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 down enough to become transparent. It took billions of years to reach us, ranging from a bright orange glow to cool, invisible microwaves. It is, of course, an excellent resource 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 fluctuations in density and temperature within the warm dense sea of the big bang. As the universe expanded, so did the fluctuations. The scale of fluctuations we see in the cosmic microwave background today therefore tells us how the universe should grow. On average, the fluctuations are about a billion light-years, and this gives us a value for the speed (the Hubble parameter) between 67.2 and 68.1 km / sec / Mpc.
Of course, the CMB is not the only way you can measure the Hubble parameter. In an earlier post, I talked about how you can use variable stars and distant supernovae to create a cosmic distance theory that tells you the rate of expansion. The problem is that this alternative method gives a greater value to the Hubble parameter. If the supernova method is right, the universe is younger and has expanded faster than the CMB scale seems. The hope was for a while that new observations and new methods of measuring cosmic expansion would solve this problem, but a new study shames that hope. This study looked at the cosmic microwave background using the Atacama Cosmology Telescope (ACT) in northern Chile.
Most detailed observations of the CMB are made with satellites such as the Planck satellite. By being in space, you give a clear picture of the cosmic residual heat, so that you can measure temperature fluctuations. The Atacama Cosmology Telescope is land-based, but it is high in the Andes, where the air is very thin and dry, so the CMB can be seen quite well. But it is also specially designed to look at the polarization of the cosmic light.
The early universe was filled with light, but because it was so hot and ionized, photons could not travel far before scattering 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 see made one last scatter before things were cleared enough to reach us. When light scatters something, it is oriented, or polarized, relative to the scattering. All the CMB lights are thus polarized and their orientation tells us about the early universe.
The team used this polarization to determine the age and expansion rate of the cosmos. Just as the magnitude of uniform temperature regions in the CMB tells us the rate of cosmic expansion, so too does the magnitude of uniform polarization regions. The team measured the polarization scale more closely than ever before and determined that the Hubble parameter is between 66.4 and 69.4 km / sec / Mpc. This gives the age of the universe 13.77 billion years, which is in line with Planck’s measurements of the CMB.
So we now have two independent exact measures of cosmic expansion from the CMB, and it agrees. But other measurements using supernovae disagree, so there is clearly something we do not understand here. At this point, it is clear that some aspect of our cosmological model needs to be revised.
Reference: Choi, Steve K., et al. “The Atacama Cosmology Telescope: A Measurement of the Cosmic Microwave Background Power Spectra at 98 and 150 GHz.” Journal of Cosmology and Astroparticle Physics 2020.12 (2020): 045.