CMB-S4 will probe the fundamental physics of the dark side of the Universe, yielding information about dark components that are difficult, or even impossible, for us to observe directly. Probes of the dark Universe enabled by CMB-S4 include exhaustive searches for new particles, guaranteed measurements to answer outstanding questions about the Standard Model of particle physics, and exploratory studies of the mysterious nature of dark matter and dark energy.
A key goal of CMB-S4 is to search for new particles, low-mass relics of the Big Bang, beyond the known particles in the Standard Model of particle physics. While these particles may have stopped interacting with, or decoupled from, photons and other known particles as the Universe cools down, such light relics would contribute to the energy density in the Universe and would change the Universe’s expansion rate. CMB-S4 will be able to detect the existence of light relic particles that contribute less than a percent to the total energy of relativistic particles present in the early Universe. Current CMB experiments, such as the Planck satellite, would detect light relic particles that stopped interacting with Standard Model particles after the first 50 micro-seconds of the Hot Big Bang. With CMB-S4 we will push back this frontier by over a factor of 10,000, to fractions of a nanosecond. CMB-S4 is the only experiment on the horizon that will detect particles that decoupled well before the quark-hadron phase transition (the epoch when the Universe cooled sufficiently that quarks became locked into hadrons like neutrons and protons). Neutrinos are an example of a known light relic that decoupled when the Universe’s temperature was around 1 MeV (the mass difference between protons and neutrons); the effect of neutrinos on the CMB has already been detected at high significance. CMB-S4 will be able to detect any particle that decoupled out to energies of 1-100GeV (the exact energy threshold depends on the spin of the particle). The measurements from CMB-S4 will be a major advance in our understanding of the particle content and thermal history of our Universe.
CMB-S4 can answer the unknown question of neutrino masses in the Standard Model by measuring the summed mass of the three neutrino species. Together with terrestrial experiments, this measurement may help us understand why neutrinos have mass. That mass is one of the biggest mysteries of the Standard Model of particle physics. When combined with neutrino oscillation data, a CMB-S4 measurement of the summed neutrino mass will unambiguously determine the absolute mass scale of neutrinos. If the sum is below a certain threshold, CMB-S4 will rule out the inverted mass ordering (2 heavier and 1 lighter neutrino species, as opposed to 2 lighter and 1 heavier). When combined with neutrinoless double beta-decay experiments, the cosmological observations from CMB-S4 can help distinguish whether neutrinos are Majorana particles (their own antiparticles) or Dirac particles (not their own antiparticle, like all the other fermonic particles in the Standard Model). CMB-S4 can also uncover evidence for unknown physics in the neutrino sector. At a bare minimum, find evidence for the neutrino mass even for the lowest mass sum compatible with neutrino oscillation data. The key observables for this measurement are maps of the matter distribution inferred from gravitational lensing of CMB temperature and polarization anisotropies, cross-correlations of these maps with external datasets from galaxy surveys, and measurements of the abundance of galaxy clusters.
Dark matter and dark energy are enormous mysteries today. On one hand, we have a wide range of astronomical and cosmological observations that require these mysterious dark components. On the other hand, we have not yet observed either dark matter or dark energy in the laboratory and so their fundamental nature eludes us. CMB-S4 will test a number of different models for dark matter and dark energy. Maps of the matter distribution at late times from gravitational lensing of CMB photons, along with maps of the hot gas in galaxy clusters inferred from scattering of CMB photons, will test the evolution of structure in our Universe and thereby predictions of different dark energy models. CMB-S4 will constrain a phenomenon known as cosmic birefringence, a rotation of CMB polarization by new scalar fields permeating our Universe, potentially constraining the microphysics of dark energy. Finally, CMB-S4 can test models of dark matter that are inaccessible to laboratory experiments. In particular, CMB observations directly probe the physics of dark matter throughout our Universe and throughout cosmic history, and unlike direct detection experiments, do not rely on assumptions about the local dark-matter distribution in the vicinity of the Earth. CMB-S4 can place constraints on a variety of scenarios, including dark matter that interacts with baryons or with dark radiation, or consists of ultra-light axion-like particles.
CMB-S4 will detect potential dark matter candidates over 33 orders of magnitude in mass, from ultralight axions at less than 10-24 eV to massive particles at the GeV scale. This figure shows the seven major classes of tests, color coded by what observations contribute to the test. “B-modes” refers to observations of the CMB B-mode polarization. “CMB lensing” refers to measurements of the gravitational lensing of the CMB. “Primary CMB” refers to the measurements of the CMB power spectra. “Low-l CMB” refers to measurements of the CMB power spectra at degree-scale or larger angular scales (l < 200). “CMB modulation” refers to a temporal modulation of the CMB photons that would be induced by a coupling with axions.