Asymptotic giant branch (AGB): A phase of stellar evolution undergone by a low to intermediate mass star (0.6 to 10 solar masses) late in its life and appearing in a second red giant phase. They have a dormant, helium-filled core surrounded by a helium-fusing shell and then a hydrogen-fusing shell.
Atacama Cosmology Telescope (ACT): A 6-meter-diameter, millimeter-wavelength telescope on Cerro Toco in the Atacama Desert of northern Chile, used to study the cosmic microwave background radiation (CMB).
Atacama Large Millimeter/submillimeter Array (ALMA): An international interferometer located at Llano de Chajnantor in the Atacama Desert of northern Chile.
Auger: The Pierre Auger Cosmic-Ray Observatory. Array of light detectors and water tanks in Argentina used to detect Čerenkov radiation produced by relativistic charged particles as they pass through Earth’s atmosphere to study extremely high energy cosmic rays.
Axion: A postulated elementary particle. One of the mysteries of the standard model of physics is the lack of CP (charge conjugation and parity) symmetry violation in the strong interaction (that holds nuclei together). One of the solutions of the “CP problem” is to postulate a new symmetry, the Peccei-Quinn symmetry. If this symmetry exists in nature and is at the appropriate scale, then copious numbers of axions would be produced in the early universe. The axion is suggested as a possible explanation of dark matter.
B-mode polarization: Component of the CMB polarization map that can be expressed as the curl of a vector field and has zero divergence. This contrasts with the E-mode polarization, which can be expressed as the gradient of a scalar field (and hence is curl free). B-mode polarization can arise because of gravity waves from the inflation at the beginning of the universe.
Baryon: Strictly speaking, “baryon” is the collective term given to particles composed of three quarks (with the neutron and proton being the two most common examples). In the present context, the term is used to encompass all normal matter that interacts via electromagnetic forces. The vast majority of dark matter is believed to be non-baryonic.