Size and age of universe [Archive]

Rajendra_Vaidya

Jun28-04, 05:33 AM

Recently I read that Universe is 156 Billion Light Years in size.
Even if every thing moves at speed of light, a 13 Billion years old universe
should not have radious more than 13 Billion years. Is it Correct?

Can the rate of expansion exceed speed of light?

kuenmao

Jun28-04, 09:08 AM

156 Billion light years in size? Isn't "light year" a measure of distance only? How can that measure size, which is volume? Do you mean cubic light years?
Please clarify your question.

Kurdt

Jun28-04, 12:13 PM

I believe he means the radius of the universe. There is a common belief amongst cosmologists that the universe when first formed underwent a very rapid expansion period that far exceeded the speed of light. It is also known as the inflationary period and lasted approximately 10 to the power negative 12 seconds. http://instruct1.cit.cornell.edu/courses/astro101/lec31.htm

The above link was about the best I found there is a little explaination as to what happened but doesn't really go into detail. I hope it helps you start to find what you are looking for.

arildno

Jun28-04, 12:36 PM

The universe is BIG, mindbogglingly BIG!!
(You may think it's a long way down to the chemist's, but that's NOTHING compared to space..)

Matrixman13

Jun28-04, 01:13 PM

Kurdt, how do you know the universe is a sphere? Recent studies of the critical density in the universe have favored a flat one, not a sphere. I'm not saying that i am definitly right or that you are wrong, just watch what you're saying.

Kurdt

Jun28-04, 02:15 PM

There are of course many theories as to the shape of the universe. A flat universe would imply one of essentially infinite size which would never collapse and never suffer heat death. My use of the word radius does imply a sherical shape, but in terms of imagining the expansion of a 3D object such as the universe a sphere is the logical tool to do so. I was not implying as the spherical model of the universe does that we live on the surface of a sphere.

You must remember that the sphere image is merely a tool to imagine a positively curved 3D universe in a 2 dimenional manner, whereas I was applying a sphere to represent a "boundary" to the universe.

toochaos

Jun28-04, 02:16 PM

do you mean the universe that has matter in it is 156 billion light years big or just in genral, because that would mean we would just hit into a barrier at some point, which makes no sence therfore the universe is infinite because space has nothing (well close to it which would diffuse forever into nothingness) in it therefor it can go on forever.

so the universe cannot have a size because it wouldnt just end

and also the space in which the universe lies cannot have an age because it is just nothingness and neither can the stuff in it because matter cannot be created or destroyed (this is insane isnt it)

Kurdt

Jun28-04, 02:22 PM

Rajendra_Vaidya

Would you lke to quote the source where you found this information and maybe it would aid any further assistance rendered?

marcus

Jun28-04, 02:36 PM

The universe is BIG, mindbogglingly BIG!!
(You may think it's a long way down to the chemist's, but that's NOTHING compared to space..)

arildno this sounds like D. Adams quoting HG to the G
bless his soul, he was a frood who knew where his towell was

marcus

Jun28-04, 02:50 PM

Rajendra_Vaidya

Would you lke to quote the source where you found this information and maybe it would aid any further assistance rendered?

we had an earlier thread at PF about that 156 billion LY
and I gave a link to the technical preprint

the study defined a "topological" size parameter which was supposed to work with a bunch of different geometries and the study was showing that the size parameter must be AT LEAST that amount

it could be 200 or 300 or 500 or infinite

I will see if I can find the old PF thread about this.

--------------

shrumeo

Jun28-04, 03:43 PM

i thought they gave the 156 ly value to the diameter of the "observable" universe

Kurdt

Jun28-04, 05:42 PM

The observable universe is only 14 billion ly in radius as the universe has only been in existence for that long and therefore light from any region beyond that has not had time to get here. The actual value varies as the hubble constant is changed, but its between the limits of 12-15 billion ly anyway.

chroot

Jun28-04, 05:53 PM

Kurdt

Jun28-04, 06:12 PM

Ok, well we have to allow for the expansion of the universe but we only observe the ojects as they were about 12-15 billion years ago so I guess its open to interpretation. In this case I guess it would be appropriate as the question is trying to put some bounds on how large the universeactually is at present. I apologise for my error.

chroot

Jun28-04, 06:19 PM

It's difficult to ask how large is the universe? because it's really open to interpretation. Cosmologists have to be very precise in the way they express sizes to avoid this sort of confusion. :smile: The universe appears to be 13.7 billion light years old, so we can only see objects that are less than 13.7 billion years old, as you said. Those objects might well be as much as 43 billion light years away, however, at the present time.

- Warren

Kurdt

Jun28-04, 06:35 PM

That is true. So it is very safe to say that the lower limit is 14 billion ly and anything after that is an open book. I think its important to note for clarities sake that galaxies very far away are all significantly red shifted. So they are travelling away from us at significant fractions of the speed of light. this is where the values of 40 odd billion ly come from. Then on top of that, if we believe in the inflationary period, there is something else beyond this value again. It would be nice to think that if we ever needed any extra mass and energy for the cosmological constant it could be found in the region we can't see.

All just wishful thinking maybe. :)

chroot

Jun28-04, 06:40 PM

I'll also note, Kurdt, that when cosmologists refer to 'the universe,' they usually mean the more qualified 'observable universe.' Ipso facto, whatever lies outside the observable universe cannot affect us in any way, not even gravitationally.

