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How dense is the concentration of dark matter? I mean, it's affected by gravity, so could it/does it come together to form "dark stars" of some form? And what would happen if I had a chunk of dark matter in a lab: what would it look like? Would it be invisible? Would I be able to pick it up in my hand, or would the particles just slip through ordinary matter like a ghost? I'm guessing the answer to most of these is "we don't know," but I'd love to know if we have any guesses.

Assuming dark matter is a WIMP, I think it would be invisible and fall through your hand (and the earth) when you tried to pick it up. Stars require the particles in question to bang around against each other and interact with each other. WIMP particles wouldn't even be able to interact with each other except gravitationally - ie. they wouldn't "collide" or create pressure in an enclosed space. A cloud of dark matter is likely to stay a cloud for a very very long time rather than collapsing in on itself.

I think of it like an N-body gravity simulation[1] with very very large N and no collision detection.

[1] http://justfound.co/gravity/

> I think it would be invisible and fall through your hand (and the earth)

Why would't it fall to the center of the earth and stay there, since that is the local gravitational center?

No, it would speed past the center until losing moment on the surface of the other side of the planet, and oscillate back and forth. Because there's no "friction" slowing down the dark matter it will continue like this forever, by conservation of energy.

Check out http://physics.stackexchange.com/questions/214950/if-dark-ma...

Wouldn't it -very- slowly lose energy due to gravitational radiation?

If the cloud was inhomogeneous, yes. However, (unless I am mistaken, which is fairly likely,) the more symmetric a bulk collection of particles (such as WIMPs), the less gravitational radiation it would emit as it oscillated. In particular, for every particle falling in from one side of the planet, you have another particle falling in from the other side of the planet, so the mass distribution of the system does not change over time. I don't know how gravetomagnetic effects would come into play.

> speed past the center

What caused these particles to accelerate up to the speeds they are at in the first place, then? Something must be interacting with them quite significantly if they have non-zero velocity?

Gravity.

Actually, almost the only thing that we know about dark mater is that gravity affect it.

(Actually, we only know that there is something that is making a gravitational pull of normal mater, and we call this mysterious thing "dark mater".)

the force of gravity

Aha, DM particles are in eternal orbits!

Thanks!

Wasn't it last year that the presumed count of red dwarf stars in the universe had to be corrected to a much larger number than previously thought?[0]

If we can be wildly wrong about the number of red dwarfs couldn't we just as well be wrong about the number of brown dwarfs, rogue planets and other massive but dim/dark objects in the universe?

edit: Last year? More like 6 years ago. I feel old now. [0]https://www.cfa.harvard.edu/news/2010-26

In principle, yes. However, there is not only evidence for dark matter from galaxy rotation curves but also from large scale structure simulations and especially from the CMB. The interesting thing is, the CMB observations, which react to the number of particles in the standard model, agree rather well with rotation curves. (And IIRC there is also a way to disentangle structure formation from simple gravitational interactions, so there are "2 and a half" independent observations which indicate dark matter or something that closely resembles dark matter.

Regarding the "no massive dark bodies" result: does this also cover dark dust? Or is that otherwise ruled out (perhaps by optical density)?

Additional dust inside a given galaxy would both absorb optical light and emit infrared light (depending on its temperature, which is set by the Galaxy's ambient starlight among other factors); both effects are readily detectable.

If you have four times of visible matter as "dark dust", this dust would clump together, creating stars, becoming visible matter again.

Baryonic dust is ruled out by BBN.

I'm not even close to a physicist, but it has always bothered me that such a huge percentage of the matter in the universe would be nearly unobservable. It's the sort of thing that wouldn't pass my normal sanity check in a scientific computing model.

But I trust that these possibilities are being well considered. Is there any significant mainstream buy-in with regard to alternative theories of gravity?

... it has always bothered me that such a huge percentage of the matter in the universe would be nearly unobservable.

