expanding universe

The non amateur stuff. Hawking, black holes, that sort of thing

Moderators: joe, Brian, Guy Fennimore

cowait
Posts: 15
Joined: Sun Dec 05, 2004 2:26 pm
Location: yorkshire uk
Contact:

expanding universe

Post by cowait »

First off may I apologise in advance if this is a dumb question. All those on here who really understand this stuff will most likely think it is just SO obvious as to be ridiculous to ask in the first place. Whatever, I do not understand for the most part, and pehaps someone will be kind enough to maybe enlighten me.
Ok well. The big bang. Seems plausible I suppose. A few things bothered me about it for some time though. If everything came into existence from a single pin point and then rapidly expanded, and carried on expanding until we have what we have today I had the idea that all matter and stars and galaxies would be expanding away from a central point and all that had come into existence would be arranged something like a thin shell. Like an expanding balloon. Simply by reverse plotting the trajectories of all the expanding heavenly bodies surely the location of the original site of the big bang could be determined. That would also open the question as to how long the big bag went on for and thus how "deep" this expanding shell was.
Well I heard an explanation on the radio which went some way to explaining that this isn't quite what happened but it still left me with much the same questions.
The explanation went something like : At the moment of the big bang it wasn't only matter that came into being, but time and space as well. Therefore matter was not being projected outwards as such. More akin to a cake expanding in a heated oven. It is all there but as it expands everything is merely altering it's relationship to everything else. The question still arises though about plotting back along the paths of moving bodies and arriving at what is in effect the "centre" of the universe. Is this "centre" at a total standstill? With everything else moving away from it either faster or more slowly depending on where in the "cake " it is. Is this in effect the only point in the entire universe that an object could actually come to rest? Is that something that is already known? Is it something we want to know? Do we care?
I hope someone can maybe put me straight on this. Maybe no one knows. If that is the case then it is carry on as before huh?
joe
Site Admin
Posts: 4382
Joined: Fri Dec 03, 2004 11:24 am
Location: Greenwich, London
Contact:

Post by joe »

Hi cowait,

It is a tricky one and it means stepping outside of our normal way of thinking to understand it, like most things when dealing with cosmology.

First of all, if the universe started from an infinitely dense, infinitely small point and then expanded, then the point is still the "same" only bigger, and that means that there isn't a centre as such. Everywhere is the centre! The centre of the universe is where you are, wherever that may be in the universe.

Secondly, the best and easiest way to think about it is the old expanding balloon analogy. The galaxies, or rather galaxy clusters, are certainly moving away from each other on the "surface of the balloon" but no matter where you are on the surface they will always look as if they are moving away from you in all directions. Now the surface of the balloon represents three dimensions, so you will then have to imagine space perpendicular to surface as well to get the proper idea.

You can't plot back along the paths of the galaxies because they will lead to you in england(?) somewhere, and not meaning to belittle England, being a Scot an'all, it probably isn't the centre of the universe. We would seem to be at a standstill simply because everything is moving (sorry expanding, it isn't moving in or through space...it is the space itself that is moving) away from us.

Can't continue because I'm being pressured to acknowledge my guests (some people don't know what's important). If any of this confuses I will try to elaborate.

Best wishes,

Joe
200mm Newtonian, OMC140, ETX90, 15x70 Binoculars.
joe
Site Admin
Posts: 4382
Joined: Fri Dec 03, 2004 11:24 am
Location: Greenwich, London
Contact:

Post by joe »

Sorry for that abrupt ending to my last post and I should also apologise in advance as I am no expert and might be talking nonsense.

Anyway, to put it simply, the expanding universe is like the SURFACE of an expanding balloon, there is no centre. Wherever you are on the expanding balloon it will look like everything is moving away from you in all directions. The two dimensional surface represents three dimensions so you have to also imagine another "balloon" perpendicular to the surface at all points. The expanding cake analogy might work if you think of a cake balloon!! The galaxy clusters are not moving away because of an explosion at a point in space, it is the space itself that is carrying the clusters with it. There is very little matter in the vast spaces between the clusters therefore gravity does not have much of a role but individual galaxies and stars are dense and are therefore affected by the force of gravity which is stronger than the expansion of space. This means that galaxies and stars can be moving in any direction, towards us or away. Asking where the centre of the universe is is a bit like asking where is the centre of the centre of a circle. The whole universe is its own centre, it just expanded to its present size.

