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: 2, Funny) by Anonymous Coward on Tuesday October 19 2021, @12:33PM (1 child)
I propose a 17th order polynomial to fit the data than the naive inverse square. It fits better, duh!
(Score: 2, Insightful) by khallow on Tuesday October 19 2021, @02:08PM
(Score: 3, Informative) by maxwell demon on Tuesday October 19 2021, @12:37PM (6 children)
The “DOI” is the URL of the article; however the actual link is a lint to the DOI web site trying to use the URL as DOI. Not surprisingly, the DOI web site doesn't give a result for this.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 1) by Snort on Tuesday October 19 2021, @02:06PM (3 children)
Trying to give a blog post an air of authority.
(Score: 5, Funny) by DannyB on Tuesday October 19 2021, @02:19PM
Blog posts already have an err of authority.
Why is it so difficult to break a heroine addiction?
(Score: 2) by FatPhil on Wednesday October 20 2021, @03:35PM (1 child)
How formal do you want your academia - is it only valid if presented by a man with a big moustache and a top hat standing behind a big oak desk at the Royal Society in London?
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 2) by driverless on Thursday October 21 2021, @04:03AM
What, some rag published in the Colonies?!?!? Damn straight it better be presented at the annual dinner of the Royal Society, preferably by someone with several groups of letters behind his name.
A large beard wouldn't go amiss either, along with the ability to explain what silly mid-off is without having to look it up.
(Score: 2) by FatPhil on Wednesday October 20 2021, @03:33PM (1 child)
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 3, Informative) by maxwell demon on Wednesday October 20 2021, @07:04PM
Thank you.
A quick search reveals that this article is also available without paywall on arXiv: https://arxiv.org/abs/2007.00082 [arxiv.org]
The Tao of math: The numbers you can count are not the real numbers.
(Score: 0) by Anonymous Coward on Tuesday October 19 2021, @01:38PM (18 children)
Why?
I am not an astrophysicist like Brian May, but I am an amateur astronomer and keep up with astronomy news, and I've never heard that dark matter was required for gravitational lensing. Is it just a "matter" that more mass is needed than what is provided from the visible components of a galaxy?
(Score: 4, Interesting) by HiThere on Tuesday October 19 2021, @01:47PM (3 children)
I think the answer is that without the "dark matter", galaxies wouldn't be heavy enough to bend the light properly. (This is well outside my specialty of programming, but...)
IIUC, even LaPlace imagined stars heavy enough to bend light into closed paths,https://blackholecam.org/john-mitchell-developed-theory-of-black-holes-already-in-1783/ , and that implies gravitational lensing. But the *degree* of gravitational lensing differs between different theories. I haven't followed MOND, so I don't know how it says light should bend, and it's been multiple decades since I looked at how GR says light should bend, so don't expect any details about the comparison from me. If I gave them they'd be wrong. But the overview is that they predict different amounts of bending by different amounts of mass (possibly it also changes with distance),
IOW, when a popularized article on science makes an absolute statement, don't trust it. The original statement was probably a comparitive statement. And interpreted strictly they are likely to have different truth values.
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(Score: 2) by driverless on Thursday October 21 2021, @04:06AM
This must be the most honest admission ever made on this site :-).
Einstein: There, done!
Cleaning lady: Naah, doesn't look right, try squaring that second term.
Einstein: Yeah, why not.
(Score: 2) by driverless on Thursday October 21 2021, @04:16AM (1 child)
Hit reply too soon:
Dark matter always struck me as the undergrad-physics exam answer for the problem, you work forwards from the question, backwards from the answer, and in the middle where you can't for the life of you figure out how to connect the two you shout DARK MATTAAAAAAAAAAAAAAAA!!!!!!.
(Score: 2) by HiThere on Thursday October 21 2021, @01:46PM
IIUC, it's not intended to be an answer. It's a name for "something needs to fit here to balance the equations, and we don't know what it is, and can't see it, but it has mass-like gravitational effects". That's why there have been so many different ideas about "What is it?". Most of them have been sort-of traditional mass like things, like axions, but others have been a lot different, like MOND. So far nothings been really convincing, so it's still just an "x marks the spot" kind of name.
