The cosmos should be ringing with dark sound, according to a recent model of dark matter that says some of the elusive particles can form unseen atoms. Now efforts to eavesdrop are refining our picture of this “shadow universe”.

Dark matter is believed to make up about 80 per cent of the matter in the universe, based on its gravitational effects on visible matter. The leading candidates for the elusive stuff are weakly interacting massive particles (WIMPs), which would only interact with normal matter via gravity and the weak force.

Most models suggest that WIMPs don’t interact with themselves to build larger structures, the way that normal matter does. But earlier this year, scientists proposed a model in which 15 per cent of dark matter can build dark atoms and larger structures, including shadow versions of galaxies.

If so, we should be able to see evidence for dark matter interactions in the large-scale structure of the universe, says Francis-Yan Cyr-Racine at the Jet Propulsion Laboratory in Pasadena, California. In the heat of the very early universe, matter existed as dense, soupy plasma. As things cooled, denser regions started to collapse, which set the gas ringing.


Fainter darkness

When the plasma cooled enough to form atoms, the sound waves became frozen in the cosmic microwave background radiation (CMB) – the first light emitted about 380,000 years after the big bang – and in the current pattern of galaxy clusters.

If some dark matter can form atoms, we should see evidence of their formation in the early universe as dark sound waves, says Cyr-Racine. We wouldn’t be able to see these ripples directly, but their gravitational pull would affect visible matter.

His team examined the latest data from the European Space Agency’s Planck spacecraft and the BOSS galaxy survey looking for ripples of dark sound in the CMB and in galaxy clusters. So far they have come up empty-handed. Self-interacting dark matter might still exist, but the current finding puts limits on how “loud” dark sound can be: the team suggest that no more than 5 per cent of dark matter should be able to build atoms. More sensitive surveys could pick this up, they say.

“We don’t rule out self-interacting dark matter,” says Cyr-Racine. “We do give the model some escape routes.”

Journal reference: arxiv.org/abs/1310.3278