constant and the proton to electron
mass ratio in each of these scenarios. When the absorption lines belong to
unscreened regions of space such as dwarf galaxies, a time variation would be
present for chameleons. For both K-mouflage and Vainshtein mechanisms, the
cosmological time variation of the scalar field is not suppressed in both
unscreened and screened environments, therefore enhancing the variation of
constants and their detection prospect. We also consider the time variation of
the redshift of distant objects using their spectrocopic velocities. We find
that models of the K-mouflage and Vainshtein types have very different
spectroscopic velocities as a function of redshift and that their differences
with the -CDM template should be within reach of the future ELT- HIRES
observations.Comment: 18 pages, 10 figure
Inflation from Tachyon Condensation, Large N Effects
Using only general properties of the tachyon potential we show that inflation
may be generic when many branes and anti-branes become coincident. Inflation
may occur because of: (1) the assistance of the many diagonal tachyon fields;
(2) when the tachyons condense in a staggered fashion; or (3) when some of them
condense very late. We point out that such inflation is in some sense a stringy
implementation of chaotic inflation and may have important applications for
``regularizing'' a lopsided or singular cosmological compact surface.Comment: 24 pages, 1 figur
Non-canonical inflation coupled to matter
We compute corrections to the inflationary potential due to conformally
coupled non-relativistic matter. We find that under certain conditions of the
matter coupling, inflation may be interrupted abruptly. We display this in the
superconformal Starobinsky model, where matter is conformally coupled to the
Einstein frame metric. These corrections may easily stop inflation provided
that there is an initial density of non-relativistic matter. Since these
additional heavy degrees of freedom generically occur in higher dimension
theories, for example as Kaluza-Klein modes, this effect can arise in multiple
scenarios.Comment: Matches published versio
Supersymmetron
We consider a supersymmetric model of dark energy coupled to cold dark
matter: the supersymmetron. In the absence of cold dark matter, the
supersymmetron converges to a supersymmetric minimum with a vanishing
cosmological constant. When cold dark matter is present, the supersymmetron
evolves to a matter dependent minimum where its energy density does not vanish.
In the early universe until the recent past of the Universe, the energy density
of the supersymmetron is negligible compared to the cold dark matter energy
density. Away from the supersymmetric minimum, the equation of state of the
supersymmetron is constant and negative. When the supersymmetron reaches the
neighbourhood of the supersymmetric minimum, its equation of state vanishes
rapidly. This leads to an acceleration of the universe which is transient
unless supersymmetry breaking induces a pure cosmological constant and
acceleration of the Universe does not end. Moreover, we find that the mass of
supersymmetron is always greater than the gravitino mass. As a result, the
supersymmetron generates a short ranged fifth force which evades gravitational
tests. On the other hand, we find that the supersymmetron may lead to relevant
effects on large scale structures.Comment: published version, 11 page
Casimir, Gravitational and Neutron Tests of Dark Energy
We investigate laboratory tests of dark energy theories which modify gravity
in a way generalising the inverse power law chameleon models. We make use of
the tomographic description of such theories which captures models in
the large curvature limit, the dilaton and the symmetron. We consider their
effects in various experiments where the presence of a new scalar interaction
may be uncovered. More precisely, we focus on the Casimir, Eot-wash and neutron
experiments. We show that dilatons, symmetrons and generalised chameleon models
are efficiently testable in the laboratory. For generalised chameleons, we
revise their status in the light of forthcoming Casimir experiments like CANNEX
in Amsterdam and show that they are within reach of detection.Comment: 30 pages, 5 figure
Cosmic String Current Stability
The stability of fermionic charge carriers on cosmic strings is considered.
We show that neutral fermion currents in cosmic strings are always chiral or
time-like, in contrast to the case of bosonic currents. The spectrum of bound
states on an abelian SO(10) string is determined both before and after the
electroweak phase transition. We determine the mass acquired by the zero mode
at this transition. A range of charge carrier scattering processes are
considered and corresponding decay rates calculated. Couplings between the
carriers and the electroweak sector generate scattering from the plasma which
can thermalise some currents. If the zero mode is isolated from the electroweak
sector, then it survives. Current-current scattering is considered, but found
to be unimportant in realistic settings where the string density is low.Comment: RevTex, 28 pages, 3 figure
Electroweak Baryogenesis with Electroweak Strings
If stable electroweak strings are copiously produced during the electroweak
phase transition, they may contribute significantly to the presently observed
baryon to entropy ratio of the Universe. This analysis establishes the
feasibility of implementing an electroweak baryogenesis scenario without a
first order phase transition.Comment: BROWN-HET-862, 12 pages; use phyzz
Electroweak Baryogenesis with a Second Order Phase Transition
If stable electroweak strings are copiously produced during the electroweak
phase transition, they may contribute significantly to the presently observed
baryon to entropy ratio of the universe. This analysis establishes the
feasibility of implementing an electroweak baryogenesis scenario without a
first order phase transition.Comment: 7 pages, use phyzzx, BROWN-HET-86
One-electron atoms in screened modified gravity
In a large class of scalar-tensor theories that are potential candidates for
dark energy, a nonminimal coupling between the scalar and the photon is
possible. The presence of such an interaction grants us the exciting prospect
of directly observing dark sector phenomenology in the electromagnetic
spectrum. This paper investigates the behavior of one-electron atoms in this
class of modified gravity models, exploring their viability as probes of
deviations from general relativity in both laboratory and astrophysical
settings. Building heavily on earlier studies, our main contribution is
threefold: A thorough analysis finds additional fine-structure corrections
previously unaccounted for, which now predict a contribution to the Lamb shift
that is larger by nearly 4 orders of magnitude. In addition, they also predict
a scalar-mediated photon-photon interaction, which now constrains the scalar's
coupling to the photon independently of the matter coupling. This was not
previously possible with atomic precision tests. Our updated constraints are
and for
the matter and photon coupling, respectively, although these remain
uncompetitive with bounds from other experiments. Second, we include the
effects of the nuclear magnetic moment, allowing for the study of hyperfine
structure and the 21 cm line, which hitherto have been unexplored in this
context. Finally, we also examine how a background scalar leads to equivalence
principle violations.Comment: Minor updates to match PRD versio
Staggered Multi-Field Inflation
We investigate multi-field inflationary scenarios with fields that drop out
of the model in a staggered fashion. This feature is natural in certain
multi-field inflationary setups within string theory; for instance, it can
manifest itself when fields are related to tachyons that condense, or
inter-brane distances that become meaningless when branes annihilate.
Considering a separable potential, and promoting the number of fields to a
smooth time-dependent function, we derive the formalism to deal with these
models at the background and perturbed level, providing general expressions for
the scalar spectral index and the running. We recover known results of e.g. a
dynamically relaxing cosmological constant in the appropriate limits. We
further show that isocurvature perturbations are suppressed during inflation,
so that perturbations are adiabatic and nearly Gaussian. The resulting setup
might be interpreted as a novel type of warm inflation, readily implemented
within string theory and without many of the shortcomings associated with warm
inflation.
To exemplify the applicability of the formalism we consider three concrete
models: assisted inflation with exponential potentials as a simple toy model (a
graceful exit becomes possible), inflation from multiple tachyons (a constant
decay rate of the number of fields and negligible slow roll contributions turns
out to be in good agreement with observations) and inflation from multiple
M5-branes within M-theory (a narrow stacking of branes yields a consistent
scenario).Comment: 29 pages, 2 figure; v2: slightly extended to match JCAP version. v3:
pre-factor corrected in (28)-(30); conclusions unchange
- …