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(Score: 1, Informative) by Anonymous Coward on Tuesday October 19 2021, @01:47PM
My take on that statement is that dark matter, being matter, will bend light via the standard theory, but MOND doesn't invoke extra matter, so it can't bend light via standard GR, so the problem for it has been to explain the observed light bending if all the matter you have in the galaxy doing the bending is what you can see. I might be misinterpreting what was written, but I haven't yet looked into it any further.
(Score: 3, Insightful) by EvilSS on Tuesday October 19 2021, @01:50PM
The extent of lensing we see from galaxies can't be explained by our current understanding of gravity and the amount of normal matter in the galaxies we are observing. So there is something we are missing or have wrong. Dark matter is one theory, and fixes the problem by addressing the mass. This theory is trying to go at it by addressing the gravity side of the equation.
(Score: 0) by Anonymous Coward on Tuesday October 19 2021, @02:53PM (8 children)
Armchair Astronomer expert like yourself here:
From Wikipedia: https://en.wikipedia.org/wiki/Dark_matter [wikipedia.org]
"Dark matter does not bend light itself; mass (in this case the mass of the dark matter) bends spacetime. Light follows the curvature of spacetime, resulting in the lensing effect."
I like the proposal that it should be named, "Transparent Matter" because it does not, in any way we know, interact with electromagnetic radiation, hence why it is so hard to detect. We only see its mass effect on spacetime warping which then indirectly creates the lensing.
(Score: 2) by Immerman on Tuesday October 19 2021, @03:21PM (6 children)
A reasonable proposal - once we rule out matter that's simply non-luminous. For example, the least-exotic possibility, black holes, are still a candidate. Though various searches have shrunk the potential mass window dramatically.
(Score: 0) by Anonymous Coward on Tuesday October 19 2021, @04:06PM (5 children)
If I understand you correctly, you are proposing the unseen Baryonic matter out there?
Read somewhere that even accounting for unseen or free roaming black holes and dark bodies like MACHOs, still can not account for the discrepancies with observations.
Here is as close as we have gotten so far:
https://www.universetoday.com/137446/astronomers-find-missing-normal-matter-universe-still-looking-dark-matter-though1/ [universetoday.com]
(Score: 4, Interesting) by HiThere on Tuesday October 19 2021, @04:43PM (1 child)
It's my understanding that this doesn't rule out "primordial black holes", on those that formed after the re-ionization. (Well, actually they need to form slightly earlier, as the current baryonic matter is needed to explain the proportion of various elements...but it's also true that I've seen those estimates claimed as a lot more imprecise than earlier claims...so...maybe?
Anyway, my personal favorite variation of that theory is that dark matter is primordial black holes that are smaller than a proton, and have too small a capture cross-section to emit Hawking radiation. (That requires swallowing half of a pair of virtual particles.) But I'm no astrophysicist of any variety, including armchair, so don't take this too seriously. And a MOND variant would be interesting, as perhaps it would allow FTL. (Maybe it wouldn't, too, but nobody's really looked, and anyway they're still flexing the theory to make it fit the data.)
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(Score: 1, Funny) by Anonymous Coward on Tuesday October 19 2021, @04:52PM
Nice additional info, thanks!
If I could mod this up I would...but then again, the moderation system on here is a difficult as looking for dark matter.
(Score: 2) by Immerman on Tuesday October 19 2021, @07:07PM
The problem is that you *can't* account for unseen matter - if there's no evidence, we have no idea how much is there, and thus whether there's a mass discrepancy. Instead you have to try to rule it out by painstakingly searching for the evidence you know must be there if X is the cause, for all possible X's.
Black holes as dark matter in contrast must have formed very early in the universe's history, I think before first light spawned the CMBR. Most possible sizes have been ruled out by careful searches for gravitational lensing, but extremely small (atomic scale) has not been ruled out (lensing would be so minor as to be undetectable to current tools). Neither has a narrow window in the range of several(IIRC) solar masses.
