Cosmology

Cosmologists at Perimeter Institute seek to help pin down the constituents and history of our universe, and the rules governing its origin and evolution. Many of the most interesting clues about physics beyond the standard model (e.g., dark matter, dark energy, the matter/anti-matter asymmetry, and the spectrum of primordial density perturbations], come from cosmological observations, and cosmological observations are often the best way to test or constrain a proposed modification of the laws of nature, since such observations can probe length scales, time scales, and energy scales that are beyond the reach of terrestrial laboratories.

Displaying 1021 - 1032 of 2119

PHYS 781 - Lecture 15

Niayesh Afshordi
University of Waterloo
November 06, 2014

PIRSA:14110053
Cosmology

Quantum Gravity

Strong Gravity
PHYS 781 - Lecture 14

Niayesh Afshordi
University of Waterloo
November 04, 2014

PIRSA:14110049
Cosmology

Quantum Gravity

Strong Gravity
How Quantum are the Cosmological Correlations?

Eugene Lim
King's College London
November 04, 2014

PIRSA:14110003
Cosmology
Beyond slow-roll - Daan Meerburg
October 28, 2014

PIRSA:14100119
Cosmology
Schroedinger method as field theoretical model to describe structure formation

Cora Uhlemann
Bielefeld University
October 21, 2014

PIRSA:14100076
Cosmology
Cosmological Constraints on theories of Modified Gravity

Alexandre Barreira
Durham University
October 16, 2014

PIRSA:14100074
Cosmology
Gravitational lensing of the CMB

Gilbert Holder
University of Illinois Urbana-Champaign
October 14, 2014

PIRSA:14100084
Cosmology
Flux Compactifications Grow Lumps

Claire Zukowski
University of California, Berkeley
October 07, 2014

PIRSA:14100081
Cosmology
The Effective Field Theory of Cosmological Large Scale Structures

Leonardo Senatore
ETH Zurich
October 02, 2014

PIRSA:14090041
Cosmology
Evolution of quantum field, particle content, and classicality in the three stage universe

Suprit Singh
IUCAA - The Inter-University Centre for Astronomy and Astrophysics
September 18, 2014

PIRSA:14090082
Cosmology
Cosmic flows: testing gravity and the matter power spectrum on very large scales

Mike Hudson
University of Waterloo
September 16, 2014

PIRSA:14090077
Cosmology
Black hole evaporation without firewalls

James Bardeen
September 02, 2014

PIRSA:14090001
Cosmology

PHYS 781 - Lecture 15

Niayesh Afshordi
University of Waterloo
November 06, 2014

PIRSA:14110053
Cosmology

Quantum Gravity

Strong Gravity
PHYS 781 - Lecture 14

Niayesh Afshordi
University of Waterloo
November 04, 2014

PIRSA:14110049
Cosmology

Quantum Gravity

Strong Gravity
How Quantum are the Cosmological Correlations?

Eugene Lim
King's College London
November 04, 2014

PIRSA:14110003
Cosmology
Beyond slow-roll - Daan Meerburg
October 28, 2014

PIRSA:14100119
Cosmology
We live in exciting times for cosmologists. There is a plethora of cosmological experiments that allow us to reconstruct the earliest moments in the Universe and test our ideas on how the Universe came into existence. Current data appear to favor an inflationary model that produces adiabatic, scale free, Gaussian fluctuations with an amplitude of 10^-5 in units of mK. WIthin the realm of cosmological models, it appears that such conditions are easily accomplished if we have a single light field slowly rolling down its potential. In this talk, I will investigate the possibility to what extend our current observations would allow for a deviation from slow-roll: several class of models predicts that the fluctuation spectra will contain superimposed features on top of their slow-roll solution. I will discuss these models and explain a novel way of extract these features from the data, both in the power spectrum as well as in the bispectrum. I will give the latest constraints from current cosmological surveys. In light of the possible detection of primordial gravitational waves, I will show that there exists evidence (3 sigma) that the data prefer a long wavelength feature driven by axion monodromy, a model that naturally predicts large tensor modes. From this I will derive a constraint on the axion decay constant. I will conclude with a discussion on how observations of higher order statistics and large scale structure could further constrain these models.
Schroedinger method as field theoretical model to describe structure formation

