How Big is the universe?

[pdq1]

Horizon should tell us on BBC2 tonight.

For an amateur couldn't they just use the time since the big bang and the expansion rate using one of Newton's Laws?

[MarkRae]

Space is big. Really big. You just won't believe how vastly hugely mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space.

[bookbinder]

For someone to be able to say how big the universe is, means that they know where the end of it (the back wall, or whatever) is, and nobody can know that. This seems to be a question that can never be answered.

[scowie]

It is generally believed that there is no "back wall". People either believe space is curved like the surface of the earth and if you go far enough in one direction you end up back where you started, or they beleive that is actually infinite in it's extent.

[pdq1]

But I thought it started with the Big Bang (and nothing) and has been expanding ever since?

[glasman]

The less said about 'big bangs' here the better. ED will be on to you.

[Arksided]

If the big bang theory is correct then the Universe will start slowing down its expansion due to matter (Dark matter has yet to be proven exists) so the measure would not be simple.

[Wharton]

If you really want to know what 'Big' is - Google 'Graham's Number'.

[jake-the-peg]

Well no - dark matter is well known to exist we've been measuring it's effect since I was at college and that's far too long a time - it's not clear exactly what it is yet - but that's rather different.

But back to the original question.

Nope sorry but Newton's laws don't stand up to the big stuff!

It's very attractive to think that the Universe is governed by the same rules that take us down to the shops on the Bus but good though they are Newton's laws only cover some of it- when Gravity gets very strong they break down a bit.

One particular example was the orbit of Mercury - if you use Newton's laws it doesn't work out right - you have to come forward a couple of hundred years and use Einstein's General relativity to get that to work.

Einstein also produced a set of so called "Field Equations" that describe the shape of the Universe. There are a number of possible solutions to these - sadly none solved before Einstein died.

Even Einstein's equations aren't really good enough - they don't mesh up with Quantum Mechanics - the laws that govern the very small

When you're looking at the big bang you're looking at the very dense and very small and so we need a new theory of "Quantum Gravity" which is something a lot of people are looking for.

There is also the issue of expansion. For some reason we don't copmpletely understand it seems the Universe expanded very rapidly (faster than the speen of light) it it's very early moments.

There's only really indirect evidence for this but it solves an awful lot of problems and doesn't generate any new ones. So while many people don't like it unless any big problem comes up with it expansion is here to stay.

The Big Bang theory does not require the Universe to be slowing down - Indeed it's rate of expansion seems to be accelerating due to something called "dark energy" - We have much less idea about dark energy than dark matter but it may be that some further modification needs to be made in our understanding of Gravity

All the same the size of the Universe is quite contentious so I doubt Horizon will come up with a satisfactory answer

[pdq1]

Thanks for that answer Jake...its much more complicated than I thought.

///There is also the issue of expansion. For some reason we don't copmpletely understand it seems the Universe expanded very rapidly (faster than the speen of light) it it's very early moments///.

As you point out I believe this was carried out in an earlier Horizon program where they showed a balloon rapidly filling up to represent the universe and called it 'inflation'

Lets hope they make tonights program quite simple.

[JonnyBoy12]

It is really really big. Nearly as big as some people's egos on here.

[Mosaic]

It's so big it's not worth worrying about. It'll take care of itself.

[Arksided]

OP, you may find this interesting:

http://www.youtube.com/watch?v=98KWR_aiK98

[pdq1]

Towards the end of Horizon a woman scientist drew a large triangle in the sand. She also sketched a triangle on a hand held globe which scanned at least two continents.

I'm not sure how this gave the size of the universe but something to see if the universe is infinite?

[Old_Geezer]

If someone does know how big the universe is, it doesn't mean they know of an edge. Merely how much it must have grown since the time when it was an infinitesimal speck. I'd suspect estimates at the moment are not likely to be too accurate.

Since it seems it was finite at some point I do not see how folk can believe it is infinite now. It would imply it underwent infinite acceleration and infinite expansion in at some point in the time since; which does not seem likely.

[jomifl]

If you do the sums re. the visible limits of the universe it will become apparent that objects at the visible limit would have had to travel at the (average)speed of light to get there in the time available since the big bang. This is obviously not likely. The expansion of the universe seems to be caused by the 'fabric' of space expanding. An expansion of space undetectable to us on earth could easily account for it's current size. Those distant objects have been rushing away from us at nearly the speed of light for so long since the light we now see left them that they are now about twice as far as they appear to be.

[Peter Pedant]

Oh God isnt there someone out there who knows how big the universe is, without answering, 'it is really BIG, pdq !'
I can imagine eye-rolling and accompanying arm flailing

If the universe is aged 15 bn years, then the observable universe is 15 light years across. There may be some more beyond that but we dont know.

I was goingto ask the posters whether Hubble 's constant was always constant or did it varywith time ? but Idont think Iwill
because I think thatthe answers I will get :
Ho is really really weeny, Peter
or perhaps
Ho is so weeny it cant get much smaller without being nought.
and you can imagine dingers and thumbs making weeny signs

The books I found good to read and easy to understand
are: a book of infinity - John Barrow
Constants in Nature, are they constant ? also John Barrow
Time travel in Einsteins universe J R Gott

Gott does a bit about the observable universe
and Barrow discusses whether g you know 9.81 m/s2 has always been that.
whcih goes some way to answering your question


PP

[DTCwordfan]

I can't resist; it is just so apropos to the thread.

http://www.youtube.com/watch?v=44DlSj6bnn4

[Nibble]

About 42!!