Light has been travelling in all directions for the amount of time that the universe has been in existence (13.7 billion years I think at last count) therefore the size of the VISIBLE universe is simply the diameter of that light "bubble" PLUS the amount of expansion in that time. Recent evidence paints a universe where the expansion has been speeding up recently so it is hard to put a figure on it but it is much bigger than 27.4 billion light years. The actual universe is bigger still because space can travel faster than light and if inflationary theories are correct then the visible universe could be just a microscopically, tiny part of the whole. Then there are the other universes.

Hope this helps a little.
Last edited by joe on Mon Dec 06, 2004 11:58 am, edited 1 time in total.
200mm Newtonian, OMC140, ETX90, 15x70 Binoculars.
stella
Posts: 1473
Joined: Fri Dec 03, 2004 2:41 pm
Location: 55° 57'N: 03° 08'W
Contact:

Post by stella »

"so it is hard to put a figure on it but it is much bigger than 27.4 light years".
I certainly hope so, since Arcturus is 36.7 light years distant, and Deneb and Polaris are further than that.
joe
Site Admin
Posts: 4382
Joined: Fri Dec 03, 2004 11:24 am
Location: Greenwich, London
Contact:

Post by joe »

Thank you Stella. The offending phrase has been edited.
200mm Newtonian, OMC140, ETX90, 15x70 Binoculars.
Kaustav
Posts: 214
Joined: Fri Dec 03, 2004 3:24 pm
Location: London, UK
Contact:

Accelerating expansion

Post by Kaustav »

Hi Joe,
Thanks for your explanation. I have read some fleeting news articles on the recent postulation that the expansion of the universe may be accelerating. Do you have any web links to any interesting papers on this? If the expansion is accelerating, then does that mean the past and current speed of expansion has not yet reached the speed of light? If I am not mistaken, nothing can travel faster than the speed of light (unless you're a Tachyon, I suppose!). Would be interesting to read up more on this. Not sure of Simon Singh's book, Big Bang, covers this. I'm on the last chapter now but still no mention of this. Probably out of the scope of the book, though.

Kaustav
Last edited by Kaustav on Mon Dec 06, 2004 11:23 pm, edited 1 time in total.
Kaustav Bhattacharya
>> http://kaustav.uk.com/unisphere/ - An online magazine about Astronomy, Science, Social Media and Society.
>> Follow me on Twitter @jupiterorbit
cowait
Posts: 15
Joined: Sun Dec 05, 2004 2:26 pm
Location: yorkshire uk
Contact:

Post by cowait »

Thanks Joe,
That explanation helps a good deal. As I understand from your explanation we are in a sort of elastic environment which is stretching in every direction all the time. Am I right to assume that stuff which is furthest away from us is travelling faster, than that which is closer? And, if it is accelerating as has been suggested, what is driving that acceleration?
You say space can travel at speeds greater than light and I suppose if it is in effect creating itself and time as it goes then I suppose the rate could be infinite.
About clusters of galaxies and the virtually empty space in between. The universe is clearly not uniform and I understand this is the topic of much debate as to where the "lumpiness" comes from. Is it possible that not one but two, or more big bangs happened all at once, thus creating ripples and eventually the random scattering of matter we see today?
Sorry to keep on with all the questions. Frankly I have dozens of them.

It is something of a shock to discover the truth about England though
Kaustav
Posts: 214
Joined: Fri Dec 03, 2004 3:24 pm
Location: London, UK
Contact:

Dark Energy

Post by Kaustav »

Cowait,

Apparently the expansion acceleration is being driven by Dark Energy. To save me typing anymore, check out http://www.nasa.gov/missions/deepspace/ ... nergy.html
Kaustav Bhattacharya
>> http://kaustav.uk.com/unisphere/ - An online magazine about Astronomy, Science, Social Media and Society.
>> Follow me on Twitter @jupiterorbit
joe
Site Admin
Posts: 4382
Joined: Fri Dec 03, 2004 11:24 am
Location: Greenwich, London
Contact:

Post by joe »

Space further away from us is indeed travelling very fast, faster than light no doubt, but only relative to us. It is still expanding at the same rate as it is near to us. If I can try a little amateur mathematics....! Forgive me, but it helps me too and I'm scared to do this sort of thing as Stella will (rightly) pull me up.

