Within the earliest moments of the Universe, the primary photons have been trapped in a sea of ionized gasoline. They scattered randomly with the recent nuclei and electrons of the cosmic fireball, like tiny boats in a stormy sea. Then, about 370,000 years after the massive bang, the Universe cooled sufficient for the photons to be free. After one final scattering, they may lastly ply interstellar house. A few of them traveled throughout 14 billion years of house and time to succeed in Earth, the place we see them as a part of the cosmic microwave background. The remnant first mild of creation.
The CMB is a central level of proof supporting the Massive Bang and the usual mannequin of cosmology. By observing the size of fluctuations throughout the CMB, we are able to measure issues reminiscent of the form of house, the distribution of matter and power, and the speed of cosmic growth. It’s that final one which has been troubling astronomers, due to the Hubble pressure downside.
Astronomers have a number of methods to measure the Hubble parameter, the worth of which tells us the speed of cosmic growth. The strategies usually fall into two sorts: these based mostly on observations of the CMB, and people based mostly on astrophysical phenomena reminiscent of supernovae. The issue is that these two forms of strategies don’t agree on the worth. They even contradict one another, main some astronomers to argue there should be one thing fallacious with the usual mannequin.
Of the 2 sorts, the CMB methodology is the one with essentially the most restricted knowledge. The very best CMB observations we’ve got come from house telescopes reminiscent of Planck, which measured fluctuations in CMB depth. One resolution to the stress downside could be to argue that the CMB observations are in some way biased. However new observations gathered by the South Pole Telescope (SPT) throw that concept out of the water.
Slightly than measuring depth fluctuations within the cosmic microwave background, the SPT noticed variations in its polarization. All of the CMB mild we observe comes from a second of final scattering, when photons scattered off an ion one final time earlier than making the billion-year journey to succeed in us. When mild is scattered, it’s polarized relative to the distribution of ionized gasoline. So these observations are a very impartial measure of cosmic growth.
One problem in working with polarized CMB knowledge is that as the primary mild traveled by way of house, it interacted with matter, house, and time. Not solely is the sunshine red-shifted resulting from cosmic growth, it’s gravitationally lensed by galaxies, which adjustments the polarization. A few of the mild scatters off interstellar gasoline, which supplies a false polarization. Even ripples of gravitational waves can have an effect on the sunshine’s orientation. So the crew checked out not simply the uncooked polarization of the CMB, but in addition what are referred to as E-mode and B-mode polarization. Every of those is delicate to completely different sorts of bias. For instance, the E-mode is extra delicate to secondary scattering, whereas the B-mode is extra delicate to cosmic inflation and gravitational waves.
By combining and contrasting these polarization modes, the crew was in a position to calculate a brand new worth for the Hubble parameter. Because it isn’t based mostly on depth fluctuations, it is freed from any bias within the space-based CMB observations. Primarily based on their knowledge, the crew bought a price of H0 at 66.0–67.6 (km/s)/Mpc. This agrees with the intensity-based observations of WMAP and Planck, which discovered a price of 67–68 (km/s)/Mpc. Compared, the astrophysical strategies discover a worth of 73–75 (km/s)/Mpc.
This research confirms that earlier CMB observations aren’t biased. The Hubble pressure could be very actual, and we presently haven’t any clear technique to resolve it.
Reference: SPT-3G Collaboration. “Cosmology From CMB Lensing and Delensed EE Energy Spectra Utilizing 2019-2020 SPT-3G Polarization Information.” arXiv preprint arXiv:2411.06000 (2024).
