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Author Topic: What is dark matter?  (Read 4851 times)

Offline J Rahman

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What is dark matter?
« on: 01/10/2009 04:04:50 »
One of my friends, Dutta, asked me about Higgs-boson particles. He said that these are dark matter. I would like to know your opinion.


 

Offline Don_1

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What is dark matter?
« Reply #1 on: 01/10/2009 07:59:16 »
Since the Higgs boson and dark matter are hypothetical, I think too much speculation could be detrimental to current research. There is evidence of something causing known phenomena to act in an unexpected manner. If you decide what that something is before you have found it, you could set back your research by narrowing your field of research.

I think an open mind is still required.
 

Offline LeeE

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What is dark matter?
« Reply #2 on: 01/10/2009 09:00:14 »
The Higgs boson and Dark matter are two different things, and as Don_1 says, both are still only theoretical/hypothetical.

The Higgs boson is a sub-atomic particle that is predicted by the Standard Model in physics, and is considered to be the mediator of gravity.

Dark matter is something that seems to be needed to account for the observed behaviour of galaxies.  When we look at other galaxies we can estimate how much mass there is in them and then work out how fast they should be spinning etc. but when we actually take measurements the two sets of results don't agree.  Although there's fair degree of guesswork involved in estimating the masses of these galaxies, the difference we get is still too great, so there's obviously something else that's not being accounted for when we try to work it out.

As it turns out, extra mass would nicely fit the bill and bring the worked out answers into line with the observed measurements, but as we can't see this extra mass, because you can only see stuff that is either generating light, or reflecting it, it's called dark matter.

Some people have taken Dark matter to be some entirely new and exotic form of matter but there's no real reason for this and I think it's just down to a few people getting a bit over excited.  It's much more likely to be ordinary matter, as we know it, very likely in the form of dust or brown dwarf stars.
 

Offline PhysBang

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What is dark matter?
« Reply #3 on: 01/10/2009 15:33:33 »
Some people have taken Dark matter to be some entirely new and exotic form of matter but there's no real reason for this and I think it's just down to a few people getting a bit over excited.  It's much more likely to be ordinary matter, as we know it, very likely in the form of dust or brown dwarf stars.
Actually, there are very good reasons to rule out dust and brown dwarf stars as significant sources of dark matter. These hypothetical sources come with predictions that are not supported by observations.

The best evidence for dark matter are the cosmological tests. These tests place limits on how much mass there should be in the form of matter that is far above how much mass there should be in the form of the standard matter (protons, neutrons, electrons). This strongly supports the claim that dark matter is some exotic particle that does not interact with electromagnetism.
 

Offline Geezer

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What is dark matter?
« Reply #4 on: 01/10/2009 17:05:48 »
It's also quite possible that there is no large amount of dark matter and our observations indicate that our understanding of gravity is in need of revision.

Sometimes it's good to accept scientific observations at face value rather than invent bizarre artifacts to force the Universe to fit existing theory that "obviously" can't be wrong.

(I'm heading for my underground bunker right now.)
 

Offline JimBob

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What is dark matter?
« Reply #5 on: 01/10/2009 18:54:44 »
Wasn't it Einstein who said the most simple idea's were the best - after commenting that throwing away his gravitational constant was the biggest blunder of his scientific life?

If I am correct, The gravitational constant would account for much if not all of the observed gravitational phenomena.
 

Offline Geezer

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What is dark matter?
« Reply #6 on: 01/10/2009 19:21:11 »
I'm not sure that a gravitational constant fixes things up. As I understand it, "dark matter" is a fairly new invention.

Without a better understanding of the nature of space, I suspect we will go around in circles on this topic for a very long time.
 

Offline PhysBang

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What is dark matter?
« Reply #7 on: 02/10/2009 18:32:21 »
Wasn't it Einstein who said the most simple idea's were the best - after commenting that throwing away his gravitational constant was the biggest blunder of his scientific life?

If I am correct, The gravitational constant would account for much if not all of the observed gravitational phenomena.
Einstein, and pretty much every other gravity theory, does use a gravitational constant. Einstein's "cosmological constant" matches some of the observations that we have, and dark matter is also required to match these observations and other observations that the cosmological constant does not bear any relation to.

