Has Dark matter really been proven?

[DontPanic]

Has Dark matter really been proven? And if, so what does this mean for the future of our understanding of physics?

 

[Pengwuino]

If by proven you mean the general trend in astrophysics is an agreement that dark matter explains certain observations that conflict with our current understanding of physics, then yes. However it doesn't really mean anything for our understanding since we have no idea what dark matter is.

 

[daumphys]

Are our Galaxy Dark matter parameters found?

 

[Garth]

Has Dark matter really been proven?

This paper in today's Physics ArXiv may be of interest A Novel Test of the Modified Newtonian Dynamics with Gas Rich Galaxies.

The data fall precisely where predicted a priori by the modified Newtonian dynamics (MOND). The scatter in the BTFR is attributable entirely to observational uncertainty. This is consistent with the action of a single effective force law but poses a serious fine-tuning problem for LCDM.

Garth

 

[Robin]

The assumption that the action of gravity is scale dependent seems to be a fudge. But then one could argue dark matter is a fudge. I can't help thinking if the action of gravity is scale dependent then the incredibly accurate measurements made by astronomers would reveal this when comparing for example the orbital behaviour of our moon or mercury to that of pluto.

 

[Garth]

The assumption that the action of gravity is scale dependent seems to be a fudge. But then one could argue dark matter is a fudge. I can't help thinking if the action of gravity is scale dependent then the incredibly accurate measurements made by astronomers would reveal this when comparing for example the orbital behaviour of our moon or mercury to that of pluto.

We seem to have the choice between two fudges!

The important point is to always be prepared to question what we hold to be true and let it be open to test and falsification. The corollary to this is to be open to alternative viable theories.

Garth

 

[cristo]

The assumption that the action of gravity is scale dependent seems to be a fudge. But then one could argue dark matter is a fudge.

Until there exists a relativistic version of MOND that agrees with data other than that which it is designed to fit, I'm more inclined to go for dark matter. After all, looking back in the past it's quite common for us to discover a particle species.

 

[Chronos]

MOND reminds me of Arp's catalogue of peculiar galaxies. It is supported by cherry picked observations that appear to match predictions of an otherwise dubious theory. I will suspend my disbelief when a version of MOND is demonstrated to better fit the preponderance of observational data than DM.

 

[Decimator]

Dontpanic, note that a form of "dark matter" is already known - http://en.wikipedia.org/wiki/Neutrino" . They're nifty little critters; read up on them if you haven't already.

 

[Vanadium 50]

Hmm...my message vanished.

Neutrinos are dark, and matter, but they can't be dark matter - they are traveling too fast to have the right properties.

I'm no MOND fan, but I think "cherry picked" is not the words I would use. MOND actually does better than LCDM in predicting galactic rotation curves (see http://www.astro.umd.edu/~ssm/mond/fit_compare.html" ), it explains the disk-halo conspiracy, and in general does a better job with galaxies than LCDM. Where LCDM does better than MOND is with structures larger than galaxies.

 

[Ameter]

Has Dark matter really been proven? And if, so what does this mean for the future of our understanding of physics?

Simple answer: no

Long answer: Observations of galactic rotation tell us that one of three things is true:

1) More mass exists inside a galaxy than we can observe
or
2) We do not understand gravity completely.
or
3) Our model of the structure of space itself is wrong.

We don't know which is true. Many (most?) astronomers believe 1) is the most likely explanation. I personally believe that the answer is a combination of 2) and 3), but couldn't support that with anything physical (yet)

 

[cristo]

3) Our model of the structure of space itself is wrong.

What do you mean by this?

 

[Ameter]

What do you mean by this?

If we think of space in four dimensions, we model it as a 3-dimensional (flat) plane relative to the fourth dimension. GR has shown that gravity can be modeled as an indentation in this plane, where the indentation is proportional to the mass of the object. However, this indentation is negligible at great distances, and so our model is essentially still a plane.

 

[cristo]

If we think of space in four dimensions, we model it as a 3-dimensional (flat) plane relative to the fourth dimension. GR has shown that gravity can be modeled as an indentation in this plane, where the indentation is proportional to the mass of the object. However, this indentation is negligible at great distances, and so our model is essentially still a plane.

GR tells us that space-time is curved, not just that the 3 spatial dimensions are curved. You seem to be describing an analogy which is often used to help describe the notion of curvature (i.e. the bowling ball on trampoline analogy). Of course, there exist spacetimes which are not asymptotically flat.

 

[Ameter]

GR tells us that space-time is curved, not just that the 3 spatial dimensions are curved. You seem to be describing an analogy which is often used to help describe the notion of curvature (i.e. the bowling ball on trampoline analogy). Of course, there exist spacetimes which are not asymptotically flat.

Of course? Are you sure? I've yet to come across a model which expressed an overall structure to space-time which was not flat. Certainly it will not be flat everywhere, due to the presence of mass, but that's not what I mean by structure.

Of course, my experience is still limited, so perhaps there is a model which covers this, but I've yet to hear of it.

 

[Pengwuino]

FLRW metric has global curvature from what little I know about it.

 

[Jack21222]

Simple answer: no

Long answer: Observations of galactic rotation tell us that one of three things is true:

1) More mass exists inside a galaxy than we can observe
or
2) We do not understand gravity completely.
or
3) Our model of the structure of space itself is wrong.

We don't know which is true. Many (most?) astronomers believe 1) is the most likely explanation. I personally believe that the answer is a combination of 2) and 3), but couldn't support that with anything physical (yet)

And what does the Bullet Cluster tell us?

 

[Decimator]

Neutrinos are dark, and matter, but they can't be dark matter - they are traveling too fast to have the right properties.

Do I misunderstand something about dark matter? Dark matter is essentially noninteractive matter, is it not? We have an example of that, neutrinos. They only interact via gravity and the weak force. Note that I do not think that neutrinos are the majority of dark matter mass, just that they're an example of dark matter.

Please tell me if I'm wrong. I like being told I'm wrong, it let's me learn things.

 

[Nabeshin]

Do I misunderstand something about dark matter? Dark matter is essentially noninteractive matter, is it not? We have an example of that, neutrinos. They only interact via gravity and the weak force. Note that I do not think that neutrinos are the majority of dark matter mass, just that they're an example of dark matter.

Please tell me if I'm wrong. I like being told I'm wrong, it let's me learn things.

I mean, I suppose you're right in that they are "a" dark matter, but they are not "the" dark matter that astrophysics chiefly concerns itself with. For a while, once we realized that neutrinos had mass they seemed like a very good candidate for the astrophysical dark matter. But as Vanadium points out, we realized they do not have the properties necessary to be the correct candidate (we could go into this more but it's not really germane to this discussion).

Similarly, for a while people thought the dark matter might be in normal objects which just emit very low levels of EM radiation -- things like brown dwarfs or black holes dubbed MACHOs (MAssive Compact Halo Objects). Again, these don't really have the right properties and studies have found that there aren't nearly enough of these objects to be the DM candidate astrophysics is currently searching for.

 

[Chronos]

The sterile neutrino is a stong contender in the dark matter sweepstakes and a regular on arxiv.

 

[ladykrimson]

Am I mistaken in assuming dark matter is simply a catch-all term for matter we cannot see or understand? If so, could dark matter be more than one type of matter?