Two new studies by Professor Stefano Profumo at the University of California, Santa Cruz, explore different ways dark matter might have formed. Dark matter is one of the biggest mysteries in science. It makes up about 80% of all matter in the universe, and its gravity explains why galaxies hold together and rotate the way they do. Observations of the large‑scale structure of the universe and the cosmic microwave background also point to its existence. But no one knows what kind of particle it is or how it came to be.
Profumo’s research looks at the idea that dark matter could have formed naturally in the very early universe, instead of being a completely new type of particle that interacts with normal matter in ways we can detect.
His most recent paper, published July 8, focuses on a “hidden sector” — a kind of mirror version of our universe with its own particles and forces that we cannot see, but which follow similar physical laws. The idea is based on quantum chromodynamics (QCD), the theory that explains how quarks are bound inside protons and neutrons by the strong nuclear force. In this hidden sector, there could be a similar “dark QCD” with dark quarks and dark gluons forming heavy particles called dark baryons.
The study models this using confining SU(N) gauge theories in the large‑N limit. It finds that while glueballs and mesons in such a sector cannot form black holes under realistic conditions, dark baryons could. For certain ranges of the confinement scale, quark masses, number of colors N, and dark sector temperature, these dark baryons could collapse into light black holes with masses up to a few hundred times the Planck mass. If these relics are stable, their abundance — both from direct formation at confinement and from dark glueball and meson annihilation — would be exponentially suppressed as N increases. This leads to an upper limit of about N ≤ 100 for models where such relics make up all of the dark matter. The paper also maps out the parameter space where this could happen.
Profumo’s other paper, published in May, looks at whether dark matter could have been created by the universe’s edge or the “cosmic horizon” during a short period of accelerated expansion after inflation. This expansion would have been less extreme than inflation but still faster than what radiation or matter alone would allow.
The analysis assumes three things: first, that the phase was driven by a fluid with equation of state P = wρ, second, that the cosmic horizon in this quasi–de Sitter universe had a temperature inversely proportional to its size; and third, that observers are static (Here w lies between −1 and −1/3).
Under these conditions, the study calculates the frozen‑in density of a stable particle of mass m produced by the cosmic horizon, assuming it does not undergo number‑changing processes later. Depending on the equation of state and the temperature when radiation domination began and the quasi–de Sitter phase ended, the mass of dark matter from this process could range from about 10 keV to nearly the Planck scale.
“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,” said Profumo.
Source: UC Santa Cruz, APS (link1, link2)
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