Cosmology

The spin-flip transition in neutral hydrogen may be used to probe large-scale structure at high redshifts, before the first luminous objects formed. The huge number of modes potentially accessible make this a very promising avenue. I will discuss several key unknowns that could be measured with high-redshift 21cm surveys: primordial non-gaussianity, the primordial small-scale power spectrum, and dark-matter-baryon interactions. I will close by discussing CMB spectral distortions, another promising probe of early Universe physics, and illustrate how they can be used to test dark-matter interactions with standard model particles.

CHIME will use the 21cm emission line of neutral hydrogen to map large-scale structure between redshifts of 0.8 and 2.5. By measuring BAO we will place constraints on the dark energy equation of state as it begins to dominate the expansion of the Universe, particularly at redshifts poorly probed by current BAO surveys. In this talk I will introduce CHIME, a transit radio interferometer designed specifically for this purpose. I will discuss its goals and describe the powerful new analysis techniques we have developed to confront the many challenges of such observations, in particular removal of astrophysical foregrounds which are six orders of magnitude larger than the 21cm signal. A smaller 40m x 37m pathfinder telescope is currently operating at the DRAO in Penticton, BC, and the construction of the full-sized 80m x 100m instrument commenced in early 2015. I will report on current progress, and the lessons already learned.

I will present results from the SDSS-III BOSS-DR11 analysis. In this talk I will focus on the analysis of the power spectrum multipoles, which allows to constrain the growth of structure through redshift-space distortions. Such measurements can be used to test GR and measure the sum of the neutrino masses. Beside RSD we also constrain the geometry of the Universe through the Alcock-Paczynski effect and Baryon Acoustic Oscillations.

New results on the observational signatures of large-scale flows during reionization, using new simulations and analytical methods, will be presented. A preliminary analysis will be described, indicating that cross-correlation of diffuse radiation with tracers of the large scale matter fluctuations can in principle provide new constraints on the timing and duration of reionization, via peculiar velocities.

I'll present a series of numerical experiments to test simple analytical predictions for large-scale Lyman-alpha forest bias parameters. Despite relying on second-order SPT, some of the predictions are surprisingly accurate, especially if thermal broadening is not taken into account. I'll also discuss details of using filtered and squared small-scale fields as robust tracers of large-scale structure that might be useful for non-Gaussianity measurements.

Present day streaming motions of neutrinos relative to dark matter and baryons are several hundred km/s, comparable with their thermal velocity dispersion. This results in a unique dipole anisotropic distortion of the matter-neutrino cross power spectrum, which is observable through the dipole distortion in the cross correlation of different galaxy populations. Such a dipole vanishes if not for this relative velocity and so it is a clean signature for neutrino mass. We estimate the size of this effect and find that current and future galaxy surveys may be sensitive to these signature distortions.

The measurement of distance has long been a fundamental challenge in astrophysics. We have developed a method of inferring distances to astrophysical sources using spatial cross-correlations with galaxies of known redshift. These “clustering redshifts” are robust to problems plaguing other distance estimates and require only knowledge of the on-sky position of the sources. We have verified the method with sources with spectroscopic redshifts, demonstrating accuracies exceeding those required for many cosmological probes. Using this technique, we have explored the SDSS photometric galaxies, characterizing their distances and discovering entirely unidentified populations within. Clustering redshifts are proving their potential in the era of large scale surveys, such as LSST and DES, and will be a new tool in unlocking the third dimension of astronomical observations from the radio to the X-ray.

This talk is based on arXiv:1506.07306, with Shun-Pei Miao. We argue that the fine tuning problems of scalar-driven inflation may be worse than is commonly believed. The reason is that reheating requires the inflaton to be coupled to other matter fields whose vacuum fluctuations alter the inflaton potential. The usual response has been that even more fine-tuning of the classical potential $V(\varphi)$ can repair any damage done in this way. We point out that the effective potential in de Sitter background actually depends in a complicated way upon the dimensionless combination of $\varphi/H$. We also show that the factors of $H$ which occur in de Sitter do not even correspond to local functionals of the metric for general geometries, nor are they Planck-suppressed.

Modeling the cosmic velocity field, and especially estimating its lowest order moment, the bulk flow, has been a popular pursuit among aficionados in the cosmological community for three decades now. Other than estimating the magnitude and direction of the flow, one of the main difficulties has been defining the scale of flow detected. There is a nearly universal agreement as to the direction of the flow, however, there is some disagreements regarding the magnitude and scale of the flow. We developed and applied the Minimal Variance (MV) formalism to optimize and clearly define the scale of a particular analysis, using the width of the survey window function as a proxy for scale. Comparing the MV ideal window function to any analysis window function gives an unbiased estimate to the survey width (or scale) and thus provides a method to directly compare various results. Further, I will introduce a new estimator of the peculiar velocity from redshift and distance estimates. This estimator results in peculiar velocity estimates that are statistically unbiased and have Gaussian distributed errors. The adoption of the new estimator significantly improves the accuracy and validity of studies of the large-scale peculiar velocity field and eliminates potential systematic biases, thus helping to bring peculiar velocity analysis into the era of precision cosmology. I will discuss the method, compare various recent analyses and show that the disagreements are not as significant as they appear.

When the primordial fluctuations are non-Gaussian and inflation lasts longer than the minimum number of e-folds, the likelihood that we observe mild deviations from isotropy increases. I will present a single framework that encompasses many of the most promising scenarios for generating a hemispherical power asymmetry. This framework allows a comparison of the observational evidence for various models, including some with a natural connection between large scale power suppression and power asymmetry.