Monday, February 18, 2019

Never again confuse Dark Matter with Dark Energy

Barnard 68 is a molecular cloud
that adsorbs light. It is dark and made
of matter, but not made of dark matter.
[Image: Wikipedia]
Dark Matter 

Dark Matter is, as the name says, matter. But “matter” is not just physicists’ way to say “stuff,” it’s a technical term. Matter has specific behavior, which is that its energy-density dilutes with the inverse volume. Energy-density of radiation, in contrast, dilutes faster than the inverse volume, because the wavelengths of the radiation also stretch.

Generally, anything that has a non-negligible pressure will not behave in this particular way. Cosmologists therefore also say dark matter is “a pressureless fluid.” And, since I know it’s confusing, let me remind you that a fluid isn’t the same as a liquid, and gases can be fluids, so sometimes they may speak about “pressureless gas” instead.

In contrast to what the name says, though, Dark Matter isn’t dark. “Dark” suggests that it adsorbs light, but really it doesn’t react with light at all. It would be better to call it transparent. Light just goes through. And in return, Dark Matter just goes through all normal matter, including planet Earth and you and me. Dark Matter interacts even less often than the already elusive neutrinos.

Dark matter is what makes galaxies rotate faster and helps galactic structure formation to get started.

Dark Energy

Dark Energy, too, is transparent rather than dark. But its name is even more misleading than that of Dark Matter because Dark Energy isn’t energy either. Instead, if you divide it by Newton’s constant, you get an energy density. In contrast to Dark Matter, however, this energy-density does not dilute with the inverse volume. Instead, it doesn’t dilute at all if the volume increases, at least not noticeably.

If the energy density remains entirely constant with the increase of volume, it’s called the “cosmological constant.” General types of Dark Energy can have a density that changes with time (or location), but we currently do not have any evidence that this is the case. The cosmological constant, for now, does just fine to explain observations.

Dark Energy is what makes the expansion of the universe speed up.

Are Dark Matter and Dark Energy the same?

Dark Matter and Dark Energy have distinctly different properties and cannot just be the same. At best they can both be different aspects of a common underlying theory. There are many theories for how this could happen, but to date we have no compelling evidence that this idea is correct.

54 comments:

  1. ...if the volume increases...

    Volume increases with the expansion of the universe, not the piston in a cylinder moving to expand the volume as is common in discussions of heat engines and thermodynamics, I suppose.

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  2. Slight edit, I think you mean name, not same.
    'In contrast to what the name says...'

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  3. ""because the wavelengths of the radiation also stretch""

    Where does the energy go when that quanta
    become more and more red? Is there really
    no piston?
    Best Georg

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    Replies
    1. Energy is not conserved. It just isn't. Get over it.

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    2. So the total energy in the Universe tends to zero as the volume of the Universe increases (taking the second law of thermodynamic as a natural process that tends to convert in time all matter into radiative heat and that even black holes will end evaporate into radiation)? Will the thermal "death" be a future state where there will not remain any energy in the whole Universe? And what kind of phenomenology could present such a zero energy Universe?

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  4. Thanks for another clear post! These are just names after all... placeholders with a hint of the nature of the problem.

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  5. Dark matter is perhaps aptly named after all? Gravitational compression of dark matter as it fall into a black hole does not make it incandesce. Dark matter never decays, producing radiation. Dark matter never explodes.

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  6. I think the better word would be 'absorb' -- although it is still a difficult analogy.

    "When a material imbibes some amount of gas or liquid into it, then it can be said that the material has absorbed the material. On the other hand, when some gas or liquid accumulates on the surface of a material, then the process can be termed as Adsorb."

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  7. "[..] but we currently do not have any evidence that this is the case."
    This is no longer entirely true (the any evidence part), though of course if one had to take a bet it was still more probable that it's a constant. Also, one could cast the different values derived from the CMB and supernovae in that light.
    https://www.space.com/43166-dark-energy-increasing-time-quasars.html

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    1. G. Bahle,

      I know, I wrote about this here. Alas, I don't think it's convincing.

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    2. I agree, but the quasar paper is newer. It will be interesting to see how more data changes the picture.
      https://arxiv.org/abs/1811.02590 (it's Nov 18)

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    3. Yes, thank you, I have seen the paper.

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  8. I believe in dark energy, as the cosmological constant exists. Dark matter continues to elude us, despite decades of costly experiments and thousands of arXiv papers. I will stick with some sort of modified Einsteinian gravity until dark matter particles start showing up all over the place.

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    1. However, if DM particles are sterile - interact via gravity only - they never will ...

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  9. 'Energy density remains entirely constant with the increase of volume.'This I do not understand. How can this happen without extra energy being added all the time? And what about the conservation of energy?

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    1. Sabine, could you please someday write a post explaining more extensively the evidences, possible causes and consequences (for the future of the Universe) of the fact that the "Energy is not conserved"?

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    2. Oh, ok. Thank you, I didn't know about that post.

