676 research outputs found
Adiabatic renormalization in theories with modified dispersion relations
We generalize the adiabatic renormalization to theories with dispersion
relations modified at energies higher than a new scale . We obtain
explicit expressions for the mean value of the stress tensor in the adiabatic
vacuum, up to the second adiabatic order. We show that for any dispersion
relation the divergences can be absorbed into the bare gravitational constants
of the theory. We also point out that, depending on the renormalization
prescription, the renormalized stress tensor may contain finite trans-Planckian
corrections even in the limit .Comment: Typos corrected; to appear in the Proceedings of IRGAC 06, Journal of
Physics
Holographic fermions in external magnetic fields
We study the Fermi level structure of 2+1-dimensional strongly interacting
electron systems in external magnetic field using the AdS/CFT correspondence.
The gravity dual of a finite density fermion system is a Dirac field in the
background of the dyonic AdS-Reissner-Nordstrom black hole. In the probe limit
the magnetic system can be reduced to the non-magnetic one, with
Landau-quantized momenta and rescaled thermodynamical variables. We find that
at strong enough magnetic fields, the Fermi surface vanishes and the
quasiparticle is lost either through a crossover to conformal regime or through
a phase transition to an unstable Fermi surface. In the latter case, the
vanishing Fermi velocity at the critical magnetic field triggers the non-Fermi
liquid regime with unstable quasiparticles and a change in transport properties
of the system. We associate it with a metal-"strange metal" phase transition.
Next we compute the DC Hall and longitudinal conductivities using the
gravity-dressed fermion propagators. For dual fermions with a large charge,
many different Fermi surfaces contribute and the Hall conductivity is quantized
as expected for integer Quantum Hall Effect (QHE). At strong magnetic fields,
as additional Fermi surfaces open up, new plateaus typical for the fractional
QHE appear. The somewhat irregular pattern in the length of fractional QHE
plateaus resemble the outcomes of experiments on thin graphite in a strong
magnetic field. Finally, motivated by the absence of the sign problem in
holography, we suggest a lattice approach to the AdS calculations of finite
density systems.Comment: 34 pages, 14 figure
Lorentz-violation and cosmological perturbations: a toy brane-world model
We study possible effects of Lorentz-violation on the generation of
cosmological perturbations at inflation by introducing a simple inflating
five-dimensional brane-world setup with violation of four-dimensional
Lorentz-invariance at an energy scale . We consider massless scalar field,
meant to mimic perturbations of inflaton and/or gravitational field, in this
background. At three-momenta below , there exists a zero mode localized on
the brane, whose behaviour coincides with that in four-dimensional theory. On
the contrary, at three-momenta above , the localized mode is absent and
physics is entirely five-dimensional. As three-momenta get redshifted, more
modes get localized on the brane, the phenomenon analogous to ``mode
generation''. We find that for , where is the inflationary Hubble
scale, the spectrum of perturbations coincides with that in four-dimensional
theory. For and time-dependent bulk parameters, the spectrum deviates,
possibly strongly, from the flat spectrum even for pure de Sitter inflation.Comment: 5 figures, iopart, minor changes, appendix adde
Incoherent thermal transport from dirty black holes.
Theoretical Physic
Where does Cosmological Perturbation Theory Break Down?
