Mapping Dark Matter on Small Scales with the Cosmic Microwave Background

Probing New Fundamental Physics withan Ultra-Deep, Ultra-High-Resolution CMB Survey P.I.: Neelima Sehgal Stony Brook University,Physics and Astronomy Department,Stony Brook, NY 11794, USA The Cosmic Microwave Background (CMB) is the afterglow of the formation and early evolution of the infant Universe.  This afterglow has revealed a wealth of information about the Universe we live in.  However, it still holds many keys that can answer fundamental physics questions. For example, the bending of the light of the CMB by intervening dark matter structure, as this light travels to us, can allow us to make a map of the dark matter distribution in the Universe on scales smaller than galaxies.  The dark matter on these scales is not necessarily lit up by stars or gas that trace its distribution, so this gravitational light-bending effect, called gravitational lensing, offers one of the best pathways to mapping it.  Once we know the distribution of dark matter on sub-galactic scales, this can inform us about the particle properties of dark matter.  The primordial CMB also contains the signatures of any new light particles (with masses less than 0.1 eV) that were in thermal equilibrium with the known standard-model particles at any time in the early Universe, all the way back to the Big Bang.  Measuring this signature can tell us if new particles exist that we have been so far unaware of. In order to access the information about dark matter and new light particles contained in the CMB, the PI is leading a proposal for a new ground-based CMB experiment, called CMB-HD.  CMB-HD would be about three times more sensitive, with six-times higher resolution, than the planned CMB-S4, over 50\% of the sky.  This would open up a qualitatively new regime of CMB science than precursor CMB surveys.  This would also allow us to probe for the first time the dark matter distribution on small scales with CMB lensing and to definitively probe light relic particles.  Opening up this novel regime of CMB data will undoubtably allow for novel probes of other fundamental physics.  Thus, an objective of this proposal is to explore the broader science gain that could be achieved with a CMB-HD-type survey.  This will be accomplished by exploring models of fundamental physics that could have unique signatures that can be detected with an ultra-deep, ultra-high-resolution CMB survey.  This would also add to the compelling science case for funding the CMB-HD project.