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The dominant Lambda-CDM model is the standard model of physical cosmology, and it has proved reasonably successful. It does, however, have problems, such as dark matter, whose true nature remains elusive. Dutch physicist Erik Verlinde has, in a recent paper, proposed that gravity might not actually be a fundamental interaction at all, but rather an emergent property of spacetime itself, and as such, what current cosmological theory considers dark matter is really an emergent gravity phenomenon. Sabine Hossenfelder has an article about several recent tests of Verlinde's theory, which show that the idea might have promise.
Physicists today describe the gravitational interaction through Einstein's Theory of General Relativity, which dictates the effects of gravity are due to the curvature of space-time. But it's already been 20 years since Ted Jacobson demonstrated that General Relativity resembles thermodynamics, which is a framework to describe how very large numbers of individual, constituent particles behave. Since then, physicists have tried to figure out whether this similarity is a formal coincidence or hints at a deeper truth: that space-time is made of small elements whose collective motion gives rise to the force we call gravity. In this case, gravity would not be a truly fundamental phenomenon, but an emergent one.
[...] Verlinde pointed out that emergent gravity in a universe with a positive cosmological constant – like the one we live in – would only approximately reproduce General Relativity. The microscopic constituents of space-time, Verlinde claims, also react to the presence of matter in a way that General Relativity does not capture: they push inwards on matter. This creates an effect similar to that ascribed to particle dark matter, which pulls normal matter in by its gravitational attraction.
[...] So, it's a promising idea and it has recently been put to test in a number of papers.
[...] Another paper that appeared two weeks ago tested the predictions from Verlinde's model against the rotation curves of a sample of 152 galaxies. Emergent gravity gets away with being barely compatible with the data – it systematically results in too high an acceleration to explain the observations.
A trio of other papers show that Verlinde's model is broadly speaking compatible with the data, though it doesn't particularly excel at anything or explain anything novel.
[...] The real challenge for emergent gravity, I think, is not galactic rotation curves. That is the one domain where we already know that modified gravity – at last some variants thereof – work well. The real challenge is to also explain structure formation in the early universe, or any gravitational phenomena on larger (tens of millions of light years or more) scales.
Particle dark matter is essential to obtain the correct predictions for the temperature fluctuations in the cosmic microwave background. That's a remarkable achievement, and no alternative for dark matter can be taken seriously so long as it cannot do at least as well. Unfortunately, Verlinde's emergent gravity model does not allow the necessary analysis – at least not yet.
Previously:
Emergent Gravity and the Dark Universe
(Score: 4, Interesting) by stormwyrm on Thursday March 02 2017, @03:25AM (4 children)
If you had a particle that only interacts by the weak interaction and gravity, or worse yet, via gravity alone, as some cold dark matter candidates postulated are, then you’ll really have a hell of a time trying to detect them, whatever they are, as these forces are the weakest of the lot by a very wide margin. Relative to the electromagnetic interaction, the weak interaction is some eleven orders of magnitude weaker, and gravity some 36 orders of magnitude weaker. That’s the trouble here.
However, we do have examples of particles in the Standard Model that, like dark matter, can interact only via the weak interaction and gravity: the various types of neutrinos, and the work on neutrinos gives some idea of the difficulties involved in trying to detect dark matter. The neutrino was hypothesised as an explanation for the missing energy in beta decay events, and it took several decades from when Wolfgang Pauli first hypothesised them as “a desperate remedy”, to their unambiguous direct detection. Even today, we can still detect only fairly high energy neutrinos. The low energy neutrinos, such as those in the cosmic neutrino background, are still resistant to all attempts at direct detection, though there is indirect evidence for them. We can expect that dark matter will be similarly difficult to directly detect barring some new breakthrough.
I will emphasise what Sabine Hossenfelder has stated in the last part of the article: the ultimate test for any alternative to the particle dark matter hypothesis is how well it holds up at galaxy cluster and cosmological scales. Wake me up when someone comes up with a modified gravity theory that can reproduce the Bullet Cluster and the CMB anisotropies. Dark matter manages to do that, so any alternative must be able to at least do the same. Galaxies and globular clusters are easy as far as that goes.
Numquam ponenda est pluralitas sine necessitate.
(Score: 2) by HiThere on Thursday March 02 2017, @08:26PM
I disagree that "ultimate test for any alternative to the particle dark matter hypothesis is how well it holds up at galaxy cluster and cosmological scales". That's certainly ONE (well, one set) of the tests it needs to pass, but it's not sufficient.
