12 ms·
(PhD student in astrophysics answering here.) This is a great question! You're addressing the idea (roughly along the lines of Occam's razor) that if there's a
by tomr_stargazer 10y ago
(PhD student in astrophysics answering here.)
This is a great question! You're addressing the idea (roughly along the lines of Occam's razor) that if there's any way to explain the "extra mass" without invoking a new form of matter, it should be preferred.
Most of these other possibilities have been largely ruled out via careful observations, which are detailed here:
https://en.wikipedia.org/wiki/Dark_matter#Composition https://en.wikipedia.org/wiki/Dark_matter#Composition
One of these possibilities, for example, is that the missing matter is actually contained in a great many small, dark, massive objects scattered throughout galaxies -- such as failed stars, planets, or even black holes -- rather than in a diffuse, invisible material. This possibility has actually been largely ruled out through a number of statistical "microlensing" surveys that are sensitive specifically to the presence of massive, dark bodies (via their gravitational lensing of background stars -- a rare event, but measurable statistically).
https://en.wikipedia.org/wiki/Massive_compact_halo_object#Detection https://en.wikipedia.org/wiki/Massive_compact_halo_object#De...
I think the wikipedia Dark Matter article is actually super well written and should address these issues, too!
- semi-extrinsic 10y agoRegarding the "no massive dark bodies" result: does this also cover dark dust? Or is that otherwise ruled out (perhaps by optical density)?
- maverick_iceman 10y agoBaryonic dust is ruled out by BBN.
- okket 10y agoIf you have four times of visible matter as "dark dust", this dust would clump together, creating stars, becoming visible matter again.
- tomr_stargazer 10y agoAdditional 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.
- tokai 10y agoWasn'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 https://www.cfa.harvard.edu/news/2010-26
- yk 10y agoIn 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.
- openasocket 10y agoHow 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.
- dandelany 10y agoAssuming 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/ http://justfound.co/gravity/
- BurningFrog 10y ago> 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?
- openasocket 10y agoNo, 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-matter-only-interacts-with-gravity-why-doesnt-it-all-clump-together-in http://physics.stackexchange.com/questions/214950/if-dark-ma...
- sloppycee 10y agoI'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?)
- klodolph 10y agoGravity 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 https://www.youtube.com/watch?v=NpEevfOU4Z8
- sloppycee 10y agoAh, so 'Gravitational waves' are ripples in spacetime. But, if gravity itself propagated as a wave (graviton + duality?) could interference explain dark matter/energy?
- powertower 10y agoGravity, in its normal sense (of matter's gravitational attraction), has no frequency or wavelength. Its only property is that of acceleration.
- evanb 10y agoThat'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 https://en.wikipedia.org/wiki/On_shell_and_off_shell
- s_q_b 10y agoI'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?
- stcredzero 10y agoit 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.
- mattmanser 10y agoIt'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.
- Synaesthesia 10y agoThere'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.
- stcredzero 10y agoThat'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.
- danielmorozoff 10y agoCould 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?
- theptip 10y agoDark 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.
- danielmorozoff 10y agoCould 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?
- theptip 10y agoSorry, 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 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.
- biot 10y agoGiven 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?
- m_mueller 10y agoas far as I understand, something that invokes gravitational force is by definition 'mass', not energy.
- evanb 10y agoWhile 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).
- m_mueller 10y agoOh 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!
- evanb 10y agoThe 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.
- wallace_f 10y agoThanks, that was insightful yet down to earth, terse yet well-written, and contained further reading and references.