According to our current theories of physics, all the matter-energy in the universe consists of about 68.5% dark energy, 26.5% dark matter, and about 5% regular matter. The dark matter is believed to be around galaxies in a halo while dark energy is everywhere. The problem is that we have not so far been able to directly detect any dark matter particles despite strenuous and expensive efforts. After each failure to detect a signal, the debate is always whether to give up the search and declare that dark matter does not exist or to build a bigger, more sensitive detector with other materials in the hope that it will work. As I discuss in my book The Great Paradox of Science, this is a recurring situation in the history science. At any given time, in addition to the dominant paradigm, there are always other competing paradigms seeking to dethrone the champion. The fortunes of the competitors depend upon he fortunes of the dominant one and in the case of dark matter, competitors see an opening in the failure to detect it.
Dark matter has, in addition to explaining the velocities of stars in the arms of spiral galaxies that originally triggered the idea, also has some supporting evidence such as some Cosmic Microwave Background (CMB) data.
There are always many possible alternative theories that can be proposed. In the case of dark matter, other particles have been proposed to the dominant idea that dark matter consists of what are known as WIMPs (Weakly Interactive Massive Particles).
Then there are those theories that say that dark matter does not exist at all and that what is needed is a modified theory of gravity. The most prominent of these is known as MOND (Modified Newtonian Dynamics). It is not easy to dethrone a dominant paradigm. As I discuss in my book, it requires that the competitor be able to explain key features that the dominant theory explains. In addition, it should make a prediction that can be tested.
In the case of MOND they have not made much headway in explaining some of the features that dark matter purports to explain. Like all new theories, it takes time to develop and it is still early days. But it has been able to gain some adherents and this month saw the appearance of a paper that proposed a relativistic form of MOND that the authors say said can explain some features of the CMB.
This month however, researchers Constantinos Skordis and Tom Zlosnik from the Czech Academy of Sciences published a paper in the journal Physical Review Letters suggesting that a new modification to the parameters of Newton’s theory of gravity could provide an answer as to why dark matter has yet to be detected. And unlike previously proposed MOND theories, this one just might stick because the new proposal can match observations of the cosmic microwave background (CMB), which is a key detail that has lacked in the previous MOND-like theories.
“This new paper suggests a way of having what is called a relativistic extension of MOND, a theory that is more general that can be applied to the universe at large,” Avi Loeb, the former chair of astronomy at Harvard University who was not involved in the paper, told Salon. “It introduces some new fields, and according to the authors, seems to satisfy both what happens on the scale of galaxies, as well as what happens on the scale of the universe and that’s quite a feat.”
You can read the paper here (paywall).
The preprint can be read here;
https://arxiv.org/abs/2007.00082
My second thought was to reading “It introduces some new fields, . . .”. Well, new fields means new particles, so they are introducing yet another set of undetectable particles to explain what we see. That’s an improvement, right? Well, no.
Then my third thought was to ask myself about its being relativisitic. OK, so it satisfies special relativity; but do they discard general relativity? So, there is a free-access (early) version available on arxiv. Here it is: New Relativistic Theory for Modified Newtonian Dynamics. If I understand it correctly, its not only relativistic, is it General Relativistic, but adds extra terms that couple to the gravitational field itself (i.e., once quantized, you’d have the new particles coupling with gravitons).
And my fourth thought was, how is this different than dark matter? Well, in dark matter, the unknown particles are a huge lumpy number that interact with gravity the normal way. In this theory, you don’t need a lot of the particles sitting in galaxies not interacting (much) with normal matter, you have these new particles interacting directly with gravity and thereby modifying a more Newtonian kind of gravity. In some ways it seems to me to be a distinction without a difference, but that has a strong possibility of being merely a measure of my ignorance.
Oh, and my first thought? This recent article in Quanta: Is the Great Neutrino Puzzle Pointing to Multiple Missing Particles? There are still neutrino mysteries (beyond the sun-puzzle of missing a third which was “solved” by the three varieties mixing into each other), and they might point to dark matter.
There’s also http://www.isfdb.org/cgi-bin/title.cgi?1484881
Well, silly atheists. Isn’t it obvious? Dark matter is JESUS. As a proponent of Intelligent Falling, I have to once again laugh at the petty efforts of atheistic “scientists” to explain things that can easily be explained by JESUS.
(I think I actually read that somewhere).
If you want to read more about the MOND side of things, astrophysicist Stacy McGaugh has an interesting blog discussing dark matter/MOND. He seems to be a lot more open minded than most dark matter proponents, with a more balanced view of that theory’s strength & weaknesses, along with a more open-minded perspective on MOND. Go to:
Triton Station -- Dark Matter
For the dark matter question, it’s not necessarily a case of either/or. There could well be as-yet undetected baryons. It could also be the case, completely independently of the undetected baryons, that GR requires modification at as-yet unexamined scales. At different times or distance scales, one or the other of these could dominate.
It’s not always a competition.
“It’s not always a competition”
And sometimes competitions, even after a couple of centuries, simply die out with “a bit of both”.
https://phas.ubc.ca/~stamp/TEACHING/PHYS340/SLIDES/PDF/P340-08--PP17-Light.pdf
Correction to #6: Dark matter particles wouldn’t be baryonic.
At least some situations (gravitational lensing around collided galaxies) are really difficult to explain without some form of dark matter. There are cases where the lensing indicates the clusters of mass aren’t congruent with the clusters of visible mass anymore, and the simplest explanation involves some sort of WIMP where the dark matter particles have maintained the original speed of the galaxies and have passed through each other while the more visible matter got slowed down in the collision. Given time the invisible and visible sources of mass will re-align with each other, but that hasn’t always happened yet.
Now, of course, I said ‘simplest’ explanation, not ‘only’ explanation. And there’s no requirement that the dark matter be WIMPs, either. It just seems to have matched things best so far. Of course, the very ‘WI’ part of ‘WIMP’ (Weakly-Interacting) makes them hard to find. We’re pretty sure that there aren’t enough neutrinos to account for dark matter, so if dark matter exists, whatever it is may be even harder to detect than a neutrino.
Is it possible that the universe is folded like a bedsheet and the unexplained mass is a star or galaxy on the other side of the fold? You can’t detect it because it isn’t there in 3d space, but overlapping. Like, if you put a lead ball on a folded bedsheet the weight of it will deform the part of the bedsheet it’s sitting on, but also on the one below that and so on. Is that a thing? I might need a calculator and more weed to figure this one out.
@10: Sounds like brane damage.
@Rob Grigjanis
lol well played
I was going to air an unsupported speculation similar (maybe identical) to A Lurker from Mexico’s -- that dark matter is just ordinary matter in “nearby” universes -- nearby in one of those extra dimensions some physicists are fond of. The typical galaxy or supercluster will be aligned with counterparts in some of these nearby universes due to gravitational attraction operating over long periods of time, without interference from other forces.