if the universe is infinite which I always thought it was as if there isn't a universe there isn't anything why..

[Ormirian]

Prof Cox insists on talking about hundreds of billions of galaxies. Because even a 1000000000 is a number and it isn't infinity.

He keeps using these BIG numbers but surely all he needs to say is 'an infinite number' or even 'lots and lots'. Both of which are more correct.

AIBU?

Twas very good this week. I liked the analogy if mountains and valleys for gravity and space time.

I don't find it alarming at all - I have always enjoyed the thought that we are only tiny little motes of matter in a huge universe. Quite reassuring really.

My brain is hurting...

Ormirian - we don't know whether the universe is infinite or not is the basic answer. When he's talking about huge numbers of galaxies, he's referring to the observable universe, the bit of it we can theoretically see, which is about 93 billion lightyears across.

The universe may well be much bigger than this, or even infinite, but we cannot know anything about it as light from outside the observable universe has not had time to reach us since the Big Bang.

Kenobi - the universe is not expanding into anything outside of itself. Rather, distances are getting longer as spacetime expands, so everything is getting progressively further away from everything else but this is intrinsic to the universe itself. There is no centre from where this expansion is taking place.

I would say that it's easier to explain this stuff with diagrams and maths but I'd be lying as the maths is really pretty scary too

The process causing the end of the sun's life is dependent on the rate of nuclear fusion within it, which in turn depends on the mass of the sun; it's not going to reach the end of its life because of a potential collision with Andromeda.

It's very likely that, during its red giant phase, the sun will expand into the orbit paths of the inner planets. I should imagine that direct collisions between stars / planetary systems in a collision with Andromeda is less likely because most of the galaxies are empty space; I have no idea, though, how you'd even begin to calculate possible perturbations in orbits due to gravitational effects.

ah ha! Thanks hogsback - 'we don't know' sounds much more sensible to me

kenobi... I'm actually reading a John Gribbin book about just that atm but have had to leave it aside due to coursework (OU)

It's called 'In Search of the Multiverse' and what i've read so far is really facinating stuff (does rather make my brain bleed a bit though!)

Umm, SummerRain (if you've finished your wine yet?), surely not the same mass? Otherwise, what about the tiny mass loss (= huge energy gain) that makes nuclear explosions such a lively event?

LB29 I have no idea! Please don't test me on what I've watched so far .

On yet another point - somebody told me yesterday that the moon is currently the closest it has been to the earth for 27 years. But I thought the moon is very slowly but constantly moving away from the earth - so has it stopped or something? Asked the 'somebody' this and they looked confused and changed the subject

Seeing as we're having big universey questions, I wonder if now would be a good time to ask if we can have Prof Brian Cox on for a webchat?

MooMooFarm: if it's not infinite, then there is no edge - if you keep going in a straight line for long enough, you'll eventually end up where you started.

I would highly recommend the Open University short course www3.open.ac.uk/study/undergraduate/course/s194.htm to any one who interested astronomy. It gives a lot of information without having to go through lots of maths.

DD1 ranted mentioned the other day that you can't use "infinity" or "infinite" to mean going on for ever and ever as in Maths they have a very specific definition. Unfortunately I had lost the will to live was a bit busy and can't remember what you have to say instead.

MooMoo -- for the moon question, I think the reason is that the moon's orbit round the earth isn't a circle but squished into an ellipse, so at its closest to the earth it's around 40 thousand km closer than at its furthest point.

On the other hand, the whole orbit may be broadening outwards slowly by a few cm a year.

I reckon the 40000 km is probably more noticeable.

but I'm meant to be working so my maths is probably vvv dodgy.

hogsback so if you can end up where you started, surely you're going round in a circle, not a straight line?

Lancelottie I am so proud that I actually understand that!

Right! Have ordered The Infinite Book and the multiverss book in an ongoing attempt to understand the incomprehensible. I read some great tome about Quantum physics last year and I would swear I understood it as I turned the page but couldn't remember a bloody word of it 5 mins later

I don't think I am thick but maybe I have the wrong sort of brain.....

It's all mind blowing isn't it. I'm sure that my 9 year old DS gets it more than I do, infact he says 'well it's only a theory' whilst watching. Haven't seen the latest episode, will watch tonight.

Lancelottie... the mass defect during a nuclear reaction is released as energy... it doesn't vanish. In order to split the elements created during nuclear fusion the exact same amount of energy would have to be appplied as was released during fusion. Thus mass and energy are interlinked.

Energy and mass are interchangable (E = mc[squared]) so mass converted to energy is not 'lost' from the universe but is still present and can be converted back to mass at any time.

MooMoo... the moon has an eliptical orbit and at times is further away and at time closer to the earth. I think it's about once a month it passes it's closest point but Saturday was the first time in a long time that occured at a full moon.

I should point out btw I'm no astrophysics expert... this stuff is just fresh in my head from last year OU science course!

MooMooFarm: it's a straight line as far as you're concerned - you have no way of knowing that spacetime has been curved back on itself.

Imagine an ant on the surface of a large balloon. The ant thinks it's walking in a straight line, but eventually it will double back on itself.

hogsback so has it then (curved back on itself)? Am not being deliberately obtuse!

Summer Agree. Lured you back through that door though, didn't it?

MooMooFarm: We don't know if the universe is finite or infinite but if it is finite, then the maths says it will indeed curve back on itself.

If spacetime were curved back on itself, we would not notice it as we have no reference points outside of spacetime to measure against.

Similar to gravity, we don't see that the Earth's mass distorts spacetime, we simply perceive it's effects which we erroneously label as a force.

I once asked the great Sir Patrick if the things he believe now would be believed in 200 years time.

He replied ...the major things would but they may have gone some minor things wrong.

Theres confidence for you 200 years is a long time.

Oh my god I bow down to you hogsback.

And thank you for answering my (probably ignorant - I don't even know ) questions!

To be semi serious moo...you are onto something. I think we know some good questions; but the professionals pretend they know answers.