Negative mass might not defy Einstein

Repulsive matter could have played a role in early universe, computational study finds

Matter with negative mass, a seeming impossibility, could actually have existed in the early universe, a new study shows.

A novel solution to Einstein’s equations for gravity permits the existence of negative mass in a rapidly expanding universe like our own, physicists Manu Paranjape and Saoussen Mbarek of the University of Montreal report November 14 in Physical Review D.

Although the research doesn’t prove that such exotic particles once floated around the cosmos, it suggests that negative mass could have played a major role in the epoch called inflation, when the universe ballooned in size just after the Big Bang.

For decades physicists have discussed the possibility of negative mass. If negative mass particles exist, they would accelerate toward someone who pushed them, and they would gravitationally repel all other matter (SN Online: 9/22/13). Unfortunately for fans of exotic matter, previous studies suggested that the existence of such particles would violate some rules of general relativity, Einstein’s theory of gravity.

But Paranjape noticed that those analyses didn’t take into account the fact that the universe is expanding at an exponentially increasing clip. So Paranjape and Mbarek explored the possibility of negative mass in simulations of a universe like our own.

Their study consisted mainly of plugging values into Einstein’s relativity equations — there were no attempts to experimentally produce or observe negative mass particles — but math can be a very powerful indicator of what’s possible in the universe (SN: 7/28/12, p. 28). Paranjape and Mbarek found that on a sheet of spacetime with an exponentially increasing expansion rate, general relativity allows for the existence of negative mass.

Under certain conditions, they discovered, regions of space would emerge that would behave like negative mass particles, at least to an observer outside the region. The existence of those negative mass bubbles, Paranjape says, is tied to the strength of the cosmological constant, which describes an intrinsic repulsive force in the vacuum of space that drives spacetime’s accelerating expansion.

“It’s an interesting result,” says Richard Hammond, a physicist at the University of North Carolina at Chapel Hill. He emphasizes that the paper shows that negative mass objects can exist, not that they do. Sabine Hossenfelder, a theoretical physicist at the Nordic Institute for Theoretical Physics in Stockholm, says that Paranjape and Mbarek still must propose a plausible and stable mechanism for actually producing particles of negative mass. Paranjape says he’s working on that now.

In the meantime, Paranjape can’t help but speculate on the effect negative matter might have had on the universe’s history. He is particularly interested in the period of inflation — a mysterious, short-lived era less than a second after the Big Bang when the universe doubled in volume dozens of times. The connection that Paranjape discovered between the cosmological constant and negative mass suggests that physicists need to consider the possible contribution of negative mass particles to the runaway expansion during inflation, he says.

Paranjape wants to look into the possibility that the very early universe contained a plasma of particles with both positive and negative mass. It would be a very strange cosmic soup, he says, because positive mass gravitationally attracts everything and negative mass repels everything. Yet the plasma surely would have had an impact on the universe’s evolution, and it almost certainly would have influenced the observational signatures of inflation that experiments such as BICEP2 are looking for (SN: 10/18/14, p. 7).

Despite her reservations about the study, Hossenfelder agrees with Paranjape’s intuition that negative mass could have played a pivotal role in the early universe. The relevance of negative mass to the evolution of the universe “has in my opinion so far not gotten the attention it deserves,” she says. “This is a promising direction of study.”

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