Dark matter, dark energy, and the Kerr metric?

Main Question or Discussion Point

I’m sorry, but I find dark matter and dark energy problematic. It’s hard to think of a Universe made up of about 95 % of stuff we have no idea about, except that maybe dark matter and dark energy have some properties.

So I’m thinking maybe there’s something wrong with the data, but I can’t see it. The Universe is expanding, and that expansion seems to be accelerating with time.

Maybe the equations of motion are wrong? Maybe Newtonian Mechanics is not the best way of be describing the Universe on a supper massive scale, something like MOND. But MOND doesn’t do it. It seems arbitrary with little insight.

So this is where I’m at. Almost every spiral galaxy has a supper massive black hole, weighing in at about a million to a billion times the mass of our sun. It’s about 10% the mass of a galaxy. The Schwarzschild radius for a super massive black hole is huge, something like the radius of our solar system. But, the average density over volume is low, not much different than the density of water.

Can some of that dark matter/dark energy come about from a Kerr metric for a spinning supper massive black hole, where Einstein’s cosmological constant is not zero? What could happen with a Kerr like metric, where the cosmological constant can vary? Is there a value which could keep galaxies together, at least for a short time?

Answers and Replies

zonde
Gold Member
I’m sorry, but I find dark matter and dark energy problematic. It’s hard to think of a Universe made up of about 95 % of stuff we have no idea about, except that maybe dark matter and dark energy have some properties.
I suppose nobody is very happy about it.

Can some of that dark matter/dark energy come about from a Kerr metric for a spinning supper massive black hole, where Einstein’s cosmological constant is not zero?
Kerr metric describes gravity around rotating black hole where cosmological constant describes average "antigravity" of the whole universe.
So your statement does not make much sense, at least for me.
Dark matter problem is more about how gravity changes from place to place around galaxy then about some overall missing mass in galaxy. So again it does not make sense.

What could happen with a Kerr like metric, where the cosmological constant can vary? Is there a value which could keep galaxies together, at least for a short time?
Kerr metric describes local effects where cosmological constant describes global effect. There isn't too much connection between the two.

If you assume uneven distribution of dark energy at galaxy scales maybe it can provide some interesting approaches to dark matter problem but it seems that you are asking about something different.

Chalnoth
Science Advisor
Can some of that dark matter/dark energy come about from a Kerr metric for a spinning supper massive black hole, where Einstein’s cosmological constant is not zero? What could happen with a Kerr like metric, where the cosmological constant can vary? Is there a value which could keep galaxies together, at least for a short time?
Black holes only make up at most about 2% of the mass of a galaxy. So no, they don't have anywhere close to enough mass to so dramatically change our equations. Furthermore, whatever the solution to Einstein's equations for a black hole in the presence of a cosmological constant, it must approach the result for a cosmological constant in the limit of large distances. So I don't see how your proposal is any different than just proposing a cosmological constant in the first place, which is the leading candidate for the solution to the accelerated expansion.

I don't see why so many people are so opposed to dark matter, though. I mean, we already know of particles that have many of the same properties required for dark matter: neutrinos. Neutrinos don't fit the bill, of course, but that doesn't mean that other, higher-energy particles that are too massive to have yet been detected in particle accelerators won't.

Nabeshin
Science Advisor
It’s hard to think of a Universe made up of about 95 % of stuff we have no idea about, except that maybe dark matter and dark energy have some properties.
Well that's a little egocentric of you!

Chronos
Science Advisor
Gold Member
'Dark' matter and 'dark' energy are unrelated. Cosmologists are fully aware of this fact. Attempting to connect the two is a symantics fallacy.

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Kerr metric describes gravity around rotating black hole where cosmological constant describes average "antigravity" of the whole universe.
Ok, the cosmological constant describes an average "antigravity" of the whole universe, dark energy. So, maybe dark energy can’t be described by a Kerr metric with a varying cosmological constant, but what about dark matter and the Kerr metric?

I see a black hole as having mass, but it also has rotation. That rotation will induce frame dragging. Can some dark matter come from frame dragging of a suppermassive black hole at the center of galaxies?

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Chalnoth
Science Advisor
Ok, the cosmological constant describes an average "antigravity" of the whole universe, dark energy. So, maybe dark energy can’t be described by a Kerr metric with a varying cosmological constant, but what about dark matter and the Kerr metric?
You do realize that a varying cosmological constant is a contradiction in terms, right?

I see a black hole as having mass, but it also has rotation. That rotation will induce frame dragging. Can some dark matter come from frame dragging of a suppermassive black hole at center of galaxies?
No, frame dragging is a local effect, and the masses of the black holes at the centers of galaxies are far, far too small to do anything of the sort.

What if frame dragging is not a local effect, but has an effect on galactic proportions?

zonde
Gold Member
I see a black hole as having mass, but it also has rotation. That rotation will induce frame dragging. Can some dark matter come from frame dragging of a suppermassive black hole at the center of galaxies?
Qualitatively it has the same effect. But of course if quantitatively it can not account for considerable part (or all) of dark matter effect it would not be very interesting.

Chalnoth
Science Advisor
What if frame dragging is not a local effect, but has an effect on galactic proportions?
Again: mass is way too small. The black holes at the centers of galaxies max out at roughly 2% of the visible mass, while the dark matter is around ten times the mass of the stars (or more).

Again: mass is way too small. The black holes at the centers of galaxies max out at roughly 2% of the visible mass, while the dark matter is around ten times the mass of the stars (or more).

Thanks Chalnoth, for putting these two in perspective.

It would be great if we could get some actual dark matter in the lab to find out what it is made of. Doesnt homogeniety require that there is at least some dark matter here and everywhere?

Thinking about it I guess we dont really know what the matter in black holes is made of either now that it is compressed almost to a true singularity.

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Think of a poll of water, with no motion. Add a spinning sphere. Eventually, with enough time, matter will begin to rotate, and that rotation will be independent of gravity.