Tristan Smith, Former BCCP Postdoc

My research lies at the interface between cosmology and gravity. Gravity has always occupied a unique place among the fundamental forces of nature: as the ‘universal force’ it plays an essential role in the evolution of the Universe, from the big bang to today; as the only known force that cannot be written within a consistent quantum theory its underlying nature is expected to help elucidate the complete ‘theory of everything’; as the weakest force it allows information to come to us to from events occurring at energy scales approaching the Planck scale. My research concentrates on using novel probes in order to address questions concerning the dynamics of inflation, the nature of dark energy, observational signatures of string-inspired modifications to general relativity, and the fundamental symmetries of gravity (such as parity and Lorentz invariance). Primordial gravitational-wave backgrounds provide an unparalleled window into physical processes occurring at energy scales as high as the Planck scale. In particular, the gravitational-wave background might allow us to infer fundamental properties of the processes that produce such backgrounds, such as inflation. To this end, along with Marc Kamionkowski and Asantha Cooray, I have studied how the direct detection of the inflationary gravitational-wave background (IGWB) would enable a probe of the fundamental nature of the inflationary epoch. Our work has shown how direct observations of the IGWB will allow us an unprecedented view into the mechanisms behind inflation and that there is a good chance the IGWB will be observable in BBO. However it is possible that a non-standard expansion history could complicate interpretations of the physical significance of the amplitude and slope of the directly observed IGWB. In future research I plan to address these issues and quantify how well we may be able to remove degeneracies with a non-standard expansion history. In addition to this, along with George Smoot and his student Noel Swanson, I have begun a study in order to explore how well we may use pulsar timing arrays in order to detect and measure the properties of gravitational waves. Currently, pulsar timing arrays are the most sensitive probe of gravitational waves and their sensitivity is expected to increase by several orders of magnitude with the construction of the Square Kilometer Array (SKA) radio telescope which plans to be in full operation by 2020. Many groups have explored how to use pulsar timing measurements in order to place limits on stochastic gravitational wave backgrounds. However, less work has focused on identifying how to use this data to explore other sources of gravitational waves such as short duration bursts and polarized backgrounds. We plan to articulate how best to use this data in order to extract as much information about gravitational waves as possible. The study of alternative theories to general relativity has recently gained a lot of interest because of the observation that the expansion of the Universe is accelerating. Many explanations for the accelerated expansion rely on proposing a new source of stress-energy within the context of general relativity (such as a cosmological constant or a scalar field). Alternatively, the observed accelerated expansion might indicate a breakdown of general relativity. Along with Adrienne Erickcek, Marc Kamionkowski, and Takeshi Chiba, I have shown that certain gravity theories [known as f(R)-gravity] that have the potential to explain the current epoch of accelerated expansion, under a particular set of conditions, severely violate observations of gravity within the solar system. Our work has made it clear that it is the non-linear behavior of the theory as well as its behavior on cosmological scales that allows it to pass solar system tests. This non-linear behavior gives f(R) gravity some unique observational consequences. In particular, measurements comparing the bending of light around an object and the object’s mass yields a determination of a parameter denoted gamma_PPN. In general relativity gamma_PPN = 1. However, in f(R) gravity the value of gamma_PPN depends on the strength of the gravitational field around that object. Current lensing surveys, such as the Sloan Lens ACS (SLACS) survey, provide us with observations that may allow us to test whether γPPN assumes a universal value or not thereby testing such alternative gravity theories as f(R) gravity. In ongoing work, I explore how well such surveys can test the universality of the value of gamma_PPN . Besides providing an explanation for the recent epoch of accelerated expansion, modifications to the Einstein-Hilbert action are expected since quantized general relativity is non- renormalizable. One common feature of these modifications is to introduce parity violation into the gravitational sector. In work done in collaboration with Adrienne Erickcek, Robert Caldwell, and Marc Kamionkowski, I have looked at how we may detect parity violation by probing the gravitomagnetic field applicable to the rotating Earth. Two current experiments seek to measure this field: LAGEOS has measured it to within 10% of its general relativistic value and Gravity Probe B will attempt to measure it to at least this accuracy. In related work I have investigated how the post-Newtonian equations of motion depend on an unspecified gravitomagnetic field. Using the fact that any alternative gravitational Ampere’s law must respect mass conservation I have been able to derive a general form that the gravitomagnetic field can take. Using these results I have found the expressions for the secular changes in the inclination, argument of pericenter, and line of nodes applicable to binary systems for a generalized gravitomagnetic field. This framework allows for a more complete analysis of gravitomagnetic effects in these systems, especially the recently discovered double pulsar system, PSR J0737-3039A. To read more, click here.