The oldest light in the universe is that of the cosmic microwave background (CMB). This light was formed when the dense matter at the beginning of the universe finally cooled enough to become transparent. It has traveled billions of years to reach us, from a bright orange glow to cool, invisible microwaves. Of course, it is an excellent source for understanding the history and expansion of the cosmos.
The CMB is one of the ways we can measure the rate of cosmic expansion. In the early universe, there were small variations in density and temperature in the hot, dense ocean of the Big Bang. As the universe expanded, so did the fluctuations. The magnitude of the fluctuations we see today in the cosmic microwave background shows us how the universe must have grown. On average, the fluctuations are about a billion light-years, and this gives a value for the rate (the Hubble parameter) between 67.2 and 68.1 km / s / Mpc.
The Atacama Cosmology Telescope. Photo credit: Jon Ward
Of course, the CMB isn't the only way to measure the Hubble parameter. In a previous post I talked about how you can use variable stars and distant supernovae to create a cosmic distance ladder that tells you the rate of expansion. The problem is that this alternative method gives a larger value for the Hubble parameter. If the Supernova Method is correct, the universe is younger and expanded faster than the CMB scale seems to support. For some time now there has been hope that new observations and new methods of measuring cosmic expansion would solve this problem, but a new study shatters those hopes. This study examined the cosmic microwave background using the Atacama Cosmology Telescope (ACT) in northern Chile.
How the CMB arises from the last scatter. Photo credit: Yacine Ali-Haïmoud
The most detailed observations of the CMB are made with satellites such as the Planck satellite. When you are in space, you have a clear view of the cosmic residual heat and can measure temperature fluctuations. The Atacama Cosmology Telescope is on land, but it's high in the Andes, where the air is very thin and dry, so it has reasonably good visibility of the CMB. But it was also specially developed to study 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 scattered 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 is largely transparent to light. The CMB light we see scattered one last time before things became clear enough to reach us. When light is scattered by something, it is oriented, or polarized, relative to that scattering. Thus, all of the CMB light is polarized and its orientation says something 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 areas in the CMB indicates the speed of cosmic expansion, so is the size of uniform polarization areas. The team measured the polarization scale more accurately than ever and found the Hubble parameter to be between 66.4 and 69.4 km / s / Mpc. This gives an age of the universe of 13.77 billion years, which is consistent with Planck's measurements of the CMB.
Now we have two independent precision measurements of cosmic expansion from the CMB, and they are in agreement. However, other measurements with supernovae do not match, so we clearly do not understand something here. At this point it is clear that some aspects of our cosmological model need to be revised.
Reference: Choi, Steve K. et al. "The Atacama Cosmology Telescope: A Measurement of Cosmic Microwave Background Power Spectra at 98 and 150 GHz." Journal of Cosmology and Astroparticle Physics 2020.12 (2020): 045.