- Warren

Kurdt

Jun28-04, 06:48 PM

marcus

Jun28-04, 07:08 PM

Hence the wishful thinking comment. I'll be sure to make my posts totally clear in the future in case anyone misinterprets. I was merely trying to establish where the value of 156 billion ly came from because I haven't read the original poster's source. I can't seem to find any reference to it.

here is a link to the original technical paper in arxiv
by Niel cornish]
and some other people
http://www.physicsforums.com/showthread.php?p=244969#post244969

there you will find an arxiv link to the paper and also to a BBC popularization article IIRC, and also a link to Niel Cornish website

his 24 gigaparsec is a "radius-like" figure so you double it to 58 gigaparsec to a "diameter-like" thing and convert parsecs to lightyears and get 156 billion LY. It didnt turn into 156 billion LY until he started talking to reporters and taking interviews

it is pretty abstruse, good luck trying to understand Niel's writing.
the main thing is it is a MINIMUM size figure----the U could be flat and infinite and that is kind of the simplest picture

but if it is finite then it has to be AT LEAST what Niel says, or we would notice it curving around----he and his buddies looked for signs of it curving around in the CMB and were able to rule it out for anything smaller than what he says

Entropy

Jun28-04, 07:30 PM

We know so little about the universe I don't know how you could make a good guess. But I guess we have to start somewhere... I'd say the universe is 20+ billion years old.

chroot

Jun29-04, 12:40 AM

WMAP, among other experiments, has determined the universe is 13.7 billion years old. We don't have to guess.

- Warren

marcus

Jun29-04, 01:21 AM

WMAP, among other experiments, has determined the universe is 13.7 billion years old. We don't have to guess.

- Warren

Yeah Entropy, Warren is right
mainstream professional cosmologists are nearly all using the same
model of the universe
and these days they have arrived at a broad agreement on the observational data to plug into the model

the error bounds have been narrowed way down

so you take the model, which is two Friedmann equations, and you take the data (which since 1998 they are generally very confident in) and put the parameters into the model and it tells you lots of stuff: like the age is 13.7

Up until the mid 1990s the field was much more vague and there were people saying things like the universe is 20+ old, like you said just now.
But mainstream people dont say that any more because it doesnt make sense.

Of course it is your privilege to believe a very fringe thing like 20+
we can all believe what we want.
but since your taxes support the cosmology profession and they have
finally agreed on some basic things about the universe I would advise you to at least try to understand what they are saying and why they are so sure about 13.7

first understand the Friedmann equations, which are nice and simple, and then disbelieve if you are inclined towards nonconformism.
no one can blame you if you understand what you are rejecting

Rajendra_Vaidya

Jun29-04, 05:57 AM

Kurdt & other friends,

The source from which I got the info is space.com

http://www.space.com/scienceastronomy/mystery_monday_040524.html

As I understand our universe is like a bubble and our three dimensional space is the
surface and the thickness of the bubble.
From the illustration shown with the article, 156 BLight Yeras is the width or diameter of the universe.

Rajendra_Vaidya

Jun29-04, 06:06 AM

That's wrong Kurdt, and would only be true if the universe had always been the same size. Of course, it has not always been the same size. The light that was emitted very early did not have very far to travel at all, you see. The "particle horizon," which is what you're talking about, is actually 43 billion light years in radius, IIRC.

- Warren
Chroot,

You have just touched the very core of my question.

With your argument the question that arises is

If the universe was at a single point 13.7 BLY ago,
then how two points in the universe got seperated by say 43 BLYs
in 13 BLYs?

hellfire

Jun29-04, 06:30 AM

If the universe was at a single point 13.7 BLY ago,
then how two points in the universe got seperated by say 43 BLYs
in 13 BLYs?
This is due to expansion of space.

First note that when talking about the radius of the observable universe a proper distance is meant. This is the distance between us and our particle horizon measured with a rod today (which is obviously unphysical according to special relativity).

Consider a radial photon beeing sent from our location at the time of big-bang. The location of the photon at each time is our particle horizon.

If it were no expansion of space, then the distance today between us and our particle horizon would be indeed 13.7 GLyr. But since space is expanding behind the particle horizon (between us and the photon) during this 13.7 Gyr, the proper distance between us and our particle horizon (after 13.7 Gyr) will be greater than 13.7 GLyr.

The actual distance depends on the cosmological model (in general it depends on Omega_matter, Omega_Lambda and the Hubble parameter).

Regards.

Rajendra_Vaidya

Jun29-04, 07:20 AM

This is due to expansion of space.

First note that when talking about the radius of the observable universe a proper distance is meant. This is the distance between us and our particle horizon measured with a rod today (which is obviously unphysical according to special relativity).

Consider a radial photon beeing sent from our location at the time of big-bang. The location of the photon at each time is our particle horizon.

If it were no expansion of space, then the distance today between us and our particle horizon would be indeed 13.7 GLyr. But since space is expanding behind the particle horizon (between us and the photon) during this 13.7 Gyr, the proper distance between us and our particle horizon (after 13.7 Gyr) will be greater than 13.7 GLyr.

The actual distance depends on the cosmological model (in general it depends on Omega_matter, Omega_Lambda and the Hubble parameter).

Regards.
If we were tracking 'This Radial Photon' would it not appeared
to have travelled faster than light?

i.e. More than 13.7BLYs distance in 13.7BLYs.

Kurdt

Jun29-04, 07:33 AM

Here, I believe it is important to note that Einstein's premise that nothing travels faster than light was not broken since the space ITSELF expanded and not matter travelling through it. We can use the inflating balloon analogy for this. Two points on the surface increase their distance from one another as the balloon expands, but neither point on the surface moves at all, it is the space expanding that moves them apart and therfore any matter embedded in spacetime does not violate this dictum.

hellfire

Jun29-04, 07:56 AM

If we were tracking 'This Radial Photon' would it not appeared to have travelled faster than light?
Yes, the Hubble sphere (objects receding at the speed of light at a distance Rh = c/H) is located between us and our particle horizon and therefore objects located at the particle horizon recede superluminally.