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On the new Astronomy.SE site, I was having a short discussion on one of my answers. The basic discrepancy was; can MACHOs like black holes/brown dwarfs/neutron stars be termed "dark matter"?

My reasoning is that these objects do not radiate EM radiation on their own but they do gravitate, and thus constitute a small part of the total dark matter in the universe. I agree that there is a lot of dark matter which doesn't

In other words, can the term "dark matter" be applied to nonradiating (or faintly radiating) bodies which still participate in the electromagnetic interaction (baryonic or otherwise)? Or is it necessary for all dark matter to not interact electromagnetically?

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    $\begingroup$ Now that's a good question. The MACHOs certainly make up part of the "missing matter" needed to explain the galactic rotations curves (albeit a minority part), but these days "dark matter" seems to be reserved for exotic stuff that does not participate in the electromagnetic interaction. Only I've never seen an authoritative definition. For that matter, who would constitute an authority at this point? $\endgroup$ Sep 28, 2013 at 14:55
  • $\begingroup$ For an authoritative definition about its influence on the evolution of the universe, see almost any modern cosmology textbook. My favorites are Scott Dodelson's Modern Cosmology, or John A. Peacock's Cosmological Physics. $\endgroup$
    – astromax
    Sep 28, 2013 at 15:22

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I know we've had this discussion on the Astronomy SE site, but let me try to elaborate on my answer.

Dark matter is an altogether different component of the universe from baryonic matter. It does cause the same overall dynamics when it comes to the universe as a whole. What I mean by this is that the hubble parameter:

$$ H(a) = H_0 \sqrt{\frac{\Omega_{m}}{a^{3}} + \frac{\Omega_{\gamma}}{a^{4}} + \Omega_{\Lambda} } $$

remains the same. Consequently, the age of the universe, the lookback time to objects in the universe, the distance to things like the cosmological horizon, the CMB, ..., these things all remain the same, since:

$$ \Omega_{m} = \Omega_{b} + \Omega_{cdm}$$

BUT, you may be asking yourself then: What does change if you change the ratio of dark matter to baryonic matter? The answer is that statistically, structure would look different. The power spectrum of the universe would look considerably different. Below is a picture of the power spectrum as measured by the Planck satellite (red are from observations, and green is the prediction from the LCDM cosmological model) - pretty nice fit, right?

Planck PS

What actually changes in this picture (if you were to, by hand, adjust the ratio of dark matter to total matter) are the relative heights of the peaks in the power spectrum. This is because the "Cold" in Cold Dark Matter arises from the fact that dark matter doesn't interact electromagnetically, and so in the early universe, it cooled off more rapidly than baryonic matter did, forming overdensities in the early universe which baryonic matter would later fall into and form the structures we see today. If dark matter were really composed of things like neutron stars and brown dwarves (like you say both here and on the Astronomy SE), which absolutely are composed of material which can be broken down into quarks, then you would be forced to conclude that the early universe had no such dark matter component. This would give you a totally different power spectrum, and would be absolutely inconsistent with the power spectrum we've measured observationally.

The alternative, is that our theories are wrong, and that there is no such component to the universe which behaves the same as regular matter gravitationally, but that doesn't interact electromagnetically. This is absolutely one possibility. Another possibility, is that dark matter is actually composed of particles which we do know to exist, but that do have mass, and additionally must be neutral (neutral particles do not interact via the electromagnetic force) - this is why various types of neutrinos (or simply neutrinos themselves if they happen to have the right masses) have been proposed as a possible dark matter particle candidate.

Things like neutron stars, brown and white dwarves, are all the end products of main sequence stars, which are composed of various gasses, which are of course, atoms. No matter how faint they actually are, they are characteristically different from what we think dark matter is. By the way, your statement that they do not emit EM radiation on their own is simply incorrect. They do emit photons. Pulsars are rapidly rotating, highly magnetized, neutron stars, and brown and white dwarves are very faint because they have moved off of the main sequence in the H-R diagram, and thus are not fusing elements at the same rate as they once were.

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    $\begingroup$ I think, again, we're coming to the same issue here. I'm not saying that DM is baryonic. The arguments you provide refute that statement; which is not one that I'm making. I'm wondering if it is OK to say that MACHOs make up a part of the DM in the universe. (see also: dmckee's comment ). However, it seems that the meaning of the term has changed over the years to exclude MACHOs. $\endgroup$ Sep 28, 2013 at 16:01
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    $\begingroup$ Apologies for the EM radiation statement, I meant "faint". Do you have any references giving a definition for DM that excludes MACHOs? (I see the books in the above comment, a quote would be nice) Thanks :) $\endgroup$ Sep 28, 2013 at 16:02
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    $\begingroup$ To clarify: This is a terminology question. Physical arguments don't really make sense in this context because they fundamentally rely on the terminology being used, which is what I'm questioning. $\endgroup$ Sep 28, 2013 at 16:04
  • $\begingroup$ I'll get back with some quotes from the books I've linked. I'm not in my office at the moment :) $\endgroup$
    – astromax
    Sep 28, 2013 at 16:17
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I don't know what exactly would constitute an acceptable answer to this question, but I have always understood dark matter to be any non-radiating matter such as black holes to be dark matter. My opinion probably doesn't count for much. Here is wikipedia. They clearly talk about baryonic dark matter, so dark matter doesn't just refer to WIMPs or more exotic stuff; it can refer to baryons too.

