37,761 research outputs found
Effective Field Theory Methods in Gravitational Physics and Tests of Gravity
In this PhD thesis I make use of the "Effective Field Theory of Gravity for
Extended Objects" by Goldberger and Rothstein in order to investigate theories
of gravity and to take a different point of view on the physical information
that can be extracted from experiments. In the first work I present, I study a
scalar-tensor theory of gravity and I address the renormalization of the
energy-momentum tensor for point-like and string-like sources. The second and
third study I report are set in the context of testing gravity. So far
experiments have probed dynamical regimes only up to order (v/c)^5 in the
post-Newtonian expansion, which corresponds to the very first term of the
radiative sector in General Relativity. In contrast, by means of
gravitational-wave astronomy, one aims at testing General Relativity up to
(v/c)^(12)! It is then relevant to envisage testing frameworks which are
appropriate to this strong-field/radiative regime. In the last two chapters of
this thesis a new such framework is presented. Using the effective field theory
approach, General Relativity non-linearities are described by Feynman diagrams
in which classical gravitons interact with matter sources and among themselves.
Tagging the self-interaction vertices of gravitons with parameters it is
possible, for example, to translate the measure of the period decay of
Hulse-Taylor pulsar in a constraint on the three-graviton vertex at the 0.1%
level; for comparison, LEP constraints on the triple-gauge-boson couplings of
weak interactions are accurate at 3%. With future observations of gravitational
waves, higher order graviton vertices can in principle be constrained through a
Fisher matrix analysis.Comment: This PhD Thesis has been conducted at the University of Geneva
(Switzerland) under the direction of Professor Michele Maggiore and the
codirection of Doctor Riccardo Sturani. Version 2: abstract slightly changed;
one typo corrected; layout issue fixe
Constraining nonperturbative strong-field effects in scalar-tensor gravity by combining pulsar timing and laser-interferometer gravitational-wave detectors
Pulsar timing and gravitational-wave (GW) detectors are superb laboratories
to study gravity theories in the strong-field regime. Here we combine those
tools to test the mono-scalar-tensor theory of Damour and Esposito-Far{\`e}se
(DEF), which predicts nonperturbative scalarization phenomena for neutron stars
(NSs). First, applying Markov-chain Monte Carlo techniques, we use the absence
of dipolar radiation in the pulsar-timing observations of five binary systems
composed of a NS and a white dwarf, and eleven equations of state (EOSs) for
NSs, to derive the most stringent constraints on the two free parameters of the
DEF scalar-tensor theory. Since the binary-pulsar bounds depend on the NS mass
and the EOS, we find that current pulsar-timing observations leave
scalarization windows, i.e., regions of parameter space where scalarization can
still be prominent. Then, we investigate if these scalarization windows could
be closed and if pulsar-timing constraints could be improved by
laser-interferometer GW detectors, when spontaneous (or dynamical)
scalarization sets in during the early (or late) stages of a binary NS (BNS)
evolution. For the early inspiral of a BNS carrying constant scalar charge, we
employ a Fisher matrix analysis to show that Advanced LIGO can improve
pulsar-timing constraints for some EOSs, and next-generation detectors, such as
the Cosmic Explorer and Einstein Telescope, will be able to improve those
bounds for all eleven EOSs. Using the late inspiral of a BNS, we estimate that
for some of the EOSs under consideration the onset of dynamical scalarization
can happen early enough to improve the constraints on the DEF parameters
obtained by combining the five binary pulsars. Thus, in the near future the
complementarity of pulsar timing and direct observations of GWs on the ground
will be extremely valuable in probing gravity theories in the strong-field
regime.Comment: 19 pages, 11 figures; accepted by Physical Review
Measuring the parameters of massive black hole binary systems with Pulsar Timing Array observations of gravitational waves
The observation of massive black hole binaries (MBHBs) with Pulsar Timing
Arrays (PTAs) is one of the goals of gravitational wave astronomy in the coming
years. Massive (>10^8 solar masses) and low-redshift (< 1.5) sources are
expected to be individually resolved by up-coming PTAs, and our ability to use
them as astrophysical probes will depend on the accuracy with which their
parameters can be measured. In this paper we estimate the precision of such
measurements using the Fisher-information-matrix formalism. We restrict to
"monochromatic" sources. In this approximation, the system is described by
seven parameters and we determine their expected statistical errors as a
function of the number of pulsars in the array, the array sky coverage, and the
signal-to-noise ratio (SNR) of the signal. At fixed SNR, the gravitational wave
astronomy capability of a PTA is achieved with ~20 pulsars; adding more pulsars
(up to 1000) to the array reduces the source error-box in the sky \Delta\Omega
by a factor ~5 and has negligible consequences on the statistical errors on the
other parameters. \Delta\Omega improves as 1/SNR^2 and the other parameters as
1/SNR. For a fiducial PTA of 100 pulsars uniformly distributed in the sky and a
coherent SNR = 10, we find \Delta\Omega~40 deg^2, a fractional error on the
signal amplitude of ~30% (which constraints only very poorly the chirp mass -
luminosity distance combination M_c^{5/3}/D_L), and the source inclination and
polarization angles are recovered at the ~0.3 rad level. The ongoing Parkes PTA
is particularly sensitive to systems located in the southern hemisphere, where
at SNR = 10 the source position can be determined with \Delta\Omega ~10 deg^2,
but has poorer performance for sources in the northern hemisphere. (Abridged)Comment: 20 pages, 12 figures, 2 color figures, submitted to Phys. Rev.