OTOH, if the theory carefully predicts a particle that there is no direct way of detecting, but passes all the other tests, then it needs to be provisionally accepted. Until some better answer comes along. But Dark Matter doesn't predict any particular particle, just that somehow there's enough invisible mass scattered in a particular distribution that could be matched by any particle having a certain broad range of characteristics. Axions, sterile neutrinos, what-all particles match the desired characteristics, but they haven't been found. This is a quite unsatisfactory situation, and since no particular particle is predicted, there's no reason to believe any particular particle is what is producing the effect. So maybe it's something else. It still needs to fit all the available evidence, but sometimes theories need a bit of fine tuning before they handle the edge cases. And sometimes the theories handle things perfectly, but it takes awhile for people to understand everything they predict. (Is the "cosmological constant" currently accepted as an appropriate adjustment? Do Black Holes have firewalls?)
This, of course, doesn't say that he's right. (Either he.) It's just that it doesn't say that he's wrong.
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 2) by Gaaark on Friday March 03 2017, @02:41PM (2 children)
But the problem is is that you have to randomly ascribe dark matter to each galaxy depending on how much is needed to make the numbers work.
4 million dark matters here and it works, but you need 6.86 million dark matters here! ( All just randomly plugged in IN ORDER to make the numbers work!)
That predicts nothing.
--- Please remind me if I haven't been civil to you: I'm channeling MDC. ---Gaaark 2.0 ---
(Score: 2) by stormwyrm on Friday March 03 2017, @03:50PM (1 child)
And how different is that from, say, inferring the existence of a planet that you know must be there based on the observed ephemeris of a known planet, but your telescopes aren’t good enough to see? How different is that from Wolfgang Pauli hypothesising a particle that seemed at the time like it would be impossible to detect, just to make conservation of energy and momentum work for beta decay? This sort of thing is done all the time in science, in the expectation that someday observations and theories will improve to explain them better. Eventually Urbain le Verrier found Neptune, and Frederick Reines and Clyde Cowan managed to detect the neutrino. Someday, we’ll be able to really figure out what’s up with dark matter, but for now, it has the status of the neutrino before 1956 or Neptune before 1846.
And no, when you get to galaxy clusters, large-scale structure formation, and the CMB anisotropies, dark matter has plenty of predictive power. Large scale structure formation theory based on dark matter predicts that galaxy clusters ought to be forming between two and three billion years after the Big Bang, and indeed, the first galaxy cluster seen in the act of formation (CL J1001+220 [soylentnews.org]) was indeed found at between two and three billion years after the Big Bang (redshift z=2.506). Dark matter could have been easily falsified if we had seen galaxy clusters instead forming later or earlier than that. The dark matter hypothesis has also managed to correctly predict just how much hydrogen, helium, and other elements are produced in primordial nucleosynthesis. Dark matter is also inferred from peaks the cosmic microwave background power spectrum that can only come from some sort of matter that does not experience pressure when compressed. I hear a lot from modified gravity theorists about galactic rotation curves but I haven’t heard too much from them about how well their theories do with large-scale structure formation, CMB anisotropies, or galaxy cluster dynamics though. What I do hear when they try to go there, is they’re generally forced to add in stuff that looks suspiciously like dark matter. In contrast, inferring dark matter has managed to successfully fit all of the data at all these varying scales.
Numquam ponenda est pluralitas sine necessitate.
(Score: 2) by Gaaark on Friday March 03 2017, @09:18PM
And how different is that from, say, inferring the existence of a planet that you know must be there based on the observed ephemeris of a known planet, but your telescopes aren’t good enough to see?
The difference is, that it CAN'T predict the existence of the planet: you can't say "Dark matter exists because if you arbitrarily plug x+1000 dark matter into this galaxy, it perfectly matches it's 'spin'. Now this galaxy, that only works if you plug x+2001 arbitrary dark matters in."
That's like saying "Planet X exists because i burped twice yesterday". See: i arbitrarily plugged my own data into the equation and got the answer i wanted."
Galaxy size = X
dark matter needed = Y
'spin' = Z
Now if they could say "Dark matter exists because if, for a size of X you could plug in Y amount of dark matter = 'spin' of Z, therefore for a galaxy of 10 size, you plug in dark matter of 100 and you get 'spin' of 1000, you could then work out the amount of data for each galaxy and have it be predictive: so for this galaxy of size 1, you need to plug in 10 dark matter and you get 'spin' 100."
Galaxy size = X
dark matter needed = Y
'spin' = Z
....
X + Y = Z.
What they have now is "we have spin 100, so for galaxy size 1 you need to plug in 10 dark matter.... except, that doesn't always work, so you need 10 + 40 for THIS galaxy, but for this other one, size 1, you need 10 + 25" and so on".
Galaxy size = X
dark matter needed = Y
'spin' = Z
....
Z= X + Y + arbitrary amounts of Y to make it work out to be Z
NOT PREDICTIVE, therefore not a theory, but a kludge.
--- Please remind me if I haven't been civil to you: I'm channeling MDC. ---Gaaark 2.0 ---