Is Dark Matter the new Phlogiston?

Hello. Cosmologist here. Before I get going I will link to http://www.talkorigins.org/faqs/astronomy/bigbang.html which will probably be useless to the questioner, but will help for anyone who doesn't have a clue what this question is about. Mostly, what has been said so far is right. The state of play on the dark matter front is that dark matter is the leading contender for explaining why we are short of luminous matter. It's not by any means considered a done deal though, and MOND, most specifically in the relativistic formulation that Bekenstein came up with called http://en.wikipedia.org/wiki/TeVeS is an alternative that receives plenty of interest. It's a great theory scientifically as it's dead easy to invalidate, at least in principle(unlike some that have been getting a lot of press *cough*ID*cough*). It's not been yet, to my knowledge, but it'd only take, say, one rotation curve that doesn't fit TeVeS to do it, or there are other routes to invalidating it. A paper I read a few days ago suggests that gravitational lensing tends to happen more often and more extremely in TeVeS for instance, and if this turns out to be true and we don't see enough such lensing events it could be a problem for TeVeS. So, alternatives to dark matter are coming in second, but they're by no means laughing stock theories or anything. They get serious consideration, at least in part because they're often able to make strong predictions we might feasibly invalidate. One last point about dark matter is that we know one kind of dark matter definitely exists - neutrinos. We detect these all the time in neutrino observatories, but it's very hard to do, and low energy neutrinos are pretty much impossible to detect but would have a gravitational effect. These particles are ghostly in the extreme, and stand a 50% chance of making it through a light year's worth of solid lead without being stopped, and were proposed to deal with the problem that energy and angular momentum seemed to go missing in certain nuclear decays. You could claim that the energy and angular momentum wasn't being conserved, was going off somewhere altogether different or whatever, but ultimately we started detecting these particles and now they're detected routinely. What we do know is that this particular brand of dark matter isn't any good. We've already ruled it out for helping with our gravitational problems because neutrinos are 'hot' - they move too fast. This means you can't get them to clump up in one place and help to hold a galaxy together. It's not that implausible to propose something that acts a bit like a neutrino but is much heavier. Also particle physicists have plenty of candidates for particles it might be which are motivated for one reason or another such as http://en.wikipedia.org/wiki/Supersymmetry. On to dark energy. What do we know about dark energy? Well, we know that our ordinary general relativity models without it don't match what we see. Something is missing from them. Noone is committing themselves to what is missing at this stage, and a lot of money is going in to figuring out some more about it. You'll hear lots about the cosmological constant, and the cosmological constant is just the simplest form of dark energy that matches the data, but we don't have any really strong reasons for thinking it is a constant right now. We have a bunch of theories competing right now, with names like phantom energy and quintessence models, but we just don't know right now which is right. We're working hard on ruling out as many as we can though. Phlogiston's an easy comparison to make, or you can compare things to the heliocentric model which suddenly did away with the bells and whistles of epicyclic orbits, but science doesn't always go that way. Sometimes you really do need extra bells and whistles. The neutrino has shown us that.

Either dark matter exists, or our understanding of gravitation is wrong. Its gravitational effects are consistently and universally observed. So absent a completely new theory of gravitation, dark matter is here to stay. But who knows: maybe gravity is the phlogiston of the 17th-21st centuries.

M-theory and string theory are generally pretty hard to test. I'm not so familiar with the particle physics side of it all, but on the cosmology side when you start talking about branes you can come up with models where there are some definite testable predictions. You can attempt to perform the necessary experiments and see if you can rule some specific cases out, but it might not be possible to rule out M-theory in general. String theory might generally make predictions about things happening at certain very high energy scales we can't achieve, but a specific braneworld cosmology could predict that something funny is going on on a small (but not subatomic) scale to gravity. You can literally do a tabletop experiment to say that if this is happening the length scale involved must be smaller than say 0.1mm, or, if in the future we're very lucky we might be able to say that on a certain length scale something funny is going on with gravity, and then that's evidence in favour of branes. http://physicsweb.org/articles/world/13/11/9/2http://physicsweb.org/articles/world/13/11/9/2 Again, this is stuff I'm less familiar with, but I can drop people more involved an email if any questions come up. No promises they will answer though.

Paris Hilton said: "Although yeah 0bvious' theory about extra dimensions hiding things doesn't really make sense." Not my theory. Try http://en.wikipedia.com/wiki/M-Theory and its many off-shoots for a glimpse into the world of higher dimensional branes and gravity being shared across universes mere nano metres away from our own.

In regard to Paris Hilton's link about using full general relativity instead of Newtonian physics: although that kind of explanation is very satisfying, there are a number of problems with it. There have been a few responses published suggesting that the authors from Paris' link actually did their math incorrectly. An even bigger problem is that galactic rotation curves are not the only means of calculating the mass of a galaxy. Other means include http://en.wikipedia.org/wiki/Gravitational_lensing and http://en.wikipedia.org/wiki/Dark_matter#Missing_matter_in_galactic_clusters. Also, the average density of the universe can be computed from a variety of cosmological sources, and compared to the amount of matter we see in traditional forms. The results from these different estimation methods all indicate that there is dramatically more matter then we would otherwise expect ---or our understanding of gravity is flawed. Since the foundations of general relativity rest on very few, seemingly simple assumptions, most physicists regard the existence of a large amount of difficult-to-detect matter as the simplest explanation.

Oh I'll add something about M-theory and branes. It's not my field at all, but as I understand it gravity tends to leak out of our brane and this is the suggested explanation for why it's weak. It's supposed to make gravity stronger on very small scales as a result. I'm not sure it's able to make gravity stronger on very large scales, which is what would be needed to explain galactic rotation curves.

I don't have anything to add to the topic but I just wanted to say that edd's comment was fantastic and that I got a real chuckle out of the phrase "In regard to Paris Hilton's link about using full general relativity instead of Newtonian physics."

Why I love ask.me

If M-Theory is based on string theory, which that wikipedia article indicates, and string theory is completely untestable, doesn't that leave M-Theory as yet another mathematical flight of fancy?