115 research outputs found
A braneworld puzzle about entropy bounds and a maximal temperature
Entropy bounds applied to a system of N species of light quantum fields in
thermal equilibrium at temperature T are saturated in four dimensions at a
maximal temperature T_max=M_Planck/N^1/2. We show that the correct setup for
understanding the reason for the saturation is a cosmological setup, and that a
possible explanation is the copious production of black holes at this maximal
temperature. The proposed explanation implies, if correct, that N light fields
cannot be in thermal equilibrium at temperatures T above T_max. However, we
have been unable to identify a concrete mechanism that is efficient and quick
enough to prevent the universe from exceeding this limiting temperature. The
same issues can be studied in the framework of AdS/CFT by using a brane moving
in a five dimensional AdS-Schwarzschild space to model a radiation dominated
universe. In this case we show that T_max is the temperature at which the brane
just reaches the horizon of the black hole, and that entropy bounds and the
generalized second law of thermodynamics seem to be violated when the brane
continues to fall into the black hole. We find, again, that the known physical
mechanisms, including black hole production, are not efficient enough to
prevent the brane from falling into the black hole. We propose several possible
explanations for the apparent violation of entropy bounds, but none is a
conclusive one.Comment: 16 page
Cosmological dynamics and dark energy from non-local infrared modifications of gravity
We perform a detailed study of the cosmological dynamics of a recently
proposed infrared modification of the Einstein equations, based on the
introduction of a non-local term constructed with ,
where is a mass parameter. The theory generates automatically a dynamical
dark energy component, that can reproduce the observed value of the dark energy
density without introducing a cosmological constant. Fixing so to reproduce
the observed value , and writing , the model provides a neat prediction for the equation of state parameters
of dark energy, and . We show that,
because of some freedom in the definition of , one can extend the
construction so to define a more general family of non-local models. However,
in a first approximation this turns out to be equivalent to adding an explicit
cosmological constant term on top of the dynamical dark energy component. This
leads to an extended model with two parameters, and .
Even in this extension the EOS parameter is always on the phantom side,
in the range , and there is a prediction for the relation
between and .Comment: 30 pages, 15 figures; v2: cross-reference to 1311.3421 adde
Supersymmetric Vacuum Configurations in String Cosmology
We examine in a cosmological context the conditions for unbroken
supersymmetry in N=1 supergravity in D=10 dimensions. We show that the
cosmological solutions of the equations of motion obtained considering only the
bosonic sector correspond to vacuum states with spontaneous supersymmetry
breaking. With a non vanishing gravitino-dilatino condensate we find a solution
of the equations of motion that satisfies necessary conditions for unbroken
supersymmetry and that smoothly interpolates between Minkowski space and
DeSitter space with a linearly growing dilaton, thus providing a possible
example of a supersymmetric and non-singular pre-big-bang cosmology.Comment: 4 pages, Latex, 2 figure
Apparent ghosts and spurious degrees of freedom in non-local theories
Recent work has shown that non-local modifications of the Einstein equations
can have interesting cosmological consequences and can provide a dynamical
origin for dark energy, consistent with existing data. At first sight these
theories are plagued by ghosts. We show that these apparent ghost-like
instabilities do not describe actual propagating degrees of freedom, and there
is no issue of ghost-induced quantum vacuum decay.Comment: 18 pages, 3 figures. v2: cross-reference to 1311.3435 adde
Effective field theory approach to the gravitational two-body dynamics, at fourth post-Newtonian order and quintic in the Newton constant
Working within the post-Newtonian (PN) approximation to General Relativity,
we use the effective field theory (EFT) framework to study the conservative
dynamics of the two-body motion at fourth PN order, at fifth order in the
Newton constant. This is one of the missing pieces preventing the computation
of the full Lagrangian at fourth PN order using EFT methods. We exploit the
analogy between diagrams in the EFT gravitational theory and 2-point functions
in massless gauge theory, to address the calculation of 4-loop amplitudes by
means of standard multi-loop diagrammatic techniques. For those terms which can
be directly compared, our result confirms the findings of previous studies,
performed using different methods.Comment: Version accepted for publication in Phys. Rev. D. Appendix C added
with details of amplitude computation
The gravitational-wave luminosity distance in modified gravity theories
In modified gravity the propagation of gravitational waves (GWs) is in
general different from that in general relativity. As a result, the luminosity
distance for GWs can differ from that for electromagnetic signals, and is
affected both by the dark energy equation of state and by a
function describing modified propagation. We show that the effect
of modified propagation in general dominates over the effect of the dark energy
equation of state, making it easier to distinguish a modified gravity model
from CDM. We illustrate this using a nonlocal modification of gravity,
that has been shown to fit remarkably well CMB, SNe, BAO and structure
formation data, and we discuss the prospects for distinguishing nonlocal
gravity from CDM with the Einstein Telescope. We find that, depending
on the exact sensitivity, a few tens of standard sirens with measured redshift
at , or a few hundreds at , could suffice.Comment: 6 pages, 3 figures; v4: minor modifications; the version to appear in
PR
Modified gravitational-wave propagation and standard sirens
Studies of dark energy at advanced gravitational-wave (GW) interferometers
normally focus on the dark energy equation of state . However,
modified gravity theories that predict a non-trivial dark energy equation of
state generically also predict deviations from general relativity in the
propagation of GWs across cosmological distances, even in theories where the
speed of gravity is equal to . We find that, in generic modified gravity
models, the effect of modified GW propagation dominates over that of , making modified GW propagation a crucial observable for dark energy
studies with standard sirens. We present a convenient parametrization of the
effect in terms of two parameters , analogue to the
parametrization of the dark energy equation of state, and we give a limit from
the LIGO/Virgo measurement of with the neutron star binary GW170817. We
then perform a Markov Chain Monte Carlo analysis to estimate the sensitivity of
the Einstein Telescope (ET) to the cosmological parameters, including
, both using only standard sirens, and combining them with other
cosmological datasets. In particular, the Hubble parameter can be measured with
an accuracy better than already using only standard sirens while, when
combining ET with current CMB+BAO+SNe data, can be measured to
. We discuss the predictions for modified GW propagation of a specific nonlocal
modification of gravity, recently developed by our group, and we show that they
are within the reach of ET. Modified GW propagation also affects the GW
transfer function, and therefore the tensor contribution to the ISW effect.Comment: 25 pages, 23 figures: v3: several significant improvement
Sensitivity of a small matter-wave interferometer to gravitational waves
We study the possibility of using matter wave interferometry techniques to
build a gravitational wave detector. We derive the response function and find
that it contains a term proportional to the derivative of the gravitational
wave, a point which has been disputed recently. We then study in detail the
sensitivity that can be reached by such a detector and find that, if it is
operated near resonance, it can reach potentially interesting values in the
high frequency regime. The correlation between two or more of such devices can
further improve the sensitivity for a stochastic signal.Comment: Revtex4, 19 pages, 3 figures. Several changes in the calculation of
the response function. Accepted by PR
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