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Concept# Accelerating expansion of the universe

Summary

Observations show that the expansion of the universe is accelerating, such that the velocity at which a distant galaxy recedes from the observer is continuously increasing with time. The accelerated expansion of the universe was discovered during 1998 by two independent projects, the Supernova Cosmology Project and the High-Z Supernova Search Team, which both used distant type Ia supernovae to measure the acceleration. The idea was that as type Ia supernovae have almost the same intrinsic brightness (a standard candle), and since objects that are farther away appear dimmer, we can use the observed brightness of these supernovae to measure the distance to them. The distance can then be compared to the supernovae's cosmological redshift, which measures how much the universe has expanded since the supernova occurred; the Hubble law established that the farther an object is from us, the faster it is receding. The unexpected result was that objects in the universe are moving away from one

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PHYS-402: Astrophysics IV : observational cosmology

Cosmology is the study of the structure and evolution of the universe as a whole. This course describes the principal themes of cosmology, as seen
from the point of view of observations.

PHYS-427: Relativity and cosmology I

Introduce the students to general relativity and its classical tests.

PHYS-428: Relativity and cosmology II

This course is the basic introduction to modern cosmology. It introduces students to the main concepts and formalism of cosmology, the observational status of Hot Big Bang theory
and discusses major physical processes in the early Universe.

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There is now very strong evidence that our Universe is undergoing an accelerated expansion period as if it were under the influence of a gravitationally repulsive “dark energy” component. Furthermore, most of the mass of the Universe seems to be in the form of non-luminous matter, the so-called “dark matter”. Together, these “dark” components, whose nature remains unknown today, represent around 96 % of the matter-energy budget of the Universe. Unraveling the true nature of the dark energy and dark matter has thus, obviously, become one of the primary goals of present-day cosmology. Weak gravitational lensing, or weak lensing for short, is the effect whereby light emitted by distant galaxies is slightly deflected by the tidal gravitational fields of intervening foreground structures. Because it only relies on the physics of gravity, weak lensing has the unique ability to probe the distribution of mass in a direct and unbiased way. This technique is at present routinely used to study the dark matter, typical applications being the mass reconstruction of galaxy clusters and the study of the properties of dark halos surrounding galaxies. Another and more recent application of weak lensing, on which we focus in this thesis, is the analysis of the cosmological lensing signal induced by large-scale structures, the so-called “cosmic shear”. This signal can be used to measure the growth of structures and the expansion history of the Universe, which makes it particularly relevant to the study of dark energy. Of all weak lensing effects, the cosmic shear is the most subtle and its detection requires the accurate analysis of the shapes of millions of distant, faint galaxies in the near infrared. So far, the main factor limiting cosmic shear measurement accuracy has been the relatively small sky areas covered. Next-generation of wide-field, multicolor surveys will, however, overcome this hurdle by covering a much larger portion of the sky with improved image quality. The resulting statistical errors will then become subdominant compared to systematic errors, the latter becoming instead the main source of uncertainty. In fact, uncovering key properties of dark energy will only be achievable if these systematics are well understood and reduced to the required level. The major sources of uncertainty resides in the shape measurement algorithm used, the convolution of the original image by the instrumental and possibly atmospheric point spread function (PSF), the pixelation effect caused by the integration of light falling on the detector pixels and the degradation caused by various sources of noise. Measuring the Cosmic shear thus entails solving the difficult inverse problem of recovering the shear signal from blurred, pixelated and noisy galaxy images while keeping errors within the limits demanded by future weak lensing surveys. Reaching this goal is not without challenges. In fact, the best available shear measurement methods would need a tenfold improvement in accuracy to match the requirements of a space mission like Euclid from ESA, scheduled at the end of this decade. Significant progress has nevertheless been made in the last few years, with substantial contributions from initiatives such as GREAT (GRavitational lEnsing Accuracy Testing) challenges. The main objective of these open competitions is to foster the development of new and more accurate shear measurement methods. We start this work with a quick overview of modern cosmology: its fundamental tenets, achievements and the challenges it faces today. We then review the theory of weak gravitational lensing and explains how it can make use of cosmic shear observations to place constraints on cosmology. The last part of this thesis focuses on the practical challenges associated with the accurate measurement of the cosmic shear. After a review of the subject we present the main contributions we have brought in this area: the development of the gfit shear measurement method, new algorithms for point spread function (PSF) interpolation and image denoising. The gfit method emerged as one of the top performers in the GREAT10 Galaxy Challenge. It essentially consists in fitting two-dimensional elliptical Sérsic light profiles to observed galaxy image in order to produce estimates for the shear power spectrum. PSF correction is automatic and an efficient shape-preserving denoising algorithm can be optionally applied prior to fitting the data. PSF interpolation is also an important issue in shear measurement because the PSF is only known at star positions while PSF correction has to be performed at any position on the sky. We have developed innovative PSF interpolation algorithms on the occasion of the GREAT10 Star Challenge, a competition dedicated to the PSF interpolation problem. Our participation was very successful since one of our interpolation method won the Star Challenge while the remaining four achieved the next highest scores of the competition. Finally we have participated in the development of a wavelet-based, shape-preserving denoising method particularly well suited to weak lensing analysis.

