What The Hell Is Going On With Dark Energy?

November 2, 2016

ast week, the science media was abuzz with reports that dark energy might not exist. The astrophysics community has largely rejected the study behind the headlines, which argues the original evidence behind dark energy is flawed. This latest dark energy kerfuffle doesn’t necessarily reveal a problem with the physics, but it does reveal the widening gulf between what our theories say the universe should be and what we actually observe.

The universe needs dark energy, the enigmatic substance that accounts for nearly 70 percent of the total energy in the cosmos and causes the universe to expand at an accelerating rate, to explain one of the most surprising discoveries of the 20th century. Two separate 1998 studies of distant supernovas found that the universe was not just expanding but that the expansion was actually accelerating, with three of the physicists involved winning the 2011 Nobel Prize for their efforts.

But that discovery didn’t fit with our understanding of gravity, which should very gradually begin to pull the matter in the universe back together. There needed to be something previously unknown that was resisting gravity and speeding up cosmic acceleration, and that something is dark energy. It could be something hardwired into the fabric of the universe – what’s known as the cosmological constant – or it could be an as yet undetected force or substance. Whatever it is, dark energy defies easy explanation.

“I mean, dark energy is weird, okay?” UCLA astrophysicist Edward Wright told Vocativ. “Having this weirdness in the universe is disturbing. And that’s why a lot of people are always on the lookout for ways to avoid it.”

At first glance, the best way to avoid dark energy would be to refute the original evidence for cosmic acceleration. That’s precisely what a new paper in Scientific Reports co-authored by Oxford University physicist Subir Sarkar purports to do. In the paper, Sarkar and his colleagues analyzed an expanded dataset of supernovas and found what they call “marginal evidence” for cosmic acceleration. They didn’t disprove an accelerating universe, but they argued the apparent data for it didn’t meet the level of statistical certainty needed for a formal discovery. In their view, dark energy might just be a fluke of the data.

To Sarkar, that’s reason enough for astrophysicists to call the entire model into question, given how strange and poorly understood dark energy is.

“Until they are absolutely certain that that is the correct theory, then they should be more conservative and leave open the possibility that there are other models that can fit the data as well,” he said.

Talking to astrophysicists, you get the sense that they wouldn’t necessarily mind being rid of dark energy, assuming the data in this paper could show that.

“I will definitely confess that when I first saw the abstract for the paper, a part of me went, ‘Yes! We don’t have to worry about this problem anymore… maybe,’” University of Washington astrophysicist Chanda Prescod-Weinstein told Vocativ. “So I can understand the impulse even from the point of view of someone who’s worked on the cosmic acceleration problem.”

Astrophysicists like Prescod-Weinstein and Wright have criticized the paper’s statistical method, arguing it makes assumptions about the universe that aren’t supported by the observational data. Sarkar pushed back against that argument, telling Vocativ that his team’s methods actively avoided the assumptions that he said tilted prior studies in favor of an accelerating universe.

The disagreements here are significant, with Sarkar calling into question a model of accelerated expansion that Wright said has been settled science for nearly 15 years. But the issues involved are also technical and nonintuitive, beyond the easy understanding of those outside the physics community. For those of us on the outside, it’s probably best to say that future research will clarify which side is right here, though it’s worth noting in the meantime that the astrophysics community is overwhelmingly in favor of the accelerated expansion model.

Either way, this is just the latest round of an ongoing debate between theoretical physicists like Sarkar and astrophysicists like Wright as to how we should best go about understanding the universe’s most challenging and perplexing mysteries. Should we believe what our instruments tell us, no matter how preposterous something like dark energy might seem, or do we first need to understand a phenomenon’s theoretical basis before accepting it as part of our understanding of the universe?

“The concern that I’ve always had is that the cosmological constant or dark energy, which is an intrinsic part of the fundamental model, has absolutely no explanation in fundamental physics,” said Sarkar. “And I would rather not bias my analysis by already assuming that it is there in the data.”

Part of Sarkar’s objection to accelerated expansion is as much political as it is theoretical. In his view, awarding the Nobel prize moved dark energy into the realm of fundamental physics, which would mean it ought to be held to the same standard of proof as the subatomic particles in the Large Hadron Collider.

“I was hoping for a simple universe and I didn’t get it.” — UCLA astrophysicist Edward Wright

“Discoveries in astronomy don’t normally get the Nobel prize,” he said. “And if it is a real discovery, if the universe’s expansion is actually accelerating, then it certainly deserves a Nobel prize. All we are saying is the criteria for that should be a little higher. The stakes should be higher.”

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