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Proof of Dark Matter

KingMerv00 said:
http://www.cnn.com/2006/TECH/space/08/21/dark.matter/index.html

Interesting, but I do have a question. If dark matter makes up 70% of all matter, why don't we feel it in the local environment? Is it because dark matter is in a giant pile elsewhere?
Click to expand...

First of all
  • Normal matter ~ 5%
  • Dark Matter ~ 20%
  • Dark Energy ~75%
of the universe. I'm not correcting you, because you said 70% of all matter, just wanted to write this so there is no confusion.

Now, dark energy is very strange and we do not have any candidates, so we'll pass it from now.

Dark matter comes in two kinds: the first one is basically everything but stars (black holes, planets, dark galaxies etc.) Difficult to see, but made of atoms and just like the stuff we have here. This is the baryonic dark matter, which makes for a small fraction of the total amount. The second kind is not made of atoms, which makes it more interesting. It is further subdivided into particles that behave ultrarelativistically, relativistically and non-relativistically. Possible candidates are neutrinos, axions and WIMPs.

As to the question of why do we not feel it here, we don't know yet... The different kinds of dark matter are all needed for several reasons (cosmological, structural, etc.) but until we know what kind of particles compose it and how were they created we cannot explain their apparent abscence on Earth.

Take neutrinos, for example. A neutrino can pass through a one light year long block of lead with a 50% probability of being undisturbed. We are continously being pierced by millions of them and we do not feel a thing. They do not interact via the electromagnetic or strong nuclear force, which amkes them very difficult to detect. The experiments currently trying to measure the neutrinos coming from the Sun are incredibly complex and big. What I mean by this is that Dark Matter, even its strangest components, may very well be present here, even though we do not detect it.

Some attemps have been made at detecting WIMPs, for example. The DAMA experiment claimed to have measured the modulation in the flux of dark matter due to the orbit around the Sun but the scientific community is still sceptic about its results.
 
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Yllanes said:
First of all
  • Normal matter ~ 5%
  • Dark Matter ~ 20%
  • Dark Energy ~75%
of the universe. I'm not correcting you, because you said 70% of all matter, just wanted to write this so there is no confusion.
Click to expand...

Thanks for the correction. Just a misread on my part.

Now, dark energy is very strange and we do not have any candidates, so we'll pass it from now.

Dark matter comes in two kinds: the first one is basically everything but stars (black holes, planets, dark galaxies etc.) Difficult to see, but made of atoms and just like the stuff we have here. This is the baryonic dark matter. The second kind is not made of atoms, which makes it more interesting. It is further subdivided into particles that behave ultrarelativistically, relativistically and non-relativistically. Possible candidates are neutrinos, axions and WIMPs.

As to the question of why do we not feel it here, we don't know yet... The different kinds of dark matter are all needed for several reasons (cosmological, structural, etc.) but until we know what kind of particles compose it and how were they created we cannot explain their apparent abscence on Earth.

Take neutrinos, for example. It is always said that a neutrino can pass through a one light year long block of lead with a 50% probability of being undisturbed. We are continously being pierced by millions of them and we do not feel a thing. They do not interact via the electromagnetic or strong nuclear force, which amkes them very difficult to detect. The experiments currently trying to measure the neutrinos coming from the Sun are incredibly complex and big. What I mean by this is that Dark Matter, even its strangest components, may very well we present here, even though we do not detect it.

Some attemps have been made at detecting WIMPs, for example. The DAMA experiment claimed to have measured the modulation in the flux of dark matter due to the orbit around the Sun. The scientific community is still sceptic about its results, however.
Click to expand...

If dark matter is so abundant and generates gravity, shouldn't our orbital calculations be off by 20% or so?
 
Yllanes said:
Dark matter comes in two kinds: the first one is basically everything but stars (black holes, planets, dark galaxies etc.).
Click to expand...

MACHOs vs WIMPs.... Who says scientists don't have a sense of humour :)
 
KingMerv00 said:
If dark matter is so abundant and generates gravity, shouldn't our orbital calculations be off by 20% or so?
Click to expand...

It's important at big scales. A neutrino has such a little mass that they were thought to be massless until recently. You need a lot of them to have an important contribution. With big scales I mean things like clusters of galaxies or galactic halos.

The galactic rotation curves indicate that the mass distribution is uniform well beyond the area were most of the stars are located. This suggests that dark matter would be more concentrated in the halo. Some globular clusters show little sign of dark matter and some galaxies seem to be made exclusively out of it.

So the distribution of dark matter is not uniform. But that doesn't mean there isn't any on Earth, or passing through Earth.
 
KingMerv00 said:
If dark matter is so abundant and generates gravity, shouldn't our orbital calculations be off by 20% or so?
Click to expand...

If you were to sit at the center of the earth, and calculate your orbit around the sun, what effect would the mass of the earth have on your calculations?

None. The mass of the earth would be evenly distributed all around you, pulling you equally in all directions at once. Even if you were to move a meter to either side, the imballance of forces would still be so small as to be unnoticeable.