But you know language can be a funny thing. Maybe somebody could generate one of these maps saying if dark matter can be baryonic in nature.

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    $\begingroup$ In recent years people have been talking about "dark-sector" physics which definitely excludes cold, dense lumps of ordinary matter. My personal feeling is that there has been a shift in usage along with the rise of that term. Your millage may, of course, vary. $\endgroup$ Sep 28, 2013 at 15:03
  • $\begingroup$ I think if you specify when you talk about the matter, and say 'baryonic dark matter', that could absolutely include MACHOs, but if you are a cosmologist, there really is a difference. See my answer below. $\endgroup$
    – astromax
    Sep 28, 2013 at 15:14
  • $\begingroup$ @astromax Ah, so it depends on context? To a non cosmologist; baryonic DM is a part of DM but to a cosmologist when DM is mentioned it is understood that it is more exotic? Interesting... $\endgroup$ Sep 28, 2013 at 16:05
  • $\begingroup$ @Manishearth No - When talking to anybody, when you put the word 'baryonic' in front of dark matter that implies that you're not talking about the dark matter component of the universe - you're talking about very cold and hard to detect clumps of the baryonic component of the universe. That's not a matter of context, that's a matter of adding the word baryonic. $\endgroup$
    – astromax
    Sep 28, 2013 at 16:12
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    $\begingroup$ To reiterate a little more clearly, my perception is that with the introduction of the idea that there is a "dark sector" there has been a shift in usage. The adjective "dark" is increasingly used to refer exclusively to the exotic and as yet not understood non-baryonic stuff. Older works are still relevant, of course, and concessions have to be made to the language used in them, but my impression is that those older usages are passing out of fashion. $\endgroup$ Sep 28, 2013 at 16:31
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There are actually TWO dark matter problems and at least two components to "dark matter".

The two dark matter problems are (i) that most of the matter in the universe appears to be non-baryonic and does not interact with baryonic matter (unless perhaps very weakly); (ii) most of the baryonic matter is still to be found; the fraction that is in luminous matter associated with galaxies etc. is too small.

The present concordance model of cosmology is that $\Omega_M \simeq 0.32$ - that is 32% of the critical density of the universe is in the form of matter, as defined for the purposes of insertion into the Friedmann equations. On the other hand, we also know from constraints provided by the cosmic microwave background and estimates of the primordial abundances of helium and deuterium, that the density of baryonic matter $\Omega_B \simeq 0.049$ (e.g. Planck collaboration 2018). We also estimate from just looking at the luminous matter in the universe and multiplying by some assumed mass-to-light ratio that the amount of "luminous" matter in the universe $\Omega_L \sim 0.01$.

Hence there are two problems - the gap between $\Omega_M$ and $\Omega_B$ and the gap between $\Omega_B$ and $\Omega_L$.

The latter gap may now be closing due to the discovery of warm-hot intergalactic medium, but compact remnants, stellar-sized black holes, cold white dwarfs, lost golf balls etc. are also candidates to fill in the $\Omega_B-\Omega_L$ gap. Numerous sources have put quite strong constraints on these things - at least as far as estimating their contribution to galactic dark matter; and they are likely to be very small contributors. They would be classed as baryonic dark matter and are dark only in these sense that they are faint/undetectable with current technology (although in the case of black holes, that will always be true). They are formed from baryonic matter that was present at the epoch of primordial nucleosynthesis. They cannot/do not make a contribution to non-baryonic dark matter.

Over the years there has been a shift, commensurate with the gradual exclusion of various baryonic candidates for dark matter, to exclusively refer to the non-baryonic component as "the dark matter". This is a more precise definition, since non-baryonic dark matter does not interact electromagnetically at all.

There are some grey areas - primordial black holes for instance. The general convention is that if the black holes were there during the epoch of nucleosynthesis then they cannot have contributed baryonically to that nucleosynthesis and they are treated as non-baryonic dark matter. If they were formed after nucleosynthesis then they would be baryonic dark matter. Further greyness arises because black holes are certainly capable of sweeping up some (but not much) non-baryonic dark matter after they have formed and it may even be that weak interactions may allow non-baryonic matter to become trapped in dense compact objects like white dwarfs and neutron stars.

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