Your article is about something unrelated - the fact that theory posits a large amount of normal matter that we didn't see direct evidence of, but now do. Exactly where the consensus expected to find it.
(Score: 3, Funny) by Mykl on Tuesday October 19 2021, @10:11PM (1 child)
Has anyone considered Phlogiston [wikipedia.org]?
(Score: 3, Funny) by kazzie on Wednesday October 20 2021, @04:41AM
Yeah, but that idea went up in flames.
(Score: 2) by driverless on Thursday October 21 2021, @04:09AM
It always struck me as a huge kludge, we've got some stuff we can't explain so we'll invent some undetectable stuff that nevertheless is there to insert and make the equations balance out.
Nice to see alternative hypotheses are being proposed.
(Score: 0) by Anonymous Coward on Tuesday October 19 2021, @03:38PM (1 child)
Yes, basically, General Relativity works great until you get to galactic scales, then suddenly there isn't enough "stuff" to account for what we observe (lensing, spinning, etc.) adding "dark matter" ie stuff with mass that doesn't do anything to light solves this issue.
So would changing the equations.
My understanding is dark matter is the solution that gives the flexibility to match observations (different galaxies need different amounts of correction). I don't think anyone is satisfied with just saying "its dark matter, don't worry about it" they are actively trying to discern what the stuff is and detect it (which is hard since its defining feature is that it doesn't interact with stuff except through gravity and gravity is a force that is weaker than Weak).
MOND is a less favored solution because the degree to which GR is off depends on the galaxy in question, which makes sense if there is "stuff" and that stuff can be there or not, but harder to account for if you need an equation that works everywhere based on what we can already see easily.
(Score: 2) by HiThere on Tuesday October 19 2021, @04:46PM
I think the "bullet ckuster" is also a high barrier to any general theory. https://science.nasa.gov/matter-bullet-cluster [nasa.gov]
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(Score: 2) by FatPhil on Wednesday October 20 2021, @03:40PM
We call that "dark matter". In so doing, no particular partical type is being named.
You're pretending that scientists have postulated a dark straw man particle that solves all their problems. They haven't. That's a fantasy inside your own head.
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(Score: 0) by Anonymous Coward on Tuesday October 19 2021, @02:59PM (1 child)
Using bottom link gives:
https://doi.org/https://physics.aps.org/articles/v14/143 [doi.org]
DOI Prefix [https:] Not Found
(Score: 2) by FatPhil on Wednesday October 20 2021, @03:42PM
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves
(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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(Score: 2, Informative) by Anonymous Coward on Tuesday October 19 2021, @03:13PM (2 children)
Search YouTube for "Dark Matter: The Situation has Changed" by Sabine Hossenfelder
To get an update on where the Dark Matter theory does not fit with observations, like densities in small galaxies, missing satellite galaxies, their alignment in planes, Baryonic Tully Fisher relation, Renzo's Rule and galaxy collisions at high velocity like the Bullet cluster.
A modified Gravity theory as proposed with MOND still has a lot of unresolved observations that Dark Matter theory correctly predicts.
From the article: However, the model, [MOND], has a lot of ingredients,” she says. “I myself would still vote for dark matter as a simpler explanation.”
Looks like we are still searching for solutions in the wrong places like Particle Physics and General Relativity.
Maybe like Sabine suggests, we need to get Condensed Matter Physicists working on the problem to fit a phase transition where two different sets of equations can be used at different scales instead.
(Score: 2) by HiThere on Tuesday October 19 2021, @04:51PM
The thing is, the dark matter theory doesn't predict those situations, it only allows them to occur. Not quite the same thing. But they *are* a really high bar to a more general theory.
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(Score: 2) by FatPhil on Wednesday October 20 2021, @03:45PM
Great minds discuss ideas; average minds discuss events; small minds discuss people; the smallest discuss themselves