Recent content by oldGhost1

Thanks phyz. Think that rounds off this thread nicely. Good work both of you.

Just to be clear, does this speed refer to the particles in the halo or those falling towards the galaxy centre? (I would expect the latter to be significantly faster than the former) I wasn't really thinking of it being trapped by somehow impacting on something. I was wondering why it...

Thanks. Yes I hadn't really considered a population of particles with varying speeds, but of course those particles from the outer edge of the halo have a longer journey and so will build up more speed than those which only make it as far as the inner edge of the halo. So these latter DM...

From other threads here I’ve gained a rough understanding of why Dark Matter forms into a halo around galaxies. It makes sense, since DM doesn’t interact and doesn’t clump together, that when a DM particle falls from the halo towards the gravitational centre of the galaxy it gradually...

I’m so glad you’re having this particular discussion bahamagreen. This is one of the issues I’ve been mulling over for a week or so now, trying to find that missing piece. For me the pertinent question was In other words, “in order for the distance between our galaxy and a distant one to...

good works guys. thanks for the quick responses all round. (like the balloonanalogy page too phinds)

I meant more specifically, within a galaxy, where the normal expansion of ‘space’ has been halted. I guess I’m thinking of it like an extremely slow build up of pressure. Just wondered what the time scale of such a (hypothetical) build up would need to be before it could compete with gravity...

And I presume there are no indications that this ‘expansion energy’ could build up within a galaxy over a long enough time and eventually start to have an effect on gravity?

Since first hearing about dark energy in the popular press I’ve always had the impression that we can’t detect it because it’s some exotic kind of energy that doesn’t interact with matter. Is it true that it doesn’t interact with matter or is that just a popular misinterpretation? I mean I...

I'm slightly annoyed with myself that I couldn't quite make you see what I meant. I know this because you've used the word 'displace' in your answer. I think this arose because I used the word 'wake' to describe the space through which the galaxy has travelled. It was not my intention to suggest...

Yes, I feared that might be the case We may be at cross purposes here. What I meant was there will be a small region of space at the trailing edge of the galaxy that has undergone no expansion throughout the last billion years because the galaxy has been passing over it. At the same time the...

Really nice posting Mordred. I get slightly lost on the gravitational redshift section but at least I now have a better understanding of how and when to apply these different flavours of redshift. Can I just ask a couple of supplementary questions related to expansion. I’m afraid they’re...

Believe it or not they’re just the sort of answers I was hoping for. My physics background is at the popular science book level so when I think of a specific problem like this it’s quite often the case that the on-line explanations go over my head. I knew the evidence for the expansion came from...

Forgive me if this is stupid or has been covered many times in the past but I have a question about dark energy and the expansion of the universe. But first, consider a question posed by Poincare in the eighteenth century. He asked, “If we woke up one morning and everything in the universe had...

ok guys, nice one. I think that finally puts this thread to rest. Thanks for the advice and for being so patient.