PhD Thesis Defenses, 2005
Jennifer Chen, "Probing scalar couplings through tests of the equivalence principle"
January 14, 2005
Online Materials

Scientific Advisor: Sean M. Carroll

It could be that our universe contains one or more nearly-massless neutral scalars, either as low energy relics of the UV complete theory, or as dynamical dark energy as called upon by observations. Here we discuss phenomenological ramifications of the coupling of a light scalar to the Standard Model. More precisely, we argue that low energy effects of this scalar are dominated by its linear couplings to gauge field kinetic terms and to fermion mass terms, which could then source fifth forces and induce variations in the "constants."

We determine the limits on each of these couplings, first by determining the strength of the source from each sector. We find that couplings to the gluon kinetic term and to the strange quark mass term are most constrained by current null results for long range composition dependent fifth forces. Should such as detection occur, it would most likely arise from couplings to these sectors. If we are fortunate enough to make multiple measurements of scalar forces with test body pairs of different compositions, it would be possible to determine each of these couplings. Since any high energy theory containing a light scalar coupled to matter and gauge fields has a four dimensional effective field theory of this form, knowing these coupling would provide an indirect test of theories beyond the Standard Model.

In addition to fifth forces arising from spatial variations from the scalar, the time variation of the scalar can drive a time variation of the constants. There has been a claimed detection of a lower value of the fine structure constant at a redshift z ~ 1.5. We find that this detection of a 0.001% change in the fine structure constant is not possible to explain with a simple quintessence model while remaining consistent with other constraints on the time variation of the fine structure constant. On the other end of the spectrum, a different group has obtained results with tight error bars consistent with no variation. From this, we place even better bounds on scalar couplings to the photon kinetic term.

Related Links:
KICP Members: Sean M. Carroll
KICP Students: Jennifer Chen

Samuel LaRoque, "Constraints on Cluster Structure & Cosmology from X-ray and Sunyaev-Zel'dovich Effect Properties of Galaxy Clusters"
June 15, 2005
Online Materials

Scientific Advisor: John E. Carlstrom

We present a study of 36 massive galaxy clusters at redshifts 0.14 <= z <= 0.89, whose properties are used to constrain cluster structure and cosmological models. The clusters are studied at both X-ray wavelengths using data from the Chandra X-ray Observatory , and at centimeter wavelengths using Sunyaev- Zel`dovich Effect (SZE) data from the BIMA and OVRO interferometric arrays. Likelihood analysis is performed using a Markov chain Monte Carlo (MCMC) method, and we motivate and adopt an isothermal b-model with central 100 kpc excluded from the X-ray data to describe the intracluster medium in all 36 clusters. This model is shown to provide consistently good fits to clusters with a wide range of morphological properties. Best-fit cluster gas masses, total masses, and integrated SZE fluxes are determined from this model with in the MCMC framework. X-ray and SZE results are then used in an effort to constrain cluster structure and cosmology. First, the integrated SZE flux is compared with X-ray derived gas mass, spectroscopic temperature, and total mass to determine how firmly connections between observables and derived cluster properties can be established in SZE surveys. These properties are found to display tight scaling relations over the full range of redshift. Cluster gas mass fractions from both X-ray and SZE data are then compared, and the implications for cluster structure are noted. The gas mass fractions are used to obtain constraints on O M and O L both by comparing the mean f gas values to O B /O M for assumed O B , and by assuming that f gas should be constant with redshift and determining what cosmology best facilitates this. Cosmological constraints are found to be consistent with CMB results, but a better understanding of the cluster baryon budget is needed to achieve the same level of precision.

Related Links:
KICP Members: John E. Carlstrom
KICP Students: Sam LaRoque
Scientific projects: Interferometric Sunyaev-Zel'dovich Effect Imaging Experiment (SZE)

Daisuke Nagai, "Sunyaev-Zeldovich Scaling Relations in Cosmological Cluster Simulations"
July 22, 2005
Picture: Daisuke Nagai, Sunyaev-Zeldovich Scaling Relations in Cosmological Cluster Simulations
Online Materials

