Dark Matter

[jomifl]

Is dark matter visible, ie. opaque or reflective or refractive?

[Mamyalynne]

What is the letter count?

[scorpiojo]

And any placed?

[Mamyalynne]

Sorry jomifl, couldn't resist - I believe it is not visible.

[bibblebub]

i don't believe that it exists, it's simply a fudge factor

[excelsior-1]

Dark matter is a type of matter hypothesized in astronomy and cosmology to account for a large part of the mass that appears to be missing from the universe.

Dark matter cannot be seen directly with telescopes; evidently it neither emits nor absorbs light or other electromagnetic radiation at any significant level. It is otherwise hypothesized to simply be matter that is not reactant to light.

Instead, the existence and properties of dark matter are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe.

[Mamyalynne]

But Fudge is visible.

[bibblebub]

only a finger of fudge at a time

[Mamyalynne]

I am prudent in that regard.

[Mamyalynne]

Think the Black hole has got jomifl.

[jomifl]

Glad to that you are all entering into the spirit of de-structuring AB :-).
It could be that we are under-estimating the mass of the matter that we can see because of an erroneous assumption somewhere earlier in the chain of logic.

[bibblebub]

If string theory is correct then there are other dimensions in which mass could reside; we wouldn't see it in the universe as we perceive it but the gravitational effects would be noticed here.

[jomifl]

Bibble, is it theoretically possible for gravity to 'leak' across dimensions? does it leak from space to time? I find it difficult to get my head around that but I suppose that as space and time are interlinked then there must be some 'leakage'.

[bibblebub]

A surface on an object in our universe is 2-dimensional but the object itself is affected by gravity with the result that that surface is affected by gravity i.e. a 2D space in our universe is affected by the gravity effects in the 3D space that surrounds it. Just extend that to >3 dimensions.

[jim360]

"It could be that we are under-estimating the mass of the matter that we can see because of an erroneous assumption somewhere earlier in the chain of logic."

I suppose that's not impossible, although it's hard to imagine such an error getting it wrong by a factor of 20, or about 1900% error. Such would be an enormous mistake!

I think the logic is based on trying to reconcile several different measurements of matter content. You can "weigh" the Universe by weighing the galaxies in a number of different ways, and compare that to a prediction based on, say, General Relativity. And they wildly disagree -- but at the same time all of the calculations are tried and tested in other contexts. After so many years, it's hard to see it being just someone forgetting to carry the one.

[jomifl]

One of the underlying assumptions in astronomy is that the universe is homogenous, is there any evicence to support this assumption or is the need to 'invent' dark matter an indicator that this assumption is wrong?

[jim360]

There's loads of evidence for that assumption, within obvious limits. The agreement is that the Universe is "nearly" isotropic and homogeneous. The evidence for that is primarily in the measurements of the Cosmic microwave background, that shows variation from a universal value at the level of about ten parts per million. So the assumption of homogeneity is almost certainly correct on large scales.

[jomifl]

Thanks jim, it seems I have some catching up to do :o)

[jim360]

You're welcome!

In the most basic sense Dark Matter reflects the fact that physics isn't finished yet. Otherwise we'd have a Standard Model and that would described everything that isn't gravity. Every theoretical model I'm aware of (and presumably several that I'm not) predicts the existence of new particles that would likely represent Dark Matter in some way. It's almost certainly impossible to extend that model in any way that will include Gravity without introducing new particles, in fact, so that we have both a strong experimental basis to theorise its existence, but also (albeit less importantly) a strong theoretical basis.