Search Talks

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.

Neutrinos occur in great number throughout the Universe, yet remain poorly understood due to their weak interactions with other matter. In particular, individual neutrino masses remain an elusive property for particle physicists and cosmologists abound. Recently, it has been proposed that individual neutrino masses may be constrained from observations of neutrino wakes that result from the relative flow between cold dark matter (CDM) and neutrinos. The only required knowledge for such a detection is the relative velocity field between CDM and neutrinos. However, since neither CDM nor neutrinos are directly observable, their relative flow must be obtained by indirect means. We modify the cosmology code, CUBEP3M, to simulate neutrino N-body particles alongside CDM. We find the result that the relative velocity field can be obtained accurately by applying linear transformations to the halo density field. Assuming prior knowledge of the halo bias, we find that the reconstructed relative velocities are highly correlated with the simulated ones, with correlation coefficients of 0.94, 0.93, 0.92 and 0.88 for neutrinos of mass 0.05, 0.1, 0.2 and 0.4 eV, respectively. We confirm that the relative velocity field reconstructed from large-scale structure observations, such as galaxy or 21 cm surveys, can be accurate in direction and, with appropriate scaling, magnitude.

Beyond its primary fluctuations, the cosmic microwave background (CMB) contains a wealth of information on the large-scale structure of the universe, which it illuminates as a backlight. The baryon momentum field is thus imprinted on the CMB through the kinematic Sunyaev-Zel'dovich (kSZ) effect. Current small-scale high-sensitivity CMB experiments make this effect detectable, providing a unique handle on peculiar velocities and baryon physics. I will report a significant detection of the kSZ effect, obtained by combining CMB intensity data with peculiar velocities reconstructed from the galaxy number density field. I will present the prospects for localizing the missing baryons, constraining baryon physics inside galaxy clusters, and measuring the growth rate of structure from the kSZ effect. I will finally explore the possibility of measuring velocities across the line of sight through the CMB moving lens effect.

I will describe a novel method for simulating nonlinear structure formation in cosmologies that have hot or warm collisionless species. After introducing the method, I will show results of our simulations for universes with massive neutrinos, and warm dark matter simulations.