In all theories that try to explain the Universe (including what existed before the Big Bang), one character is omnipresent: dark matter. It is responsible for expanding the cosmos and also the focal point of a mystery in physics.

The cosmos is getting bigger and bigger, but you can’t tell how quickly; different measurements produce different results. The rate of expansion is determined by the Hubble constant (H0) and it varies according to Hubble’s Law, which says that the further a galaxy is away from us, the faster it moves away.

To calculate H0, we can observe galaxies and measure how much they move or take measurements of the cosmic microwave background (CMB), the echo of the Big Bang that remains in space, keeping information about how it has expanded since its birth.

two weights and two measures
The mystery that still remains is that if you do both, you will find different answers. Nothing explains why the results are incompatible with each other – at least until now, thanks to cosmologist and particle physicist Dan Hooper, who specializes in dark matter, cosmic rays and neutrino astrophysics.

Hopper is head of the Theoretical Astrophysics group at the Fermi National Accelerator Laboratory and an associate professor of Astronomy and Astrophysics at the University of Chicago – where they also teach Nikita Blinov and Celeste Keith, co-authors of the work now published in the arXiv scientific article repository.

For the trio of cosmologists, the Universe may have undergone profound changes since the first moments after the Big Bang, which originated both the visible mass and an ancient form of dark matter, which ended up falling apart; it would then be a significant part of the total mass of the cosmos.

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Primordial matter would have fallen
If so, the cosmic microwave background refers to a different rate of expansion, a value linked to a matter that no longer exists. Billions of years later, what would determine the Hubble constant would be the current mass of the Universe.

The study does not determine what makes up lost dark matter, but strongly suggests that it can be made of sterile neutrinos, theoretical ghostly particles that interact weakly with matter.

Another assumption is what primordial dark matter has become, since, by the laws of physics, it cannot cease to exist; necessarily, it changed to something else. If the final product was distributed differently in the universe or interacted differently with other particles in the cosmos, it would change the way it expands.

Lack of methods, leftovers
“We would be bathed in this dark radiation just as we are by neutrinos: a flood that fills space today with extremely inert forms of matter,” Hooper told LiveScience.

For now, researchers have no methods to investigate this type of hidden radiation, so the idea remains speculation. “There are other possibilities. This is one that definitely requires the least number of fairies to prove correct,” joked the scientist.

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