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Dark Matter/Energy

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grizzly
52948.  Sat Feb 18, 2006 5:18 pm Reply with quote

Dark Matter and Dark Energy are both theoretical ideas.

Virtually all the matter in the Universe is dark, i.e. it does not emit radiation that can be observed by astronomers. But it can be detected by measuring the velocity of objects in the galaxy's outlying regions. If there were no additional matter to hold objects at the edge of galaxies in place then these objects would simply fly off into space.

Dark Energy is a theory to account for the accelerating expansion of the universe. The accelerating expansion of the universe implies that some form of energy, as yet unknown is forcing the universe to expand.

As to precisely what dark matter and dark energy are there is as yet no definite answer. Some explanations for the dark energy include neutrinos (already known to exist) and many other more exotic particles (tachyons for example are particles that travel faster than the speed of light so also travel back in time), a quantum soup or the cosmological constant.

 
Quaintly Ignorant
52974.  Sat Feb 18, 2006 7:27 pm Reply with quote

It is also possible that our understanding of gravity is wrong:

Quote:
There's something wrong with our understanding of spiral galaxies such as our own Milky Way. The stars in their outer parts are being whirled around far too fast. Like children on a speeded-up roundabout, they should be flung into intergalactic space.
To explain why this does not happen, astronomers have been forced to propose that the visible stars and nebulae are supplemented by at least 10 times more invisible stuff. The gravity of this "dark matter" holds on to the fast-orbiting stars and stops them going AWOL. But not everyone is happy with this picture.

"If Newton were alive today and saw the evidence," says Mordechai Milgrom, of Israel's Weizmann Institute, "he would have come up with a different law of gravity." In Newton's absence, Milgrom has obliged. And a sizeable minority of astronomers think he may be on to something. For Milgrom, it began at Princeton's Institute for Advanced Study in the early 1980s.

Quote:
Mond has made predictions that have been confirmed by subsequent observations of galaxies. But it is yet to come up with the killer prediction that will cause a stampede of theorists to the Mond camp. Milgrom doubts such a prediction exists. He believes it more likely that Mond will be accepted because of general dissatisfaction with dark matter - which wrongly predicts stellar motion in the centre of galaxies.

http://www.guardian.co.uk/print/0,3858,4604208-111019,00.html

 
AndyE
52982.  Sat Feb 18, 2006 7:56 pm Reply with quote

Once upon a time I wanted to be an astrophysicist, and I studied this kind of stuff.

Observable matter (i.e. stuff that we can see) is reckoned only to make up about 4% of the mass of the Universe.

23% is reckoned to be dark matter. What that is is not known for certain, but the current favourite is that it's made of neutralinos (different from neutrinos). Much as I'd love to tell you all what a neutralino is, to do so would require the use of terms such as "the superpartner of the neutral higgsino" and "conservation of R parity". So I'll settle for saying that they are extremely small, have a mass infinitely close to zero, and travel at the speed of light.

Neutralinos are basically impossible to detect; it is only possible to infer their presence from the absence of other things. Particle physics is a bit like that, which is at least partly why I stopped trying to become a physicist and specialised in geography instead ...

The other 77% of the mass of the Universe is reckoned to be dark energy. What that is is even harder to get a handle on, because nobody really has a clue. Dark matter is at least "stuff", in a vague sense of the word, but dark energy just is. We know how much it ought to weigh: the amount of dark energy required to fill a sphere the size of the Earth would weigh one ten millionth part of a gram. Therefore, there's an awful lot of it about! A group in 1982 claimed to have detected some dark energy, but not many physicists really believe them ...

There are all sorts of theories for dark energy, which rely on things like "the cosmological constant", "quintessence", "it doesn't exist, ergo Einstein's Theory of General Relativity was wrong", or the wonderfully ludicrous "brane cosmology".

I'd love to go into brane cosmology, but I'm two series too late ...

 
dr.bob
53369.  Mon Feb 20, 2006 6:53 am Reply with quote

AndyE wrote:
Observable matter (i.e. stuff that we can see) is reckoned only to make up about 4% of the mass of the Universe.

23% is reckoned to be dark matter.

The other 77% of the mass of the Universe is reckoned to be dark energy.


You've just accounted for 104% of the mass of the universe :)

 
djgordy
53373.  Mon Feb 20, 2006 6:58 am Reply with quote

Just sweep the excess 4% under the carpet. Nobody will notice.

 
AndyE
53408.  Mon Feb 20, 2006 8:21 am Reply with quote

dr.bob wrote:
You've just accounted for 104% of the mass of the universe :)


Oh dear so I have. I meant 73% of course, but I was distracted by my fiancée's post about lesbians! That's my excuse anyway!

 
dr.bob
53414.  Mon Feb 20, 2006 8:46 am Reply with quote

Yeah yeah, that's what they all say :)

 
Tas
53434.  Mon Feb 20, 2006 9:39 am Reply with quote

Just a few quick questions. I know very little about astro-physicals, neutrinos, neutralinos, neutragenas and all that stuff.

According to astronomers and physicists and so on, the observable universe is missing 'x' amount of matter.