We demonstrate that the combination of the ideas of unimodular gravity, scale invariance, and the existence of an exactly massless dilaton leads to the evolution of the universe supported by present observations: inflation in the past, followed by the radiation and matter dominated stages and accelerated expansion at present. All mass scales in this type of theories come from one and the same source. © 2008 Elsevier B.V. All rights reserved.

2009The LCDM model has emerged as the concordance model of cosmology for its ability to explain a variety of observations, ranging from the anisotropies of the Cosmic Microwave Background (CMB) to the accelerated expansion of the Universe. However, with the ever-increasing precision of the measurements, tensions have recently emerged between the latest CMB observations and other cosmological probes. The most prominent one concerns the expansion rate of the Universe, that is the Hubble constant, H0, with a statistically significant discrepancy between local and early measurements. If not arising from unaccounted systematic errors, this discrepancy is an exciting opportunity to explore missing physics beyond the standard LCDM model.Time-delay cosmography has emerged as a competitive and single-step method to measure locally the Hubble constant, providing an alternative to the cosmic distance ladder. The two techniques do not share any known source of errors, allowing us to obtain truly independent estimates. This technique is based on the strong lensing phenomenon, occurring when the light coming from a background source is split into multiple images by a massive foreground galaxy. In asymmetric configurations, the travel time is then slightly different for each of the lensed images, leading to measurable time delays in the signals received from the background object. The Hubble constant can then be directly inferred from these time delays, provided that an accurate reconstruction of the mass distribution of the lens galaxy is available.The first part of my PhD work focuses on the precise determination of the time delays in strongly lensed quasars, an essential ingredient of the method. The COSMOGRAIL collaboration has been monitoring lensed quasars for two decades to record enough quasar variations that can be unambiguously matched in all light curves. I compiled these data obtained from the Euler 1.2m Swiss telescope in La Silla, Chile, and measured the time delays in 18 systems, more than doubling the current sample of lensed quasars with known delays. This work required the development and automation of curve-shifting algorithms presented in a dedicated chapter. As the sample of known lensed quasars is growing rapidly, I also present a novel monitoring strategy to obtain the time delays efficiently, in only one or two monitoring seasons. Thanks to this long-term monitoring effort, the TDCOSMO collaboration published the most precise determination of the Hubble constant obtained with time-delay cosmography from a sample of seven lensed quasars. We found H0 = 73.7+/- 1.5 km/s/Mpc, at 2% precision. This result confirmed the existing tension, now reaching a high significance when combined with the distance ladder results. "As extraordinary claims require extraordinary evidence", the second part of my work focused on the verification of the assumptions made to obtain this result. In the last chapter of this thesis, I present my work on the search for unaccounted systematic errors in every step of the analysis. I also show the results of our participation to a blind data challenge designed to test lens modelling codes and the assumptions used to reconstruct the mass distribution of the galaxies. I conclude this thesis by presenting the new approach proposed by the TDCOSMO collaboration to relax all assumptions about the density profile of massive elliptical galaxies and adopt a parametrisation entirely constrained by stellar kinematic.

Related concepts (64)

Dark energy

In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurement

Big Bang

The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolu

Universe

The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description