Is there really dark matter?

[Frank1985]

It hasn`t been proved to exist, as far as I know. But, if there`s no dark matter to bend, and we can`t travel faster than the speed of light, can we ever see aliens?

 

[promytius]

Since so many more people believe in dark matter than believe in aliens, I don't see the connection.
No "proof" of either, yet one is part and parcel of modern physics, and the other is still fictionalized.

 

[Valeriy Tarasov]

We don't know why peripheral stars in spiral galaxies have constant velocity (by the way in early universe it is not observed phenomena) - speculate about dark matter, particles which are not detected. Atomic nuclei cannot be explain for proton/neutrons composition - speculate about strong force, quarks which don't exist as all other particles. It looks as both cases are nothing more than speculations.

 

[Catastrophe]

What I do not like about dark matter (although I can see why it might have been introduced) is that it has no comparative components of "ordinary" matter. That is, it is nothing but a label for an unknown?

Or am I missing something? Are we really in touch with only 5% of thr Universe? - or is 5% far too high an estimate anyway?

Cat

 

[Valeriy Tarasov]

That is, it is nothing but a label for an unknown?

Yes, IMHO.

 

[Helio]

What I do not like about dark matter (although I can see why it might have been introduced) is that it has no comparative components of "ordinary" matter. That is, it is nothing but a label for an unknown?

I think it is a label for a discovered phenomena. There have been consistent observations (indirect) of its presence around all sizes of galaxies, and clusters. The Bullet Cluster adds to the evidence with its more unique dynamics.

But, to your point, it is an unknown as to what sort of particle this might be, assuming it is a particle.

Thus, until the particle is found for it, "Dark Matter" should be taken as a label for both the known (gravity-like phenomena) and the unknown (particle?).

 

[Catastrophe]

How Dark Matter Could Be Measured in the Solar System - NASA

Pictures of the Milky Way show billions of stars arranged in a spiral pattern radiating out from the center, with illuminated gas in between. But our eyes can

www.nasa.gov

Helio, what I take exception to, on a strictly scientific basis is this:

Thus, until the particle is found for it, "Dark Matter" should be taken as a label for both the known (gravity-like phenomena) and the unknown (particle?).

You, yourself, finish with a ?

I believe that science is better served by accuracy, and an attempt to be semantically, and General Semantically accurate.
You post "Thus, until the particle is found for it" and "particle?".
To me, this is like saying "we will continuue to believe in Phlogiston until we find it".
OK, there is probably a better future for dark matter than for phlogiston, but it is not scientific to assume it must exist until it is found.

I feel a responsibility here to try to "tell it as it is", and not give the impression (and I do not mean you personally) that science "knows it all". Back to phlogiston.

Please accept this in the way that I intend it. When applicable, I would like a valid example to be set to any new (possibly younger) enquirers in these threads.


Cat

 

[Helio]

You, yourself, finish with a ?

Yes. I think everyone agrees we don't have a scientific answer for Dark Matter..... on a microscopic level. Some believed it would be a WIMP. There are many hypotheses that are still being tested, no doubt.

What's different about DM and Phlogiston (or whatever) is the solid objective evidence of the gravitational phenomena it represents. Zwicky in 1933 could only explain the speed of galaxies found in the Coma Cluster by assuming some additional matter had to be there that was unobservable, hence he called it dunkle Materie (dark matter). Vera Rubin brought it to the fore front with her data on the unexpected non-Keplerian stellar revolution rates in Andromeda. No subjective faith is required to recognize it.

But that's on a macro level. Microscopically, since all the normal suspects have been gathered (Casa Blanca?) and are all innocent, then something special is at hand. [I won't quote Spock ]

This isn't unlike gravity where the search for the graviton continues. It seems to have been Galileo, around 1603, who broke away from the Aristotelians. They assigned "heaviness" to objects, thus since their essence of this wouldn't change then their free-fall would remain at a constant speed.

But Galileo had done enough inclined plane measurements of rolling objects to declare that the distance traveled varied as the square of the time in motion.

Of course, Newton quantified it, but stated he didn't know what gravity actually is. Similarly, we don't know what DM is, but how things are found to "fall" makes it clear it deserves a label. Finding the particle, if it is a particle, will be extra bonus, at least to particle physicists.

BTW, I enjoy learning from your literary skills.

 

[Catastrophe]

Helio, you have posted yourself that DM is a label, mostly down to Zwicky.

From a General Semantic point of view, "the map is not the territory".

Just because Zwicky called this phenomenon "matter" does not make it matter.
Apart from a word of six letters, there is nothing to suggest that it IS matter.
To be scientific about it, it might be and it might not.

With respect, you are "giving a dog a good name, and giving it a bone"
No one knows whether it likes bones. It may not be a dog.

I am not against you. The DM guess of Zwicky may be correct. we don't know.
What we do know, is that there is an effect. Gravity may well be the same galaxies away as it is here. Don't forget galaxies away is billions of years away.
That is a further assumption that gravity was the same then, as now.
Are you suggesting that gravity was the same during BB and inflation?
I am not suggesting that it wasn't. I am a scientist saying that we don't know.
We cannot observe or experiment. We can only theorise.