It is important to realize that scientists have taken seriously the idea that our theory of gravity might be wrong. So far, modified theories of gravity cannot match the observations to the extent that standard gravity, together with dark matter, can.

It is likely that we can come up with a more accurate gravity theory, but it looks increasingly unlikely that we can throw out dark matter.
 

Offline LeeE

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What is dark matter?
« Reply #8 on: 02/10/2009 18:39:45 »
Some people have taken Dark matter to be some entirely new and exotic form of matter but there's no real reason for this and I think it's just down to a few people getting a bit over excited.  It's much more likely to be ordinary matter, as we know it, very likely in the form of dust or brown dwarf stars.
Actually, there are very good reasons to rule out dust and brown dwarf stars as significant sources of dark matter. These hypothetical sources come with predictions that are not supported by observations.

The best evidence for dark matter are the cosmological tests. These tests place limits on how much mass there should be in the form of matter that is far above how much mass there should be in the form of the standard matter (protons, neutrons, electrons). This strongly supports the claim that dark matter is some exotic particle that does not interact with electromagnetism.

Everything about this issue has a large helping of guesswork, and the 'good reasons' for saying that the extra mass cannot be ordinary matter and must therefore be some new form of exotic matter are equally based upon guesswork.  The uncertainty in the guesswork can be manipulated or presented to satisfy either argument.

There is also though, the very strong argument against exotic dark matter in that until now there's been no evidence, or requirement, or place for it, and it still cannot be defined in terms other than 'not ordinary matter', which doesn't tell us anything.

In fact, we can only really define exotic dark matter as being: 'that which is required to make our sums work', which I think is a pretty flimsy reason for inventing it.
 

Offline PhysBang

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What is dark matter?
« Reply #9 on: 03/10/2009 15:01:12 »
Everything about this issue has a large helping of guesswork, and the 'good reasons' for saying that the extra mass cannot be ordinary matter and must therefore be some new form of exotic matter are equally based upon guesswork.  The uncertainty in the guesswork can be manipulated or presented to satisfy either argument.
All science is guess work to some extent. However, we do not dismiss scientific results because of this.

In this case, there is no uncertainty that will allow the dark matter detected in cosmological tests to be the result of brown dwarfs or dust. These tests are based on the behaviour of the density of matter and on the behaviour of the density of baryons, what we think of as normal matter. A number of different tests indicate that there is a far greater density of matter than there is of baryons. Since dust and brown dwarfs would be made of baryons, they cannot be the source of dark matter in these tests.

Additionally, modified theories of gravity cannot fit the cosmological tests. It may be that some future, yet undiscovered modified theory of gravity will fit the observations, but we can always come up with such imaginary hypotheses in science. What matters is tests. Right now, different cosmological tests measure the amount of dark matter, in a form other than baryons, in a number of different ways. These tests measure the same amount.
Quote
There is also though, the very strong argument against exotic dark matter in that until now there's been no evidence, or requirement, or place for it, and it still cannot be defined in terms other than 'not ordinary matter', which doesn't tell us anything.
This is not completely correct. We have agreeing measurements of just how much there is in the universe and on roughly how fast it must be moving relative to ordinary matter.
Quote
In fact, we can only really define exotic dark matter as being: 'that which is required to make our sums work', which I think is a pretty flimsy reason for inventing it.
But it doesn't make just one sums work, it makes a lot of measurements work. And it's always the same amount. So far, no alternative theory can do that. As far as its invention, as most of our previous mass estimates were based on counts of stars and galaxies and measurements of dust. But mass that did not interact with electromagnetism would not show up in such surveys. Thus it was always a very viable possibility that there was such mass, though it was fairly pointless to assign a probability that there was such mass until such a time as it was possible to measure the amount of such mass. Now it is possible.
 

Offline Geezer

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What is dark matter?
« Reply #10 on: 03/10/2009 17:24:56 »
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A number of different tests indicate that there is a far greater density of matter than there is of baryons.

