The cosmic microwave background (CMB) is a faint glow in microwave radiation that is almost perfectly uniform across the sky. This thermal radiation was emitted when the Universe became transparent to photons for the first time, when the Universe was about 400,000 years old.
The CMB spectrum peaks at a wavelength of about 2 mm with a nearly perfect blackbody spectrum corresponding to a temperature of 2.73 K. It is the uniformity of the CMB that established the Big Bang paradigm: the Universe used to be in a hot, dense state and had since expanded and cooled. Although the CMB is extremely uniform, there are slight variations in temperature throughout. The temperature anisotropies of the CMB provide the evidence for the now standard cosmological model LCDM---that our Universe is mostly dark energy and dark matter and only ~5% of its energy content is made up of normal matter.
Not only is the CMB one of the observational pillars of modern cosmology, it continues to provide one of the best handles to some of the biggest unanswered questions. This is because the answers can lie in several fainter aspects of the CMB: polarization of the CMB, CMB lensing, and secondary anisotropies arising from the kinematic and thermal Sunyaev-Zel’dovich effects.
Polarization of the CMB and primordial gravitational waves
Inflation denotes a period of near-exponential expansion of the Universe in the earliest 10^-33 seconds. It is the leading early-universe paradigm amongst cosmologists. Inflation generically predicts the existence of primordial gravitational waves (PGW). These PGWs source a divergence-free polarization signature on the CMB, the B-mode polarization. These PGW-generated B modes are extremely faint. The search of them drives the design of many current- and next-generation CMB experiments, including those that KIPAC scientists are actively involved in.
Lensing of the CMB and the sum of neutrino masses
As CMB photons travel to us, their paths are deflected by the intervening gravitational potentials. Given the distorted (lensed) image of the CMB, KIPAC scientists reconstruct the integrated gravitational potential and use that to infer properties of matter (dark or otherwise). Specifically, one of the big questions in particle physics is what is the sum of neutrino masses, which change the integrated gravitational potential.
The Sunyaev-Zel’dovich effect and dark energy
When CMB photons travel through dense regions such as galaxies, groups, and clusters, some of them undergo inverse-Compton scattering off of electrons in hot, ionized gas. This distorts the blackbody spectrum of the CMB and provides a way to search for galaxy clusters, even out to high redshifts where it is difficult for optical and X-ray surveys. KIPAC scientists use the number density of galaxy clusters to understand the late-time acceleration of the expansion of the Universe, driven by dark energy.
Millimeter-wave observations and galactic science: with CMB telescopes, KIPAC scientists also get to integrate the mm-wave emissions of the interstellar medium to observations from other wavelengths to study the magnetic field of the Milky Way.
Studying the CMB at KIPAC
KIPAC researchers lead and contribute to multiple CMB experiments that enable the multifaceted science given observations of the millimeter-wave sky. We design and fabricate detectors, integrate telescope receivers, reduce data collected with the telescopes, and test our models of the Universe with these observations. With these observations, we push our boundaries on our understanding of the Universe. Major efforts are focused on the search of primordial gravitational waves from inflation using the small-aperture telescopes BICEP/Keck Array at the South Pole and the large-aperture South Pole Telescope. In addition, KIPAC scientists are active in planning upcoming and next-generation CMB experiments such as the Simons Observatory, AliCPT, CMB-S4, and LiteBird.