Cora Uhlemann
Bielefeld University
October 21, 2014

PIRSA:14100076
Cosmology
We investigate large-scale structure formation of collisionless dark matter in the phase space description based on the Vlasov equation whose nonlinearity is induced solely by gravitational interaction according to the Poisson equation. Determining the time-evolution of density and velocity demands solving the full Vlasov hierarchy for the cumulants of the distribution function. In the presence of long-range interaction no consistent truncation is known apart from the dust model which is incapable of describing the formation of bound structures due to the inability to generate higher cumulants like velocity dispersion. Our goal is to find a simple ansatz for the phase space distribution function that approximates the full Vlasov distribution function and can serve as theoretical N-body double to replace the dust model. We present the Schroedinger method which is based on the coarse-grained Wigner probability distribution obtained from a wave function fulfilling the Schroedinger-Poisson equation as sought-after model. We show that its evolution equation approximates the Vlasov equation in a controlled way, cures the shell-crossing singularities of the dust model and is able to describe multi-streaming which is crucial for halo formation. This feature has already been employed in cosmological simulations of large-scale structure formation by Widrow & Kaiser (1993). We explain how the coarse-grained Wigner ansatz allows to calculate higher cumulants like velocity dispersion analytically from density and velocity in a self-consistent manner. On this basis we show that instead of solving the Vlasov-Poisson system one can use the Schrödinger method and solve the Schrödinger-Poission equation to directly determine density and velocity and all higher cumulants. As a first application we study the coarse-grained dust model, which is a limiting case of the Schrödinger method, within Eulerian and Lagrangian perturbation theory.
Cosmological Constraints on theories of Modified Gravity

Alexandre Barreira
Durham University
October 16, 2014

PIRSA:14100074
Cosmology
Recently, research in cosmology has seen a growing interest in theories of gravity beyond General Relativity (GR). From an observational point of view, there are two main reasons for this. Firstly, the law of gravity has never been directly tested on scales larger than the Solar System. Hence, by understanding better the various signatures that different gravity models can leave on cosmological observables, one can improve the chances of identifying any departures from GR, or alternatively, extend the model's observational success into a whole new regime. Secondly, theories of modified gravity can arise also as an alternative to the cosmological constant (or any other form of dark energy) to explain the current accelerated expansion of the Universe. Using my results from suitably modified Boltzmann, N-body codes and semi-analytical models of structure formation, I will describe the way modified gravity models typically impact a series of cosmological observables. I will use two popular models as examples, which are known as Galileon and Nonlocal Gravity. In the Galileon model, the modifications to gravity on large scales are driven by nonlinear derivative interactions of a scalar field, which can nevertheless be suppressed in the Solar System by means of a mechanism known as Vainshtein screening. This model can provide a good fit to the latest CMB, BAO and SNIa data, although with different cosmological parameters than the standard LCDM model. Specifically, unlike LCDM, the Galileon model predicts nonzero neutrino masses (over 5sigma) and the constraints on the Hubble rate are compatible with its local determinations. The results from my N-body simulations and Halo Occupation Distribution analysis also show that the model can describe the measured clustering amplitude of Luminous Red Galaxies and that the screening mechanism can be very efficient in "hiding" the modifications to gravity on small scales. However, these results also show that the observational viability of the model may be under pressure due to the combined constraints derived from the sign of the ISW effect and from Solar System tests. In the Nonlocal model, the acceleration of the universe is driven by terms that involve the inverse of a derivative operator acting on curvature tensors. This model is also likely to pass large-scale structure constraints with the same flying colors as Galileon gravity, but the lack of a screening mechanism in this model makes it unclear on whether or not it is able to satisfy Solar System constraints. These steps I will describe for the cases of the Galileon and Nonlocal models can be viewed as guidelines for one to place constraints on other (or not yet invented) models of modified gravity. References: The results I will describe are based on the following papers: arXiv:1208.0600, arXiv:1302.6241, arXiv:1306.3219, arXiv:1308.3699, arXiv:1401.1497, arXiv:1404.1365, arXiv:1406.0485, arXiv:1408.1084
Gravitational lensing of the CMB

Gilbert Holder
University of Illinois Urbana-Champaign
October 14, 2014

PIRSA:14100084
Cosmology
Gravitational lensing of the cosmic microwave background is emerging as a useful cosmological tool. Recent measurements have been made by several experiments (including the South Pole Telescope, which will be featured), with rapidly improving precision. These measurements can be used for many purposes, including studying the connection between dark matter and galaxies on large scales, measuring the clustering of matter at z~3, and improving the precision of possible measurements of gravitational radiation from inflation.
Flux Compactifications Grow Lumps

Claire Zukowski
University of California, Berkeley
October 07, 2014

PIRSA:14100081
Cosmology
The simplest flux compactifications are highly symmetric—a q-form flux is wrapped uniformly around an extra-dimensional q-sphere. I will discuss a family of solutions that break the internal SO(q+1) symmetry of these solutions down to SO(q)×Z_2, and show that often at least one of them has lower vacuum energy, larger entropy, and is more stable than the symmetric solution. I will describe the phase diagram of lumpy solutions and provide an interpretation in terms of an effective potential. Finally, I will provide evidence that the perturbatively stable vacua have a non-perturbative instability to spontaneously sprout lumps; generically this new decay is exponentially faster than all other known decays of the model.
The Effective Field Theory of Cosmological Large Scale Structures

