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Dark matter has been proposed to explain two phenomena -

  1. Uniform velocity curve of spiral galaxies
  2. Observed gravitational lensing.

If something explains these two, would we still need dark matter?

Please see what could be wrong with it -

Space can ripple, curve, distort, squeeze, expand, fall into a black hole at FTL.. Then why it can not swirl around the massive black hole at the center of spiral galaxies. Can it be a space hurricane (SP-HU, in short) which, not only can explain the above two phenomena, but it would also explain the shape of these galaxies to begin with.

A SP-HU would keep the velocity curve pretty much uniform, It would also explain gravitational lensing because light would bend along the direction of swirl.

Aren't the lensed images (of background) always stretched along the direction of spiral?

Do DM models have any requirements to be fulfilled in addition to "causing excess gravity" and "being cold"? If not, SP-HU has both these and in a more natural way, without any mystery, it should fit all the models where DM fits.

I know dark matter is proposed in some non-spiral galaxies as well, but a SP-HU would also explain that, except the shape. So it can be a lighter/slower hurricane in those. That would translate in to lesser percent of proposed dark matter in those galaxies. I do not have the data but I guess if that is the case, then the hurricane hypothesis makes sense for these galaxies as well.

It is not frame dragging -

SP-HU is not same as frame dragging, it is a space swirl that would have been created at the time of big bang as a micro swirl blowing up into galactic/clusteric levels. This would explain structure formation.

Frame dragging is caused by rotation of the bodies. SP-HU is reverse - it caused the rotation of spiral galaxies and so, also their shape.

Comparison with MOND -

MOND can explain some phenomena and fails at others because, it tries to bridge the gap by changing the laws of gravity. Changing laws effects (fails) explanation of other phenomena that have already been explained by current laws.

SP-HU on the other hand does not even touch the laws. SP-HU describes the excess gravity effect without changing the laws. Therefore, it would bridge the gap without effecting other phenomena that have been already explained with existing laws.

Comparison with Dark matter -

SP-HU would bridge all gaps that can be bridged with the excess gravity effect. I.e. it would fill all the gaps that dark matter does, plus one (shape of the spiral galaxies itself). Therefore, it can be a marginally (but importantly) better fit than dark matter. Does dark matter explain why the galaxies are shaped the way they are (spiral).

Is being cold a requirement -

Dark matter is cold - Not sure, but what it means is that dark matter does not absorb the EM waves and so, does not acquire any energy that way, EM would pass right through DM. Whatever it means, we can easily see that SP-HU would have same cold nature as well. e.g. SP-HU would not absorb EM. So even if being cold is a requirement, it is not an issue with the SP-HU hypothesis.

Same logic would apply to being dark, we can not see the SP-HU too. Even though being dark (or transparent) does not seem to be a requirement, it rather appears to be a way out.

But, in case of SP-HU, we do not have to invent this (transparent) way out, we know for sure that it would be transparent.

SP-HU does not conflict with Bullet cluster observation.

What are other requirements that would prefer "dark matter" over SP-HU?

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closed as off-topic by user10851, Sebastian Riese, Kyle Kanos, ACuriousMind, Gert Feb 21 '16 at 3:02

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    $\begingroup$ "Dark matter" is the name for excess gravity without the presence of visible matter. It does not have any other implications beyond that. I have no idea how you came up with the "space hurricane" idea... but it's funny. Maybe someone can work it into a scifi show? $\endgroup$ – CuriousOne Feb 13 '16 at 10:12
  • $\begingroup$ The excess gravity has been observed only in terms of the two phenomena I mentioned. Actually that was the reason, they had to come up with the dark matter as far as I know from the youtube documentaries. And accelerated expansion of universe is the reason they come up with dark energy. $\endgroup$ – kpv Feb 13 '16 at 10:15
  • $\begingroup$ Google "dark matter" "frame dragging" to see discussions of this idea. $\endgroup$ – Mitchell Porter Feb 13 '16 at 10:21
  • $\begingroup$ The observation of excess gravity without the finding of enough matter was all there was ever needed to coin the phrase... if you want to go beyond that, we are talking about WIMPs and searches thereof. $\endgroup$ – CuriousOne Feb 13 '16 at 10:21
  • $\begingroup$ My point is - how the excess gravity was figured in the first place - Due to the two phenomena as far as I know. There was not enough baryonic matter to explain the gravity needed for two phenomena and so dark matter came into being. $\endgroup$ – kpv Feb 13 '16 at 10:24
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There's a couple more points of evidence:

  • From cosmology: the universe is (very nearly at least) asymptotically flat, which is neatly explained by the presence dark matter (and dark energy). And dark matter neatly ties into the formation of large scale structure in the universe, where otherwise there's just not enough stuff to form the sort of structures we see.
  • From galaxy clusters: it's not just galaxies that need dark matter to explain their motion, but clusters of galaxies as well.

And I'm sure more that doesn't come to mind immediately.

So, dark matter neatly works for things on a relatively small scale (galaxies) to the very largest of scales. Of course, if a coherent theory that could explain all of these things just as well were created, we don't need dark matter, but it is a remarkably simple solution that solves several different- apparently unrelated- problems. Even if space hurricanes were resolved into a coherent, predictive theory, you would still need then to come up with explanations for the other problems that dark matter solves for us.