Example; space expands in all directions at 100 miles per second.

Draw a line A____B____C with you standing by A. In one second A expands or travels to B, but during that second, B also travelled at 100 mps to C. From your point of view B would be travelling at 200mps and of course C would seem to be travelling at 300mps. Now imagine C___B___A___B___C . If this was an elastic band and you pulled from each end C, A would stay still, the Bs would move away from A and the Cs would move away even faster from A. It's the same with space. You can therefore see that space very far from you would be travelling very fast.


There are some good books that explain this much better that I could refer to but they are all still in boxes, having just moved.

The ripples that are spoken of come from quantum fluctuations at the time of the big bang. I dare not go near quantum mechanics although it is fascinating. Alan Guth's book "The Inflatonary Universe" is essential reading as far as I'm concerned if you want to find out about this stuff. It's a little dated but it also explains ideas about negative pressure and repulsive gravity, a possible source of dark energy.

Regards,
200mm Newtonian, OMC140, ETX90, 15x70 Binoculars.
stella
Posts: 1473
Joined: Fri Dec 03, 2004 2:41 pm
Location: 55° 57'N: 03° 08'W
Contact:

Post by stella »

Very well explained, Joe. No need to pull you up on anything, except your spelling of 'Inflationary' :lol:
joe
Site Admin
Posts: 4382
Joined: Fri Dec 03, 2004 11:24 am
Location: Greenwich, London
Contact:

Post by joe »

Still haven't got a desk and therefore typing with my elbows on my knees. Causes many typoos!

Best wishes,
200mm Newtonian, OMC140, ETX90, 15x70 Binoculars.
cowait
Posts: 15
Joined: Sun Dec 05, 2004 2:26 pm
Location: yorkshire uk
Contact:

Post by cowait »

Yes thanks again Joe. Even I understood it.
May I impose on your powers of elucidation one more time? Yes? Thanks.
The observable universe is as you put it, within the light “bubbleâ€
joe
Site Admin
Posts: 4382
Joined: Fri Dec 03, 2004 11:24 am
Location: Greenwich, London
Contact:

Post by joe »

Hi cowait,

I think what you might be refering to is something called the Cosmological Principle. This states that; assuming we are not in any spacial place in the universe, observational evidence shows that the universe is isotropic and homogenous, i.e. on the largest scale it looks the same no matter where you are and that it is made up of the same stuff. Scientists have then no reason to believe that things change dramatically just because we can't see them. And we of course believe them. I think also that their calculations about the amount of matter in the universe is based on density, how many protons in a cubic metre, that sort of thing. If there were more galaxies there would need to be more space to house them and the density wouldn't change.

There is a good debate about expansion and contraction of the universe and quite amusing too. Our brains are hard wired to a direction of time and entropy. We could just as well be living in a universe that is contracting and our brains are perceiving everything backwards! We wouldn't be able to tell the difference. :P 8)


Regards,
200mm Newtonian, OMC140, ETX90, 15x70 Binoculars.
cowait
Posts: 15
Joined: Sun Dec 05, 2004 2:26 pm
Location: yorkshire uk
Contact:

Post by cowait »

Joe, Have you read Times Arrow by Martin Amis?
It is an interesting book where everything happens in the reverse. It starts with the main character awakening from his death and then it follows the story of his life right back until he enters his mother at birth. People for example, begin to develop scars and they bit by bit become more vivid until they go into hospital where the surgeons open them up and , for instance, send them back out to a road crash where, in their damaged states they are then placed in the mangled cars. The cars then race away from the crash site to "repair" the damage to the cars and occupants. And so on.
A bit off the topic but an interesting story anyway.
joe
Site Admin
Posts: 4382
Joined: Fri Dec 03, 2004 11:24 am
Location: Greenwich, London
Contact:

Post by joe »

My wife has got it Cowait but I haven't read it yet. Funnily enough a number of writers from Amis's generation have all flirted with new ideas in cosmology and neo-darwinism, writers like Ian McEwan and Julian Barnes. Probably been spending too many late nights together on the brandy.

Regards,
200mm Newtonian, OMC140, ETX90, 15x70 Binoculars.
Post Reply