Science & Technology

A New Theory Says Dark Energy and Dark Matter Might Be the Same Weird Substance

There's a problem with our theory of the universe: Even after more than a century of research, we can only explain 5 percent of it. The rest is "dark" — invisible, unidentified matter that we can only detect from the pull of its gravity. While scientists get to work on understanding these mysterious substances — called dark energy and dark matter — one researcher is trying something different. What if dark energy and dark matter aren't two substances, but one? It might sound overly simplistic, but there are some good reasons to believe that this negative-mass "dark fluid" could really exist.

The Missing 95%

The leading theoretical model for the makeup of our universe is known as the Lambda cold dark matter model, or ΛCDM. It says that 25 percent of the universe is made of dark matter, which exists in halos around galaxies and galaxy clusters, exerting a gravitational push that keeps them from flying apart. (The model calls it "cold" because it didn't move very fast as the universe was developing).

Another 70 percent of the universe is made up of dark energy, which seems to be exerting some sort of reverse gravity that's making the universe expand faster and faster. In the Lambda cold dark matter model, dark energy is the lambda — the symbol Albert Einstein used for his cosmological constant, which dealt with the expansion of the universe.

The final 5 percent is baryonic matter. It's the ordinary stuff that makes up literally everything you know: stars, planets, people, the little stickers they put on apples. We can see it, hear it, often touch it and smell it. It's the matter we know the most about, and it's the smallest fraction of our universe. Wouldn't it be great if there were an easy way to explain everything else?

Why Not Both?

In December, Dr. Jamie Farnes of the University of Oxford in England published a theory that may get us closer to that goal. It says that the invisible portion of the universe is made up of not dark matter and dark energy, but one "dark fluid." This fluid has two key characteristics. One is that it possesses negative mass — while the regular mass of stars and planets exerts a gravitational pull that attracts other objects, something with negative mass would exert a gravitational push. If you were to throw a negative-mass bowling ball, it would roll toward you instead of away from you. Negative mass isn't a new idea, but previous research had ruled it out as an explanation because, while the force of dark energy has remained constant over time, a negative-mass substance would thin out as the universe expands.

That brings us to the second characteristic. To fix this thinning-out issue, Farnes's theory says that this negative-mass substance would be continuously created through time. If this dark fluid is constantly produced like floss from an infinite cotton-candy machine, it never dilutes and it acts identically to dark energy.

But what about dark matter? Farnes developed a 3D computer model of a universe suffused with this dark fluid to see if it could explain that, too. As mentioned earlier, dark matter is believed to be an invisible substance that exerts a gravitational attraction that can keep galaxies clumped together in a way that isn't explained by their visible mass. But if a galaxy with positive mass is surrounded by a dark fluid with negative mass, their opposing forces would hold that galaxy together and keep it from flying apart.

3D Simulation of an Expanding Negative-Mass Universe

It also solves a nagging problem with the universe's expansion rate. That rate depends on a number known as the Hubble constant — but it's anything but constant. Measurements of it keep changing, and that's causing more than a little drama in cosmology circles. Farnes's theory predicts that this number should change over time. Easy peasy.

"Clearly," Farnes wrote in an article for The Conversation, "there is evidence that this weird and unconventional new theory deserves our scientific attention."

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Learn more about the search for the invisible majority of the universe in "The 4 Percent Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality" by Richard Panek. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer January 4, 2019

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