Weighing Dark Matter Halos with the Cosmic Microwave Background

Weighing something that is partially invisible is very challenging. But this is exactly what astronomers contend with when estimating masses of galaxies and their clusters. These structures contain significant amounts of still-mysterious dark matter residing in a “halo” extending beyond the standard, baryonic (and mostly luminous) matter. Gravitational lensing allows astronomers to get around this weighing problem. The effect arises when photons emitted by some distant (background) object are bent by the gravitational force of some intervening (foreground) object, known as the lens. Astronomers observe gravitational lensing as a distortion or brightening of the image of the background source, and from that, they estimate the total mass of the lens. Distant galaxies are typically used as the background objects [1], but astrophysicists suggested nearly a decade ago [2] that the cosmic microwave background (CMB) could be a particularly suitable alternative. Mathew Madhavacheril of Stony Brook University, New York, and his colleagues have now put this idea into practice, using high-resolution CMB data from the Atacama Cosmology Telescope Polarimeter (ACTpol) [3]. The team found evidence for gravitational lensing by dark matter halos on the CMB anisotropies and from that derived an estimate of their average mass. With further refinement, this approach could take advantage of the CMB’s ubiquity to provide accurate mass estimates for more objects and at greater distances (higher redshifts) than the background of distant galaxies can do.