1,313 research outputs found
Wormholes in spacetimes with cosmological horizons
A generalisation of the asymptotic wormhole boundary condition for the case
of spacetimes with a cosmological horizon is proposed. In particular, we
consider de Sitter spacetime with small cosmological constant. The wave
functions selected by this proposal are exponentially damped in WKB
approximation when the scale factor is large but still much smaller than the
horizon size. In addition, they only include outgoing gravitational modes in
the region beyond the horizon. We argue that these wave functions represent
quantum wormholes and compute the local effective interactions induced by them
in low-energy field theory. These effective interactions differ from those for
flat spacetime in terms that explicitly depend on the cosmological constant.Comment: 10 pages, LaTeX 2.O9, no figure
The envelope of the power spectra of over a thousand \delta Scuti stars. The - scaling relation
CoRoT and Kepler high-precision photometric data allowed the detection and
characterization of the oscillation parameters in stars other than the Sun.
Moreover, thanks to the scaling relations, it is possible to estimate masses
and radii for thousands of solar-type oscillating stars. Recently, a \Delta\nu
- \rho relation has been found for \delta Scuti stars. Now, analyzing several
hundreds of this kind of stars observed with CoRoT and Kepler, we present an
empiric relation between their frequency at maximum power of their oscillation
spectra and their effective temperature. Such a relation can be explained with
the help of the \kappa-mechanism and the observed dispersion of the residuals
is compatible with they being caused by the gravity-darkening effect
Phonon background versus analogue Hawking radiation in Bose-Einstein condensates
We determine the feasibility of detecting analogue Hawking radiation in a
Bose-Einstein condensate in the presence of atom loss induced heating. We find
that phonons created by three-body losses overshadow those due to analogue
Hawking radiation. To overcome this problem, three-body losses may have to be
suppressed, for example as proposed by Search et al. [Phys. Rev. Lett. 92
140401 (2004)]. The reduction of losses to a few percent of their normal rate
is typically sufficient to suppress the creation of loss phonons on the time
scale of a fast analogue Hawking phonon detection.Comment: 4 pages, no figures, revised versio
Analogue model for quantum gravity phenomenology
So called "analogue models" use condensed matter systems (typically
hydrodynamic) to set up an "effective metric" and to model curved-space quantum
field theory in a physical system where all the microscopic degrees of freedom
are well understood. Known analogue models typically lead to massless minimally
coupled scalar fields. We present an extended "analogue space-time" programme
by investigating a condensed-matter system - in and beyond the hydrodynamic
limit - that is in principle capable of simulating the massive Klein-Gordon
equation in curved spacetime. Since many elementary particles have mass, this
is an essential step in building realistic analogue models, and an essential
first step towards simulating quantum gravity phenomenology. Specifically, we
consider the class of two-component BECs subject to laser-induced transitions
between the components, and we show that this model is an example for Lorentz
invariance violation due to ultraviolet physics. Furthermore our model suggests
constraints on quantum gravity phenomenology in terms of the "naturalness
problem" and "universality issue".Comment: Talk given at 7th Workshop on Quantum Field Theory Under the
Influence of External Conditions (QFEXT 05), Barcelona, Catalonia, Spain, 5-9
Sep 200
Entropy bounds in terms of the w parameter
In a pair of recent articles [PRL 105 (2010) 041302 - arXiv:1005.1132; JHEP
1103 (2011) 056 - arXiv:1012.2867] two of the current authors have developed an
entropy bound for equilibrium uncollapsed matter using only classical general
relativity, basic thermodynamics, and the Unruh effect. An odd feature of that
bound, S <= A/2, was that the proportionality constant, 1/2, was weaker than
that expected from black hole thermodynamics, 1/4. In the current article we
strengthen the previous results by obtaining a bound involving the (suitably
averaged) w parameter. Simple causality arguments restrict this averaged
parameter to be <= 1. When equality holds, the entropy bound saturates at the
value expected based on black hole thermodynamics. We also add some clarifying
comments regarding the (net) positivity of the chemical potential. Overall, we
find that even in the absence of any black hole region, we can nevertheless get
arbitrarily close to the Bekenstein entropy.Comment: V1: 14 pages. V2: One reference added. V3: This version accepted for
publication in JHE
Acoustic geometry for general relativistic barotropic irrotational fluid flow
"Acoustic spacetimes", in which techniques of differential geometry are used
to investigate sound propagation in moving fluids, have attracted considerable
attention over the last few decades. Most of the models currently considered in
the literature are based on non-relativistic barotropic irrotational fluids,
defined in a flat Newtonian background. The extension, first to special
relativistic barotropic fluid flow, and then to general relativistic barotropic
fluid flow in an arbitrary background, is less straightforward than it might at
first appear. In this article we provide a pedagogical and simple derivation of
the general relativistic "acoustic spacetime" in an arbitrary (d+1) dimensional
curved-space background.Comment: V1: 23 pages, zero figures; V2: now 24 pages, some clarifications, 2
references added. This version accepted for publication in the New Journal of
Physics. (Special issue on "Classical and Quantum Analogues for Gravitational
Phenomena and Related Effects"
Evidence of amplitude modulation due to Resonant Mode Coupling in the delta Scuti star KIC5892969
A study of the star KIC5892969 observed by the Kepler satellite is presented.