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  • $\begingroup$ One has to be careful with the naming here. "Dark matter" and "dark energy" are the collective names for these phenomena (no matter what causes them), while WIMPs (and other "cold dark matter" particles) would be the necessary non-baryonic matter based mechanism to detail the mechanism of "dark matter" phenomena. We may not need WIMPs to explain "dark matter", but that won't make the "dark matter phenomenon" disappear. $\endgroup$ – CuriousOne Feb 13 '16 at 10:45
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    $\begingroup$ Chris, if space hurrican can explain even one phenomeon, then in a way, it explains the excess gravity effect. With that effect explained, other explanations should fall in place because they are solved by dark matter in terms of excess gravity caused by its presence, if I am not wrong. The cluster issue is certainly complex to explain, but how about a more clusteric hurricane on top of galactic hurricanes? $\endgroup$ – kpv Feb 13 '16 at 10:51
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    $\begingroup$ @kpv No, not at all. For instance a Modified Newtonian Mechanics theory can apply pretty well to explain galactic dynamics, but it completely fails when taken to other scales. Also, frankly, your theory is not well-posed, and doesn't work on even galactic scales. While rotating black holes can have a drag effect around them, this effect is very localized and does not lead to a constant velocity curve in any case. $\endgroup$ – Chris Feb 13 '16 at 11:31
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    $\begingroup$ I can assure you that it wasn't "mostly semantics" for generations of astronomers who were looking for the baryonic matter that simply isn't there. Many people spent their careers on searching for brown dwarf candidates and cataloging dust and gas clouds inside and outside of the galaxy and we had to build deep infrared satellites to make sure that this wasn't just really cold baryonic matter. Physicists tend to forget how long it took to rule out the "obvious" suspects. $\endgroup$ – CuriousOne Feb 13 '16 at 11:32
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    $\begingroup$ Chris, it is more like an hypothesis rather than a theory. A theory can not be put in place like this and I am not claiming it to be a theory at all. However, MOND is bound to fail at many places because it tries to solve the issue by changing basic laws of gravity, doing so messes with other phenomena - which is explained using current laws. Space Hurricane (SP-HU) is not changing the basic laws at all and so, should fit all places, if fits one. Does dark matter explain why the galaxies are spiral shaped? SP-HU kind of does explain the shape too! $\endgroup$ – kpv Feb 13 '16 at 15:41
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Your concept of space hurricane seems to be a type of geon. A geon is a gravitational soliton, a gravitational wave that doesn't disperse because of nonlinear self-interaction. You want a geon that rotates, that was blown up to galactic size by inflation, and which produces the velocities seen in the galactic rotation curves.

Geons are very little studied, I think because the full nonlinear equations of general relativity are difficult. People prefer the linear approximation, but a geon is inherently nonlinear... However, the magnitude of the gravitomagnetic precession that a galaxy-size space hurricane must produce, if it is to account for the rotation curves, is so enormous that it must be possible to reason about whether such an object is consistent with general relativity.

I guess I'll come back to this answer if no-one else does the work first.

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Dark matter is needed to explain how galaxies hold together. There is not enough gravity to hold galaxies together in the visible stars we observe, therefore there must be something else that keeps things together, that must interact like matter with the force of gravity but not with the other forces (therefore it is invisible). Hence its name "dark" (it's invisible) "matter" (acts as matter). Of course, like every theory, it could be proven false one day, but so far I don't think any physicist disputes its existence.

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Is dark matter really there?

We don't know for sure. But something is causing those flat galactic rotation curves, and we can't see it. Personally I dislike the word matter in dark matter because a concentration of energy causes gravity. Matter only causes gravity because it is a concentration of energy. And as Einstein said "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy". You can't see this spatial energy, it's "dark", but it causes gravity, and it isn't matter. As to why a lot of people seem convinced that dark matter consists of some kind of particle I just don't know.

Dark matter has been proposed to explain two phenomena - 1. Uniform velocity curve of spiral galaxies 2. Observed gravitational lensing. If something explains these two, would we still need dark matter?

Not per se. But see Einstein talking about space as the ether of general relativity here, and saying "the contrast between ether and matter would fade away". You can find papers such as http://arxiv.org/abs/gr-qc/9610066. It's by Christoph Schiller, and the title is Does matter differ from vacuum? And space is dark.

Space can curve, distort, squeeze, expand, fall into a black hole at FTL...

Space can curve and distort and squeeze and expand. But it doesn't fall into a black hole. This is a fairy tale that's arise from Gullstrand-Painlevé coordinates, which Einstein dismissed for good reason. Because a gravitational field is a place where space is "neither homogeneous nor isotropic", this being modelled as curved spacetime. We do not live in some Chicken-Little world where the sky is falling down, despite what you may have heard on the Discovery Channel. The waterfall analogy is a bad analogy, and an example of lies to children.

Then why it can not swirl around the massive black hole at the center of spiral galaxies. Can it be a space hurricane which, not only can explain the above two phenomena...

No, sorry. The nearest thing we have to a space hurricane is a "space time vortex", which is the gravitomagnetic field. See this NASA article:

enter image description here Image copyright James Overduin, Pancho Eekels, and Bob Khan.

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