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A paper published last week proposed a hypothesis that identified 95% of the missing stuff in the universe, but the headlines have been a little over-hyped.

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Sources:

https://www.eurekalert.org/pub_releases/2018-12/uoo-bbt120318.php
https://www.aanda.org/articles/aa/pdf/2018/12/aa32898-18.pdf
https://www.wired.co.uk/article/dark-fluid-theory-of-dark-matter-energy
http://backreaction.blogspot.com/2018/12/no-negative-masses-have-not.html
https://twitter.com/seanmcarroll/status/1071066682234261504

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https://svs.gsfc.nasa.gov/12307
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https://svs.gsfc.nasa.gov/11894
[♪ INTRO].

Maybe you’ve seen some of the headlines over the last week or so: “Mystery ‘dark fluid’ could make up 95% of the universe.” “We might have finally discovered where the missing 95% of the universe is.” They’re referring to a paper published last week in Astronomy & Astrophysics, where a postdoctoral fellow from Oxford proposed a hypothesis that identified 95% of the missing stuff in the universe. According to the author, Jamie Farnes, the missing stuff in question takes the form of a fluid with quote-unquote “negative gravity,” which permeates all of space.

And his work is based on the combination of two older ideas in physics. But that’s the thing: these are all just ideas, nowhere close to proven theories. And some of the coverage has been a little, you know, over-hyped.

So here’s what the paper actually argues, and how likely it really is. Our current understanding of the universe says that only about 5% of everything is the stuff we can actually see, including regular matter and light. Another 26% is matter we can’t directly observe; what we call dark matter, which is not so much “dark” as “invisible.” The other 69% or so is made up of dark energy, which is basically the biggest mystery cosmology has to solve at the moment.

All we really know is it’s a sort of pressure that keeps the collapsing power of gravity at bay, and even overpowers it on an intergalactic scale. Right now, dark energy is making the universe’s expansion accelerate. Cosmologists have spent the past several decades trying to figure out what either or both of these actually are.

Sometimes it feels like they’re just throwing ideas at a wall and waiting to see what sticks. Or, rather, what the observed data don’t contradict. This new hypothesis claims to not just explain what dark matter or dark energy is.

It tries to unite them both into a single substance, called a “negative mass fluid,” or “dark fluid,” which would be everywhere, with more of it being generated all the time. The idea incorporates two older, also-hypothetical concepts: negative mass and matter creation. Negative mass is basically like the gravity version of a negative electrical charge.

As far as we know, there’s no reason it shouldn’t exist, we’ve just never seen any outside of special lab experiments. As this new paper explains it, positive mass attracts positive mass, what we’re used to seeing from gravity. On the other hand, if you pushed something with negative mass away, it would actually move toward you.

But Farnes also claims that two negative mass particles should repel one another, which runs totally counter to the established rules of general relativity, the physics that governs much of the universe. That’s one obvious problem some cosmologists have with this hypothesis, especially because. Farnes put the repulsion idea in there because he thought it made sense, rather than getting it from mathematical equations.

But it’s a fundamental change that this whole new model depends on, so we’re just gonna have to take it as-is, for the moment. Physicists have dismissed negative mass as a potential dark energy candidate because as far as we can tell, the density of dark energy stays constant as the universe expands, whereas the density of negative mass would go down as it spread out. So Farnes’ hypothesis includes matter creation: the idea that this negative matter is continuously created throughout the universe, keeping its density constant.

This is actually an idea that early 20th-century astronomers invoked for positive mass to argue that the universe had an infinite age and was always the same size, meaning it didn’t start with the Big Bang. Matter creation doesn’t jibe with observations, though. So Farnes only allows it for negative mass, not for the regular positive stuff, and suggests we could find evidence of it using future telescopes that will map out the distribution and movement of galaxies through the history of the universe.

All together, he argues it’s a sensible hypothesis because it simplifies two mysteries into one, and introduces some mathematically attractive symmetry to the matter in our universe. But is all the regular matter out there really just a bunch of rubber ducks floating along in a universe-sized ocean of “dark fluid”? I mean, it’s way too soon to declare an answer to that question, but naturally, many cosmologists have arguments against this proposal.

One problem is that if the vacuum of space were capable of spontaneously generating negative mass, quantum mechanics would dictate that it, and therefore everything, would become really unstable. That doesn’t seem to have happened. Then there’s the lack of evidence for matter creation.

But one of the biggest problems with this whole situation is that the published press release failed to emphasize how preliminary it is, and then some media outlets just kind of ran with the hype. Farnes has taken issue with that overhyping, too, but then again, he wrote a similar article himself. Maybe this paper is onto something, and we really do need to investigate the idea that dark matter and dark energy are a single fluid with negative mass.

But when you’re working with the biggest mysteries of the universe, the answers are never solved by a single piece of research. Thanks for watching this episode of SciShow Space News, and thanks especially to our community on Patreon for your support. Because of you, we’re able to explore complicated cosmology like this, and spread scientific literacy and a passion for science with anyone who wants to learn.

If you’re not yet a patron and want to learn more about how you can join us, just check out patreon.com/scishow. [♪ OUTRO].