Maybe think of it this way: expecting matter to be easily observable is an anthropocentric point of view, because human intuition defines matter as something that can be observed by the senses.

But the universe was not designed to be observed by us. There's no reason to expect that the majority of matter should be observable.

> I'm not even close to a physicist, but it has always bothered me that such a huge percentage of the matter in the universe would be nearly unobservable.

It turns out that in some ways we actually are precious snowflakes. What we are made off (baryonic matter) makes up less than 5% of the whole "stuff" of the Universe. A mere froth on the surface of existence.

Well, just think about the Dark Scientist who says "OK, we make up 27% of the universe, and there's a missing 5% we don't see because it doesn't interact with us."

And then some dark model builder says, "Guys, guys. I bet it's SU(3)xSU(2)xU(1) with 3 generations and the parameters tuned so that..."

it has always bothered me that such a huge percentage of the matter in the universe would be nearly unobservable. It's the sort of thing that wouldn't pass my normal sanity check in a scientific computing model.

How about this? -- A large fraction of bugs in a large, long lived computer system will be difficult to recreate.

It's not the same thing.

Imagine being told there's loads of bugs in your system and it's constantly breaking, even though no-one can show you a single one of these bugs and you've not got a single user complaint.

That's dark matter.

It's obviously bullshit, but it's the 'simplest' explanation.

That's not quite right. A better analogy would be:

"Imagine being told there's loads of bugs in your system and it's constantly breaking, and even though no-one can show you a single one of these bugs, tons of users are complaining."

That would fit the situation better: There's only one kind of indication that these things exist.

There's been some pretty specific observations of dark matter like galaxies which have been shown to have varying proportions of DM, there have been measurements where scientists have actually localised the dark matter, i.e. mapped where it should be concentrated based on the perturbation of nearby stars.

Given that E=mc^2, how do we know that there isn't simply a form of energy which is overly abundant? If the energy and mass are interchangeable, could it be that a massive amount of energy could be undetectable yet still have the same gravitational effects as what we call dark matter?

as far as I understand, something that invokes gravitational force is by definition 'mass', not energy.

While that's the common understanding, general relativity tells us that the spacetime curvature is dictated by the stress-energy tensor, which is accounts for all different types of mass and energy. For example, if you have a crystal at very low temperature it will gravitate less than the same crystal at a high temperature (though for familiar materials it is a VERY slight effect).

Oh wow TIL. So, say a neutron star has stronger gravity than given by its mass alone, due to its energy density? What fraction of gravitational force are we talking roughly for such an object?

Edit: Do photons have gravity even though they are massless?

Edit2: Is that why gravity can bend photons?

GRT suddenly makes a bit more sense to me if the answer to these are 2x yes, so thank you!

The issue is the speed of light is so big. So, you need a LOT of energy in order to source gravity comparably to just a little bit of mass (essentially, because E=mc^2 -- or perhaps phrased more clearly in this case, m = E/c^2).

Even Newtonian Gravity can bend light classically, by Galilean relativity. But, weirdly, it essentially hinges on the fact that the the mass m of the photon cancels from m*a = GmM/r^2. Of course, that is really the equivalence principle---the acceleration of all things under gravity's influence is the same. Whether or not light is a source of Newtonian gravity... I'm not sure. It's a tricky question because m=0. I want to say no because the "equal and opposite" forces should both be 0, even though one of them effects an acceleration on the other. I should emphasize that I'm not sure!

In Einsteinian gravity, the paths of photons (and indeed all things) are bent because spacetime itself is curved. Classical electromagnetic static fields and waves certainly have an energy density that can source gravity, and individual photons do too. But their energy is on the order of hbar. So you're talking a source of gravity like hbar/c^2. THIS IS REALLY TINY unless the photon's frequency is ENORMOUS.

To me, the speed of light is actually really slow - meaning, if you think classically you assume c infinite (at which point, as far as I understand, relativity theory essentially behaves like classical physics).

