What is dark matter?
Physicists have been trying to answer this question since the first observations of distant galaxies suggested that only a fifth of all the “stuff” in the universe is the visible matter with which we interact every day. The other 80% is the enigmatic dark matter which has so far eluded direct observation.
Dark matter’s existence would account for the gravitational forces behind how galaxies bind together and rotate.
There are hundreds of experiments around the world trying to be the first to detect a dark matter particle. But all these experiments rely on theory to understand what it is we might be looking for.
Most of these experiments hinge on the idea that dark matter is made of some new or exotic form of particle, often referred to as weakly interacting massive particles (WIMPs).
Now, a physicist at the University of California, Santa Cruz (UCSC) in the US has published 2 studies which put forward a new approach to explain where dark matter comes from.
Mirror universe at the beginning of time
Professor Stefano Profumo has drawn from the well-established quantum chromodynamics (QCD). This is the fundamental theory of the strong interaction which governs how quarks and gluons bind together, including to form protons and neutrons in the nuclei of atoms.
Profumo proposes a “mirror world”, or dark sector, with its own “dark” versions of fundamental particles and forces. This hypothetical dark sector is a part of our universe but remains tantalisingly unobserved. This, Profumo argues, might have emerged out of conditions in the first seconds of the universe.
The first paper suggests that there is a mirror “dark QCD” in the dark sector with its own dark quarks and dark gluons. These bind together to form heavier particles. In the very early universe, Profumo argues, these heavy dark particles could become dense and heavy enough to collapse into extremely small, stable black hole-like objects which only interact with normal matter through gravity.
If there were enough of these, they could account for the effects of dark matter seen around the universe today.
Is the universe like one big black hole?
Profumo’s other paper looks at another mechanism for dark matter creation – a hypothetical, brief period of accelerated expansion of the universe after the initial phase of inflation which followed the Big Bang.
An artistic illustration of the mechanism proposed by Professor Stefano Profumo where quantum effects near the rapidly expanding cosmic horizon after the Big Bang gravitationally generate dark matter particles. Credit: Stefano Profumo.
If the universe in this period was expanding faster than radiation or matter would allow, it could have radiated new particles into existence. This “cosmic horizon” theory draws inspiration from a black hole’s event horizon.
Stephen Hawking in 1974 showed that an event horizon should theoretically emit pairs of particles due to the interplay of quantum mechanics and thermodynamics with the extreme curvature of spacetime around the black hole.
Now called “Hawking radiation”, there may have been an equivalent effect on the “cosmic horizon” which could have led to the creation of dark matter particles.
“Both mechanisms are highly speculative, but they offer self-contained and calculable scenarios that don’t rely on conventional particle dark matter models, which are increasingly under pressure from null experimental results,” Profumo says in a USCS media release.