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    3. Sabine,

      above you mentioned to Samu your dark energy post – just read it – great as always and no questions about Λ and energy not conserved. Only a side issue where you say “This gives rise to a local conservation of stress-momentum when coupled to gravity (the stress-momentum tensor is covariantly conserved).“
      So far I thought that ∇,T’’=0 with ,’’=μ,ν does allow that energy (and momentum) can flow out of a spacetime volume and thus T’’ is not locally conserved.
      In contrast to e.g. a current J’ where 0=∇,J’=1/√g ∂,(√g J’) integrated over a volume indeed leads to a zero surface integral. But ∇,T’’ has the additional term Γ’,,T’’ and therefore the gravitational field can carry out energy.
      I hope only my wording is confused and the math is correct. I checked Zee here and Weinberg here and I know that energy in GR is a pain and we probably speak about the same.

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    4. Reimond,

      I think this is a miscommunication. The "local conservation of stress-momentum" refers to the very equation you mention. Maybe I should better have written "conservation of the stress-momentum tensor". But I don't really see how it helps. You either know the equation already, in which case you know what I am talking about anyway, or you don't, in which case adding "tensor" will not help.

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  10. Well, the statement you make above is too categorical. There are indications that it might be time dependent.

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    1. "Indications" is not the same as evidence.

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    2. Energy in the universe is defined from the Hamiltonian constraint H = 0 with

      0 = ½(da/dt)^2 + 4πGρa^2/3 + k,

      from memory here. This leads to the expansionary universe. I derive this using Newtonian mechanics here.

      https://physics.stackexchange.com/questions/257476/how-did-the-universe-shift-from-dark-matter-dominated-to-dark-energy-dominate/257542#257542

      There is an issue with phantom energy creeping up on us the Hubble factor from CMB is H = 67km/sec-Mpc and from more local galaxy expansion H = 72km/s-Mpc. There appears to be a change in energy density

      dρ/dt = -3(1 - w) ρ

      which increases for w < -1. Data is starting to weigh in favor of this and the so called big rip. It might sound horrible, but on the other hand it does make things maybe more interesting.

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  11. "But its name is even more misleading than that of Dark Matter because Dark Energy isn’t energy either."

    As Sean Carroll pointed out, "dark energy" is a really stupid name: essentially everything has energy, and many things are dark. His "smooth tension" would be a better name, but it never caught on.

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    Replies
    1. Sounds like something green you get to drink at a spa.

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  12. That’s a lawyer’s response.

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  13. In one sense dark matter and dark energy are related. All quantum fields and matter are derived from the quantum vacuum. The raising operator a^† acts on the vacuum state |0> to give a^†|0> = |1>. If dark matter is a particle then it is derived from the vacuum. Dark matter would be no exception, at least within the standard ΛCDM theory. Dark energy is likely a vacuum physics. It is not hard to derive an accelerated expansionary spacetime in the FLRW framework given a constant energy density in all space.

    There are obstructions to understanding this. The inflationary vacuum had an enormous energy density, and in some ways is similar to the vacuum energy from what Weinberg called a “stupid calculation.” Sum over all the vacuum modes and you get this huge vacuum energy density up to the Planck energy density ρ ≈ 10^{76}GeV^4. The inflationary vacuum was only a few orders magnitude smaller. This collapsed into the vacuum energy of the accelerated universe or dark energy with ρ ≈ 10^{-47}GeV^4. In quantum field theory this sort of free vacuum is largely ignored, and there is a procedure to do this called normal ordering. However, with gravitation we can't get away with this. A vacuum energy density is a source of curvature and gravitation. So we might say that in some ways this huge false vacuum of inflation converted itself into mass-energy of the world we observe around us. This may include dark matter.

    How this happens is not that clear. Standard QFT does not have the vacuum “used up” this way. For that matter standard QFT throws out nonlocality of quantum physics with Wightman conditions and other things. So we have tied our hands with some limitations that are excellent approximations for particle physics at the energy we test with. However, this is not entirely adequate.

    This is said not to be quantum gravitation; the occurrence of matter and for that matter dark matter are not thought of as quantum gravitation. However, it is similar to Hawking radiation and it does involve a relationship between gravitation and quantum physics. The ideas of Verlinde or the prospect proposed by Hossenfelder that dark matter has a superfluid phase suggests this might be more strange than we think.

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  14. From the first time I read about dark matter a (maybe stupid) question puzzle my mind. if dark matter rarely interact with other matter, how can it influence e.g. the average velocity of galaxies in the clusters? is it not a gravitational effect?

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    1. Sounds,

      The velocity of a star on its orbit around the galactic center depends on the *total* amount of mass inside that orbit. Ie, it depends on the amount of dark matter. I explained this here.

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    2. that was clear, I read most part of your post, but...I guess the total amouunt of matter influence the velocity through gravity and this happens only if the dark matter interact.

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    3. Dark matter interacts gravitationally.

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    4. So, in which way dark matter does not interact? or, "Dark Matter interacts even less often than the already elusive neutrinos" ?