Learning that information travels way slower than the universe expands is quite unnerving. Similarly, learning that earth's fate could be determined already since hundreds of thousands of years through a hypernova directed at it - that we'll only know about when it hits us and wipes out our atmosphere. Well... light speed is far too slow for my taste ;).

Edit: It still doesn't answer my first question though: If you have something really energy dense like a neutron star - what fraction of gravity does energy make out then? 1E-3? 1E-10? 1/2? I'd find that interesting to know. According to wiki, neutron stars fall in temperature within years of creation from up to 1E12 K to 1E6 K. Six orders of magnitude. Depending on how much this decreases gravity I could imagine this effect alone influencing stellar orbits (I assume that a supernova would still allow other stars in a multi star system to continue existing). Has such a thing ever been measured?

Sorry.. there's just a whole can of worms opened about this in my head right now. Need to find an astro physicist to shake down :D.

Watch that youtube link in my sibling comment first, but I actually wanted to take a crack at an answer. What's the change in mass, ∆m, as the star cools? We'll assume the mass of the neutron star is 1.5 solar masses.

    E = mc^2
    ∆m = ∆E/c^2

So really, what's ∆E? A hyper hand-wavy estimate:

    ∆E = Q = mc∆T (c being specific heat)

Specific heat by mass is really hard to predict, but by moles it is fairly constant, well within an order of magnitude. So we'll discuss mass in moles.

    m = neutron star moles
    m = (mass of neutron star / mass of neutron) / Avogadro's #
    m = 2.9580163e33 mol

    c = 24 J / (mol * K)

    ∆T = 1e12 K - 1e6 K

→

    ∆E = 24 * 2.9580163e33 * (1e12 - 1e6) J = 7.0991681e45 J

Substituting that back into the original, as a neutron star cools:

    ∆m = 7.0991681e45 J / c^2 = 7.89888982e28 kg

Which is like 2.6% of the mass of the original star, so a pretty solid chunk. But that number is pulled out of my ass—I am not a physicist.

But there are even more weird effects going on, due to the warping of gravity the mass of neutron stars can be up to 20% less than you'd expect based on its baryonic (neutron) constituents (questions 4 and 7):

https://www.astro.umd.edu/~miller/teaching/questions/neutron...

Thank you!

About your last equation, wouldn't J/m^2/s^2 come out as kg? 7.9E2 would be very low then, no?

I wonder how much energy is stored electromagnetically and through nuclear forces though. These things are supposed to have extremely strong EM fields and I imagine every piled up nucleus like a little atomic spring that has been depressed as much as possible. Wouldn't most of the stored energy be in there?

"The True Nature of Matter and Mass" - https://www.youtube.com/watch?v=gSKzgpt4HBU

This whole channel will be up your alley, but this video and "The Real Meaning of E=mc^2" one directly answer your question.

Could you speak a bit more to the idea of gravitational lensing as a proxy to show no regular matter but rather dark matter. If dark matter accounts for the missing 'mass' holding galaxies together, wouldn't you expect it to cause lensing of light?

Dark matter wouldn't cause lensing because it's diffuse; only very dense and massive astronomical objects cause light to bend enough for us to detect it.

Why is dark matter diffuse? Shouldn't it clump together? I guess if there's no Coulomb force the stuff just moves past itself. However there should be some kind of collision cross-section shouldn't there? Since there's so much of it, what would be the result of even a small probability of collision?

That's a very good question. The current 'best candidate' model predicts dark matter to be collisionless, i.e. the particles do not (often) collide with each other [1]. In other words, considering a 2-body system, the dark matter particles would accelerate towards each other, and then shoot straight past and decelerate on the other side, continuing to oscillate.

