Dark Mattter

[wollery]

Movement is relative, so, if those galaxies are receding away from us at that speed, it is logical tu assume that we are receding from them at that speed... do I have a point? if not, why?

You have to remember that if you observe a galaxy that's 6 billion light years away then you're also seeing it as it was 6 billion years ago. It's observed velocity, relative to us, is therefore not the same as the velocity it has relative to us today, or the velocity that we have relative to it.

 

[Badly Shaved Monkey]

On yer bike

Just a quicky.

I've been talking about bike-riding with the kids and found I was not completely confident on this question.

Why is it easy to balance on a bike that is moving and hard to balance on one that is stationary? Is it because of the conservation of angular momentum of the wheels, i.e. they act like gyroscopes?

 

[Badly Shaved Monkey]

Re: On yer bike

Just a quicky.

I've been talking about bike-riding with the kids and found I was not completely confident on this question.

Why is it easy to balance on a bike that is moving and hard to balance on one that is stationary? Is it because of the conservation of angular momentum of the wheels, i.e. they act like gyroscopes?

Oops! This was meant to be the opener of a new thread. Sorry!

 

[SpaceFluffer]

Well, BSM, your answer is correct anyway.

If you've ever done that experiment where you hold a bicycle wheel horizontally and spin it, you can feel how hard it is to rotate the wheel away from it's axis. Pretty cool.

 

[SpaceFluffer]

Why modifying newton?
Wouldn't genral relativity have to be modified to account for the difference?

Well, sure - but any deviation from an inverse-square law is going to modify Newtonian gravity and everything that came afterwards.

And how is the 50 GeV limit supported?

Simply, if it were any lighter we would expect to have seen it in accelerator collisions already. We've discovered the top quark and that's almost 200GeV in mass.

 

[SpaceFluffer]

Sorry to add to the barrage of questions

Ahhh, it's already a barrage - one more question isn't going to make it any worse. Best be careful though, ten more and it'll turn into an onslaught.

...I think you are confirming that the cosmological constant- dicating the fate of the Universe, and omega- dictating its geometry are independent parameters. Is that right? Or do you just mean that it is geomterically flat in our epoch, but that its geometry may vary with time?

The plot I posted above shows the various possible evolutions of the universe by varying the contribution of the cosmological constant. i.e. the only difference between each line is the amount of dark energy in the universe.
Omega and the cosmological constant are not truly independent parameters. A nonzero CC implies that it must contribute in some way to omega, but by how much is completely arbitary and must be determined via other data.

 

[Carn]

Simply, if it were any lighter we would expect to have seen it in accelerator collisions already. We've discovered the top quark and that's almost 200GeV in mass.

Even if WIMP production had a low cross section?

And how to you judge the detection efficiency for WIMPs?
For this you could probably take neutrino detection efficiency as a estimation, but afaik detection efficiency for neutrinos is not that good.

If both are too low, then we wouldn't see any WIMPs.

Carn

 

[Badly Shaved Monkey]

A nonzero CC implies that it must contribute in some way to omega, but by how much is completely arbitary and must be determined via other data.

I think that's what I needed. Ta!