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Dark Matter

Ian

Unregistered
I
I'm skeptical about whether or not dark matter exists. What evidence is there for it?
 
Froum ought I can tell it's all indirect at best, or circumstantial.

from here:

http://www.space.com/scienceastronomy/dark_matter_021023.html


Most astronomers already view dark matter as the only logical way to explain the orbits of stars and shapes of galaxies. Nobody has ever seen dark matter, and scientists don’t know exactly what it is, but without it galaxies would fly apart.
Still, a competing theory suggests the universe contains plenty of regular matter but that its effects at the outskirts of a galaxy are less than what most scientists predict.
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It's kind of like being able to tell that there are two stars in a binary plantary system by observing the wobble of the orbits. You can't actually see that there are two stars but the indirect evidence from the "wobble" in light frequencies as they reach us indicates a phenomenon that is best explained by two stars in binary.

In the same manner the universe appears to be expanding at a rate that is too slow for the amount of observable matter in the universe. That is, the rate at which things are moving apart would imply a much higher gravitational attraction than would be accounted for by the known mass in the universe.

So ther theory which starts pretty simplistically, is, "well, there must be some matter out there that for various reasons, we can't observe. So much in fact that the universe is expanding as slowly as it is, based on what we know about gravity. Since we can't see, it, let's call it dark matter."
 
Ian said:
I'm skeptical about whether or not dark matter exists. What evidence is there for it?
Click to expand...

the general argument is that when you look at distant galaxies and gauge thier roatation, there is not enough visible matter to account for the speed and compactness of rotation. Visible matter does include the interstellar matter than can be detected by radio waves. So there isn't enough visble matter to account for the gravitation as observed.

Current speculation also include dark energy.
 
U think that the gravitational attraction attributed to dark matter is the result of the mass, or matter curving space-time. What would dark matter look like if space was or is curved?
 
I don't know, i just gave the theoretical structure of the search for dark matter, personaly iI think that the vacum energy is probably the culprit, or that we don't really understand how the curvature of space time might work.
 
Sorry, Ian , I am just a hack at physics, I like it but it is not like a field that I really understand.

Vacum energy is the basic energy in the universe, in some cases it is causing the universe to expand, in others it is the energy out of which virtual particles materialize.

basicaly, I think it is the idea that 'empty space' is not empty but that there is this pervasive energy in that space, maybe left over echos of the big band.
 
To a cosmologist, there are only two components to the universe, matter and energy, and they're unable to account for the vast majority of either.

As far as I'm concerned, we basically don't have the foggiest notion of what's going on at a galactic or larger level.
 
Ian said:
I'm skeptical about whether or not dark matter exists. What evidence is there for it?
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I am a long long way from knowing anything on this matter, but I do have an observation based on my high school science.

It seems to me that whenever science comes up with a "missing" substance in order to complete a theory, it is usually wrong.

eg.

phlogiston (spelling?) - the undetectable substance that supposedly made things combustable.

ether - the undetactable substance that allowed light waves to propogate in a vacuum.

I am sure there are more. I wonder if "dark matter" will join them one day?
 
Skeptoid said:


I couldn't help but chuckle at the typo. :D
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It was a typo I made many weeks ago, and have found great pleasure at rrepeating it ver since. It seems to generate as many useful notions as the big bang, I also was raised on big band music in my childhood. I got to see Stan Kenton and Buddy Rich.
 
Caveat, the following is printed in hopes one of the real physicist will comment/correct it. Weight the truth of this accordingly.

A neutrino "shortage" was observed. Less of a certain type (electron?) of neutrino was detecting than expected by theory. It was theorized that neutrinos may occasionally change flavours as they pass through other matter. For this to happen theory also required neutrinos to have mass.

I am under the impression that most physicts are leaning heavily to the neutrinos having non-zero mass idea. I believe measurements of neutrinos have now found a mass, but the error value is on the order of the measured value.

I believe if the neutrino has mass, it qualifies as dark matter or a WIMP to be precise (weekly interacting matter particle). The reason it is hailed as a breakthrough is, even if it is dark matter, it does not account a significant amount of mass compared to what theory requires.

Walt

So how did I do experts?

Edit: Did a search on neutrino mass. While theory shows a positive mass, experiment seen to put an upper limit of a few eV on the mass. The search turns up several pages which briefly discuss it but I didn't see a mention of dark matter.
 
Dark matter is not as sweet as regular matter , but has a fuller flavor and is ideally suited to baking.



Dancing David said:


I got to see Stan Kenton and Buddy Rich.
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Man, Buddy Rich...Met him once when I went to my brothers job at Franks Drum Shop on Walbash in Chicago had to be 69-70.

Right around that time he had an appearence on Johnny Carson
I was expecting that monster drum set he played but the curtain opens up and there he is , drum sticks in his hands and sitting in a wooden chair...he goes on to play the chair, the boom mike, the floor, Johnny's desk and it was brilliant.
 
