Talks & Events
Ph.D. Thesis Defenses: 2007
Modified gravity as dark energy
We study the effects of introducing modifications to general relativity ("GR") at large scales as an alternative to exotic forms of matter required to replicate the observed cosmic acceleration. We survey the effects on cosmology and solar-system tests of Dvali-Gabadadze-Porrati ("DGP") gravity, f ( R ) he changes to the background expansion history of the universe, these modifications have substantial impact on structure formation and its observable predictions.
For DGP, we develop a scaling approximation for the behaviour of perturbations off the brane, for which the predicted integrated Sachs-Wolf ("ISW") effect is much stronger than observed, requiring new physics at around horizon scale to bring it into agreement with data. We develop a test based on cross-correlating galaxies and the ISW effect which is independent of the initial power spectrum for perturbations and is a smoking-gun test for DGP gravity.
For f ( R ) models, we find that, for the expansion history to resemble that of Lambda-CDM, it is required that the second derivative of f with respect to R be non-negative. We then find the conditions on f ( R ) which allow this subset of models to pass solar-system tests. Provided that gravity behave like GR in the galaxy, these constraints are weak. However, for a model to allow large deviations from GR in the cosmology, the galactic halo must differ significantly from that predicted by structure evolution in GR. We then discuss the effect that these models have on structure formation, and find that even in the most conservative of models, percent-level deviations in the matter power spectrum will exist and should be detectable in the future.
Finally, for MSG, we investigate the cosmology of a theory of gravity with a modified constraint structure. The acceleration era can be replicated in these models; however, linear perturbations become unstable as the universe begins to accelerate. Once the perturbations become non-linear, the model reverts to GR, regaining stability. This leaves a significant imprint on structure-formation probes, but one which we cannot calculate in the linear approximation.
The Climate Dynamics of Titan
We study the climate dynamics of Titan by developing a hierarchy of planetary climate models and theories. We begin with a one-dimensional radiative- convective model of Titan's atmosphere including the greenhouse and antigreenhouse effects and a generalized moist convection scheme. Our simulations indicate the thermodynamics of methane evaporation and condensation play fundamental roles in establishing deep, precipitating convection while maintaining surface energy balance with the weak solar forcing at Titan's surface.
We then derive an extension to a steady, analytic theory for the large-scale circulation of an atmosphere and apply the theory to Titan. The theory predicts Titan's meridional overturning circulation, or Hadley cell, spans the globe. Titan's Hadley cell tends to eliminate latitudinal temperature gradients, which is consistent with the observed weak equator-to-pole surface temperature gradients. We expect Titan's Hadley cell to globally converge moisture into the large-scale updraft and suppress convection everywhere else; resulting cloud patterns should appear sparse and isolated in latitude.
We then study the seasonal cycle in a zonally symmetric general circulation model of Titan's climate with an unlimited surface supply of methane. This model produces condensation consistent with the position and timing of observed clouds, but only with the thermodynamic effect of methane condensation and evaporation included. The large-scale circulation in our simulations latitudinally oscillates with season, which in the annual mean dries the low- latitude surface. However, self-consistent drying of the surface requires an accounting of the methane reservoir.
Finally, we present zonally symmetric general circulation model simulations with a soil model for the lower boundary and a finite reservoir of methane. Due to annual-mean moisture divergence of the oscillating large-scale circulation, more than 50 m of liquid methane is removed from the low-latitude surface and deposited at mid and high latitudes. Simulations with total reservoir depth below 50 m completely dry the low latitude surface. All simulations with the soil model produce condensation at positions and times consistent with observed clouds.
Cosmic Microwave Background Analysis for CAPMAP and Future Experiments
A major frontier for cosmology in the coming decade will be making precision measurements of the cosmic microwave background (CMB) polarization, complementing existing measurements of the CMB temperature anisotropies. The E- mode, or gradient-like component, of CMB polarization will break parameter degeneracies from CMB temperature alone and improve constraints on reionization history and initial conditions in the standard cosmological model. The B-mode, or curl-like component will permit strong constraints on growth of structure from CMB lensing, and probe new physics by measuring the gravity wave content of the early universe.
