27 research outputs found
Clockwork Cosmology
The higher order generalisation of the clockwork mechanism to gravitational
interactions provides a means to generate an exponentially suppressed coupling
to matter from a fundamental theory of multiple interacting gravitons, without
introducing large hierarchies in the underlying potential and without the need
for a dilaton, suggesting a possible application to the hierarchy problem. We
work in the framework of ghost free multi-gravity with "nearest-neighbour"
interactions, and present a formalism by which one is able to construct
potentials such that the theory will always exhibit this clockwork effect. We
also consider cosmological solutions to the general theory, where all metrics
are of FRW form, with site-dependent scale factors/lapses. We demonstrate the
existence of multiple deSitter vacua where all metrics share the same Hubble
parameter, and we solve the modified Einstein equations numerically for an
example clockwork model constructed using our formalism, finding that the
evolution of the metric that matter couples to is essentially equivalent to
that of general relativity at the modified Planck scale. It is important to
stress that while we focus on the application to clockwork theories, our work
is entirely general and facilitates finding cosmological solutions to any ghost
free multi-gravity theory with "nearest-neighbour" interactions. Moreover, we
clarify previous work on the continuum limit of the theory, which is
generically a scalar-tensor braneworld, using the Randall-Sundrum model as a
special case and showing how the discrete-clockwork cosmological results map to
the continuum results in the appropriate limit.Comment: 48 pages, 4 figure
Zipping and unzipping in string networks: dynamics of Y-junctions
We study, within the Nambu-Goto approximation, the stability of massive string junctions under the influence of the tensions of three strings joining together in a Y-type configuration. The relative angle β between the strings at the junction is in general time dependent and its evolution can lead to zipping or unzipping of the three-string configuration. We find that these configurations are stable under deformations of the tension balance condition at the junction. The angle β relaxes at its equilibrium value and the junction grows relativistically. We then discuss other potential “unzipping agents” including monopole/string forces for long strings and curvature for loops, and we investigate specific solutions exhibiting decelerated zipping and unzipping of the Y junction. These results provide motivation for incorporating the effects of realistic string interactions in network evolution models with string junctions
Cosmic superstring trajectories in warped compactifications
We explore the generic motion of cosmic (super)strings when the internal
compact dimensions are warped, using the Klebanov-Strassler solution as a
prototypical throat geometry. We find that there is no dynamical mechanism
which localises the string at the tip of the throat, but rather that the motion
seems to explore both internal and external degrees of freedom democratically.
This indicates that cosmic (super)strings formed by inflationary
brane-antibrane annihilation will have sufficient internal motion for the
gravitational wave signals from the string network to be suppressed relative to
the signal from a `standard' cosmic string network.Comment: 31 pages, 8 figure
CMB constraints on cosmic strings and superstrings
We present the first complete MCMC analysis of cosmological models with evolving cosmic (super)string networks, using the Unconnected Segment Model in the unequal-time correlator formalism. For ordinary cosmic string networks, we derive joint constraints on ΛCDM and string network parameters, namely the string tension Gμ, the loop-chopping efficiency cr and the string wiggliness α. For cosmic superstrings, we obtain joint constraints on the fundamental string tension GμF, the string coupling gs, the self-interaction coefficient cs, and the volume of compact extra dimensions w. This constitutes the most comprehensive CMB analysis of ΛCDM cosmology + strings to date. For ordinary cosmic string networks our updated constraint on the string tension, obtained using Planck2015 temperature and polarisation data, is Gμ < 1.1 × 10^(−7) in relativistic units, while for cosmic superstrings our constraint on the fundamental string tension after marginalising over gs, cs and w is GμF < 2.8 × 10^(−8)
Cosmological effects of scalar-photon couplings: dark energy and varying-α models
We study cosmological models involving scalar fields coupled to radiation and discuss their effect on the redshift evolution of the cosmic microwave background temperature, focusing on links with varying fundamental constants and dynamical dark energy. We quantify how allowing for the coupling of scalar fields to photons, and its important effect on luminosity distances, weakens current and future constraints on cosmological parameters. In particular, for evolving dark energy models, joint constraints on the dark energy equation of state combining BAO radial distance and SN luminosity distance determinations, will be strongly dominated by BAO. Thus, to fully exploit future SN data one must also independently constrain photon number non-conservation arising from the possible coupling of SN photons to the dark energy scalar field. We discuss how observational determinations of the background temper- ature at different redshifts can, in combination with distance measures data, set tight constraints on interactions between scalar fields and photons, thus breaking this degeneracy. We also discuss prospects for future improvements, particularly in the context of Euclid and the E-ELT and show that Euclid can, even on its own, provide useful dark energy constraints while allowing for photon number non-conservation
Extending the velocity-dependent one-scale model for domain walls
We report on an extensive study of the evolution of domain wall networks in Friedmann-Lemaˆıtre- Robertson-Walker universes by means of the largest currently available field-theory simulations. These simulations were done in 40963 boxes and for a range of different fixed expansion rates, as well as for the transition between the radiation and matter eras. A detailed comparison with the velocity-dependent one-scale (VOS) model shows that this cannot accurately reproduce the results of the entire range of simulated regimes if one assumes that the phenomenological energy loss and momentum parameters are constants. We therefore discuss how a more accurate modeling of these parameters can be done, specifically by introducing an additional mechanism of energy loss (scalar radiation, which is particularly relevant for regimes with relatively little damping) and a modified momentum parameter which is a function of velocity (in analogy to what was previously done for cosmic strings). We finally show that this extended model, appropriately calibrated, provides an accurate fit to our simulations
Gravitational shocks as a key ingredient of Gamma-Ray Bursts
We identify a novel physical mechanism that may be responsible for energy
release in -ray bursts. Radial perturbations in the neutron core,
induced by its collision with collapsing outer layers during the early stages
of supernova explosions, can trigger a gravitational shock, which can readily
eject a small but significant fraction of the collapsing material at
ultra-relativistic speeds. The development of such shocks is a strong-field
effect arising in near-critical collapse in General Relativity and has been
observed in numerical simulations in various contexts, including in particular
radially perturbed neutron star collapse, albeit for a tiny range of initial
conditions. Therefore, this effect can be easily missed in numerical
simulations if the relevant parameter space is not exhaustively investigated.
In the proposed picture, the observed rarity of -ray bursts would be
explained if the relevant conditions for this mechanism appear in only about
one in every core collapse supernovae. We also mention the
possibility that near-critical collapse could play a role in powering the
central engines of Active Galactic Nuclei.Comment: 9 pages, 3 figure