Are modified theories of gravity credible?

Question

I'm a statistician with a little training in physics and would just like to know the general consensus on a few things.

I'm reading a book by John Moffat which basically tries to state how GR makes failed predictions in certain situations. I know GR is well extremely tested, but I imagine all physicist are aware it doesn't always hold up.

The book tries to put forth modified theories of gravity that make do without the need of dark matter and dark energy to make GR match real world observations. (ie speed of galaxy rotations etc)

Are modified theories of gravity creditable?

Is dark energy/matter the 'ether' of the 20th/21st century? Is it likely scientists are looking for something that simply doesn't exist and there are unknown fundamental forces at work? What's the best evidence for it's existence other than observations based on the 'bullet' cluster?

Any insight would be great. Cheers :)

Answer 1

Excellent question!

In short, there are two logical possibilities to explain the data:

1. There is dark matter and a cosmological constant (standard model)
2. Gravity needs to be modified

Interestingly, both possibilities have historical precedent:

• The discovery of Neptune (by Johann Gottfried Galle and Heinrich Louis d’Arrest) one year after its prediction by Urbain le Verrier was a success-story for the dark matter idea.
  (Of course, after its discovery by astronomers it was no longer dark...)
• The non-discovery of Vulcan was the a failure of the dark matter idea - instead, gravity had to be modified from Newton to Einstein.
  (Funnily, Vulcan actually WAS observed by Lescarbault a year after its prediction by Urbain le Verrier, but this observation was never confirmed by anyone else.)

So basically you are asking: are we in a Neptune or a Vulcan scenario? And could not the Vulcan scenario be more credible?

The likely answer appears to be no. Modifications of gravity that seem to explain galactic rotation curves are usually either in conflict with solar system precision tests (where Einstein's theory works extraordinarily well) or they are complicated and less predictive than Einstein's theory (like TeVeS) or they are not theories to begin with (like MOND).

Besides the gravitational evidence for dark matter, there is also indirect evidence from particle physics. For instance, if you believe in Grand Unification then you must also accept supersymmetry so that the coupling constants merge in one point at the GUT scale. Then you have a natural dark matter candidate, the lightest supersymmetric particle. There are also other particle physics predictions that lead to dark matter candidates, like axions. So the point is, there are no lack of dark matter candidates (rather, there an abundance of them) that may be responsible for the galactic rotation curves, the dynamics of clusters, the structure formation etc.

Note also that the Standard Model of Cosmology is a rather precise model (at the percent level), and it requires around 23% of dark matter. There are a lot of independent measurements that have scrutinized this model (CMB anisotropies, supernovae data, clusters etc.), so we do have reasonable confidence in its validity.

In some sense, the best evidence for dark matter is perhaps the lack of good alternatives.

Still, as long as dark matter is not detected directly through some particle/astro-particle physics experiment it is scientifically sound to try to look for alternatives (I plead guilty in this regard). It just seems doubtful that some ad-hoc alternative passes all the observational tests.

Answer 2

While it is possible that gravity still needs to be modified, it is looking increasingly unlikely that there ISN'T some form of dark matter. In particular, the observation of the bullet cluster is a tall order for the various modified gravity theories (though, arguably, the extra fields in something like bimetric gravity or TeVeS could be self-coupling in a different way than the ordinary fields and this could be what is inducing the seperation, but this is contrived, at best). The basic MOND idea has be be stretched pretty desperately to explain dark matter in galactic clusters, also.

Furthermore, looking at cosmological 'freeze-out' scenarios where dark matter falls out of equilibrium with ordinary matter at some high temperature in the early universe produces a model that is consistent with current nucleosynthesis data and with the observed abundance of dark matter. This is pretty strong evidence that there is something to the beyond the standard model dark matter hypothesis.

But once again, it's not wholly impossible that there is some sort of modified gravity. After all, quantum effects are going to need to modify gravity at some point. It would be wrong minded to not look for it just because another hypothesis seems better at the time.

Answer 3

Keep in mind that while GR is well extremely tested, all those tests have been in a teeny tiny range of what it predicts. For example, GR predicts a gravitational redshift that can range from 0% (no redshift) to 100% (no photons received, a black hole). The largest gravitational redshift detected to date is less than 0.01%.