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Monday, September 12th 2016

12:15 pm:

About 4.6 billion years ago, some event disturbed a cloud of gas and dust, triggering the gravitational collapse that led to the formation of the solar system. A core-collapse supernova, whose shock wave is capable of compressing such a cloud, is an obvious candidate for the initiating event. This hypothesis can be tested because supernovae also produce telltale patterns of short-lived radionuclides, which would be preserved today as isotopic anomalies. Previous studies of the forensic evidence have been inconclusive, finding a pattern of isotopes differing from that produced in conventional supernova models. Here we argue that these difficulties either do not arise or are mitigated if the initiating supernova was a special type, low in mass and explosion energy. Key to our conclusion is the demonstration that short-lived 10Be can be readily synthesized in such supernovae by neutrino interactions, while anomalies in stable isotopes are suppressed.

Monday, September 19th 2016

12:15 pm:

The theoretical modeling of the statistical observables of the large-scale structures of the Universe, like galaxy clustering, weak lensing etc., is necessary in order to derive any constraints on the cosmological parameters. One of the most important ingredients of the theoretical model is the two-point correlation function, or its Fourier transform the matter power spectrum. I will discuss the precision in its calculations based on a modified halo model, and the systematic effects due to the baryonic processes. Further, I will also discuss the covariance matrix of the matter power spectrum and its estimators based on the halo model and the perturbation theory. We find the agreement with the simulations is at a 10% level up to k ∼ 1 h/Mpc. We show that all the connected components are dominated by the large-scale modes (k < 0.1h/Mpc), regardless of the value of the wavevectors of the covariance matrix. Finally, I will provide a prescription for how to evaluate the covariance matrix from small box simulations without the need to simulate large volumes.

Monday, September 26th 2016

12:15 pm:

Monday, October 24th 2016

12:45 pm:

Monday, October 31st 2016

12:15 pm:

A new systematic analysis technique was developed using the Effective Field Theory (EFT) approach for direct dark matter detection and folding in energy-dependent information, when possible, about the detected events, experimental efficiencies, and backgrounds. The Bayesian inference tool, MultiNest, efficiently calculates highly-dimensional likelihoods over WIMP mass and multiple EFT coupling coefficients. The resulting likelihoods can then be used to set limits on these parameters and choose models (EFT operators) that best fit the direct detection data. Expanding the parameter space beyond the standard spin-independent isoscalar cross-section and WIMP mass reduces tensions between previously published experiments. Combining these experiments into a single likelihood leads to stronger limits than when each experiment is considered on its own. Simulations using two non-standard operators (operators 3 and 8) are used to test the proposed analysis technique in up to five dimensions and demonstrate the importance of using multiple likelihood projections when determining the limits on WIMP mass and EFT coupling coefficients.

Monday, November 14th 2016

12:15 pm:

General Relativity can be described as a theory of a massless spin-2 particle (the graviton), whose interactions are local and Lorentz-invariant. It is a fundamental and long-standing question as to whether the graviton may acquire a small mass. Additionally, a small graviton mass can potentially explain cosmic acceleration in a way that avoids a technically unnatural cosmological constant. In this talk, I will describe recent progress in constructing a healthy, non-linear theory of massive gravity. I will also describe observational tests from solar system and from binary pulsar observations.

Monday, November 28th 2016

12:15 pm:

Monday, December 5th 2016

12:15 pm:

Clusters of galaxies form at the intersection of the enormous filamentary structures of the cosmic web, and evolve through the continued infall of material along those filaments, with occasional major mergers between clusters. Or so the numerical simulations to be discussed next week by Tom Jones, tell us. The history of that evolution should be embedded in the current dynamical state of the intracluster medium (ICM), but our best tools, the thermal X-ray emission from the hot plasma, are often blind to these structures. I will discuss some of the indicators of ICM dynamics that are available using synchroton emission, what we have learned so far, and some prospects for the future.

Monday, December 12th 2016

12:15 pm:

Galaxy clusters are the most massive bound systems in the universe and the last stage of hierarchical structure formation. They occupy intersections of cosmic filaments, where they come together through accretion from the filaments and incredibly violent mergers with other clusters. While dark matter accounts for about 85% of cluster total mass, ~ 90% of the remaining, baryonic matter is in the form of diffuse, keV temperature plasma, known as the "intracluster medium" or ICM. Being dissipative on "small" scales, ICM plasma captures important characteristics of cluster formation dynamics that are otherwise obscure. At the lowest order, an ICM is virialized and relaxes to a hydrostatic equilibrium with the cluster's gravitational potential. But the cluster formation process is unsteady, non-symmetric and subject to frequent external gravitational distortion, so, at the next level ICMs are expected to be crossed with transonic winds, gravity waves, concentrated, infalling streams, weak to moderate strength shocks and turbulence, all of whose distributions should reveal much about the cluster history and its environment. The links between these patterns and cluster formation are especially strong outside of the central cores of the clusters, where thermal emissions are weak, but where non-thermal processes can be strong enough to provide discriminating signatures of ICM dynamics. I will review these issues, including some current puzzles, focusing on our local simulation and modeling efforts, past, current and planned.

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