Scientific Advisors: John E. Carlstrom, Andrey V. Kravtsov

We study the effects of radiative cooling and galaxy formation on the Sunyaev- Zel'dovich (SZ) observable-mass relations using high-resolution cosmological simulations. The simulations of eleven individual clusters spanning a decade in mass are performed with the shock-capturing eulerian adaptive mesh refinement N-body+gasdynamics ART code. To assess the impact of galaxy formation, we compare two sets of simulations performed in the adiabatic regime (without dissipation) and with radiative cooling and several physical processes critical to various aspects of galaxy formation: star formation, metal enrichment and stellar feedback. We show that the SZ signal integrated to sufficiently large fraction of the cluster volume correlates strongly with the enclosed cluster mass, regardless of the details of the cluster physics or dynamical state of the cluster. The slope and redshift evolution of the SZ flux-mass relation are also insensitive to the details of the cluster gas physics, and they are well characterized by the simple self-similar cluster model. While the tightness, slope and redshift evolution are relatively unaffected, we show that the radiative cooling and galaxy formation significantly modify the normalization of the SZ scaling relations. In our simulations, we find that the gas cooling and associated star formation suppress the normalization by [approximate]30- 40%. The effect is due to the decrease in the hot gas fraction, which is offset slightly by the increase in the gas temperature. The baryon dissipation also slightly modifies the cluster mass and affects the normalization by non- negligible amount. Finally, we show that the simulations that include gas cooling and star formation are in good agreement with the recent observational results on the SZ scaling relations obtained using 36 OVRO/BIMA SZ+Chandra X- ray cluster observations, while the simulations neglecting galaxy formation are inconsistent with the observed correlation. The comparison highlights the importance of galaxy formation in theoretical modelling of clusters and shows that the current generation of simulations produce clusters with gross properties quite similar to their observed counterparts.

Related Links:
KICP Members: John E. Carlstrom; Andrey V. Kravtsov
KICP Students: Daisuke Nagai

Vikram Duvvuri, "Modified-Gravity as an Alternative to Dark Energy"
August 18, 2005 | 1:30 PM | LASR conference room
Online Materials

Scientific Advisor: Michael S. Turner

We consider general curvature-invariant modifications of the Einstein-Hilbert action that become important only in regions of extremely low space-time curvature. We investigate the late-time evolution of the universe in such models, examining the possibilities for cosmic acceleration and other ultimate destinies. The models generically possess de Sitter space as an unstable solution and exhibit an interesting set of attractor solutions which, in some cases, provide alternatives to dark energy models. We show that modifications of the form f ( R ) are ruled out by solar system tests of gravitation. In addition, we also review the Palatini method of variation for such theories and contrast it with the metric variation approach.

Related Links:
KICP Members: Michael S. Turner
KICP Students: Vikram V. Duvvuri

Argyro Tasitsiomi, "Lyman-alpha radiative transfer in cosmological simulations and application to a z [approximate] 8 Ly-alpha emitter"
October 20, 2005
Picture: Argyro Tasitsiomi, Lyman-alpha radiative transfer in cosmological simulations and application to a z [approximate] 8 Ly-alpha emitter
Online Materials

Scientific Advisor: Andrey V. Kravtsov

I develop a Lyman-a radiative transfer (RT) Monte Carlo code for cosmological simulations. High resolution, along with appropriately treated cooling, can result in simulated environments with very high optical depths. Thus, solving the Lyman-a RT problem in cosmological simulations can take an unrealistically long time. For this reason, I develop methods to speed up the Lyman-a RT. With these accelerating methods, along with the parallelization of the code, I make the problem of Lyman-a RT in the complex environments of cosmological simulations tractable.

I test the RT code against simple Lyman-a emitter models, and then I apply it to the brightest Lyman-a emitter of a gas dynamics+N-body Adaptive Refinement Tree (ART) simulation at z ~= 8. I find that recombination rather than cooling radiation Lyman-a photons is the dominant contribution to the intrinsic Lyman- a luminosity of the emitter, which is ~= 2.3 x 10 44 ergs/s. The size of the emitter is pretty small, making it unresolved for currently available instruments. Its spectrum before adding the Lyman-a Gunn-Peterson absorption (GP) resembles that of static media, despite some net inward radial peculiar motion. This is because for such high optical depths as those in ART simulations, velocities of order some hundreds km/s are not important.

I add the GP in two ways. First I assume no damping wing, corresponding to the situation where the emitter lies within the HII region of a very bright quasar, and second I allow for the damping wing. Including the damping wing leads to a maximum line brightness suppression by roughly a factor of ~ 62. The line fluxes, even though quite faint for current ground-based telescopes, should be within reach for JWST.

Related Links:
KICP Members: Andrey V. Kravtsov
KICP Students: Argyro Tasitsiomi