Cosmology & Gravitation

This series consists of talks in the areas of Cosmology, Gravitation and Particle Physics.

Seminar Series Events/Videos

Currently there are no upcoming talks in this series.

 

Tuesday Sep 28, 2010

The quantum spin Hall effect relates seemingly unrelated degrees of freedom, i.e., charge and spin degrees of freedom. We will discuss such "duality" can be extended to much wider class of quantum numbers, and the corresponding order parameters. In particular, two valleys in graphene can be viewed as an SU(2) pseudo spin degree of freedom, which turns out to be "dual" to the charge degree of freedom, pretty much in the same way as spin in the quantum spin Hall effect is closely tied with charge. I.e., graphene can host "the quantum valley Hall effect" (QVHE).

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Tuesday Jun 29, 2010
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Using a formulation of the post-Newtonian expansion in terms of Feynman graphs, we discuss how various tests of General Relativity (GR) can be translated into measurement of the three- and four-graviton vertices. The timing of the Hulse-Taylor binary pulsar provides a bound on the deviation of the three-graviton vertex from the GR prediction at the 0.1% level.

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Tuesday Jun 08, 2010
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Constraints on the formation of primordial black holes - especially the ones which are small enough to evaporate - provide a unique probe of the early universe, high energy physics and extra dimensions. For evaporating black holes, the dominant constraints are associated with big bang nucleosynthesis and the extragalactic photon background, but there are also other limits associated with the cosmic microwave background, cosmic rays and various types of relic particles. For larger non-evaporating black holes, important constraints come from their gravitational and astrophysical effects.

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Tuesday May 18, 2010
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I consider some of the issues we face in trying to understand dark energy. Huge fluctuations in the unknown dark energy equation of state can be hidden in distance data, so I argue that model-independent tests which signal if the cosmological constant is wrong are valuable. These can be constructed to remove degeneracies with the cosmological parameters. Gravitational effects can play an important role. Even small inhomogeneity clouds our ability to say something definite about dark energy.

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Tuesday May 11, 2010
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Although inflation is, by far, the best known mechanism to explain the observed properties of our Universe, there is still some room for alternative models, most of which implying a contracting phase preceding the current expanding one. Both phases are connected by a bounce at which the expansion rate must vanish. General relativity can only produce such a phase provided the spatial curvature is positive, in contradiction with the current observations.

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Monday May 03, 2010
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Dark matter, constituting a fifth of the mass-energy in the Universe today, is one of the major "known unknowns" in physics. A number of different experimental and observational techniques exist to try to identify dark matter. However, these techniques are not only sensitive to the "physics" of dark matter (mass, cross sections, and the theory in which the dark matter particles live) but to the "astrophysics" of dark matter as well, namely the phase-space density of dark matter throughout the Milky Way and other galaxies and its evolution through cosmic time.

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Tuesday Apr 27, 2010
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We have announced the results from 7 years of observations of the Wilkinson Microwave Anisotropy Probe (WMAP) on January 26. In this talk we will present the cosmological interpretation of the WMAP 7-year data, including the detection of primordial helium, images of polarization of microwave background around temperature peaks, and new limits on inflation and properties of neutrinos. We also report a significant detection of the Sunyaev-Zel'dovich effect and discuss implications for the gas pressure in clusters of galaxies.

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Tuesday Apr 20, 2010
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We present a holographic description of four-dimensional single-scalar inflationary universes in terms of a three-dimensional quantum field theory. The holographic description correctly reproduces standard inflationary predictions in their regime of applicability. In the opposite case, wherein gravity is strongly coupled at early times, we propose a holographic description in terms of perturbative QFT and present models capable of satisfying the current observational constraints while exhibiting a phenomenology distinct from standard inflation.

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