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I’m still here, but it’s an exam week, so I bring you another guest blogger, Ethan Siegel, who knows way more about dark matter than I probably ever will (a theorist even!). Enjoy his guest post!

This is Ethan from Starts With A Bang! over here at Pamela’s blog, Star Stryder. I was pretty pleased that she came across a couple of press releases and actually thought of me… so let’s have at it! First off, what do the press releases say?

Dr HongSheng Zhao, of the University’s School of Physics and Astronomy, has shown that the puzzling dark matter and its counterpart dark energy may be more closely linked than was previously thought.

Dr Zhao reports that, “Dark energy has already revealed its presence by masking as dark matter 60 years ago if we accept that dark matter and dark energy are linked phenomena that share a common origin.”

Really? Well, this is a possibility, but this is certainly not necessarily true. We do know that about 4% of the stuff in the universe is normal matter: what we know as protons, neutrons, and electrons. But although you need dark matter to explain things like

  • the internal velocities in galaxies and galaxy clusters,
  • the gravitational lensing data, and
  • the formation of structure in the universe,

dark energy is only needed to explain why the cosmic expansion rate is larger than it would be if the Universe were made exclusively of matter. They could be related, but they aren’t necessarily. Dark matter affects how gravitational collapse happens, but dark energy affects how things move apart on much larger scales.

But the press release goes on, and contends:

However, the Universe might be absent of dark-matter particles at all. The findings of Dr Zhao are also compatible with an interpretation of the dark component as a modification of the law of gravity rather than particles or energy.

Oh! Well now I know what they’re talking about, and it’s incompatible with the data. Let’s walk you through it, but first, so you know where I’m coming from, let me link you to the scientific article that reviews Dr. Zhao’s work, as well as his works that were published in the Physical Review and the Astrophysical Journal.

Basically, people don’t like the idea that we only understand 4% of the Universe, and that 96% of the energy in the Universe is made up of stuff (dark matter and dark energy) that we don’t understand at all. So what some scientists attempt to do is to modify the laws of gravity that we know, General Relativity, into something that can mimic the effects of dark matter and dark energy without actually requiring them. But we do need dark matter, and here’s why:

See this picture? This is a picture of two merging galaxy clusters, known as the Bullet Cluster. The bright pink in the picture is X-rays, which come from normal matter. But the blue? That’s a measurement of mass, though gravitational lensing. Normal matter and dark matter both have mass. You tell me, from the image above: where is most of the mass?It isn’t where the normal matter is, that’s for sure. In fact, there isn’t even a consistent way to make a theory of gravity that explains this. Let me quote from this paper that Zhao wrote:

Angus et al. [of which Zhao is one of the coauthors] found that the lensing peaks of the Bullet Cluster could be explained by adding neutrinos in a TeVeS-like modified gravity; the phase space density of neutrinos at the lensing peaks requires 2eV mass to in order not to violate exclusion principle for fermions.

Putting the language issues aside, he’s saying that even if you modify gravity in the best way possible for the Bullet Cluster, you still need 2eV mass neutrinos. From this page, we know that’s more massive than neutrinos are allowed to be. So you still need to add an extra, dark, non-interacting mass to this theory of modified gravity. Know what we call that? Dark Matter!

So the answer to my question is, emphatically, we still need dark matter to be consistent with all of the observations we make. Could dark matter be related to dark energy? Sure, but there’s no evidence that it is. People have also speculated that dark energy is related to neutrinos, axions (a dark matter candidate), and inflation. But at this point, all of it, including this press release, is still just speculation.

I’m happy that people are thinking about it, because these are two of the most interesting topics out there: figuring out what makes the Universe form structure and expand as it does. And yet, the observational evidence for dark matter is overwhelming, even if it still has trouble explaining all of the galactic rotation curves. There’s a lot we still have to learn, but throwing dark matter away isn’t going to be the answer.