One of the most remarkable facts about our universe is that most of its contents are invisible. In particular, more than 80% of the mass in our universe is not made of atoms, but instead of invisible dark matter, distributed in a filamentary dark matter network that underlies all visible mass. 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, its gravitational pull 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 traveled through 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.
Over the next five years, new experiments will provide CMB data of extremely high quality; these data have immense potential for high-precision lensing measurements of the dark matter distribution. However, the lensing features are minuscule, so measuring them reliably from noisy data can be challenging at this level of precision. This ERC project involves work in theory, simulation, statistical methods, and data analysis that will enable such powerful lensing measurements. Analyzing data from state-of-the-art CMB surveys known as AdvancedACT and Simons Observatory, we are working to extract the lensing signal at unprecedented precision and construct a high-resolution map of the dark matter distribution across much of the universe.
The ERC project is organized within three themes; each theme focuses on using our CMB lensing mass maps to gain insight into a certain type of new physics. The goal of the first theme is to measure the unknown neutrino mass with our new lensing measurements. In particular, 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 significant fraction of the known elementary particles, their masses are unknown. The shape of the distribution of dark matter depends on the masses of these particles; therefore, with high-precision CMB lensing maps, we can 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.
A second theme focuses on combining our lensing maps, which are two-dimensional projections, with maps of the distribution of galaxies, for which we know the full three-dimensional distribution. This allows us to probe the growth of dark matter structures in three dimensions, which in turn can shed light on the mysterious phenomenon of cosmic acceleration; in particular, our goal is to test whether cosmic acceleration is due to a constant or time-varying form of energy.
Precise knowledge of the CMB lensing signal will also allow us to learn more about the beginning of the universe, which is the focus of our third project theme. 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 its energy scale and other details. 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 (in a process known as delensing). Using the precise lensing maps my group is constructing and the new methodology we are developing, we are aiming to provide more powerful constraints on inflation and the beginning of our universe.
