A long-lost type of dark matter may resolve the biggest disagreement in physics
Source: livescience.com
Date: 29 April, 2020
One of the deepest mysteries in physics, known as the Hubble tension, could be explained by a long-since vanished form of dark matter.
The Hubble tension, as Live Science has previously reported, refers to a growing contradiction in physics: The universe is expanding, but different measurements produce different results for precisely how fast that is happening. Physicists explain the expansion rate with a number, known as the Hubble constant (H0). H0 describes an engine of sorts that’s driving things apart over vast distances across the universe. According to Hubble’s Law (where the constant originated), the farther away something is from us, the faster it's moving.
And there are two main ways of calculating H0. You can study the stars and galaxies we can see, and directly measure how fast they're moving away. Or you can study the cosmic microwave background (CMB), an afterglow of the Big Bang that fills the entire universe, and encodes key information about its expansion.
As the tools for performing each of these measurements have gotten more precise, however, it's become clear that CMB measurement and direct measurements of our local universe produce incompatible answers.
Researchers have offered different explanations for the disparity, from problems with the measurements themselves to the possibility we live in a low-density "bubble" within the larger universe. Now, a team of physicists is suggesting that the universe might have fundamentally changed between the time after the Big Bang and today. If an ancient form of dark matter decayed out of existence, that loss would have changed the mass of the universe; and with less mass, there would be less gravity holding the universe together, which would have impact the speed at which the universe expands — leading to the contradiction between the CMB and the direct measurements of the universe's expansion rate.
https://www.livescience.com/amp/lost-dark-matter-hubble-tension.html