Its three highest amplitude modes present a strong amplitude modulation. The
aim of this work is to investigate amplitude variations in this star and their
possible cause. Using the 4 years-long observations available, we obtained the
frequency content of the full light curve. Then, we studied the amplitude and
phase variations with time using shorter time stamps. The results obtained are
compared with the predicted ones for resonant mode coupling of an unstable mode
with lower frequency stable modes. Our conclusion is that resonant mode
coupling is consistent as an amplitude limitation mechanism in several modes of
KIC5892969 and we discuss to which extent it might play an important role for
other delta Scuti stars
Quasi-particle creation by analogue black holes
We discuss the issue of quasi-particle production by ``analogue black holes''
with particular attention to the possibility of reproducing Hawking radiation
in a laboratory. By constructing simple geometric acoustic models, we obtain a
somewhat unexpected result: We show that in order to obtain a stationary and
Planckian emission of quasi-particles, it is not necessary to create an
ergoregion in the acoustic spacetime (corresponding to a supersonic regime in
the flow). It is sufficient to set up a dynamically changing flow either
eventually generating an arbitrarily small sonic region v=c, but without any
ergoregion, or even just asymptotically, in laboratory time, approaching a
sonic regime with sufficient rapidity.Comment: 30 pages, 16 figure
Evidence of chaotic modes in the analysis of four delta Scuti stars
Since CoRoT observations unveiled the very low amplitude modes that form a
flat plateau in the power spectrum structure of delta Scuti stars, the nature
of this phenomenon, including the possibility of spurious signals due to the
light curve analysis, has been a matter of long-standing scientific debate. We
contribute to this debate by finding the structural parameters of a sample of
four delta Scuti stars, CID 546, CID 3619, CID 8669, and KIC 5892969, and
looking for a possible relation between these stars' structural parameters and
their power spectrum structure. For the purposes of characterization, we
developed a method of studying and analysing the power spectrum with high
precision and have applied it to both CoRoT and Kepler light curves. We obtain
the best estimates to date of these stars' structural parameters. Moreover, we
observe that the power spectrum structure depends on the inclination,
oblateness, and convective efficiency of each star. Our results suggest that
the power spectrum structure is real and is possibly formed by 2-period island
modes and chaotic modes
A Biolinguistic Approach to the Vocalizations of H. Neanderthalensis and the Genus Homo
This paper revisits the old question about the possibilities and aptitudes H. neanderthalensis had for vocalization. New evidence will be discussed that moves the discussion beyond traditional interest in the presence and inter-pretation of the fossil record and its comparison with the closest species to H. sapiens, like chimpanzees and gorillas. An interdisciplinary perspective on the analysis coupled with information gathered from neuropsychology, genetics, and comparative psychology will prove useful for obtaining a new vision in biolinguistics, so that neurocognitive activity becomes important thanks, above all, to the comparison with other species. Finally, it will be argued that it is still plausible to accept the hypothesis on the formation of a vocal capacity prior to the cognitive faculty of language, and independent of it, so that Neandertals were probably able of vocalizing voluntarily, with communicative intentions and in a sophisticated way
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