Dark matter does much the same at scales as small as that of a single solar system. It is evenly distributed all around, so it's net effect is zero (or close to zero). Only when you get to the scale of galaxies do you see a sufficiently non-uniform distribution for there to be an effect, and that effect is one we have observed.
 
Also, dark matter is located in large quantities by the observation of bodies which are moving in odd paths. Recalculating their surroundings based on their movements will often predict the presence of an unnoticed source of gravity. This can be confirmed by other nearby bodies acting oddly.
 
Yllanes said:
First of all
  • Normal matter ~ 5%
  • Dark Matter ~ 20%
  • Dark Energy ~75%
of the universe. I'm not correcting you, because you said 70% of all matter, just wanted to write this so there is no confusion.

Now, dark energy is very strange and we do not have any candidates, so we'll pass it from now.

Dark matter comes in two kinds: the first one is basically everything but stars (black holes, planets, dark galaxies etc.) Difficult to see, but made of atoms and just like the stuff we have here. This is the baryonic dark matter, which makes for a small fraction of the total amount. The second kind is not made of atoms, which makes it more interesting. It is further subdivided into particles that behave ultrarelativistically, relativistically and non-relativistically. Possible candidates are neutrinos, axions and WIMPs.

As to the question of why do we not feel it here, we don't know yet... The different kinds of dark matter are all needed for several reasons (cosmological, structural, etc.) but until we know what kind of particles compose it and how were they created we cannot explain their apparent abscence on Earth.

Take neutrinos, for example. A neutrino can pass through a one light year long block of lead with a 50% probability of being undisturbed. We are continously being pierced by millions of them and we do not feel a thing. They do not interact via the electromagnetic or strong nuclear force, which amkes them very difficult to detect. The experiments currently trying to measure the neutrinos coming from the Sun are incredibly complex and big. What I mean by this is that Dark Matter, even its strangest components, may very well be present here, even though we do not detect it.
Click to expand...

Doesn't the test for neutrinos involve heavy water? it makes it spark basically?
 
KingMerv00 said:
If dark matter is so abundant and generates gravity, shouldn't our orbital calculations be off by 20% or so?
Click to expand...


Since the calculations were created before darkmatter became an issue why would it?

Sounds like a deepwater fish asking another fish (if it could talk) if water is so heavy why doesn't it crush us.
 
KingMerv00 said:
.... why don't we feel it in the local environment? Is it because dark matter is in a giant pile elsewhere?
Click to expand...

You can only feel it when it hits the fan. Otherwise, yes it is in a giant pile elsewhere.
 
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KingMerv00 said:
Is it because dark matter is in a giant pile elsewhere?
Click to expand...

I'd hope not, given that many people want to say that the universe is essentially uniform.

IMO, currently dark matter/energy is just like that constant term in a differential equation, where it sops up everything.

THat is, dark stuff are ways to excuse our inadequate theory by sopping up the trouble parts.
 
Ceritus said:
Doesn't the test for neutrinos involve heavy water? it makes it spark basically?
Click to expand...

Yes, but it is more complicated than it sounds.

The first neutrino detector involved the reaction

[latex]$\footnotesize ^{37}Cl + \nu_e \to ^{37}Ar + e^-$
[/latex]

But a detector based on a chlorine solution had some problems, so the next generation used the Cerenkov effect. This is the equivalent of a bullet breaking the sound barrier, but with electrons as bullets and the speed of light instead of the speed of sound.[1] A shockwave is produced and a system of photomultipliers detects the radiation. The most famous detector (SuperKamiokande) is a cylinder with a height and diameter of 40m, filled with heavy water and surrounded by 13000 photomultiplier tubes. Even with so large a tank, very few neutrinos are detected.

This is a very nice article about the solar neutrino problem, solved a few years ago after 4 decades.


_____
[1] You can't go faster than the speed of light in a vacuum, but light is slower in other mediums.
 
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T'ai Chi said:
I'd hope not, given that many people want to say that the universe is essentially uniform.
Click to expand...

The universe is only uniform at huge scales, beyond superclusters of galaxies. A galaxy is obvioulsy inhomogeneous. Dark matter being 'piled up' in some regions of galaxies or clusters wouldn't be any problem for the concept of a homogeneous universe.
 
Dark energy is ~70% of all energy.... just not the energy around us? That is a little hard to believe. More likely our understanding of energy is inadeuate.
 
T'ai Chi said:
Dark energy is ~70% of all energy.... just not the energy around us? That is a little hard to believe. More likely our understanding of energy is inadeuate.
Click to expand...

~90% of the atoms in the universe are hydrogen. However, hydrogen is only 1 ppm in our atmosphere. That is a little hard to believe. More likely our understanding of atoms is inadequate.
 
Yllanes said:
~90% of the atoms in the universe are hydrogen. However, hydrogen is only 1 ppm in our atmosphere. That is a little hard to believe. More likely our understanding of atoms is inadequate.
Click to expand...

Our atmosphere, sure. Not in our solar system.
 
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