39 research outputs found
Zero-point length, extra-dimensions and string T-duality
In this paper, we are going to put in a single consistent framework
apparently unrelated pieces of information, i.e. zero-point length,
extra-dimensions, string T-duality. More in details we are going to introduce a
modified Kaluza-Klein theory interpolating between (high-energy) string theory
and (low-energy) quantum field theory. In our model zero-point length is a four
dimensional ``virtual memory'' of compact extra-dimensions length scale. Such a
scale turns out to be determined by T-duality inherited from the underlying
fundamental string theory. From a low energy perspective short distance
infinities are cut off by a minimal length which is proportional to the square
root of the string slope, i.e. \sqrt{\alpha^\prime}. Thus, we provide a
``bridge'' between the ultra-relativistic string domain and the low energy
arena of point-particle quantum field theory.Comment: 28 pages, Latex, no figures; two references adde
Tackling Higher Derivative Ghosts with the Euclidean Path Integral
An alternative to the effective field theory approach to treat ghosts in
higher derivative theories is to attempt to integrate them out via the
Euclidean path integral formalism. It has been suggested that this method could
provide a consistent framework within which we might tolerate the ghost degrees
of freedom that plague, among other theories, the higher derivative gravity
models that have been proposed to explain cosmic acceleration. We consider the
extension of this idea to treating a class of terms with order six derivatives,
and find that for a general term the Euclidean path integral approach works in
the most trivial background, Minkowski. Moreover we see that even in de Sitter
background, despite some difficulties, it is possible to define a probability
distribution for tensorial perturbations of the metric.Comment: 21 page
Higher Derivative Gravitational Systems and Ghost Fields
Effective Field Theory (EFT) is one of the most powerful theoretical tools in the hands of cosmologists, it allows them to come up with testable effective descriptions of the universe even when a fundamental theory is missing. EFT though, is not the only possible answer for pushing our knowledge beyond the limits of what has already been established. Applying EFT and other alternative methods has become an important part of a cosmologist’s work, particularly in the last few years when a vast plethora of extensions of the Standard Model of Cosmology has been proposed and needs to be tested against experimental results. In this work we mainly investigate the limits of EFT in the context of cosmic acceleration, and the possibility of calculating corrections to the low energy standard cosmological results by re-interpreting the meaning of higher derivative terms in perturbation expansions
Can Cosmic Parallax Distinguish Between Anisotrophic Cosmologies?
In an anisotropic universe, observers not positioned at a point of special symmetry should observe cosmic parallax—the relative angular motion of test galaxies over cosmic time. It was recently argued that the nonobservance of this effect in upcoming precision astrometry missions such as GAIA may be used to place strong bounds on the position of off-center observers in a void-model universe described by the Lemaitre-Tolman-Bondi metric. We consider the analogous effect in anisotropic cosmological models described by an axisymmetric homogeneous Bianchi type I metric and discuss whether any observation of cosmic parallax would distinguish between different anisotropic evolutions
Realistic fluids as source for dynamically accreting black holes in a cosmological background
We show that a single imperfect fluid can be used as a source to obtain the
generalized McVittie metric as an exact solution to Einstein's equations. The
mass parameter in this metric varies with time thanks to a mechanism based on
the presence of a temperature gradient. This fully dynamical solution is
interpreted as an accreting black hole in an expanding universe if the metric
asymptotes to Schwarzschild-de Sitter at temporal infinity. We present a simple
but instructive example for the mass function and briefly discuss the structure
of the apparent horizons and the past singularity.Comment: 5 pages, 2 figures. Updated references and minor changes to match the
version accepted for publishing in PR
Can Cosmic Parallax Distinguish Between Anisotropic Cosmologies?
In an anisotropic universe, observers not positioned at a point of special
symmetry should observe cosmic parallax - the relative angular motion of test
galaxies over cosmic time. It was recently argued that the non-observance of
this effect in upcoming precision astrometry missions such as Gaia may be used
to place strong bounds on the position of off-center observers in a void-model
universe described by the Lemaitre-Tolman-Bondi metric. We consider the
analogous effect in anisotropic cosmological models described by an
axisymmetric homogeneous Bianchi type I metric and discuss whether any
observation of cosmic parallax would distinguish between different anisotropic
evolutions.Comment: 24 pages, 6 figure
Zero-point length from string fluctuations
One of the leading candidates for quantum gravity, viz. string theory, has
the following features incorporated in it. (i) The full spacetime is higher
dimensional, with (possibly) compact extra-dimensions; (ii) There is a natural
minimal length below which the concept of continuum spacetime needs to be
modified by some deeper concept. On the other hand, the existence of a minimal
length (or zero-point length) in four-dimensional spacetime, with obvious
implications as UV regulator, has been often conjectured as a natural aftermath
of any correct quantum theory of gravity. We show that one can incorporate the
apparently unrelated pieces of information - zero-point length,
extra-dimensions, string T-duality - in a consistent framework. This is done in
terms of a modified Kaluza-Klein theory that interpolates between (high-energy)
string theory and (low-energy) quantum field theory. In this model, the
zero-point length in four dimensions is a ``virtual memory'' of the length
scale of compact extra-dimensions.
Such a scale turns out to be determined by T-duality inherited from the
underlying fundamental string theory. From a low energy perspective short
distance infinities are cut off by a minimal length which is proportional to
the square root of the string slope, i.e. \sqrt{\alpha^\prime}. Thus, we bridge
the gap between the string theory domain and the low energy arena of
point-particle quantum field theory.Comment: 7 pages, Latex, no figures, one reference adde