PDF Ebook Weak lensing and cosmic acceleration
Cosmology is commonly thought to have entered its “precision era”, a threshold of every science. Some of the first - and essential in order to promote it to the rank of science - developments happened almost by chance, such as Hubble’s discovery of expansion, or Penzias and Wilson’s casual observation of the Cosmic Microwave Background, or CMB). Afterwards, the level of sophistication reached by the auxiliary pure sciences, like physics or mathematics, allowed room for fast and spectacular improvement. In the Sixties, after the couple of extraordinary events I mentioned, the new-born Cosmologists not only had a fantastic playground - the Universe itself - but were also supported by a well-established mathematical setting like Einstein’s theory of General Relativity and by a good knowledge of most of the required physics of fluids; also, high-energy physicists were at the same time developing the standard model of particles.
From the Hot Big Bang Cosmology to the formulation of the theory of inflation to the discovery of the CMB anisotropies, there were years that revolutioned the way people thought about the Universe. Then, after some tens of years, Cosmology has become a precision science, in the sense that the guidelines are written, and now we are cross-stitching on models, more often limited by the sensitivity of the instruments than by the cosmological community’s capacity of having better ideas. We have quite a converging view (so converging, in fact, that we call it Concordance model) of the numbers that form the Universe matter-energy content, and still we profoundly lack a theoretical interpretation of these numbers. I think that the fact that we are conceiving more and more sophisticated experiments, while at the same time the nature of a good 96% of the Universe is obscure to us, renders the present era for Cosmology more thrilling than ever. This peculiar state-of-the-art sets as a crucial challenge the search for unusual point of views, in the sense of new observables, able to resolve the present degeneracies among different parameters or give us new independent - in space or in time - constraints. We are in the intriguing era of “tricking the Universe” to force it to tell us something about itself... rather than the elegant Universe, I would call it the reluctant one.
The work I have been doing during my PhD tried to go in the direction that I outlined above - the choice of an observable which was capable to select one particular epoch in the history of the Universe, and the exploration of the information that we can infer from it. Our target is to shed some light on the mechanism giving rise to the observed cosmic acceleration, a relatively recent cosmological process, started a few billions years ago. The easiest theoretical interpretation is in terms of a non-zero energy density in the vacuum, the Cosmological Constant; however, its required value is extremely low with respect to any conceivable (in the sense of “natural”) fundamental physical scale, and the corresponding riddle is motivating a huge theoretical and experimental effort for the understanding of this component. If we generally (although a bit inappropriately) call “dark energy” the component responsible for the acceleration, and constituting the 75% of the matter-energy content of the universe, one of the major present challenges, that motivated the present work, is to determine whether the dark energy has a constant behavior, or is instead characterized by some dynamics.
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