1,944 research outputs found
Consistency relations and degeneracies in (non)commutative patch inflation
The consistency equations of patch inflation are considered in a
next-to-leading-order slow-roll (SR) expansion. Some general aspects of
braneworld degeneracy are pointed out, both with an ordinary scalar field and a
Born-Infeld tachyon. The discussion is then extended to the maximally symmetric
noncommutative case.Comment: 6 pages; v3: version to appear in Phys. Lett.
Observational effects from quantum cosmology
The status of quantum cosmologies as testable models of the early universe is
assessed in the context of inflation. While traditional Wheeler-DeWitt
quantization is unable to produce sizable effects in the cosmic microwave
background, the more recent loop quantum cosmology can generate potentially
detectable departures from the standard cosmic spectrum. Thus, present
observations constrain the parameter space of the model, which could be made
falsifiable by near-future experiments.Comment: 16 pages, 3 figures. Invited review article also containing original
material. v2: new section on holonomy corrections, introduction extended,
references added, typos corrected; v3: includes corrections of eqs. (3), (62)
and (64c) of the erratu
ABC of multi-fractal spacetimes and fractional sea turtles
We clarify what it means to have a spacetime fractal geometry in quantum
gravity and show that its properties differ from those of usual fractals. A
weak and a strong definition of multi-scale and multi-fractal spacetimes are
given together with a sketch of the landscape of multi-scale theories of
gravitation. Then, in the context of the fractional theory with
-derivatives, we explore the consequences of living in a multi-fractal
spacetime. To illustrate the behavior of a non-relativistic body, we take the
entertaining example of a sea turtle. We show that, when only the time
direction is fractal, sea turtles swim at a faster speed than in an ordinary
world, while they swim at a slower speed if only the spatial directions are
fractal. The latter type of geometry is the one most commonly found in quantum
gravity. For time-like fractals, relativistic objects can exceed the speed of
light, but strongly so only if their size is smaller than the range of
particle-physics interactions. We also find new results about log-oscillating
measures, the measure presentation and their role in physical observations and
in future extensions to nowhere-differentiable stochastic spacetimes.Comment: 20 pages, 1 figure. v2: typos corrected, minor improvements of the
tex
Complex dimensions and their observability
We show that the dimension of spacetime becomes complex-valued when its
short-scale geometry is invariant under a discrete scaling symmetry. This
characteristic can generically arise in quantum gravities, for instance, in
those based on combinatorial or multifractal structures or as the partial
breaking of continuous dilation symmetry in any conformal-invariant theory.
With its infinite scale hierarchy, discrete scale invariance overlaps with the
traditional separation between ultraviolet and infrared physics and it can
leave an all-range observable imprint, such as a pattern of log oscillations
and sharp features in the cosmic microwave background primordial power
spectrum.Comment: 6 pages, 1 figure. v2: discussion slightly expande
Lorentz violations in multifractal spacetimes
Using the recent observation of gravitational waves (GW) produced by a
black-hole merger, we place a lower bound on the energy above which a
multifractal spacetime would display an anomalous geometry and, in particular,
violations of Lorentz invariance. In the so-called multifractional theory with
-derivatives, we show that the deformation of dispersion relations is much
stronger than in generic quantum-gravity approaches (including loop quantum
gravity) and, contrary to the latter, present observations on GWs can place
very strong bounds on the characteristic scales at which spacetime deviates
from standard Minkowski. The energy at which multifractal effects should become
apparent is (thus improving previous bounds by 12
orders of magnitude) when the exponents in the measure are fixed to their
central value . We also estimate, for the first time, the effect of
logarithmic oscillations in the measure (corresponding to a discrete spacetime
structure) and find that they do not change much the bounds obtained in their
absence, unless the amplitude of the oscillations is fine tuned. This feature,
unavailable in known quantum-gravity scenarios, may help the theory to avoid
being ruled out by gamma-ray burst (GRB) observations, for which or greater.Comment: 12 pages, 1 figure. v2: discussion expanded at several points,
comparison with the Lorentz-violating Standard-Model extension added,
references adde
Detecting quantum gravity in the sky
Getting signatures of quantum gravity is one of the topical lines of research
in modern theoretical physics and cosmology. This short review faces this
challenge under a novel perspective. Instead of separating quantum-gravity
effects of a specific model between UV and IR regimes, we consider a general
feature, possibly common to many frameworks, where all scales are affected and
spacetime geometry is characterized by a complex critical exponent. This leaves
a log-oscillating modulation pattern in the cosmic microwave background
spectrum and gives a unique opportunity, illustrated with the example of a
multi-fractional theory, to test quantum gravities at cosmological scales.Comment: 4 pages, 1 figure. In Proceedings of EPS-HEP 2017, European Physical
Society conference on High Energy Physics, July 5-12, 2017, Venice, Italy.
v2: minor corrections to match the published version
(https://pos.sissa.it/314/033/
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