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General Relativity Resolves Galactic Rotation

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vincent    154
A galaxy is modeled as a stationary axially symmetric pressure-free fluid in general relativity. For the weak gravitational fields under consideration, the field equations and the equations of motion ultimately lead to one linear and one nonlinear equation relating the angular velocity to the fluid density. It is shown that the rotation curves for the Milky Way, NGC 3031, NGC 3198 and NGC 7331 are consistent with the mass density distributions of the visible matter concentrated in flattened disks. Thus the need for a massive halo of exotic dark matter is removed. For these galaxies we determine the mass density for the luminous threshold as 10^{-21.75} kg.m$^{-3}.

Now i'm not sure what this means and Dark Matter has been a subject of some skepticism among some scientists, but if im right, when it is factored into a few situations and/or calculations its presence is noticed. so far as far as galactic rotation is concerned i've only so far read on how they are detected to be rotating and that is through red shifting, clearly i need to read and learn more.

here is an interesting article on the subject of dark matter:

Galaxies don't have enough regular matter to keep them from flying apart, scientists have been telling us for years. So there must be a bunch of unseen "dark matter" lurking in every galaxy.

But dark matter has never been directly detected, and nobody knows what it might be made of. A few scientists remeain skeptical. To a lay person, it might sound downright crazy.

Now a new study suggests there may be no such thing as dark matter.

Fred Cooperstock of Northeastern University and Steven Tieu at the University of Victoria say Einstein's theory of general relativity can explain the cohesiveness of individual galaxies including our Milky Way.

Here's the thinking:

Newton's laws of physics explain why our solar system stays together. But the planets are negligible in the overall gravitational scheme, with the Sun being the total ruler and containing 99.86 percent of all the mass.

The same Newtonian physics were long ago applied to galaxies, and the rotation of stars couldn't be explained, so dark matter was invented to make theory work.

But a galaxy is much different than the solar system, Cooperstock explains. The conglomeration of all the matter -- stars, black holes, gas, and dust -- is collectively the source of the galactic gravity. Even a black hole at a galaxy's center typically packs less than 1 percent of the galaxy's overall mass.

The overall galaxy's gravity "feeds its own motion ... unlike the case of the solar system," Cooperstock told SPACE.com.

The science of the new argument is complex, but here goes:

"In the galaxy case, having rotation, we have found that general relativity provides a very important potential that is connected to the density of the galactic matter in what we call a 'nonlinear' manner,'" Cooperstock says. "This is unlike Newtonian physics."

This nonlinear effect has been noted before. "The interesting twist is that this holds also for the simpler steady rotational motion under gravity as in the galaxy," he said.

The upshot: The motions of stars in galaxies "is realized in general relativity's equations without the need to invoke massive halos of exotic 'dark matter' that nobody can explain by current physics," Cooperstock said.

A small percent of what used to be considered dark matter is made of burned-out stars that are hard to see. Predictions for how much of that material exists would not change.

Also, the new idea does not yet explain how large clusters of galaxies bind together. Further research by other theorists might solve that problem too, however, Cooperstock said. The new analysis has been submitted to the Astrophysical Journal but has yet to be reviewed by other scientists.

If it is right?

"This would remove about 25 percent of the mass of the universe, the ultimate weight-reduction program," Cooperstock

Edited by ripgut

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neowin_hipster    0

Using general relativity to solve these problems is too damn hard. These guys tried to do something like that but they traded one set of assumptions for another. IANAP but i think in the end its the right approach and will yield the need for less dark matter, however not completely. Why is the universe accelerating? That's still an odd question relativity doesn't address (afaik).

The thing is, that in science is that when given something simple ( a base ideal case ) the solution can be extremely computationally expensive if at all. Then if you want to make it realistic the system becomes impractical or NP-hard. Aerodynamics is a perfect example.

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raid517    0

He's right Riput, this does seem to be the best way forward. I always thought in my gut that Einstein had given us the key - and that we just needed to do more work to fully understand what he told us. 25% is certainly good, but I'm not sure if this explains it all yet. Dark matter is a good explanation, but like many others I would be happy if perhaps one day it could be replaced by a better theory. The problem with dark matter is that it could literally be anything (even green cheese) so even if we do find some candidates, it's by no means certain that we will be able to confirm the validity of these candidates on a galactic scale. The difficulty with dark matter is that we can't ever really test it - because none of our current models predict it - and also because we can never truly hope to go to the stars and gallaxies to perhaps one day collect a sample - and given this limitation everything in my bones says something that can't be tested, or which carries no real direct observational evidence for it is not science. (A good example of this is evolution where we have quite a lot of direct observational evidence, in the form of the geological record and the genetic code and in our understanding of the process of inheritance and so on. But in the case of dark matter all of the evidence we have is indirect, in that we can only observe it's effects).

We need something water tight - in the same way that Einstien's General Relativity is pretty water tight (there have been some very good tests during total solar elclipses to test it) and which holds open the prospect that it too one day might be open to being studied directly.

Still, what we need and what we will get may be two different things. It is perfectly possible that approaches such as the above may be judged fruitless - and that we might hit a limit of what is actually knowable through science. It seems unlikely to me that we will, but given the vast distances involved between us and the stars, it is certainly possible that we might reach a point where it is genuilnely just not possible to test conclusively any of the theories we might come up with. For me that would be very sad, because we would enter a stage where we simply had to 'believe' in our theories - which in the end would also mean that it would be virtually impossible to singificantly distinguish much of the field of astrophysics from religion.

It would almost in a sense be the end of science (given that astrophysics is likely to be the ultimate expression of science) where we would hit a point at which it was no longer possible for us to go beyond. (That is not to say however that dark matter does not exist, only that it's nature might prove unknowable). Which is why I very much personally do hope that a better theory than dark matter will one day emerge.

Nonetheless I am confident that it will, which is why I find stories such as this to be extremely encouraging. We will have to wait and see what this fellow's peers say - and if there response is positive, we may well have moved a significant step forward in our understanding.

Best regards,

GJ

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raid517    0

Of course realistically we shouldn't have to travel anywhere to see dark matter, because if previous predictions are right (and given that the Universe and the laws of physics are said to be the same regardless of where we might be in the Universe) we should have to travel anywhere to see this intergalactic 'green chesse', because in actual fact there should be so much of it that we should literally be surrounded by it everywhere we look. The fact that we haven't found any candidate in a sufficient degree of abundance yet, locally or in any other sense, seems to point to the possibility that either our models are wrong, or that we don't have the full picture yet. Personally I would say it is probably a mix of both of these. Science never claims that it's understanding of anything is ever 100% complete (or in other words there are no absolutes) because fundamentally there will always be a limit to how much we can know at any one time.

GJ

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