May I politely ask you why you consider DM more than a theory, even if it has some merit as a good theory. I am sure phlogiston was once considered a good theory. You have to consider how it was considered then, not how we see it now.

May I remind you:

Yes. I think everyone agrees we don't have a scientific answer for Dark Matter

I agree with you.

Cat

 

[Helio]

Helio, you have posted yourself that DM is a label, mostly down to Zwicky.

From a General Semantic point of view, "the map is not the territory".

Agreed, especially in this case.

But, it must be recognized that we actually have a map that we know roughly represents something substantial (ie DM). We can observe its presence but we only see it indirectly, similar to the fact we can't directly observe a BH but we have lots of indirect evidence that gives us confidence they exist.

A prior post of yours stated you see DM as a label for an unknown. So are you saying the unknown is not even on a map? I'm not understanding clearly your view of DM.

Just because Zwicky called this phenomenon "matter" does not make it matter.
Apart from a word of six letters, there is nothing to suggest that it IS matter.
To be scientific about it, it might be and it might not.

Agreed. [You know how tempted I am to pun-handle this...matter. ]

Recall that, above, I more than alluded to that fact noting it might not be a particle. GR says spacetime can warp, so maybe something else is involved other than a particle that is causing this extra bump in GR. This is why I say there are two aspects to DM, one being only the observational phenomena, the other its essence.

MOND has attempted to take this route but it seems to always need a little extra something, DM, to make the equations fit both galaxies and clusters, IIRC.

I am not against you. The DM guess of Zwicky may be correct. we don't know.
What we do know, is that there is an effect.

Ah, good. you seem to agree that DM does qualify as a recognized phenomena. This, of course, justifies giving it a name of some kind.

So does this mean you would prefer to call it Dark ????, in lieu of adding "matter" to it? I assume this is the real issue for you, because labels are important.

If we think about the dilemma Zwicky faced, he likely thought there was simply regions of matter that was not bright enough for his 1933 telescope observations. This could be dark nebuale, later neutrino densities, etc. All being particles hence matter.

But, though far less likely but not ruled-out, it could be energy of some kind. If this is more likely we could call it Dark Energy, but, obviously, that won't work.

Physics has a hard time, I think, with trying to address anything that isn't either some form of matter or energy. Am I forgetting something? Perhaps in GR one could address greater variation in spacetime to "save the appearances", but that will likely be taken as ad hoc since a causal view will likely be required to be believable.

Gravity may well be the same galaxies away as it is here. Don't forget galaxies away is billions of years away.
That is a further assumption that gravity was the same then, as now.
Are you suggesting that gravity was the same during BB and inflation?
I am not suggesting that it wasn't. I am a scientist saying that we don't know.
We cannot observe or experiment. We can only theorise.

Einstein was the first in his cosmological model, followed quickly by de Sitter. They both chose to assume homogeneity for the cosmos, which greatly simplified their equations. It was a logical guess. Observations, especially the near perfect isotropy of the CMBR, greatly supports this argument. There are some recent papers, I think, that are playing with less homogeneity, but these are unlikely to go mainstream. [As if I'm smart enough to know.]

May I politely ask you why you consider DM more than a theory, even if it has some merit as a good theory.

I'm fine even if and when your curt, Cat.

Are the SM boxes checked?
1) Do we have any observations to suggest DM (phenomena) exists?
2) Do we have multiple lines of independent evidence of its presence?
3) Can it be observed at any time by others, thus being objective evidence?
4) Can mathematics be employed to describe its behavior? [Think Newton and gravity.]
5) Can we propose testable predictions of our hypotheses?

All these are a "yes", right?

I am sure phlogiston was once considered a good theory. You have to consider how it was considered then, not how we see it now.

Yes, I think it was indeed a bonafide scientific theory. It allowed others to test the predictions. I assume they were tested and were found to be false. [ A theory can only be proven false, never proven to be true.]

Another example is the Vulcan theory. [I was watching Star Trek last night.] This was a theory to explain the anomaly regarding Mercury's orbit. It suggested that a planet was causing the anomaly. A planet so close to the Sun it was too hard to have been observed by others without special care. One or two astronomers claimed to have seen it, giving it some plausibility. It was named Vulcan.

Einstein demonstrated brilliantly that his theory eliminated the Vulcan theory.

Changing "Dark Matter" to something like "Dark Phlogiston" might prove more appropriate someday, but IMO it's less likely than getting astronomy to dump the "yellow Sun" label. [I've tried, though gently, to get astronomy to move past it but it may be another couple of decades before it happens. ]

 

[Catastrophe]

You, yourself, finish with a ?

Yes. I think everyone agrees we don't have a scientific answer for Dark Matter..... on a microscopic level. Some believed it would be a WIMP. There are many hypotheses that are still being tested, no doubt.

***************************************************************************************

We are agreed. That is all we need.

A phenomenon is not necessarily material.

Just giving an idea a name does not make it material.

"The map is not the territory" is equally expressed by:
An abstraction is not the underlying reality.
I think we can agree that a verbal description is not the object described.

Cat

 

[Helio]

Yes. But a thought arose and it involved Shakespeare.