Wine and Cheese Fall 2015

This page records the schedule, titles and abstracts of the JHU/STScI CAS Astrophysics Wine & Cheese Series in Fall 2015.

If not specified otherwise, the talks are a 25-min presentation plus a 5-min Q/A session.

August 28th

Hans Böhringer

Testing Cosmological Models with X-ray Galaxy Clusters (full-hour)

We use a large, statistically very well characterised X-ray flux-limited sample of galaxy clusters to study the large-scale structure of the Universe out to redshifts of ~ 0.4. We use the cluster mass function to obtain tight constraints on the matter density and amplitude parameter of the density fluctuation power spectrum. We find some tension in the resulting amplitude parameters with the prediction from the PLANCK results in the frame of the standard Lambda-CDM cosmological model. The tension implies a less pronounced fluctuation amplitude of nearby large-scale structure as compared to the predictions based on Planck and a pure LCDM model. The results can be reconciled, however, by for example introducing massive neutrinos. We also use the cluster sample to study the matter distribution in the local Universe in a cosmographical fashion. One of the findings of this research is a locally underdense region in the Southern Galactic Cap region, with interesting consequences for local measurements of cosmological parameters, like the Hubble constant.

September 14th

Shadab Alam

Testing Gravity using Galaxy Redshift Surveys and CMB

The Redshift Space Distortions (RSD) in galaxy redshift surveys can probe the local dynamics at a given epoch of galaxy. I will discuss how redshift can help us learn the local dynamics and hence measure the nature of gravity at the epoch of the galaxy. I will show results from our recent analysis of SDSS-III high redshift sample (CMASS). I will then talk about combining similar RSD measurements from various other surveys to learn more about cosmology and modified gravity. I will end with a discussion on combining these measurements with CMB lensing in order to probe gravity to better precision and earlier time.

Laurent Pueyo

Discovery and spectroscopy of the young Jovian planet 51 Eri b with the Gemini Planet Imager

Directly detecting thermal emission from young extrasolar planets allows measurement of their atmospheric composition and luminosity, which is influenced by their formation mechanism. Using the Gemini Planet Imager, we discovered a planet orbiting the ~20 Myr-old star 51 Eridani at a projected separation of 13 astronomical units. Near-infrared observations show a spectrum with strong methane and water vapor absorption. Modeling of the spectra and photometry yields a luminosity of L/L⦿=1.6-4.0 × 10−6 and an effective temperature of 600-750 K. For this age and luminosity, “hot-start” formation models indicate a mass twice that of Jupiter. This planet also has a sufficiently low luminosity to be consistent with the “cold-start” core accretion process that may have formed Jupiter.

September 21st

Tim Brandt

Disrupted Globular Clusters as the Source of the Galactic Center GeV Excess

The Fermi satellite has recently detected excess gamma ray emission from the central regions of our Galaxy. This may be evidence for dark matter particles, a major component of the standard cosmological model, annihilating to produce high-energy photons. I will show that the observed signal may instead be generated by millisecond pulsars that formed in dense globular clusters in the Galactic halo. Most of these clusters were ultimately disrupted by evaporation and gravitational tides, contributing to a spherical bulge of stars and stellar remnants. The gamma ray amplitude, angular distribution, and spectral signatures of this source may be predicted without free parameters, and are in remarkable agreement with the observations. The gamma rays are then from the fossil remains of dispersed clusters, and constitute the first direct evidence for the former existence of a much larger globular cluster population.

Simeon Bird

Title and Abstract coming soon

October 12th

Moritz Münchmeyer

Oscillations in the CMB bispectrum

Oscillating signatures in the correlation functions of the primordial density perturbations are predicted by a variety of inflationary models. A theoretical mechanism that has attracted much attention in recent years is a periodic shift symmetry in the inflaton potential, which allows to protect large field models from quantum corrections, as implemented in axion monodromy inflation. This symmetry leads to so called resonance non-gaussianities, whose key feature are logarithmically stretched oscillations in the power spectrum and bispectrum. Oscillations are also a generic consequence of excited (non Bunch-Davies) states during inflation. A number of possible excited states have been proposed, leading to a variety of possible power spectrum and bispectrum shapes. As a third example, sharp features in the potential induce oscillations as the inflaton relaxes back to its attractor solution. Oscillating shapes are therefore a very interesting experimental target. After giving an overview of these theoretical motivations, I will discuss how to search for these signatures in the CMB data. Fast oscillations are very difficult to search for with traditional estimation techniques, and I will demonstrate how targeted expansions, that exploit the symmetry properties of the underlying shapes, allow to circumvent these difficulties. As a member of the Planck collaboration, I will discuss the Planck results that have been obtained using these methods in the bispectrum, as well as a joint search combining bispectrum and power spectrum. Due to their low overlap with well constrained non-gaussian shapes, as well as their low overlap among each other, oscillating bispectrum shapes are not exhaustively constrained and a potential discovery is therefore not yet ruled out. My talk will be based in particular on arxiv:1412.3461, arxiv:1505.05882 and Planck publications on inflation and non-gaussianities.

Johannes Sahlmann

Title and abstract coming soon.

October 26th

Tom Brown

Full seminar talk

Title and Abstract coming soon