General relativity

We have a member who, in the great tradition of Internet crackpottery, is derisive of general relativity.

This thread is about the evidence for General Relativity and the absence of evidence against it.

Perhaps the best of all is the way that General Relativity predicts that all orbiting objects gradually radiate energy as gravitational waves, causing orbits to decay, just like accelerating charges radiate electromagnetic radiation. This effect is quite large in pulsars, leading to a precise prediction of the rate of slowing of their rotation over time.

Here is the comparison between this prediction and the measured change in the rate of rotation of the Hulse-Taylor Pulsar.

Ain't science beautiful?

Very good.

Sticking my neck out here. I reckon gravitational waves will be detected directly in the next few years.

See http://m.bbc.co.uk/news/science-environment-31642215

http://relativity.livingreviews.org/Articles/lrr-2014-4/title.html

A hundred pages review about tests of GR, by one of the best in the field. There are several technical parts, but also a lot of explanatory prose. There should be enough to convince anybody that GR is extremely successful, within its applicability range (i.e. when quantum effects of gravity are negligible).

There are viable alternatives to GR, including modifications of the relevant field equations, that can be useful to explain certain effects on cosmological scales. However, all alternatives have to essentially "reduce" to GR in all the tested regimes (e.g. solar system scales), precisely due to the massive experimental success of the theory. So no serious Modified Gravity theory out there tries to argue that GR is "wrong". Likewise, nobody can argue that Newtonian gravity is "wrong"; it is perfectly right and working in the specific regime of weak gravitational fields; GR extends it, it does not prove it wrong, of course. Actually, GR must reduce to Newtonian gravity in the appropriate limit. That's how Physics works. Teories are right when they are experimentally verified in a given range of scales and energies. They can be eventually extended and incorporated into more complete theories when probing higher energies or new scale ranges. 

So, if you say that GR is just an effective theory and it is incomplete, since it cannot be consistently quantized (so that e.g. at certain energy scales we cannot use it), that's well known. If you claim GR is "wrong", that's nonsense, since a ridicolously large amount of extremely accurate experimental tests, developed over a span of nearly a century, proves irrefutably the opposite.    

RPaulB, Michele78 does a better job than me of explaining why you are misguided: I suggest you read what he wrote more carefully.

There is a long history in astronomy of detecting the location of mass by the effect of gravity. This technique has been used to detect outer planets, orbiting objects of many types, and other invisible objects. Now is it used to make inferences about the distribution of dark matter. Why do you object to astronomers observing the Universe using gravity rather light?

Your statements about the current state of scientific knowledge remains just as inaccurate however many times you repeat it. There is only one standard model of cosmology, and alternatives to the theory of gravity are constrained by the fact that there is literally no empirical data that favours any of them over general relativity.

Indeed, as Elroch points out, you can use gravity to infer the presence of dark matter, as done in the past for many other objects.

And in this respect, RPaulB, it is funny that you mention the bullet cluster, because that was considered for a long time as the definitive prove that dark matter exists, and it gave a lot of problems not to GR, but to modified gravity theories which do not include dark matter!

The bullet cluster is formed, as you say, by two different merging clusters. In the merging, all the gas, that can interact in ways different from just gravity, is heated up and slows down due to the friction generated by collisions. If there is additional "dark" matter, then one expects it to keep moving, passing the gas, and stay around the galaxies, since it does not interact collisionally and it feels no friction, by definition. 

So the prediction of "GR with dark matter" is: gas in the middle of the bullet cluster, dark matter farther away. This is *exactly* what you see with X-ray and gravitational lensing observations (X-rays are emitted only from hot visible matter in the outskirt; due to lensing, instead, light rays from background sources are bent when they cross the bullet cluster, and the bending is determined by the entire matter content, including DM). It is very hard to explain lensing in the bullet cluster for theories without dark matter (because you see light rays bending when they pass well away from visible matter, but you assume there is nothing there!). So, for long time, people considered the bullet cluster as a sort of smoking-gun for dark matter. But then ways to fit the data also in other theories were found.

Independently of this, surely the bullet cluster does not disprove GR, or show that DM does not exist. If anything, it does the opposite, and it poses challenges to other approaches!

Regarding Abell, that is a similar system to the bullet cluster, and there was at some point the issue that dark matter did not seem to be around the galaxies, but in a separate region. That was however solved with more accurate measurements a while ago (http://arxiv.org/abs/1209.2143), and now everything looks consistent (in passing, let me also mention that GR skeptics also mention the "Pioneer anomaly" all the time; that is another issue that was solved ago, and all in the Pioneer is consistent with GR, no worries).

