Challenges in probing inflation with primordial gravitational waves

Thesis

It is still not yet understood how structure in the Universe was seeded. The theory of inflation provides a framework to understand the origins of structure as quantum fluctuations of the vacuum, whilst maintaining an attractive simplicity, and giving a natural explanation of how the Universe is so flat, homogeneous, and isotropic. If inflation occurred, it would have produced a gravitational wave background, indirectly observable today through its effect on the polarization of the cosmic microwave background.

We present a new model for the polarized microwave sky, combining data from the Planck and WMAP satellites, and legacy radio surveys, with theoretical models for Galactic foreground SEDs. We present an example use-case of this model by forecasting the effect of Galactic foregrounds on a future satellite mission.

We then move on to consider an extension to the simplest inflationary scenario in which an additional set of SU(2) gauge fields are present during inflation, and may source a chiral gravitational wave background. We show that future CMB missions will not be able to distinguish such a scenario from the simplest single field slow-roll model using standard two- point statistical techniques, and that space-based laser interferometers will provide a complementary probe by constraining the short wavelength behavior of the blue-tilted gravitational wave spectrum.

Finally, we demonstrate the efficacy of a new method to account for complex Galactic foregrounds in the analysis of high-sensitivity polarized CMB data from the upcoming Simons Observatory. We first show that ignoring the spatial variation could lead to a false detection of a gravitational wave background, and that this can be corrected for by introducing additional parameters describing the spatial variation of the foreground spectral energy dependence.

Authors: Thorne, BL

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Keywords: cosmology; primordial gravitational waves; cosmic microwave background; foregrounds; inflation
• Subjects: Astrophysics; Cosmology