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Author Topic: What is the difference between Dark Matter & Dark Energy ?  (Read 4941 times)

Offline neilep

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Well, the title says it all.

Are the differences akin to ' regular ' matter and ' regular ' energy ' ?


 

Offline Heliotrope

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Well at the moment they are both conjectural fantasy.
However, dark matter is matter that interacts only via the gravitational force. Not by the electromagnetic force. So it emits no light at any wavelength. Hence dark matter.
Dark energy is poorly named. A better name would perhaps be 'vacuum energy'.
It is supposedly the energy of spacetime itself.
It only got the name 'dark' energy because it's an unknown.

 

Offline Soul Surfer

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Dark matter has mass and creates a gravitational attraction that tends to make the universe collapse and contract.

Dark energy is in some respects the opposite of this it tends to make the universe expand.  It is a bit like a lot of energetic photons bashing around the place and tending to speed things up but it can't be photons because we can detect all types of photons. 

As heliotrope suggests dark energy is very much the most speculative feature of cosmologically significant observations of the universe
 

Offline neilep

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Thank you both so very much.

Would it be silly to assume then that Dark matter is something we could collect and put in a jar to observe...ie: is it tangible to us ?
 

Offline Heliotrope

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Assuming it actually exists then yes, you should be able to put it in a jar and poke it with a stick.
The definition of matter is : That which is acted upon by gravity.
Now that doesn't mean to say that things can't be acted upon by anything else also.
Electromagnetic radiation affects some matter. Specifically the matter inside my radiometer on my windowsill etc...
But dark matter is only affected by gravity.
It is still matter though.

Actually thinking about it...
You couldn't put it in a jar because the reason things stay inside jars is because they are kept there by electromagnetic forces.
Electrons bouncing off electrons etc...
The dark matter could only be contained inside a gravitational bottle.
Heard of magnetic bottles ? Magnetic levitation ?
Well we'd have to be able to do the same thing with gravity in order to be able to keep hold of some dark matter to study it.
Otherwise it'd just slip straight down through the Earth and orbit around the centre of mass of the planet blissfully unaffected by the normal matter it was passing through.

Think about this : Neutrinos are almost dark matter.
They only (99.999999999999999999999999999999999999999999999999999999999999999999999% of the time) interact via the gravitational force.
They do occasionally interact with matter but it's exceptionally rare.
So they could be considered an analogue of dark matter to help someone get their head around what the stuff might actually be.
 

Offline lightarrow

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But dark matter is only affected by gravity.
Wait a moment, where did you get this information? Are you sure about it? According to Wikipedia: http://en.wikipedia.org/wiki/Dark_matter
it could also be some simple form of ordinary matter that it's simply not visible (neutrinos and hidden planets, for example).
 

Offline Heliotrope

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As far as I'm aware, and I'll say right up front that obviously I'm not aware of everything, most of the alternate candidates for dark matter have been ruled out. Or as ruled out as they can be when after looking for them you haven't found anything.
Mini black holes, neutrinos, brown dwarfs, MACHOS etc...
A variety of methods have been used to rule them out.
Gravitational lensing, occultation etc... Neutrinos are too massive when you consider their numbers, primordial nucleosynthesis rules out most of the really exotic stuff.

Basically the only candidates for the constituents of dark matter are not covered by the Standard Model.
So we have a couple of options :

1) New physics to find out what it is. Then of course you have to do experiments to prove it. The only theoretical candidate for a new physics is String Theory (in general) and we won't be doing any experimental verification of it for many, many decades to come.

2) Change the existing theories to account for the observations.

Personally I think it may well be a combination of the two with the greater emphasis placed on modification of existing theories.
Of course there are things out there that we don't know about yet which is why I think that part of the cange will come from new particles.

 

Offline Heliotrope

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Oh, most of that information comes from the same article you quoted, by the way.
 

Offline syhprum

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1) New physics to find out what it is. Then of course you have to do experiments to prove it. The only theoretical candidate for a new physics is String Theory (in general) and we won't be doing any experimental verification of it for many, many decades to come.
It has been suggested that mini black holes will be generated by the new CERN accelerator despite the energy being way below what is needed by QED which can be explained by string theory.
 

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