Leonardo Senatore
ETH Zurich
October 02, 2014

PIRSA:14090041
Cosmology
The Effective Filed Theory of Large Scale Structures provides a novel framework to analytically compute the clustering of the Large Scale Structures in the weakly non-linear regime in a consistent and reliable way. The theory that describes the long wavelength fluctuations is obtained after integrating out the short distance modes and adding suitable operators that allow to correctly reconstruct the effect of short distance fluctuations at long distances. A few observables have been computed so far, and the results are extremely promising. I will discuss the formalism and the main results so far.
Evolution of quantum field, particle content, and classicality in the three stage universe

Suprit Singh
IUCAA - The Inter-University Centre for Astronomy and Astrophysics
September 18, 2014

PIRSA:14090082
Cosmology
I will discuss the evolution of a quantum scalar field in a toy universe which has three stages of evolution, viz., (i) an early (inflationary) de Sitter phase (ii) radiation-dominated phase and (iii) late-time (cosmological constant dominated) de Sitter phase. Using the Schr\"odinger picture, the scalar field equations are solved separately for the three stages and matched at the transition points. The boundary conditions are chosen so that field modes in the early de Sitter phase evolve from the Bunch-Davies vacuum state. I shall look the (time-dependent) particle content of this quantum state for the entire evolution of the universe and describe the various features both numerically and analytically. I shall also describe the quantum to classical transition in terms of a classicality parameter which tracks the particle creation and its effect on phase space correlation of the quantum field.
Cosmic flows: testing gravity and the matter power spectrum on very large scales

Mike Hudson
University of Waterloo
September 16, 2014

PIRSA:14090077
Cosmology
Peculiar velocities - deviations from Hubble expansion - are the only practical probe of the growth of matter density fluctuations on very large scales in the nearby Universe. I will discuss recent measurements of quantities of cosmological interest from our group and others. One is the "bulk" flow of nearby galaxies with respect to the frame defined by the Cosmic Microwave Background, and what this tells us about fluctuations on large very spatial scales. The second is the measurement of the growth rate of fluctuations, a critical test of Lambda CDM and modified gravity models.
Black hole evaporation without firewalls

James Bardeen
September 02, 2014

PIRSA:14090001
Cosmology
There need not be any conflict between unitarity, locality, and regularity of the horizon in black hole evaporation. I discuss a scenario in which the initial collapse that forms the black hole results in a small non-singular core inside an inner event horizon. This core grows as the result of quantum back-reaction associated with the increasing entanglement entropy of Hawking radiation quanta and their partners trapped inside the core. By the Page time the inner and outer apparent horizons either merge into a degenerate horizon, shutting off the Hawking radiation and leaving a massive remnant, or they disappear completely, allowing the trapped quantum information to escape. The scenario is justified by appeals to the Bousso covariant entropy bound and the ER=EPR conjecture. The talk is largely based on arxiv.org/1406.4098.

Title	Speaker(s)	Date	Talk Type	Info link
PHYS 781 - Lecture 15	Niayesh Afshordi	2014-11-06	Course	View details
PHYS 781 - Lecture 14	Niayesh Afshordi	2014-11-04	Course	View details
How Quantum are the Cosmological Correlations?	Eugene Lim	2014-11-04	Scientific Series	View details
Beyond slow-roll - Daan Meerburg		2014-10-28	Scientific Series	View details
Schroedinger method as field theoretical model to describe structure formation	Cora Uhlemann	2014-10-21	Scientific Series	View details
Cosmological Constraints on theories of Modified Gravity	Alexandre Barreira	2014-10-16	Scientific Series	View details
Gravitational lensing of the CMB	Gilbert Holder	2014-10-14	Scientific Series	View details
Flux Compactifications Grow Lumps	Claire Zukowski	2014-10-07	Scientific Series	View details
The Effective Field Theory of Cosmological Large Scale Structures	Leonardo Senatore	2014-10-02	Scientific Series	View details
Evolution of quantum field, particle content, and classicality in the three stage universe	Suprit Singh	2014-09-18	Scientific Series	View details
Cosmic flows: testing gravity and the matter power spectrum on very large scales	Mike Hudson	2014-09-16	Scientific Series	View details
Black hole evaporation without firewalls	James Bardeen	2014-09-02	Scientific Series	View details

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