/sci/ - Science & Math

I am not as familiar with the dark energy side so will skip that, but for dark matter the issue is that the current theory of gravity, which works so well within our solar system (see all the complicated gravity assist maneuvers space probes have had plotted) fails hard at predicting how stars rotate around the center of galaxies, or how galaxies interact with each other on galactic clusters. Stars in the outer part of galaxies move much faster than expected based on the visible mass, likewise say (most) dwarf galaxies move too fast around bigger galaxies.

This has lead to two competing explanations - modified gravity theories, which suggest that gravity acts differently at large scales, and dark matter, which suggests there is a very large amount of non-visible matter holding things together. Modified gravity theories have fallen out of favor as no one has been able to come up with one that fits all the available data; for example dwarf galaxies with similar amounts of visible matter can have very different motions. The dark matter theory can explain those dwarf galaxies as having more or less dark matter, due to odd events separating the two. Likewise, there are regions like the bullet cluster where the gravitational lensing does not match the visible matter, which indicates well some sort of invisible matter.

As for dark matter, if it exists, it is still unknown exactly what it is. What it isn't has been well tested - various astronomical surveys have ruled ordinary matter, like planets, brown dwarfs, large black holes, and so on. Particle detector tests have ruled a host of hypothetical particles. So that leaves some sort of exotic particle that makes neutrinos seem trivial to detect (axions being a leading one) or primordial black holes in a specific size range. Or possibly something else no one has thought of, or maybe even the Modified gravity guys will have the last laugh. It just isn't known yet.

>>14739787
Where do you get the idea that it is "illegal" to question general relativity? Tons of different modified gravity hypothesis have been published & given serious consideration. Its just that so far all of them have ran into major issues with explaining certain observations.

One big issue with any modified gravity theory are the so called "dark matter deficient" galaxies. That is, there are a small number of dwarf galaxies that do behave like relativity says they should, based on their visible matter, while most other visually similar dwarf galaxies do act as if they have a bunch of extra mass. The dark matter hypothesis explains those galaxies as having some odd event separate the visible matter from the dark matter, while pretty much all of the modified gravity ones fall flat on their face. Well, aside maybe from the idea that gravity works differently in different parts of the universe, but that would be a bitch to test, to say the least.

>>14656866
Sometimes invisible stuff throwing things off is the answer though. Look at how Neptune was discovered when astronomers noticed Uranus's orbit did not match their theory, or how neutrinos were predicted to exist when physicists noticed that beta decay seemed to violate the conservation of momentum.
When a previously well tested theory does not match new observations, both new theories and new, currently invisible shit throwing things off needs to be considered. And so far, all the new gravity theories have not been able to explain various observations as well as the dark matter hypothesis.

>>14540238
Because we do not know if the model is actually broken and needs to be replaced, or if it just needs a patch job (i.e. adding in a particle(s) that are a bitch to detect). And various lines of evidence are pointing more towards the patch job being the correct solution. Lots of very smart people have proposed new models to replace relativity, but all of them so far have failed to fit the data as well as the cold dark matter hypothesis. And as others have mentioned, there have been plenty of times in the past where the patch solution (like neutrinos patching up beta decay & conservation of momentum) has been the correct solution.

>>14494702
To be fair, sometimes "making up magic numbers" actually is the correct solution. For example, back in the 1920s physicist noticed that beta decay violated the conservation of momentum; there were some who did try to "fix the theory" so speak, with assorted adjustments, while Pauli proposed that maybe there were some super tiny particle that barely interacts with normal matter carrying away the excess momentum. It took decades of looking before said particles (neutrinos) could be directly detected.

That said, sometimes new theories are needed. Plenty of alternative theories have been put forth, its just that so far they have all had major issues.

Anyways, whatever is causing the phenomena that have lead to the dark energy & dark matter theories is still very poorly understood, and shows that physics is still nowhere near complete.