1. How do we know the calculation on the amount of matter is correct?

2. Could the matter be beyond other matter. EG hiding behind other stars, nebulae, supernovae and so on?

3. Maybe the speed at which matter was flung away from the big bang or whatever is greater than supposed, and thus beyond the limits of the visible universe. People say that the universe is 'x' billion years old, but this number keeps jumping around. Shouldn't it be that the observable universe is 'x' billions of years old, as we can only see as far as 'x' billion light years, and matter could be 'x+1' billion light years away?

4. If matter=energy (I think I read that somewhere before), couldn't the missing matter be all the radiation and so on that is flying around the cosmos?

Sorry for seeming a bit thick, but the other half tries to cut down on the amount of Space I watch on the televisual box, and tries to inundate it with reality (*SPIT*) tv!

:-)

Tas

 
Quaintly Ignorant
53443.  Mon Feb 20, 2006 9:46 am Reply with quote

I had virtually the same thoughts.

The missing matter, as I understand it, is infered due to the motion of the visible universe i.e the rate of spin of the galaxies and what have you. The fact that we can only see bright shining objects implies to me that alot of the matter missing to tie in with calculations could be the general everyday non-shining matter i.e asteroids, planets, dead stars, black holes et al.

 
grizzly
53479.  Mon Feb 20, 2006 10:22 am Reply with quote

Quaintly Ignorant wrote:
I had virtually the same thoughts.

The missing matter, as I understand it, is infered due to the motion of the visible universe i.e the rate of spin of the galaxies and what have you. The fact that we can only see bright shining objects implies to me that alot of the matter missing to tie in with calculations could be the general everyday non-shining matter i.e asteroids, planets, dead stars, black holes et al.


I think the suggestion made by a lot of astrophysicists is that even this non-visible normal matter could not make up what is missing. Considering the huge mass that we can see (that 4% afterall is a hell of a lot of matter) it would seem nearly impossible to say that this type of matter made up the whole of the rest. I suppose this is the reason that there is the search for these other particles that would ensure that all of space had some sort of intrinsic mass that would make up for the missing value.

 
Kevino7
53483.  Mon Feb 20, 2006 10:27 am Reply with quote

Another problem is the fact that 90% of the universe cannot be seen by us and could contain anything.

 
dr.bob
53485.  Mon Feb 20, 2006 10:30 am Reply with quote

Tas wrote:
Just a few quick questions. I know very little about astro-physicals, neutrinos, neutralinos, neutragenas and all that stuff.


I want to do a cosmological investigation of neutragenas now :)

Tas wrote:
According to astronomers and physicists and so on, the observable universe is missing 'x' amount of matter.

1. How do we know the calculation on the amount of matter is correct?


It's not. It's a rough guess based on extrapolating from an incomplete data set.

Tas wrote:
2. Could the matter be beyond other matter. EG hiding behind other stars, nebulae, supernovae and so on?


Not really. As I understand it most of these quotes come from observing the motion of galaxies. The outer reaches of these move in a manner which, according to Newtonian theories of gravity, imply that there's a lot of matter beyond the visible edge. This occurs in galaxies even where there are no other matter obscuring the view.

The quotes about "missing 'x' amount of matter" then comes from comparing the gravitational effects of the bits you can see to the bits you can't see, and extrapolating that across the entire universe.

Tas wrote:
People say that the universe is 'x' billion years old, but this number keeps jumping around.


That's because it's really hard to date the universe, so most ages people come out with have pretty hefty error bars on them. They don't usually mention those, though.

Tas wrote:
4. If matter=energy (I think I read that somewhere before), couldn't the missing matter be all the radiation and so on that is flying around the cosmos?


I don't think so, though I can't think of a good way to explain why off hte top of my head.

Tas wrote:
Sorry for seeming a bit thick, but the other half tries to cut down on the amount of Space I watch on the televisual box, and tries to inundate it with reality (*SPIT*) tv!


That's cruel and unusual punishment :)

 
Kevino7
53488.  Mon Feb 20, 2006 10:33 am Reply with quote

Neurinos are very interesting. From what I know is that they have bizarre behaviour.

 
dr.bob
53492.  Mon Feb 20, 2006 10:34 am Reply with quote

Neutrinos are freaky.

Some people have suggested they have mass. I didn't even know they were catholic.

(sorry, old joke!)

 
grizzly
53505.  Mon Feb 20, 2006 10:57 am Reply with quote

dr.bob wrote:
Tas wrote:
4. If matter=energy (I think I read that somewhere before), couldn't the missing matter be all the radiation and so on that is flying around the cosmos?


I don't think so, though I can't think of a good way to explain why off hte top of my head.


I assume that by refering to matter=energy that Tas is refering to the E=MC2. Matter can be changed into energy and back again through the processes of nuclear fussion and nuclear fission. It is not radiation itself that changes into matter. So it isn't a candidate for the missing matter in the universe, neither is background radiation much of a candidate for the missing dark energy. Dark energy needs to be causing the force that is causing the expansion of the universe to accelerate and I don't believe radiation is a candidate for this.

 

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