Dark Matter from Light Itself

March 5, 2019
Hertz Staff
Livermore, Calif

Let there be light – and dark matter?

Dark matter makes up 80% of our Universe’s mass, but it barely interacts at all with regular matter – including the atoms that make up scientific instruments – so it’s thus far been undetectable. Now, Hertz Fellow Katelin Schutz has proposed a new origin story for the mysterious substance – a way for dark matter to be born from light itself in the first minutes after the Big Bang. If this is indeed where dark matter came from, scientists may soon have the technology to find it in the lab and a map for searching for it in the heavens.

“In some sense we know less about dark matter than we did twenty years ago,” says Schutz. At the turn of the twenty-first century, it seemed likely that dark matter was made up of massive particles – hundreds of times heavier than protons – but scientists have searched in vain for these particles for decades.

“We built all these experiments that worked exactly as we expected, and they didn’t find the dark matter,” she says. “It’s really opened up a lot of possibilities, and there’s a lot of room for creativity right now.”

One possibility – that dark matter is made from particles in a mass range far lighter than the mass of an electron – stood out to Schutz, a PhD student at UC Berkeley, and her collaborators, UCSD physicist Tongyan Lin and Harvard physicist Cora Dvorkin. There was just one problem: there was no satisfying model for the origin of dark matter particles in this mass range that was also consistent with our observations of the Universe today.

The cosmic microwave background
The cosmic microwave background, whose characteristics Hertz Fellow John Mather won the 2006 Nobel Prize for studying, may yet reveal more secrets about dark matter

But Schutz, Dvorkin, and Lin realized that there was an additional way for dark matter to form: from light. Photons – the particle form of light – normally have a mass of zero, but when moving through highly conductive environments, like the dense plasma that suffused the universe in its earliest moments, they pick up some effective mass, like the extra drag you’d feel swimming through syrup. If dark matter originated from the decay of these massive photons the effect on the Universe’s development would be small enough to be consistent with our observations, but large enough to pick up on with some careful cosmological research.

One tool for matching theoretical predictions like Schutz’s with reality is the microwave radiation left over from the first million years after the big bang. This cosmic microwave background radiation comes streaming at us from all directions, but it is more intense in some parts of the sky than others (Hertz Fellow John C. Mather won the 2006 Nobel Prize in Physics with George F. Smoot for measuring these irregularities). Schutz, Lin, and Dvorkin are now working to model exactly what features of the cosmic microwave background radiation this new theory predicts.

But we need not look only to the skies for dark matter. If it is indeed made of particles in this mass range, it would be very hard to detect in the lab, but not impossible. Schutz says that, with focused development, scientists might be able to detect this sort of dark matter within a few years. “I think it’s definitely worth pursuing the necessary R&D for science reasons,” she says, “and maybe we could even find something useful to do with this tech for ‘everyday’ purposes.”

“Now is a really exciting time to be thinking about dark matter,” says Schutz.