Cosmology and Fundamental Physics from High Precision CMB Lensing Science

One of the most striking facts about our universe is that most of its contents are invisible. More than 80% of the matter in our universe is not made of atoms, but instead of invisible dark matter, distributed in an enormous filamentary dark matter network that underlies all visible objects. The form of this dark matter distribution encodes a wealth of information about the contents, origin, and evolution of our universe.

How can we see such invisible dark matter structures? Though dark matter does not emit or scatter light directly, it exerts a gravitational pull that allows us to observe its presence indirectly: a clump of dark matter gravitationally attracts rays of light that are passing by, deflecting their paths and causing everything that lies behind to appear magnified. Observation of this gravitational lensing effect allows us to infer the presence of dark matter. To map out the matter distribution, we can search for subtle lensing features in the most distant source of light: the afterglow of the hot big bang, the cosmic microwave background radiation (CMB). This CMB radiation has traversed the entire cosmic web of dark matter before reaching our telescopes. By finding lensing features in this CMB light, we can reconstruct maps of the matter distribution projected across the entire observable universe.

In past work, I made some of the first measurements of this CMB lensing effect. Now, for the first time, new experiments will provide CMB data of extremely high quality, which have immense potential for high-precision lensing measurements of the dark matter distribution. However, the lensing features are more than a hundred thousand times smaller than the mean brightness of the CMB, so measuring them reliably from noisy data can be challenging at this level of precision. My research program will involve work in theory, simulation, statistical methods and data analysis that will enable such powerful lensing measurements. Analyzing data from upcoming CMB surveys known as AdvancedACT and Simons Array, I will extract the lensing signal at unprecedented precision and construct a high-resolution map of the dark matter distribution across much of the universe.

Such highly precise CMB lensing mass maps will be powerful probes of new physics. For example, the form of the cosmic dark matter distribution is affected by the presence of neutrinos, a type of particle with poorly understood properties. Though neutrinos make up a quarter of the known elementary particles, their masses are completely unknown. The shape of the distribution of dark matter depends on the masses of these particles, because the more massive neutrinos are, the more their motions smooth out fine features in the cosmic dark matter distribution. With precise CMB lensing maps, I will measure the detailed shape of the matter distribution and hence determine how massive neutrinos are. This will elucidate the properties of this mysterious type of particle and give insight into the physical origin of their masses.

Precise knowledge of the CMB lensing signal will also allow us to learn more about the beginning of the universe. Our leading theory for the cosmic origin is inflation -- a mechanism that causes the universe to initially expand exponentially fast. However, this mechanism has not been definitively established, and little is known about the energy with which it took place. While a certain characteristic pattern (B-modes) in the polarization of the CMB would be definitive evidence for inflation and would determine its energy, measurements of this inflationary pattern are currently limited because inflationary effects can be confused with similar effects from lensing. However, if we can directly measure the CMB lensing signal, we can disentangle the lensing effects from the inflationary effects. With the precise lensing maps I construct, I will thus enable more powerful constraints on inflation and the beginning of our universe.