I wouldn't get overly-excited; modified gravity theories are ten-a-penny.
However, philosophically this is very appealing. If dark matter is a gravitational superfluid, i.e. deformations in space-time that behave just like a bunch of massive [1] particles, then the hitherto[2] impossible middle ground of both missing mass and modified gravity become possible. It wouldn't be a particle, so direct detection experiments are doomed to failure. But you could absolutely still call it mass. Could you call it matter ? Probably not, it would be completely unlike conventional matter. You could also call it modified gravity. Doesn't mean it would be anything much like MOND - it might, or it could be something different. So both sides get to point and jeer and should "I TOLD YOU SO !" and go home for tea and biscuits. And if it's unified along with the hitherto[3] completely-separate-yet-also-mysterious dark energy, without breaking poor old Einstein, then so much the better.
[1] Meaning, "they've got mass", not that they're especially heavy.
[2] Not meaning to imply that this particular variant is novel in this aspect - it isn't.
[3] I believe this aspect is genuinely novel.
Not that "philosophically appealing" is much of a guide to truth, mind you. It'd be nice though. And I'm not in the least bit qualified to judge this latest theory, before anyone asks. What I don't get - no idea at all if this is a press release issue - is how a negative mass fluid could explain dark matter, which normally behaves very much as though it has positive mass.
EDIT : Thanks Eli Fennell for pointing out that this particular theory isn't about spacetime superfluids, it's just another sort of (presumably more conventional) matter that has negative mass.
https://phys.org/news/2018-12-universe-theory-percent-cosmos.html
"negative mass"?
ReplyDeleteGreat, now how do we harvest it? I have wormholes to build.
Just TBC, this isn't a Modified Gravity Theory. In a Modified Gravity Theory, they're Modifying Gravity relative to the conventional models thereof. Here, Gravity itself remains unchanged, but mass is also (re)given a Negative Sign, where Negative Mass responds the opposite way to Gravity, and allows Negative Mass to be generated just as Positive Mass is generated (that whole e=MC2 thing), rather than all of it having come to be early in cosmic history. Rather elegantly, it also parsimonizes one of the biggest issues I've always had with Dark Whatever Theories: they never seem to prevent the existence of Macro Dark Masses, e.g. Dark Planets, Stars, Galaxies, etc..., yet if such Dark Macro Masses existed, then from time-to-time their acceleration through space would overcome antigravity and hit us.
ReplyDeleteConsequently, we'd expect to be impacted by things like Dark Asteroids, Dark Comets, etc...
Here, the Negative Masses repel both Matter and each other, so that nothing beyond maybe elementary Dark Particles if even that can overcome the repulsive force to form a true Macro Mass.
I'm not saying this is right. But it has an elegance to it that conventional Dark Stuff Theories don't, without also undertaking yet another in the as-yet-wholly-failed attempts to find where Newton and/or Einstein got Gravity itself wrong (so far, they haven't).
I also like this because it harmonizes well with the 'Push Theories' of Gravity, which also don't Modify Gravity in a mathematical sense, but just flip its direction. According this perspective, a Graviton (the presumed but as-yet-undetected particles which mediate gravity) are more-or-less equally scattered and flying about randomly in empty space, and are so tiny they pass right through most matter (like Neutrinos). But a very small number strike matter, imparting movement in the direction opposite the one they impacted (as it works with normal matter). Place two Masses in this field, and consequently, there will be slightly fewer Gravitons between them than behind them. Put very large masses, and the number grows. Consequently, the objects get pushed together.
ReplyDeleteIf Negative Mass, as the article says, responds in the reverse direction, where the impacted object moves towards the direction of impact instead of away from it (i.e. push it and it moves towards you instead of away), then Gravitons would push them apart, instead of pulling them together.
Eli Fennell Thanks for the correction ! I jumped the gun when I read the word "fluid" in there; post edited.
ReplyDeleteI don't know much about dark matter on the scales of stars and planets. My naive guess would be that in the standard assumptions of a collisionless, non-dissipative particle, objects of that size would be hard to form. IIRC this might be possible with some models of self-annihilating dark matter, however.
On the scale of dark galaxies though, that happens to be my speciality (it may have been a throwaway comment, but I can't let it pass :P). There are indeed very good reasons to suspect such objects should exist according to the standard model. There are also candidate objects of varying plausibility, some of which do seem to have interacted with other galaxies.
Here, have a bunch of links :
https://astrorhysy.blogspot.com/2014/05/the-importance-of-being-idle.html
https://astrorhysy.blogspot.com/2016/06/into-darkness-i.html
https://astrorhysy.blogspot.com/2016/06/into-darkness-ii-attack-of-flying-snakes.html
https://astrorhysy.blogspot.com/2016/08/my-dark-galaxy-is-darker-than-your-dark.html
https://astrorhysy.blogspot.com/2017/01/check-out-my-kinky-curves.html
https://www.extremetech.com/extreme/221086-a-dark-matter-galaxy-just-buzzed-the-milky-way
There are also hints of dark matter interacting on smaller scales with stellar streams :
https://astrobites.org/2018/05/08/stellar-streams-the-nature-of-dark-matter/
Elie Thorne Alcubierre Drive...