In that paradigm, considering a bulk of dark matter particles, the particles will be attracted to each other, and will fall towards the center of mass of the clump -- but there's nothing to stop them, and so they pass out to the other side of the cloud, where they decelerate. This puts a limit on how dense the cloud can become (I haven't studied the details of the mechanics here, but look into the Virial Theorem if you want the equations that describe these limits). In a normal cloud of gas in space, the particles would collide with something as they fall into the center of the cloud, which would convert their linear motion into random motion, and so they would essentially be trapped.

Note that while dark matter doesn't form dense objects, it does clump to some extent, and this is actually involved in galaxy formation [2]; based on initial small perturbations in the densities of matter before the inflationary period, the Cold Dark Matter forms clumps (halos) which act as the initial seeds of attraction for the baryonic matter (H/He) that formed the first galaxies.

[1]: http://www.ncbi.nlm.nih.gov/pubmed/10828999 [2]: https://en.wikipedia.org/wiki/Dark_matter_halo

Could you explain diffuse please? Relativistically, space-time deformation causes both galactic formation and lensing. Collections of stars millions of light years apart(dense locally but not on an average), acting together on light passing near the conglomerate will lens.

So what do you mean by diffuse and could you point me to some source I can read up on this?

For dark matter to explain the missing mass it has to be everywhere and spread evenly - hence defused.

If it was concentrated in only specific spots it would cause different gravitational effects such as lenseing.

The lack of lenseing isn't the only issue it's also the general mass distribution across the galaxy for example the stars in the outer parts of the Milky Way move at nearly the same speed as the stars in the center. Since the center has much more mass the stars should move faster but they don't which means there is a lot of more mass that we do not see and that is distributed evenly across our own galaxy and not clustered in the center like the normal matter.

So to match the observation the dark matter has to be every where think of it like the air around you.

Now it doesn't have to be actual dark matter but it has to gravitationally affect the rest of the matter in the universe one of the version of string theory has dark matter as gravity leaking into our universe from higher dimensions or parallel universes the problem with that is that it still does not explain the diffusion unless the brain it's leaking from for some reason unlike our universe is diffused.

Sorry, I wasn't specific enough in my explanation. GP was referring to micro-lensing experiments, i.e. lensing around a star or other such object. That experiment is essentially selecting a bright point light source behind a galaxy, and looking for the sort of distortion that would occur if there was an object near the light's path as it passed through the galaxy. If you see lensing but no object, you've found a dark, dense object.

https://en.wikipedia.org/wiki/Gravitational_microlensing

You're right that all the dark matter in a galaxy would contribute to gravitational lensing around that galaxy, but that's a different signal; you'd be looking for the lensing at different scale, e.g. Einstein Rings around the edge of the galaxy, not inside it.

I meant 'diffuse' literally, in the physical sense, as in sparse, not dense -- the 'missing mass' that we're trying to explain is, under the dark matter hypothesis, spread out more thinly than if it was accounted for by dense dark objects like planets or stars.

I've always wondered, if gravity is a wave (has it been confirmed?), then there must be areas in space where different waves superimpose.

Would the constructive interference between gravity waves be significant enough to account for 'dark matter'? And what would destructive interference look like (dark energy?)

Gravity waves are caused by moving objects, and the waves themselves move. So if you did see constructive interference between gravity waves, it would be temporary. Gravity waves also tend to be extremely small, small enough that it was only this year (2016) that we were finally able to detect them (at LIGO).

Here's a video of what standing waves look like in water: https://www.youtube.com/watch?v=NpEevfOU4Z8

Ah, so 'Gravitational waves' are ripples in spacetime.

But, if gravity itself propagated as a wave (graviton + duality?) could interference explain dark matter/energy?

Gravity, in its normal sense (of matter's gravitational attraction), has no frequency or wavelength. Its only property is that of acceleration.

That's right. Quantum mechanically, potentials are generated by the exchange of off-shell mediators (the clearest explanation I have seen is in Zee's QFT in a Nutshell).

https://en.wikipedia.org/wiki/On_shell_and_off_shell

Thanks, that was insightful yet down to earth, terse yet well-written, and contained further reading and references.