Measuring information-transfer delays
In complex networks such as gene networks, traffic systems or brain circuits it is important to understand how long it takes for the different parts of the network to effectively influence one another. In the brain, for example, axonal delays between brain areas can amount to several tens of milliseconds, adding an intrinsic component to any timing-based processing of information. Inferring neural interaction delays is thus needed to interpret the information transfer revealed by any analysis of directed interactions across brain structures. However, a robust estimation of interaction delays from neural activity faces several challenges if modeling assumptions on interaction mechanisms are wrong or cannot be made. Here, we propose a robust estimator for neuronal interaction delays rooted in an information-theoretic framework, which allows a model-free exploration of interactions. In particular, we extend transfer entropy to account for delayed source-target interactions, while crucially retaining the conditioning on the embedded target state at the immediately previous time step. We prove that this particular extension is indeed guaranteed to identify interaction delays between two coupled systems and is the only relevant option in keeping with Wiener’s principle of causality. We demonstrate the performance of our approach in detecting interaction delays on finite data by numerical simulations of stochastic and deterministic processes, as well as on local field potential recordings. We also show the ability of the extended transfer entropy to detect the presence of multiple delays, as well as feedback loops. While evaluated on neuroscience data, we expect the estimator to be useful in other fields dealing with network dynamics
Unveiling the Dynamics of the Universe
We explore the dynamics and evolution of the Universe at early and late
times, focusing on both dark energy and extended gravity models and their
astrophysical and cosmological consequences. Modified theories of gravity not
only provide an alternative explanation for the recent expansion history of the
universe, but they also offer a paradigm fundamentally distinct from the
simplest dark energy models of cosmic acceleration. In this review, we perform
a detailed theoretical and phenomenological analysis of different modified
gravity models and investigate their consistency. We also consider the
cosmological implications of well motivated physical models of the early
universe with a particular emphasis on inflation and topological defects.
Astrophysical and cosmological tests over a wide range of scales, from the
solar system to the observable horizon, severely restrict the allowed models of
the Universe. Here, we review several observational probes -- including
gravitational lensing, galaxy clusters, cosmic microwave background temperature
and polarization, supernova and baryon acoustic oscillations measurements --
and their relevance in constraining our cosmological description of the
Universe.Comment: 94 pages, 14 figures. Review paper accepted for publication in a
Special Issue of Symmetry. "Symmetry: Feature Papers 2016". V2: Matches
published version, now 79 pages (new format
Probing the physical and mathematical structure of gravity by PSR
There are several approaches to extend General Relativity in order to explain
the phenomena related to the Dark Matter and Dark Energy. These theories,
generally called Extended Theories of Gravity, can be tested using observations
coming from relativistic binary systems as PSR . Using a class of
analytical -theories, one can construct the first time derivative of
orbital period of the binary systems starting from a quadrupolar gravitational
emission. Our aim is to set boundaries on the parameters of the theory in order
to understand if they are ruled out, or not, by the observations on PSR
. Finally, we have computed an upper limit on the graviton mass
showing that agree with constraint coming from other observations.Comment: 6 pages, 1 figure, accepted in International Journal of Geometric
Methods in Modern Physic
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