Is the term 'matter' in 'dark matter' misleading?

Question

If the only evidence for dark matter we have is gravitational, why call it 'matter' rather than 'observed space-time geometry that does not match predictions'?

General Relativity tells us that mass distorts local space-time but as far as I know, it doesn't say that it is the only thing that does so - isn't for all we know space-time is more fundamental than mass and can have local distortions caused by the big bang / inflationary epoch / flying-spaghetti-monster blueprints?

It's not just mass. GR says that stress-energy induces spacetime curvature. — PM 2Ring, Oct 22 '18 at 12:35
+1 for invoking the flying-spaghetti-monster. Einstein must have missed that one ;) — Time4Tea, Oct 22 '18 at 15:06
Your question has a common incorrect assumption that "the only evidence for dark matter we have is gravitational". The strongest evidence for dark matter by far is encoded in the power spectrum of cosmic microwave background https://en.wikipedia.org/wiki/Lambda-CDM_model — thegreatemu, Oct 22 '18 at 17:57
Simple version: the term "dark matter" pre-dates the term "space-time geometry". — Mark, Oct 22 '18 at 21:01
"misleading" is a subjective term, so I voted to close this question. Can you change it to reflect the real question about whether we are sure if dark matter is actually 'matter'? — , Oct 23 '18 at 12:30
I posed the same question but in different words. It was treated as ridiculous. Basically I said that dark gravity or extra gravity could be a term for dark matter.... — Alchimista, Jul 31 '20 at 21:52

score 23 · Answer 1 · answered Oct 22 '18 at 11:10

23

As the universe expands you would expect the density of the stuff in it to decrease for the obvious reason that the amount of stuff remains constant while the volume increases. So you'd expect something like:

$$ \rho \propto \frac{1}{V} \propto \frac{1}{a^3} $$

where $a$ is the linear scale factor (i.e. $a^3$ is the volume scale factor).

However things are more complicated than this. For radiation and highly relativistic matter the energy density decreases as $a^{-4}$, while for a cosmological constant the energy density doesn't decrease at all but remains constant. It is only matter, and specifically pressureless matter, that decreases as $a^{-3}$.

And the stuff that we call dark matter decreases as $a^{-3}$ just like regular matter, so whatever it is it behaves like matter. Hence the name dark matter.

answered Oct 22 '18 at 11:10

John Rennie

355,118

What results are considered to observe this 'stuff' decrease as $a^{-3}$ rather than not-enough regular matter decreasing as $a^{-3}$ ? – AlternativelyBaryonic Oct 22 '18 at 12:13
2

@AlternativelyBaryonic the current best measurements of the dark energy density and behaviour come from the Planck experiment to observe the microwave background. – John Rennie Oct 22 '18 at 13:14
Dark matter or dark energym Is this really dark matter? – Vladimir F Героям слава Oct 22 '18 at 16:14
@JohnRennie What has the dark energy density measurement to do with dark matter? – Vladimir F Героям слава Oct 23 '18 at 14:13
@VladimirF I'm not sure what you are asking about. Can you expand your question a bit to make it clearer what you are asking? Are you asking about the Planck experiment? – John Rennie Oct 23 '18 at 14:24
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Well there are to very distinct concepts, the dark matter, which is about orbital velocities of galaxies, and the dark energy, which is about the cosmological constant. So I am very confused about your answer and how the dark matter properties are inferred from dark energy measurements. – Vladimir F Героям слава Oct 23 '18 at 14:38
But I see it is probably just my confusion and the answer will be in the link mentioned in the comments https://en.wikipedia.org/wiki/Lambda-CDM_model – Vladimir F Героям слава Oct 23 '18 at 14:41

score 13 · Answer 2 · answered Oct 22 '18 at 21:00

The name, both the "dark" and the "matter" parts of it, is a historic accident.

The difference between the motion of stars in the Milky Way and that predicted by Newtonian gravity has been known since at least 1884, pre-dating General Relativity by three decades. The obvious candidate to explain this was ordinary matter such as planets, dust clouds, and burnt-out stars, hence "dark (too cold to give off light) matter".

Non-luminous baryonic matter remained the leading candidate even after Einstein developed GR (at the distances and scales involved, there isn't much difference between the Newtonian and relativistic motion predictions), so there was no real reason to change the name. It's only very recently that astronomers have found solid evidence that there isn't enough conventional matter to explain things, and with a century of usage behind it, it's going to be very hard to change the name.

MadMax · Answer 3 · 2018-10-22T14:57:00.080

1

According to the Einstein-Cartan version of gravity, there are two sources which "distort" local space-time:

Mass (or more generally energy-momentum tensor), which induces space-time curvature.
Spin current (from Dirac fermions), which induces space-time torsion.

As an alternative to missing/dark mass, we could attribute the "observed space-time geometry that does not match predictions" to dark spin current (or dubbed as " flying-spaghetti-monster", if you will).

As per Poplawski, the spin current/torsion approach has another nice property that "averts the unphysical big-bang singularity, replacing it with a cusp-like bounce at a finite minimum scale factor, before which the Universe was contracting".

edited Oct 22 '18 at 14:57

answered Oct 22 '18 at 14:49

MadMax

3,737

If you're going to mention this, you should also mention exotic smooth structure, which fits into the paradigm of Einstein-Cartan gravity (since it's part of the underlying manifold structure), and can distort local space-time in exactly the way mass can. But, I also don't think either of these things answer the question, "should we call it dark matter?" – levitopher Oct 25 '18 at 15:41

Is the term 'matter' in 'dark matter' misleading?

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