Talks & Events
KICP Friday Noon Seminars: 2001
How CDM explains Milgrom's Law
The Dark Galaxy Problem and the Effects of Substructure on Gravitational Lenses
I argue that the cold dark matter (CDM) model requires that even within a few kpc of the center of a galactic halo a significant fraction (greater than a few percent) of the surface density is contained in substructures with masses > 10^3 solar masses. These structures should be light enough to avoid dynamical friction and dense enough to avoid tidal disruption. I then show using the results of numerical simulations that this substructure will significantly alter the flux ratios of multiply imaged quasars (QSOs) without changing the image positions. The degree to which this occurs will depend on the angular size of the QSO and thus the wavelength of the observations.
Are cosmic rays a cause of climate change?
Recent observations suggest that cosmic rays may play a significant role in the climate. In particular, satellite data have revealed a surprising correlation between cosmic ray intensity and the fraction of the Earth covered by low clouds. Since the cosmic ray intensity is modulated by the solar wind, this could provide an important clue to the long-sought mechanism connecting solar and climate variability. Moreover, if this connection were to be established, it could have significant consequences for our understanding of the solar contributions to the present global warming, since the cosmic ray intensity has fallen during the 20th century due to a more-than-doubling of the strength of the solar wind. In order to the test whether cosmic rays and clouds are causally linked and, if so, to understand the microphysical mechanisms, a novel particle-accelerator experiment known as CLOUD has been proposed. The talk will discuss the scientific case for a connection between cosmic rays and clouds, and describe the proposed CLOUD facility.
Towards a Complete Picture of Damped Lyman Alpha Systems
Damped Lyman alpha Absorption systems (DLAs) contain most of the neutral hydrogen in the high-redshift universe and are the likely progenitors of typical galaxies like the Milky Way. Nonetheless, it is not yet clear whether DLAs are massive galaxies or low-mass protogalaxies, or whether they are connected with the Lyman break galaxies or represent a separate population. I will present results from an observational program designed to clarify the DLA picture. Neither the fraction of the matter density comprised by neutral hydrogen nor the metallicity of the universe as probed by DLAs evolves from z=4 to z=2. Dust extinction does not appear to bias these measurements significantly. The cosmic star formation rate measured in DLAs is comparable to that of Lyman break galaxies and may explain the observed excess of the Near Infrared Background radiation. Finally, I will describe an ongoing effort to determine the mass of DLAs by studying their cross-correlation with Lyman break galaxies.
Cosmological Insights from TeV Gamma Rays
The present and future generations of TeV gamma-ray detectors can provide interesting probes of a variety of cosmological issues. The technique of ground-based gamma ray astronomy and some of the cosmological questions which it can address will be presented. Recent results from the Whipple 10 m telescope will be discussed, as well as prospects for the new VERITAS 7-telescope array, currently under construction.
Inflation, Braneworlds and Quintessence
Inflationary cosmology is developed in the second Randall-Sundrum braneworld scenario, where the accelerated expansion arises through potentials that are too steep to drive inflation in conventional cosmology. A relationship between the scalar and tensor perturbation spectra is derived that is independent of both the inflaton potential and the brane tension. It is found that a single field with an inverse power law potential can act as both the inflaton and the quintessence field for suitable values of the brane tension.
k-Essence and late time cosmic acceleration
Several different experiments seem to imply that the universe is presently undergoing a stage of accelerated expansion. If this expansion is due to the late dominance of a cosmological constant or a slowly evolving scalar field, it is hard to understand why cosmic acceleration has started just recently rather than at a much earlier epoch. k-Essence attempts to address this puzzle by linking late time cosmic acceleration to the moment of matter-radiation equipartition. k-Essence relies on a very general family of scalar fields with non-linear kinetic terms. We discuss how the requirement of a consistent cosmology constrains these kinetic terms and how due to dynamical reasons k-essence happens to drive cosmic acceleration today. We also point out how, in principle, it is possible to experimentally distinguish k-essence from other dark energy forms such as a cosmological constant or quintessence.
Large scale structure and voids
Whereas most of the luminous mass in the universe is in dense clusters of galaxies, most of the volume is in regions which are much less dense. I'll discuss analytic models for the number and spatial distribution of massive clusters, and show how the models can be extended to describe voids.