In the first half of this thesis, we describe design and implementation of the analysis pipeline for the 2005 observing season of CAPMAP, an experiment to measure CMB polarization on small angular scales using coherent polarimeters and the Lucent 7 meter telescope in Crawford Hill, New Jersey. Although the results of the analysis are not completely finalized, we present partial results obtained from the data, and full results for a full-season simulation, in order to illustrate the measurement that will be obtained.
The CAPMAP analysis pipeline uses a likelihood formalism which is computationally expensive, but results in measurement uncertainties which are provably optimal. An optimal analysis will be computationally infeasible for upcoming generations of CMB polarization experiments, in which the problem size will be larger by several orders of magnitude. Therefore, fast approximate methods have been proposed. In the second half of this thesis, we show that in their originally proposed form, these methods fail to preserve the E- B decomposition, and this failure ultimately acts as a limiting source of noise when measuring B-modes, and propose modifications which solve this problem.
A Catalog of Slow-Moving Objects Extracted from the SDSS: Compilation and
I have compiled the Slow-Moving Object Catalog of Known minor planets and comets ("the SMOCK") by comparing the predicted positions of known bodies with those of sources detected by the Sloan Digital Sky Survey (SDSS) that lack positional counterparts at other survey epochs. For the ~50% of the SDSS footprint that has been imaged only once, I have used the Astrophysical Research Consortium's 3.5-meter telescope to obtain reference images for confirmation of Solar System membership.
The SMOCK search effort includes all known objects with orbital semimajor axes a > 4.7 AU, as well as a comparison sample of inherently bright Main Belt asteroids. In fact, objects of all proper motions are included, resulting in substantial overlap with the SDSS Moving Object Catalog (MOC) and providing an important check on the inclusion criteria of both catalogs. The MOC does not contain any correctly-identified known objects with a > 12 AU, and also excludes a number of detections of Main Belt and Trojan asteroids that happen to be moving slowly as they enter or leave retrograde motion.
The SMOCK catalog is a publicly-available product of this investigation. Having created this new database, I demonstrate some of its applications. The broad dispersion of color indices for transneptunian objects (TNOs) and Centaurs is confirmed, and their tight correlation in ( g - r ) vs ( r - i ) is explored. Repeat observations for more than 30 of these objects allow me to reject the collisional resurfacing scenario as the primary explanation for this broad variety of colors. Trojans with large orbital inclinations are found to have systematically redder colors than their low-inclination counterparts, but an excess of reddish low-inclination objects at L5 is identified. Next, I confirm that non-Plutino TNOs are redder with increasing perihelion distance, and that this effect is even more pronounced among the Classical TNOs. Finally, I take advantage of the byproducts of my search technique and attempt to recover objects with poorly-known orbits. I have drastically improved the current and future ephemeris uncertainties of 3 Trojan asteroids, and have increased by 20%-450% the observed arcs of 10 additional bodies.
Scatter in the Galaxy Cluster Mass-Observable Relations
We use numerical simulations of cosmological structure formation to study the distribution and evolution of galaxy clusters. We employ simulations in both WMAP1 and WMAP3 cosmologies, and simulated with and without the physics of galaxy formation, resulting in a sample of nearly 300 galaxy clusters spanning two decades in mass at the present epoch. We show that the mass weighted temperature of the intracluster medium and the Sunyaev-Zel'dovich (SZ) Compton Y integrated within a radius enclosing 500 times the critical density of the universe each correlates strongly with total cluster mass, and the mean relation is reasonably well described by a simple self-similar model. These relations exhibit remarkably little scatter (10-15%) independent of cluster mass and redshift. We find that the distribution of these quantities about the best fit scaling relations is not well fit by a log-normal distribution, but instead exhibits significant positive kurtosis. Additionally, we find that the residual from the best fit mass-temperature relation correlates with halo temperature, indicating a connection between halo merger history and the properties of the cluster gas. These results have significant implication for the ability of future SZ galaxy cluster surveys to self-calibrate the mass- observable relations and thus constrain cosmological parameters.