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    5. Neutrinos interact with the weak interaction. Dark matter interacts as weak as that or weaker both with itself and with normal dark matter. I said this in the video I already referred you to.

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    6. Sorry for hassling you...if I have understood: the Earth could have a dark matter moon orbiting around and we wouldn't see it (dark moon would not reflect the Sun light; a "dark" asteroid passing by would be attracted but...instead of hit the Earth, would pass through! Is this correct?

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    7. Dark matter, for all we currently know, doesn't clump enough to form objects the size of the moon or asteroids. Dark matter hovers in galaxies in form of thinly dispersed clouds.

      The probability of dark matter to interact with normal matter is not zero. Just how high the probability of a number of dark matter particles is to interact depends on the assumptions you make about the interaction strength, which is not known. We only know it's weak.

      I collected some numbers about the interaction probability for currently used dark matter models here.

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    8. So we are sure the probability of dark matter to interact with normal matter is not zero? There is so a coupling constant between ordinary matter and dark matter? And what kind of interaction was that experimented, weak interaction?

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    9. I know that dark matter does not condensate in massive objects. Mine was just an hyperbole to be sure that I have correctly understood the interaction behaviour of dark matter.

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  15. Bill1:21 PM, February 18, 2019
    "I believe in dark energy, ..."

    That's the problem of modern science, I think, ... or do you believe e.g. in gravitational forces in the same way, like you believe in "dark energy"? Where ends knowing, where starts believing?

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  16. Sabine said… The cosmological constant, for now, does just fine to explain observations.

    When put in the context of the bigger picture, science history, unification, human behaviors' influence continually exhibited in the science's; I don’t see how that belief isn’t more troubling to professionals in the field.

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    1. It's not a belief, it's a fact. We have a model, it fits the data, and that's that.

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    2. Just because it's a fact it works doesn't mean you should be unwavering it is the correct solution. There are good reasons to think some mathematics, while making good accurate predictions are not mimicking how nature does it and that could be the cause for many of physics current problems. In my opinion math should be viewed as our best physics communication tool but still fallible, even when making good predictions.

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    3. No one is "unwavering". You use it until something better comes up.

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  17. "Dark Matter and Dark Energy have distinctly different properties and cannot just be the same. At best they can both be different aspects of a common underlying theory"

    there's Verlinde and Smolin. dark matter could be result of some form of MOND, and since MOND is on the same order as dark energy, you have a unification of sorts.

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  18. “if you divide dark energy by Newton’s constant, you get energy density”
    I tried it and it appeared that the dimension of dark energy is (m^2)/(sec^4). This does not make too much sense for me and moreover I can hardly imagine that this stuff interacts gravitationally.

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    Replies
    1. Gerben,

      The cosmological constant has dimension 1/length^2, Newton's constant has dimension 1/mass^2, so the CC divided by Newton's constant is mass^2/length^2, which is an energy-density as it should be. (Modulo c and hbar - disperse as needed).

      Easiest way to see this is to look at Einstein's field equations and divide away Newton's constant. You'll get a factor \Lambda/G * g_{\mu\nu} which is the dark energy term (for the specific case in which it's constant, needless to say).

      It couples to gravity by definition. (If it didn't, there would be no point talking about it because it would be unobservable.)

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  19. Inventing undetectable dark stuff instead of admitting your theory is broken. That can never end well. Einstein would have been appalled by all this.

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    1. Samuel,

      Dark matter is not undetectable. The very reason we have introduced dark matter to begin with is that we do have evidence for its existence. I have listed the evidence in my slides here. You may instead be talking about directly detecting a particle. That only makes sense if you think it's a particle to begin with.

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    2. I agree with Samuel. The best we can say about Dark Matter, until we can narrow down *what* it is, is that it is inferred - not detected.

      And Einstein would absolutely be appalled. I believe he would go so far as to retract the concepts that infer Dark Matter (Inertial/Gravitational Mass equivalence) and start again on a theory of everything.
      regards
      Marco

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    3. Dark matter is a parameter in an equation. That's all we say about it. If you think anything else, you haven't been following. The scientific case is clear.

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    4. Einstein would have changed (fixed) the equations so they would be DM parameter free. He would have made a connection between a 0 and the cosmic expansion acceleration. Thus connecting the issues driving the need for DE as well as DM in galactic rotations.

      Regards Marco

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  20. Are you aware of any meaningful work looking at the possibility that phenomena attributed to dark energy could arise from gravity becoming weaker than predicted by GR at intergalactic scales, in lieu of solutions like a cosmological constant or quintessence?

    Conceptually, dark energy imagines that stars are flying apart from each other faster than GR without a cosmological constant because there is more stuff in the vacuum providing a source to pull them apart.

    But, wouldn't a weaker than expected in GR tug of gravity between galaxies and galaxy clusters lead to the same result?

    I imagine that this might be harder to model analytically, but it seems like this could give rise to the same phenomena of galaxies flying away from each other faster than we would expect with straight GR, that is amplified and accelerates as the average intergalactic distances between galaxies and galaxy clusters grows greater over time.

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