Having said that, I think that my main point was before just to point out that, leaving aside cosmology for a second, GR has passed a large amount of very stringent tests on scales from fraction of millimeters up to hundreds of millions of km. The GPS in your car uses it, and it works, doesnt'it? If this is not the definition of an extremely successful theory, I don't know what that is. One day GR will be included into a more general theory that can be consistently quantized, used at the Planck scale etc. That will not make it less successful at all. Exactly as GR did not turn Newtonian Physics into a "wrong" theory.

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Addendum: post edited and corrected at some point, since I was swapping by mistake the position of baryonic and dark matter in the previous version, as rightly pointed out by RPaulB

Michele78 has given a characteristically thorough explanation. Many unusual objects in the Universe have improved understanding of the behaviour of large systems.

Apologies, stupid mistake on my side. In fact, baryons collide and gets stuck in the middle due to interactions (the collitions act as a friction force), while non-colliding matter just goes through, without altering its motion, and keeping the same speed. So, the theoretical prediction is that baryons in heated gas have to end up closer to the center, and dark matter has to stay around galaxies, and farther away from the center. This is the correct prediction (and it is the contrary of what I wrongly claimed above). And that is what the experiments exactly see. I swapped the correct positions of the two components in my post above, but everything else stays the same. GR+dark matter predictions are in perfect agreement with the observations.

Since you are using wiki as your source of information, just read two lines further, and you see that the person that wrote in the wiki says the same:

"This accords with predictions of dark matter as only weakly interacting, other than via the gravitational force."

If you keep reading the wiki, you will also see how the bullet cluster has been for long time considered the best evidence in favour of dark matter.

Then (I am reading now) the wiki follows with an objection, that people made at some point, that the velocity of the merger is too large and unlikely in the standard cosmological model. However that point is outdated. Recent accurate simulations show no friction between LCDM cosmology (the "standard model") and observed infall speed:

http://cosmo.nyu.edu/lage/support_files/astro_papers/Lage_Farrar_1406.6703.pdf

So, sorry again for swapping both the predicted and observed relative positions of the two components, but the main point stays the same: bullet cluster observations are perfectly consistent with the standard cosmological model, assuming GR and including DM. If anything, the bullet cluster poses a challenge to alternative theories. This is clearly stated in the scientific literature, and also clearly  written in the same wiki post you mention (first 8 lines of the "overview" paragraph).

I had already discussed Abell in my previous post, please have a look. Let me repost the relevant part:

"Regarding Abell, that is a similar system to the bullet cluster, and there was at some point the issue that dark matter did not seem to be around the galaxies, but in a separate region. That was however solved with more accurate measurements a while ago (http://arxiv.org/abs/1209.2143), and now everything looks consistent (in passing, let me also mention that GR skeptics also mention the "Pioneer anomaly" all the time; that is another issue that was solved ago, and all in the Pioneer is consistent with GR, no worries)."

Let me also point out that, in general, you cannot take a very complicated physical system, such as colliding clusters, with a lot of measurement  uncertainties, complex hydrodynamics etc, and, if there are strange results, just conclude that there are problems with a theory which already underwent a huge amount of experimental confirmation. I would check data and simulations first. That's what happened here, and improved simulations and data show again good agreement with expectations.

If you do not want DM, you need to explain lensing by modifying the relation between the distribution of visible matter and the bending of light rays. That can be done (the formalism remain that of GR anyway, covariant approach and differential geometry, just additional ingredients and different relation between "matter" and "curvature") but it is very contrived generally. And these alternative theories find it hard to work well at all scales, for all systems. For example, MOND/TeVeS can work well with galaxies, but then it is hard to fit Cosmic Microwave Background and cluster data.

With GR and *just one single value* of the parameter which characterizes density of dark matter, in Cosmology you explain:

- Rotation curves of galaxies

- Gravitational lensing observations

- Cosmic Microwave Background data

- Abundance of galaxies and clusters, their distributions, correlations and  more

- Supernovae data

...and I am probably still forgetting something.

All this covers a huge range of scales (kiloParsecs to Gigaparsecs, roughly) and times (from ~13 billion years ago, CMB, to essentially today). All observations of these totally different physical systems, at different times, on different scales, point to a perfectly consistent number for dark matter density. The same for all systems.

Then you have to add that ~100 pages review, that I was linking before, for many other high-precision tests of GR, from mm to solar system scales.

Please link the paper you are referring to

And please read replies to your arguments entirely and not selectively. I mention the importance of the reliability of the datasets, the large amount of cosmological observations explained perfectly with the GR + DM paradigm, the ridiculously large amount of successful tests of GR at mm to solar system scales. Please explain the reason for the success of these tests with GR being wrong... These crazy physicists must be really lucky in predicting experimental results with fraction of percent accuracy, starting from a totally wrong theory...

Your (out of date in the light of improved data) quote was:

Moffat said, " For Abell 520, it may turn out only MOD can ... explain" .

This was a hypothesis. In this case, it was a hypothesis that was proven wrong (not unlike Moffat's contrarian prediction that the Higg's boson did not exist).