Walter Wayne said:
I am under the impression that most physicts are leaning heavily to the neutrinos having non-zero mass idea. I believe measurements of neutrinos have now found a mass, but the error value is on the order of the measured value.
Click to expand...
Experiments have shown that there is a non-zero difference between masses of the three neutrinos. The measurements are accurate enough to rule out `zero mass difference'. This implies that at least two of the neutrinos have a non-zero mass.
I believe if the neutrino has mass, it qualifies as dark matter or a WIMP to be precise (weekly interacting matter particle). The reason it is hailed as a breakthrough is, even if it is dark matter, it does not account a significant amount of mass compared to what theory requires.
Click to expand...
Correct. The upper limits on neutrino mass are so small that they cannot account for the dark matter (at least not the whole of it).
[BTW, the `M' in WIMP stands for `massive', so strictly speaking neutrinos do not qualify for this title. :)]
 
Dorman said:

Experiments have shown that there is a non-zero difference between masses of the three neutrinos. The measurements are accurate enough to rule out `zero mass difference'. This implies that at least two of the neutrinos have a non-zero mass.

Correct. The upper limits on neutrino mass are so small that they cannot account for the dark matter (at least not the whole of it).
[BTW, the `M' in WIMP stands for `massive', so strictly speaking neutrinos do not qualify for this title. :)]
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Thanks for the additional info.

And d'oh, I knew that M stood for massive, but somehow matter got lodge into my brain and comes out everytime. However, I thought massive meant "having mass" in this context?
 
Theres another less exotic side of the dark matter question which contrasts with WIMPs , called MACHOS...Massive Compact Halo Objects. I say less exotic because we have actually observed them. The class of objects include massive planetary objects ( like Jupiter only much larger ) and Brown dwarfs. Truly dark matter.

Like I said they have been observed but the extrapolated statistical model still doesn't provide enough mass for the galaxies to behave in the way they do , hence....Dark energy, the newest postulate on the block. Dark Energy was predictid by Einstien and discarded so if it exists it can either be an exerted force on our spacetime ( which of course can't be detected directly) or it can be a static background charge, which if it was would not have to be very large to account for the " MISSING MASS" as it permeates the Hubble volume ( the existing universe ) If you just remember that mass and energy are equivalent ( sez Einstein) then it's not a large jump to understand, but like the others just a theory =)
 
There is another theory mentioned in this month's Discover magazine which is not very accepted, but I thought it was interesting. It basically states that the Newtonian gravity equations do not apply to particles that have a small acceleration. Because we are so close to the earth, the gravitational acceleration it imposes on us is about 32 ft/s/s which is pretty high. The gravitational acceleration under which the Newtonian equations don't work is about 1 Angstrom/s/s which is damn low. In case you want to find out more here is a link: MOdified Newtonian Dynamics (MOND)
and the ubiquitous FAQ

Anyway, the idea is that, much like Newtonian physics don't apply when traveling at speeds approaching c, perhaps they don't apply at accelerations approaching 0.

More cool links here: http://www.discover.com/oct_03/featgravity.html
 
JesFine:
Anyway, the idea is that, much like Newtonian physics don't apply when traveling at speeds approaching c, perhaps they don't apply at accelerations approaching 0.
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Interesting. Wouldn't that result in "dark energy" rather than dark matter? (As you'd effectively be throwing out some gravitational effects.)

If gravity were quantized, it seems to me that there should be a jump-to-zero point, but I'd expect that point to be much greater than, say, the current size of the universe.

EDIT: Most of those links were dead ends. This looked interesting, but basically went over my head, I'm afraid:

http://zebu.uoregon.edu/special/mond.html

What bothers me about it all is that it's all in relation to Newtonian equations. Why Newtonian? I thought relativity trumped those?

And is this claim about Pioneer 10 true:

http://www.discover.com/oct_03/featgravity.html
http://www.geocities.com/solarstormmonitor/Pioneer.html

EDIT AGAIN:

Hmm, looks like it is:

http://www.lanl.gov/worldview/news/releases/archive/98-140.shtml

EDIT YET MORE:

But is explained here:

http://www.newtonphysics.on.ca/Anomalous/Acceleration.html
 
JesFine said:
Anyway, the idea is that, much like Newtonian physics don't apply when traveling at speeds approaching c, perhaps they don't apply at accelerations approaching 0.
Click to expand...

Another problem highlighted in a recent article is that the "newtonian" physics don't apply in weak gravitationfields.

The laws of physics as we currntly know them have been extrapolated from experimental results here on earth, and by observing the planets in the solarsystem. The earth and all those stellar bodies are in the gravitational field of the sun. (else they wouldn't be part of our solar system)

The probes that has been sent out deep into space have proved the laws of gravity and mass to be incorrect in weaker gravitational fields.
 
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