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>particles invented before being directly observed as support of dark matter hypothesis Correct me if I'm wrong, which is quite likely, but isn't it one of the
by corey_moncure 10y ago
>particles invented before being directly observed as support of dark matter hypothesis
Correct me if I'm wrong, which is quite likely, but isn't it one of the qualities ascribed to dark matter that it doesn't interact with 'ordinary' matter or photons in any way other than gravity, and therefore can't be directly observed?
Or to turn the question around, is the dark matter hypothesis actually falsifiable?
- mikeash 10y agoIt obviously doesn't interact with ordinary matter very much, or else it would have been observed non-gravitationally by now. It could interact a little bit, though. Consider neutrinos: they almost don't interact with ordinary matter at all, but they can be observed with enough patience and care, and they are definitely real. Neutrinos were actually an early candidate for dark matter. The numbers didn't work out, but it's an example of something that would interact weakly. And sure, dark matter is falsifiable. If observations are made where gravity appears to deviate from what's visible and can't be explained by adding invisible mass, that would falsify it. Or if someone finally comes up with a change to the known laws of physics which explains the observations better than dark matter does, that would do it. Remember, nothing is truly "directly observed." All we can do to observe stuff is to see how it affects other stuff. Whether it's with photons or with gravity, it's all ultimately indirect.
- EGreg 10y agoHow do we know dark matter isn't just really transparent matter? Like, we can't see it in space but it's there, not reflecting or deflecting enough light for us to detect it.
- TheHegemon 10y agoLike some kind of dark matter?
- EGreg 10y agoJUST transparent to light and nothing more
- XorNot 10y agoBecause it would collide with itself. Dark matter isn't just "not visible" - it's non-electromagnetic. As observed it didn't aggregate when colliding with itself, so there aren't big dark matter made black holes around, nor gravity anomalies from dark matter nebula or the like. Two dark matter particles headed directly at each other would pass through each other and be only slightly deflected by their gravitational interaction.
- btilly 10y agoNot necessarily. Dark matter could be made of small dense objects. Because they are small, we wouldn't see absorption of light. And because they are small, they will very seldom collide. These kinds of object are called MACHOs. Based on observed dimming events while monitoring many stars, we have ruled out brown dwarfs as more than 20% of the total. Other possibilities still remain, but none have shown evidence of being sufficient to explain dark matter. However we have not ruled out the possibility of it being something that we haven't thought of yet.
- scatters 10y agoYes, that's what cosmologists mean by "dark": not interacting with light of any observable wavelength, to any detectable degree.
- yati 10y agoFrom what I understand, if dark matter is stuff that only interacts gravitationally, by definition it is transparent to light. Also collisions, being ultimately electromagnetic interactions, aren't observed. So it's not transparent in the sense of glass. But I'm not a physicist, and this thread is interesting :)
- Y_Y 10y agoIt could be[0]. It's just not very likely, since you'd have to have very special circumstances, but it's not logically impossible, at least in cosmology. [0] http://abyss.uoregon.edu/~js/cosmo/lectures/lec17.html http://abyss.uoregon.edu/~js/cosmo/lectures/lec17.html
- Paul-ish 10y ago> And sure, dark matter is falsifiable. If observations are made where gravity appears to deviate from what's visible and can't be explained by adding invisible mass, that would falsify it. Whats to stop us from "adding epicycles" when that happens?
- mikeash 10y agoThe same thing that stopped the addition of epicycles to the geocentric model of the solar system: a better theory that explains the observations more naturally.
- T-A 10y agoThere is a (waning?) hope that dark matter interacts weakly with ordinary matter, which would make it detectable: https://en.wikipedia.org/wiki/Weakly_interacting_massive_particles https://en.wikipedia.org/wiki/Weakly_interacting_massive_par...
- thechao 10y agoYou can observe it via gravitational lensing. For instance: http://scienceblogs.com/startswithabang/2011/04/20/how-gravitational-lensing-show/ http://scienceblogs.com/startswithabang/2011/04/20/how-gravi... .
- lngnmn 10y agoYou can observe nothing but the fact that mass distorts light. Everything else is imagination.
- chongli 10y agoWe can observe the effect of gravitational lensing. The claim that dark matter causes it is part of the same claim that dark matter is responsible for galactic rotation curves looking the way they do. Thus it does not provide any further support for the dark matter hypothesis.
- deleted 10y ago[deleted]
- dogma1138 10y agoWe've seen instances already where gravitational lensing was explained through other means (very dim clusters, dust, and even camera issues), we also don't know if the space time on those scales is "smooth", if it isn't then light will not travel in a straight line which would account for the lensing effects. The spacetime curvature, mass/energy, and gravity have quite a few interesting interactions as in gravity as a force, vs gravity as a feature of spacetime, vs potential other forms of gravity. We aren't even sure at this point if a curved spacetime without mass/energy that would generate that curve, would actually produce gravity or not since we aren't able to observe this nor do we have a complete theory of gravity.
- XorNot 10y agoExcept we have good evidence that the universe is very close to flat, space-time wise. A hypothesis that there's local unexplained curvature...Well that's really what dark matter is all about. What's curving it?
- yk 10y agoThe dark matter hypothesis may not actually be falsifiable, however that is not what physicists are working on. Physicists work on dark matter models, precisely defined theories which incorporate dark matter and these are falsifiable. For example roughly ten years ago models of weakly interacting particles (that is, particles that interact via the weak force) were very popular, but they have a influence on the abundance on nucleons in the early universe. Turning the argument around, one gets a interaction probability of the particles, and that part of the parameter space was excluded a few years ago. (You can of course try to mask the signal, but then you are generating different models, the original very simple models were falsified.)
- deleted 10y ago[deleted]
- dogma1138 10y agoThat only is if dark matter is indeed "WIMPS" or weakly interactive massive particles. But then again there are multiple contenders for dark matter.
- wyager 10y agoThere are several ways that dark matter might exhibit detectable interactions. For example, it could undergo weak interactions with itself or with ordinary matter. (See WIMPs.) It could also undergo some antimatter-annihilation interaction, emitting e.g. a Z boson, which could decay to a detectable particle-antiparticle pair. We might be able to observe Z bosons from matter interactions decaying to dark matter pairs, which would manifest as an unexplained shortage of matter after a collision. We're still looking for any such interactions.