We apply the effective field theory approach to the coupled metric-inflaton
system, in order to investigate the impact of higher dimension operators on the
spectrum of scalar and tensor perturbations in the short-wavelength regime. In
both cases, effective corrections at tree-level become important when the
Hubble parameter is of the order of the Planck mass, or when the physical wave
number of a cosmological perturbation mode approaches the square of the Planck
mass divided by the Hubble constant. Thus, the cut-off length below which
conventional cosmological perturbation theory does not apply is likely to be
much smaller than the Planck length. This has implications for the
observability of "trans-Planckian" effects in the spectrum of primordial
perturbations.Comment: 25 pages, uses FeynM
Effects of Nonlinear Dispersion Relations on Non-Gaussianities
We investigate the effect of non-linear dispersion relations on the
bispectrum. In particular, we study the case were the modified relations do not
violate the WKB condition at early times, focusing on a particular example
which is exactly solvable: the Jacobson-Corley dispersion relation with quartic
correction with positive coefficient to the squared linear relation. We find
that the corrections to the standard result for the bispectrum are suppressed
by a factor where is the scale where the modification
to the dispersion relation becomes relevant. The modification is {\it mildly}
configuration-dependent and equilateral configurations are more suppressed with
respect to the local ones, by a factor of one percent. There is no
configuration leading to enhancements. We then analyze the results in the
framework of particle creation using the approximate gluing method of
Brandenberger and Martin, which relates more directly to the modeling of the
trans-Planckian physics via modifications of the vacuum at a certain cutoff
scale. We show that the gluing method overestimates the leading order
correction to the spectrum and bispectrum by one and two orders, respectively,
in . We discuss the various approximation and conclude that for
dispersion relations not violating WKB at early times the particle creation is
small and does not lead to enhanced contributions to the bispectrum. We also
show that in many cases enhancements do not occur when modeling the
trans-Planckian physics via modifications of the vacuum at a certain cutoff
scale. Most notably they are only of order O(1) when the Bogolyubov
coefficients accounting for particle creation are determined by the Wronskian
condition and the minimization of the uncertainty between the field and its
conjugate momentum.Comment: v1: 11 pages, 2 figures; v2: references update
Finding Nemo’s clock reveals switch from nocturnal to diurnal activity
Timing mechanisms play a key role in the biology of coral reef fish. Typically, fish larvae leave their reef after hatching, stay for a period in the open ocean before returning to the reef for settlement. During this dispersal, larvae use a time-compensated sun compass for orientation. However, the timing of settlement and how coral reef fish keep track of time via endogenous timing mechanisms is poorly understood. Here, we have studied the behavioural and genetic basis of diel rhythms in the clown anemonefish Amphiprion ocellaris. We document a behavioural shift from nocturnal larvae to diurnal adults, while juveniles show an intermediate pattern of activity which potentially indicates flexibility in the timing of settlement on a host anemone. qRTPCR analysis of six core circadian clock genes (bmal1, clocka, cry1b, per1b, per2, per3) reveals rhythmic gene expression patterns that are comparable in larvae and juveniles, and so do not reflect the corresponding activity changes. By establishing an embryonic cell line, we demonstrate that clown anemonefish possess an endogenous clock with similar properties to that of the zebrafish circadian clock. Furthermore, our study provides a first basis to study the multi-layered interaction of clocks from fish, anemones and their zooxanthellae endosymbionts
Boundary Effective Field Theory and Trans-Planckian Perturbations: Astrophysical Implications
We contrast two approaches to calculating trans-Planckian corrections to the
inflationary perturbation spectrum: the New Physics Hypersurface [NPH] model,
in which modes are normalized when their physical wavelength first exceeds a
critical value, and the Boundary Effective Field Theory [BEFT] approach, where
the initial conditions for all modes are set at the same time, and modified by
higher dimensional operators enumerated via an effective field theory
calculation. We show that these two approaches -- as currently implemented --
lead to radically different expectations for the trans-Planckian corrections to
the CMB and emphasize that in the BEFT formalism we expect the perturbation
spectrum to be dominated by quantum gravity corrections for all scales shorter
than some critical value. Conversely, in the NPH case the quantum effects only
dominate the longest modes that are typically much larger than the present
horizon size. Furthermore, the onset of the breakdown in the standard
inflationary perturbation calculation predicted by the BEFT formalism is likely
to be associated with a feature in the perturbation spectrum, and we discuss
the observational signatures of this feature in both CMB and large scale
structure observations. Finally, we discuss possible modifications to both
calculational frameworks that would resolve the contradictions identified here.Comment: Reworded commentary, reference added (v2) References added (v3
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