ReplyDeleteRhys Taylor It sounds like you may be using Galaxy somewhat differently than how we use it to refer to a Matter Galaxy. Are you saying there could be a diffuse mass of this Dark Fluid Stuff on the scale of a Galaxy, but with no actual Macro Masses, e.g. no Dark Stars, Dark Planets, Dark Comets, etc...? If so, what would make that a Galaxy, since my understanding is that containing Stars is actually definitional to a Galaxy?
ReplyDeleteEli Fennell Rhys has written about it at length in various semi-recent posts, but the upshot is that these things have only a tiny amount of visible stuff (stars, gas) but appear to have a much larger gravitational gradient, implying that most of their mass is dark matter of some sort. I haven't looked up the modern definition of "galaxy," but a big part of it is going to be mass-based. (The one tricky bit of which I'm aware is that some globular clusters are as large as or larger than some dwarf galaxies; the former are "cuspier," but I don't know whether that's the key differentiator.)
ReplyDeleteI'm still reading through the paper, but my naive Newtonian worldview is having trouble with the idea of negative mass + positive mass having the usual dark-matter behavior in seeding galaxies (rather than, say, cancelling each other out). I'm pretty sure the answer is General Relativity, i.e., you don't add the two together to get mass density (at least, not with opposite signs); instead you curve space according to the absolute value of each one. But I haven't found the place where he explicitly says something along those lines, and my memory of GR is really poor at this point.
ReplyDeleteEli Fennell Basically yes.
ReplyDeleteConventional simulations contain particles of dark matter and normal, baryonic matter. The dark matter particles interact with gravity only - their own gravity and that of the normal matter equally - but don't experience nuclear or electromagnetic forces. They also cannot dissipate energy. Any mass of pure dark matter can only be bound together by its own gravity. So it's easy to form really massive clumps ("halos") but difficult to form smaller ones (there may be exceptions but that's another story).
The normal matter, on the other hand, can lose energy and interacts with all the other forces. It's much easier for it to form very low-mass, stable structures. But there isn't nearly as much of it as the dark matter.
What you can get in these simulations is a large population of dark matter halos without any stars and gas at all. Most people wouldn't call these dark galaxies, they'd just call them dark halos. But there are also some which have a little bit of gas as well - not enough to form any stars, and nowhere near enough for it to be gravitationally self-bound. Here the gas relies on the mass of dark matter to keep itself from flying apart, just like in a normal galaxy, but without any stars. That's what makes it a galaxy : normal matter bound together primarily by the associated dark matter halo.
"Needs dark matter to stop exploding" is a pretty decent working definition of a galaxy. It's not perfect, because tidal dwarf galaxies (stellar systems torn out during galaxy-galaxy interactions) don't have dark matter, but it generally works. Another option is the "Gilmore gap", which is a size-mass relation and not directly related to the kinematics. See slide 21 :
https://astro.mff.cuni.cz/vyuka/AST021/2017-2018/Dabringhausen-1.pdf
And tidal dwarfs are on slide 19 : https://astro.mff.cuni.cz/vyuka/AST021/2017-2018/Dabringhausen-2.pdf
It's a bit messy, but it seems to work.
If we look at galaxies which have significantly more stars than gas, then the definition becomes a bit like the philosophical question of the nature of a heap. Starting from the assumption that a Milky Way-type object is definitely a galaxy, if I take away one star at a time but leave the dark matter untouched, at what point does it stop being a galaxy ? It's not entirely speculative either; there are galaxies known which are Milky Way size but with far less stars. They are not totally dark but they're certainly very dim. So if a an object with just a few stars but lots of dark matter gets to be called a galaxy, then one made up of just gas and dark matter could be called a galaxy as well.
There is no formal definition of "galaxy", however, thankfully the IAU hasn't tried to do that.
Farnes : "Observations clearly indicate that the Universe is not empty".
ReplyDeleteThat ought to win some kind of award.
... followed closely by "This implies that our Universe is just one of those things that happen on occasion..." which is worthy of Pratchett or Adams, no less.
ReplyDeleteRhys Taylor imagine the outcry: according to the IAU, the Milky Way is no longer a galaxy.
ReplyDeleteTorch&pitchfork time for sure...
From the article: "a fluid which possesses 'negative mass." If you were to push a negative mass, it would accelerate towards you." Riiiigghhttt.
ReplyDeletephys.org - Bringing balance to the universe: New theory could explain missing 95 percent of the cosmos
Jack Martinelli The universe doesn't obey Human Intuitions. If it did, there'd be no such thing as gravity in the first place. An invisible force, caused by an invisible, undetectable particle, that pulls/pushes masses together? And accelerates a boulder towards the ground at the same speed as a feather? Hardly an intuitive notion, to begin with.
ReplyDelete