Search Talks

How can we rule out whole classes of dark energy models? And what quantities, at what redshift, and with what accuracy, should be measured in order to rule out these classes of models? I present answers to these questions by discussing an approach that utilizes the principal component parametrization of dark energy. I show results based on current data, and future forecasted data from SNAP and Planck.

We show that the existence of the bullet cluster, 1E0657-56, is incompatible with the prediction of the standard Lambda CDM cosmology. The probability of finding the large infall velocity (3000 km/s) necessary for explaining the X-ray and weak lensing data of 1E0657-56 is between 3.3x10^{-11} and 3.6x10^{-9}. The existence of the bullet cluster poses a serious challenge to LCDM cosmology, unless a lower infall velocity solution for 1E0657-56 with <1800 km/s is found.

The Atacama Cosmology Telescope (ACT) has recently measured the CMB angular power spectrum from maps with arcminute resolution at 148 GHz. By fitting to a template for the SZ angular power spectrum, we constrain the model's amplitude A_SZ < 1.63 (95% confidence level) and the amplitude of matter perturbations sigma_8 < 0.86 (95% CL). In this talk, we review the fiducial template and present additional templates for the SZ angular power spectrum based on different models for the hot gas in dark matter halos. We also discuss how the TSZ, KSZ, and SZ power spectra scale with sigma_8.

X-ray surveys and deep Chandra observations recently provided ~10% accurate measurements of the total mass in nearly 100 galaxy clusters at z=0-0.9. These data clearly show the effect of Dark Energy on slow-down of the structure growth at z<1. THe combination of the structure growth measurements with other cosmological observations substantially improves the constraints on the Dark Energy equation of state parameter. More advanced applications include constraints on the deviations from General Relativity on 10-100 Mpc scales. Will the situation improve in the future when samples of 10^3-10^5 clusters are available? I will review the current ''bottlenecks'', and discuss possible strategies for using future cluster data for ''precision cosmology''.

The arrow of time dilemma: the laws of physics are invariant for time inversion, whereas the familiar phenomena we see everyday are not (i.e. entropy increases). I show that, within a quantum mechanical framework, all phenomena which leave a trail of information behind (and hence can be studied by physics) are those where entropy necessarily increases or remains constant. All phenomena where the entropy decreases must not leave any information of their having happened. This situation is completely indistinguishable from their not having happened at all. In the light of this observation, the second law of thermodynamics is reduced to a mere tautology: physics cannot study those processes where entropy has decreased, even if they were commonplace. I will discuss the possible limitations that stem from recent typicality results in employing this as a complete self-consistent solution to the arrow of time dilemma.

I will discuss distinctions between dark energy and modified gravity explanations of cosmic acceleration from the horizon scale to the deeply non-linear regime using the modified action f(R) and braneworld DGP models as worked toy examples.

Observations are opening new windows to test general relativity at cosmological scales. In this talk, I will discuss how gravity determines the expansion and structure formation of the universe, what smoking guns of gravity in the cosmos we are expecting, what difficulties we are facing to perform unambiguous tests of gravity and what are possible ways to overcome these difficulties.

Non-linear structures in the universe leave characteristic imprints in the cosmic microwave background. These include Compton scattering (Sunyaev-Zeldovich effects) and gravitational lensing. The South Pole Telescope now has a catalog of massive galaxy clusters that were discovered this way, along with a measure of the background fluctuations generated by smaller clusters, that can be used to chart the growth of structure in the universe.

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.