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?

closed as primarily opinion-based by AccidentalFourierTransform, A.V.S., Jon Custer, Kyle Kanos, Chair Oct 23 at 11:41

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    It's not just mass. GR says that stress-energy induces spacetime curvature. – PM 2Ring Oct 22 at 12:35
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    +1 for invoking the flying-spaghetti-monster. Einstein must have missed that one ;) – Time4Tea Oct 22 at 15:06
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    Yes, but it doesn't matter. – Evargalo Oct 22 at 15:36
  • 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 en.wikipedia.org/wiki/Lambda-CDM_model – thegreatemu Oct 22 at 17:57
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    "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'? – Chair Oct 23 at 12:30

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.

  • 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 at 12:13
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    @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 at 13:14
  • Dark matter or dark energym Is this really dark matter? – Vladimir F Oct 22 at 16:14
  • @JohnRennie What has the dark energy density measurement to do with dark matter? – Vladimir F Oct 23 at 14:13
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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 Oct 23 at 14:38

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.

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

  1. Mass (or more generally energy-momentum tensor), which induces space-time curvature.
  2. 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".

  • 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 at 15:41

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