216 research outputs found
On the renormalization procedure for quantum fields with modified dispersion relation in curved spacetimes
We review our recent results on the renormalization procedure for a free
quantum scalar field with modified dispersion relations in curved spacetimes.
For dispersion relations containing up to powers of the spatial momentum,
the subtraction necessary to renormalize depends
on . We first describe our previous analysis for spatially flat
Friedman-Robertson-Walker and Bianchi type I metrics. Then we present a new
power counting analysis for general background metrics in the weak field
approximation.Comment: Talk given at the 7th Alexander Friedmann International Seminar on
Gravitation and Cosmology, Joao Pessoa, Brazil, July 200
Superconductivity in carbon nanotubes coupled to transition metal atoms
The electronic structures of zig-zag and arm-chair single-walled carbon
nanotubes interacting with a transitional-metal atomic nanowire of Ni have been
determined. The Ni nanowire creates a large electron density of states (DOS)at
the Fermi energy. The dependence of the enhanced DOS on the spin state and
positioning of the transition-metal wire(inside or outside the nanotube) is
studied. Preliminary estimates of the electron-phonon interaction suggest that
such systems may have a superconducting transition temperature of 10-50
K. The signs of superconductivity seen in ``ropes'' of nanotubes may also be
related to the effect of intrinsic transition-metal impurities.Comment: 4 pages and two figure
Nonlinear effects of dark energy clustering beyond the acoustic scales
We extend the resummation method of Anselmi & Pietroni (2012) to compute the
total density power spectrum in models of quintessence characterized by a
vanishing speed of sound. For standard CDM cosmologies, this
resummation scheme allows predictions with an accuracy at the few percent level
beyond the range of scales where acoustic oscillations are present, therefore
comparable to other, common numerical tools. In addition, our theoretical
approach indicates an approximate but valuable and simple relation between the
power spectra for standard quintessence models and models where scalar field
perturbations appear at all scales. This, in turn, provides an educated guess
for the prediction of nonlinear growth in models with generic speed of sound,
particularly valuable since no numerical results are yet available.Comment: 28 pages, 12 figure
Vector fuzzy dark matter, fifth forces, and binary pulsars
We study the secular effects that an oscillating background ultralight (fuzzy) cosmological vector field has on the dynamics of binary systems; such effects appear when the field and the binary are in resonance. We first consider the gravitational interaction between the field and the systems, and quantify the main differences with an oscillating background scalar field. If the energy density of such a field is sufficiently large, as required if it is supposed to be all of the dark matter, we show that the secular effects could yield potentially observable signatures in high precision time of arrival measurements of binary pulsars. We then analyse the secular effects that arise when the field is directly coupled to the bodies in the binary. We show that this study is particularly relevant for models where fuzzy dark matter mediates a baryonic force B (or B-L, with L the lepton number), due to the stellar amount of nucleons present in the stars. The constraints we obtain from current data are already competitive with (or even more constraining than) laboratory tests of the equivalence principle.Fil: Lopez Nacir, Diana Laura. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Urban, Federico R.. Czech Academy of Sciences; República Chec
Extreme parameter sensitivity in quasidilaton massive gravity
We reanalyze the behavior of Friedmann-Lema\^itre-Robertson-Walker
cosmologies in the recently proposed quasidilaton massive-gravity model, and
discover that the background dynamics present hitherto unreported features that
require unexpected fine-tuning of the additional fundamental parameters of the
theory for an observationally consistent background cosmology. We also identify
new allowed regions in the parameters space and exclude some of the previously
considered ones. The evolution of the mass of gravitational waves reveals
non-trivial behavior, exhibiting a mass squared that may be negative in the
past, and that presently, while positive, is larger than the square of the
Hubble parameter. These properties of the gravity-wave mass have the potential
to lead to observational tests of the theory. While quasidilaton massive
gravity is known to have issues with stability at short distances, the current
analysis is a first step toward the investigation of the more stable extended
quasidilaton massive gravity theory, with some expectation that both the
fine-tuning of parameters and the interesting behavior of the gravity-wave mass
will persist.Comment: 10 pages, 7 figure
Renormalized Stress Tensor for trans-Planckian Cosmology
Finite expressions for the mean value of the stress tensor corresponding to a
scalar field with a generalized dispersion relation in a
Friedman--Robertson--Walker universe are obtained using adiabatic
renormalization. Formally divergent integrals are evaluated by means of
dimensional regularization. The renormalization procedure is shown to be
equivalent to a redefinition of the cosmological constant and the Newton
constant in the semiclassical Einstein equations.Comment: 14 pages. Minor changes; version published in Physical Review
model in Euclidean de Sitter space: beyond the leading infrared approximation
We consider an scalar field model with quartic interaction in
-dimensional Euclidean de Sitter space. In order to avoid the problems of
the standard perturbative calculations for light and massless fields, we
generalize to the theory a systematic method introduced previously for a
single field, which treats the zero modes exactly and the nonzero modes
perturbatively. We compute the two-point functions taking into account not only
the leading infrared contribution, coming from the self-interaction of the zero
modes, but also corrections due to the interaction of the ultraviolet modes.
For the model defined in the corresponding Lorentzian de Sitter spacetime, we
obtain the two-point functions by analytical continuation. We point out that a
partial resummation of the leading secular terms (which necessarily involves
nonzero modes) is required to obtain a decay at large distances for massless
fields. We implement this resummation along with a systematic double expansion
in an effective coupling constant and in 1/N. We explicitly
perform the calculation up to the next-to-next-to-leading order in
and up to next-to-leading order in 1/N. The results reduce to
those known in the leading infrared approximation. We also show that they
coincide with the ones obtained directly in Lorentzian de Sitter spacetime in
the large N limit, provided the same renormalization scheme is used.Comment: 31 pages, 5 figures. Minor changes. Published versio
Renormalization in theories with modified dispersion relations: weak gravitational fields
We consider a free quantum scalar field satisfying modified dispersion
relations in curved spacetimes, within the framework of Einstein-Aether theory.
Using a power counting analysis, we study the divergences in the adiabatic
expansion of and , working in the weak field
approximation. We show that for dispersion relations containing up to
powers of the spatial momentum, the subtraction necessary to renormalize these
two quantities on general backgrounds depends on in a qualitatively
different way: while becomes convergent for a sufficiently large value
of , the number of divergent terms in the adiabatic expansion of
increases with . This property was not apparent in previous
results for spatially homogeneous backgrounds.Comment: 12 pages, no figure
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