11
$\begingroup$

I just read a text about astronomy and when talking about dark matter the author says:

[...], the dark matter responsible for the orbits of the stars in the Milky Way is probably different from the dark matter responsible for the orbit of the Milky Way within the local super-cluster of galaxies.

Is this true? How would it be different? And why?

For context, this is the whole paragraph:

Since the 1930's astronomers have measured the orbits of galaxies in clusters of galaxies, clusters of galaxies in clusters of clusters, and so forth. They have found similar anomalies in the angular velocity of galaxies at these larger scales. Again the anomalous high angular velocities of the galaxies and clusters of galaxies may be explained by postulating a mysterious dark matter that fills the universe. It is doubtful that one can explain the anomalous angular velocities at different scales by the same type of dark matter. Thus, the dark matter responsible for the orbits of the stars in the Milky Way is probably different from the dark matter responsible for the orbit of the Milky Way within the local super-cluster of galaxies.

And the article can be found here: https://mathblog.com/wp-content/uploads/2009/12/Keplers-New-Astronomy.pdf

$\endgroup$
  • $\begingroup$ As far as I know, this is not a mainstream view. In general, you don’t want to read about physics on math blogs. In the standard cosmological model, there is only one kind of dark matter. $\endgroup$ – G. Smith Oct 18 at 16:31
  • $\begingroup$ Well in his defense, the rest of the text is an interesting review of Kepler's work and, according to the article, the author is a Ph. D. $\endgroup$ – Chegon Oct 18 at 16:35
  • $\begingroup$ This may be a reference to the variaous musing on the possibility of there being both warm and cold populations of dark matter present. I haven't the faintest idea how seriously those musing are taken right now. There was a point when they were, if not actually accepted, at least something serious people could talk about. $\endgroup$ – dmckee Oct 18 at 16:50
  • 9
    $\begingroup$ There are a lot of wrong statements coming from PhDs. Getting a degree doesn't magically make you infallible. You remain almost exactly as fallible about everything outside the very very narrow domain of your PhD as before. $\endgroup$ – knzhou Oct 18 at 17:13
  • 1
    $\begingroup$ I should have added this before: being a contrarian without an advanced degree doesn't make you infallible either. Nothing does. Understanding what's correct is a hard task for anyone. It shouldn't be taken lightly. $\endgroup$ – knzhou Oct 19 at 4:28
13
$\begingroup$

The standard model of cosmology (for now) is called Lambda-Cold-Dark-Matter. It has only one kind of dark matter, and it agrees well with the observational data.

Other types of dark matter, such as “warm” or “hot” rather than “cold”, have been considered, and some people have considered models in which more than one kind of dark matter exist. These more complicated models do not appear to be necessary to explain what we observe.

The precise nature of dark matter doesn’t matter in cosmology. What matters is the relationship between its energy density and its pressure, because in General Relativity both energy density and pressure cause gravity. What is important is how relativistic dark matter is, because that determines its pressure. Cold dark matter is non-relativistic and has negligible pressure. All of its gravity comes from its energy density.

$\endgroup$
14
$\begingroup$

Others have made the point that the current concordance cosmology includes only one variety of dark matter, which is cold. This is correct.

If dark matter is indeed some as-yet unknown particle, it would be surprising if it was the only unknown particle. The standard model of particle physics is basically complete, without room for extra particles without changing some of the underlying principles. There are some anomalies which point to our knowledge of particle physics being incomplete, so eventually discoveries will likely be made. Anyway, all this is to say that there are probably many species of dark particles, but it is currently thought (mostly by invocation of Occam's razor) that the 'astrophysical dark matter', the stuff that explains the motions within and between galaxies, is either predominantly due to one particle, or a family of particles which behave similarly as far as the relevant astrophysics is concerned.

Now I hesitate to make this next point in this way, but I think there is some merit to the statement that dark matter isn't "the same" on all scales. The standard cold dark matter has been hugely successful in explaining the physics of the large scale Universe, but fails in some areas on small scales. There are other examples of ideas to explain astrophysical dark matter which work very well on small scales but struggle on large scales. One example is the modified gravity theory MOND. I bring up this particular theory only because there is a nice, reasonably accessible article contrasting it and LCDM available here: https://arxiv.org/abs/1404.7525

As yet no one theory has succeeded in explaining the dark matter effect on all scales, which is why we still have an astrophysical dark matter problem, in addition to the question of what particle dark matter might be.

So I agree with some interpretation of (the spirit of?) the statement, but I don't think it was especially well articulated in the article cited in the question.

$\endgroup$
  • $\begingroup$ "As yet no one theory has succeeded in explaining the dark matter effect on all scales" - Has TeVeS been falsified? $\endgroup$ – safesphere Oct 19 at 5:51
  • 1
    $\begingroup$ @safesphere TeVeS, at least the simpleet formulation, made a strong prediction thay gravitational waves do not propagate at the speed of light. The LIGO measurements with optical counterparts strongly rule out the theory. $\endgroup$ – Kyle Oman Oct 19 at 6:57
1
$\begingroup$

Nobody knows what dark matter is, or even whether it exists (see for instance this link). So asking how many kinds of dark matter there are is like asking how many angels can dance on the head of a pin.

$\endgroup$
  • 1
    $\begingroup$ I think the other answers demonstrate, that asking this question can be useful, and can have meaningful, observation based answers. $\endgroup$ – hyde Oct 20 at 12:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.