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from the as-long-as-it-predicts-apples-dropping dept.
https://physics.aps.org/articles/v14/143
"Researchers pursuing an unconventional view of cosmology that dispenses with dark matter have developed a model that can match observations of the cosmic microwave background (CMB), the leftover glow of the big bang [1]. This dark-matter-free model is an extension of the so-called MOND (modified Newtonian dynamics) theory, which assumes that the gravitational force on galaxy scales is different from the standard Newtonian force. Previous MOND-based models could not reproduce the CMB. The researchers say that their model can be further tested with observations of galaxy clusters and gravitational waves."
The MOND theory was devised more than 30 years ago as a way to explain galactic rotation data without invoking the existence of the mysterious dark matter [2]. MOND proponents offered an alternative mystery in which the gravitational force changes for accelerations smaller than a threshold of 10−10m/s2. The idea did not spring from any underlying theory, but surprisingly, the same acceleration threshold works for nearly all galaxies—small and large, young and old.
The main reason that dark matter has been favored over MOND is that dark matter is consistent with a much larger range of astrophysical observations. For example, dark matter can explain galaxies' bending of light from distant sources (gravitational lensing), whereas MOND in its initial form could not. Researchers have devised so-called relativistic MOND models that can fit the lensing observations [3], but until now, none of these revised versions of the theory were able to reproduce CMB data. "If the theory can't do that, then it's not worth considering further," says Constantinos Skordis from the Czech Academy of Sciences in Prague.
The APS Physics article is a review of this paper:
C. Skordis and T. Złośnik, “New relativistic theory for modified Newtonian dynamics” Phys. Rev. Lett. 127, 161302 (2021).
(Score: 0) by Anonymous Coward on Tuesday October 19 2021, @03:12PM (8 children)
Which theories have a non-infinite speed of gravity?
(Score: 0) by Anonymous Coward on Tuesday October 19 2021, @04:19PM (7 children)
Ummmm...General Relativity?
https://archive.org/details/specialrelativit0000chan/page/332 [archive.org]
(Score: 2) by HiThere on Tuesday October 19 2021, @04:49PM (6 children)
Well....sort of. in GR space can expand, and therefore presumably contract, at faster than light speed. But gravitational waves propagate at the speed of light, so it depends on just what is meant. (After all, in GR space flexing is the implementation of gravity, and gravity waves is only how you directly detect it.)
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(Score: 2) by maxwell demon on Wednesday October 20 2021, @07:15PM (5 children)
The “speed of gravity” means the speed at which gravitational effects propagate. Expansion doesn't propagate at all (as far as we can tell), it just happens everywhere. The apparent speed of far-away objects exceeds the speed of light (apparent because the growing distance is not due to movement, but due to additional space “produced” in between), but that's not related to the speed of gravity.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by HiThere on Wednesday October 20 2021, @07:45PM (4 children)
If gravity *is* the bendiness of space, then the bending of space *is* the gravity. Gravitational wave are just how we know about it, like the light from the sun is not the sun.
Of course, with that interpretation even talking about the "speed of gravity" is a bit weird. It's some fractional number of m/parsec except near dense chunks of mass, and it's not localized, though effects like the Schwartzhild radius can be localized. Still, if something is far enough away is collapsing then the effect can be receeding from us at faster than the speed of light, so it's only weird, not totally meaningless.
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(Score: 2) by maxwell demon on Thursday October 21 2021, @06:31AM (3 children)
The far-away effect is not receding from us, the space in between the effect and us is growing. Literally.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by HiThere on Thursday October 21 2021, @01:48PM (2 children)
I think the space between us growing is the same thing as the thing far away receding. If it isn't I'm not sure just what the difference is. Perhaps something about relative momentum?
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(Score: 2) by maxwell demon on Thursday October 21 2021, @01:56PM (1 child)
To start with, the accelerated expansion does not give rise to inertial forces.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by HiThere on Thursday October 21 2021, @05:34PM
I'm not sure why stabile inertial forces should have much to do with whether something is receeding. If it's receeding as a constant velocity even close up you don't get any change in inertial forces (that is measureable without doing something like climbing out of a gravity well). Recession is generally measured by doppler shifts.
OTOH, this whole area isn't dealt well with in human languages, because in common human experience we don't directly notice the expansion (or contraction) of space. Ball is relatively well defined, because everybody has experience with balls and other roughly sphereical objects. You can point to a few and that say "it's like those things". Things that are harder to observe have fussier definitions. E.g., "like". Does it mean acceptable or preferable? Well, it depends on context, and how I'm feeling when I say it. I like a cup of left-over coffee in the morning, but given a choice I'd prefer fresh hot coffee. But that would mean I'd have to make it. "Like" has a rather fluid meaning, and people don't commonly notice that they way they do with, e.g., "love". Now we usually think we know way we mean by receeding, but generally it has something to do with something like "If I went to pick it up, I'd have farther to go to reach it".
In that sense I claim that if the space between us and something else is expanding, then it is receeding. If you want to argue for a different meaning, I'd like to know what meaning you're arguing for. It clearly isn't dealt with by red shift, because that's how we noticed that the stuff is receeding.
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