PhysBang, Can you please point me at a reference for some of the tests you mention?
Thanks, Geezer
 

Offline PhysBang

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What is dark matter?
« Reply #11 on: 03/10/2009 22:10:36 »
Quote
A number of different tests indicate that there is a far greater density of matter than there is of baryons.

PhysBang, Can you please point me at a reference for some of the tests you mention?
Thanks, Geezer
The trifecta of tests are the supernova tests, the background radiation tests, and the galaxy survey tests. Here are the main website for these projects. Ideally, they should point you to their results, though not all the websites are equal.

Supernova tests:
Supernova Legacy Survey: http://www.cfht.hawaii.edu/SNLS/
Supernova Cosmology Project: http://supernova.lbl.gov/
High-z Supernova Search Team: http://www.cfa.harvard.edu/supernova//public.html

Background radiation:
http://map.gsfc.nasa.gov/

Galaxy Survey tests:
Sloan Digital Sky Survey: http://www.sdss.org/
2dF Survey: http://www2.aao.gov.au/2dfgrs/
 

Offline DoctorBeaver

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What is dark matter?
« Reply #12 on: 04/10/2009 02:22:21 »

The Higgs boson is a sub-atomic particle that is predicted by the Standard Model in physics, and is considered to be the mediator of gravity.


Are you sure of that? I thought the Higgs boson was postulated as the mechanism by which particles gain their mass and that it is the graviton that is hypothesised to mediate gravity.
 

Offline graham.d

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What is dark matter?
« Reply #13 on: 04/10/2009 11:22:51 »
I think you are correct, DB, as far as I know.

PhysBang has explained the current ideas regarding dark matter quite well. I believe the known facts are:
1. The Virial theorem relates the relationship between kinetic and potential energy in a closed system.
2. Galaxies are not completely closed systems, but can be considered close enough to not affect the analysis.
3. We are able to observe rotational velocities of stars (by Doppler effect) in elliptic galaxies (and similarly in galaxy clusters) and relate this to the distance from the centre of rotation.
4. The velocities, away from the galactic centre, are much higher than would be estimated from the observed brightness (which is usually related to the estimate of total mass). This implies more mass than can be observed.
5. The distribution of velocities is very flat, except close to the centre, which implies that the extra mass must be distributed throughout the region and not just in some extra large central body.
6. Assuming our own galaxy is representative, it is estimated that there must be about 10x the amount of matter that we can observe.
7. Explanations that this is in the form of dust is not consistent with measurements, in our own galaxy, of distant stars which would have much lower luminosity as a result; unless the dust did not interact electromagnetically (some form of dark matter).
8. Similarly, there seems to be insufficient numbers of non-radiating stars (or explanations for their formation) to fit the observations.
9. The existence of dark matter also allows for simulations of big bang models of the universe to give results that are consistemt with observation.

There are other theories that involve modifying various laws of physics, and one of these may be right, but it is probably fair to say that the simplest explanation so far is dark matter.


 

Offline graham.d

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What is dark matter?
« Reply #14 on: 04/10/2009 13:20:53 »
Quite a good verbal explanation of the Higgs field, Higgs Boson and mass is:

http://www.fnal.gov/pub/inquiring/questions/higgs_boson.html

 

Offline LeeE

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What is dark matter?
« Reply #15 on: 06/10/2009 16:54:01 »

The Higgs boson is a sub-atomic particle that is predicted by the Standard Model in physics, and is considered to be the mediator of gravity.


Are you sure of that? I thought the Higgs boson was postulated as the mechanism by which particles gain their mass and that it is the graviton that is hypothesised to mediate gravity.

Oops! - yes.  My bad.
 

Offline Geezer

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What is dark matter?
« Reply #16 on: 07/10/2009 07:03:03 »
Particle models work well on very small scales, but just because they work well on small scales does not mean they will work well on large scales. Our current experience of forces and distance might lead us to conclude that particle models have limitations.

Just one person's totally unsubstantiated opinion, but I think we still have much to learn about mass, gravity and space.
 

Offline graham.d

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What is dark matter?
« Reply #17 on: 07/10/2009 08:51:05 »
There's one thing we can be certain of - everything we know is wrong.
 

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What is dark matter?
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