Champ gravitationnel

Résumé

En physique classique, le champ gravitationnel ou champ de gravitation est un champ réparti dans l'espace et dû à la présence d'une masse susceptible d'exercer une influence gravitationnelle sur tout autre corps présent à proximité (immédiate ou pas). L'introduction de cette grandeur permet de s'affranchir du problème de la médiation de l'action à distance apparaissant dans l'expression de la force de gravitation universelle. Il est possible de montrer que le champ gravitationnel \vec{\mathcal{G}}(\vec{r}) créé en un point quelconque par un corps ponctuel dérive d'un potentiel scalaire newtonien noté \Phi(\vec{r}) en 1/r, analogue au potentiel électrostatique. En fait, il existe une analogie formelle entre champ électrostatique et champ gravitationnel, et leurs potentiels scalaires respectifs. La théorie de la relativité générale interprète le champ gravitationnel comme une modification de la métrique de l'espace-temps. À la limite, les équatio

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https://fr.wikipedia.org/wiki/Champ_gravitationnel

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Gravitational lensing - the deflection of light by gravity - has greatly developed since its famous first observation in 1919, which validated Einstein's General Theory of Relativity. The strength of this effect does not depend on the nature of the mass which produces the gravitational field and thus it is a great tool to weigh both the visible and the invisible parts of the universe. Consequently gravitational lensing has become a pillar of observational cosmology over the last decades, and it is used to study the nature of Dark Matter and Dark Energy, the two mysterious quantities which dominate our universe, but are not yet understood by physics theory. The success of this endeavor rests on a thorough understanding of lensing theory and observations, including their systematics, the availability of a sufficient amount of precise data, and the development of efficient software to precisely measure these lensing effects in digital astronomical images. This thesis presents advanced techniques which can improve several of these areas. In the first part, we develop a new theoretical method to break the mass-sheet degeneracy, which prevents accurate mass determinations from lensing observations. The second part focuses on spectroscopic data from MUSE, a second-generation Integral-Field Spectrograph installed on one of the largest ground-based telescopes on earth. We present a pipeline which permits the efficient determination of the redshift of a source observed by MUSE. The redshift indicates the distance of the source from us and depends on the expansion of the universe. In addition, we use MUSE observations of a galaxy cluster to improve the determination of its total weight, including the dominant Dark Matter component. In the third part of this thesis, we investigate how we can accelerate the computation of these mass maps. In the era of big data and large surveys, computing efficiency is key to obtaining new scientific insights. We use High Performance Computing techniques like graphics card acceleration to improve the code performance and we develop a method which harnesses extra performance from using single precision without loosing the required accuracy. In the last section, we present first results from measuring flexion, a higher order lensing effect which could substantially increase the resolution of lensing mass maps and thus lead to a sharper view of structure in the universe.

EPFL2018

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Scale-invariant alternatives to general relativity

Diego Blas Temino

We study the general class of gravitational field theories constructed on the basis of scale invariance (and therefore absence of any mass parameters) and invariance under transverse diffeomorphisms, which are the 4-volume conserving coordinate transformations. We show that these theories are equivalent to a specific type of scalar-tensor theories of gravity (invariant under all diffeomorphisms) with a number of properties, making them phenomenologically interesting. They contain, in addition to the dimensionless coupling constants of the original theory, an arbitrary dimensionful parameter Lambda(0). This parameter is associated with an integration constant of the equations of motion, similar to the arbitrary cosmological constant appearing in unimodular gravity. We focus on the theories where Newton's constant and the electroweak scale emerge from the spontaneous breaking of scale invariance and are unrelated to Lambda(0). The massless particle spectrum of these theories contains the graviton and a new particle-dilaton. For Lambda(0) = 0, the massless dilaton has only derivative couplings to matter fields and the bounds on the existence of a 5th force are easily satisfied. As for the matter fields, we determine the conditions leading to a renormalizable low-energy theory. If Lambda(0) not equal 0, scale invariance is broken. The arbitrary constant Lambda(0) produces a "run-away" potential for the dilaton. As a consequence, the dilaton can act as a dynamical dark energy component. We elucidate the origin of the cosmological constant in the class of theories under consideration and formulate the condition leading to its absence. If this condition is satisfied, dark energy is purely dynamical and associated to the dilaton.

2011

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Ultralight Dark Matter Resonates with Binary Pulsars

Diego Blas Temino, Sergey Sibiryakov

We consider the scenario where dark matter (DM) is represented by an ultralight classical scalar field performing coherent periodic oscillations. We point out that such DM perturbs the dynamics of binary systems either through its gravitational field or via direct coupling to ordinary matter. This perturbation gets resonantly amplified if the frequency of DM oscillations is close to a (half-) integer multiple of the orbital frequency of the system and leads to a secular variation of the orbital period. We suggest using binary pulsars as probes of this scenario and estimate their sensitivity. While the current accuracy of observations is not yet sufficient to probe the purely gravitational effect of DM, it already yields constraints on direct coupling that are competitive with other bounds. The sensitivity will increase with the upcoming radio observatories such as the Square Kilometer Array.

American Physical Society2017

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EPFL2018