The oldest light in the universe is that of the microwave cosmic background (CMB). This light formed when dense matter at the beginning of the universe finally cooled enough to become transparent. It has traveled billions of years to reach us, going from a vivid orange glow to cold, invisible microwaves. Of course, this is an excellent source for understanding the history and expansion of the cosmos.
One way to measure the rate of cosmic expansion is the CMB. In the early universe, there were small fluctuations in density and temperature in the dense, warm sea of the Big Bang. As the universe has grown, the fluctuations have also increased. So, the magnitude of the fluctuations we see in the cosmic microwave background today tells us how the universe must have grown. On average, the fluctuations are about a billion light years in diameter, giving us a value for the rate (the Hubble parameter) between 67.2 and 68.1 km / s / Mpc.
Of course, the CMB is not the only way to measure the Hubble parameter. In a previous article, I talked about how you can use variable stars and distant supernovae to create a cosmic distance scale that tells you the rate of expansion. The problem is that this alternate method results in a larger value for the Hubble parameter. If the supernova method is correct, then the universe is younger and has grown faster than the CMB scale seems to support. For a while, the hope was that new observations and new methods of measuring cosmic expansion would solve this problem, but a new study overshadows those hopes. This study examined the microwave background using the Atacama Cosmological Telescope (ACT) in northern Chile.
The most detailed observations of the CMB are made with satellites like the Planck satellite. Being in space gives you a clear view of residual cosmic heat, allowing you to measure fluctuations in temperature. The Atacama Cosmology Telescope is based on land, but it is high in the Andes, where the air is very thin and dry, allowing it to have a fairly good view of the CMB. But it is also specially designed to watch the polarization of cosmic light.
The early universe was filled with light, but because it was so hot and ionized, photons couldn’t travel far before they dispersed onto a proton or electron. But about 380,000 years after the Big Bang, matter in the early universe cooled enough to become neutral hydrogen and helium, which are largely transparent to light. The CMB light we are seeing made one last scatter before things cleared up enough to reach us. When light diffuses something, it is oriented or polarized in relation to that dispersion. Thus, all CMB light is polarized and its orientation tells us about the early universe.
The team used this polarization to determine the age and rate of expansion of the cosmos. Just as the size of uniform temperature regions in the CMB tells us the rate of cosmic expansion, so does the size of uniform polarization regions. The team measured the polarization scale more accurately than ever and determined the Hubble parameter to be between 66.4 and 69.4 km / s / Mpc. This gives the age of the universe at 13.77 billion years, which is consistent with the CMB’s Planck measurements.
So now we have two independent precision measurements of CMB cosmic expansion, and they agree. But other measurements using supernovae disagree, so there is clearly something we don’t understand here. What is clear at this point is that some aspects of our cosmological model need to be revised.
Reference: Choi, Steve K. et al. “The Atacama Cosmological Telescope: A Measurement of the Microwave Cosmic Background Power Spectrum at 98 and 150 GHz.” Journal of Cosmology and Astroparticle Physics 2020.12 (2020): 045.