Thread: how big is the universe?

I know that the known universe is roughly 27 billion light years in diameter, but what about the unknown universe - that is, the universe even beyond what is visible?

I used to assume it just wasn't known how big it was until I came across a chart the other day depicting the rate of expansion of the early universe. It was like this:

Time: size:
10^-40s 1mm
10^-30s 100m
10^-20s Earth
10^-10s solar system
1s 10 Ly

Now I'm not sure how reliably we can extrapolate this chart to today's universe. The rate of expansion has been changing after all. From what I understand according to the inflationary theory, the universe started out by accelerating, then decelerated, and is now into an accelerating phase once again.

But given that physicists seem to have some kind of grasp on its rate of expansion and size at various points in its history, and that we know the age of the universe, can we estimate with any confidence its size today?

I calculated awhile back the maximum possible size for the universe, unfortunately I've lost my result I think it was around 26.4B ly's


the universe ends at what we see, there is no universe beyond what we see (assuming we can see everything, tech is currently too weak to see as far as possible) simply the universe consists only of matter and energy, beyond what we can see, beyond the oldest light in the universe, matter ends, beyond the matter is nothing, no matter, no energy, no up no down, just nothing, you can't measure nothing, nothing doesn't age so has no time and can't exist, nothing isn't part of the universe

Something tells me this is pseudoscience.

Originally Posted by gib65

I know that the known universe is roughly 27 billion light years in diameter, but what about the unknown universe - that is, the universe even beyond what is visible?

I used to assume it just wasn't known how big it was until I came across a chart the other day depicting the rate of expansion of the early universe. It was like this:

Time: size:
10^-40s 1mm
10^-30s 100m
10^-20s Earth
10^-10s solar system
1s 10 Ly

Now I'm not sure how reliably we can extrapolate this chart to today's universe. The rate of expansion has been changing after all. From what I understand according to the inflationary theory, the universe started out by accelerating, then decelerated, and is now into an accelerating phase once again.

But given that physicists seem to have some kind of grasp on its rate of expansion and size at various points in its history, and that we know the age of the universe, can we estimate with any confidence its size today?

Big Bang theorist who are able to estimate the time when the Bang occurred, and its rate of expansion since, would also be able to predict how far it has expanded.

As for my personal view, the BBT just looks like another non-finisher, like the theory of Luminiferous Aether. I think scientists 100 years from now will laugh themselves silly at the idea that we really believed something so ridiculous.


... but the BBT can probably predict the size of the universe.

Originally Posted by Booms

I calculated awhile back the maximum possible size for the universe, unfortunately I've lost my result I think it was around 26.4B ly's


the universe ends at what we see, there is no universe beyond what we see (assuming we can see everything, tech is currently too weak to see as far as possible) simply the universe consists only of matter and energy, beyond what we can see, beyond the oldest light in the universe, matter ends, beyond the matter is nothing, no matter, no energy, no up no down, just nothing, you can't measure nothing, nothing doesn't age so has no time and can't exist, nothing isn't part of the universe

With the same argument you could say that there is nothing beyond the Earth's horizon. What you see at the visible horizon of the universe is its status at a distant past. As far as we know, this horizon has the same distance from any other point in the universe. It is not a real barrier. There is no way telling how big the universe really is.

How big is the universe and what is behind there? You have to complete your question. You can ask me this question and I try to answer.

Originally Posted by kojax

Big Bang theorist who are able to estimate the time when the Bang occurred, and its rate of expansion since, would also be able to predict how far it has expanded.

The going model has it that the BB began in an accelerating expansion, then started decelerating, and only recently started accelerating again. They would have to know not only the current rate of expansion or the average rate of expansion throughout its history, but the exact rates of acceleration/deceleration and the points at which the expansion switched from one to the other.

Originally Posted by kojax

Big Bang theorist who are able to estimate the time when the Bang occurred, and its rate of expansion since, would also be able to predict how far it has expanded.

As for my personal view, the BBT just looks like another non-finisher, like the theory of Luminiferous Aether. I think scientists 100 years from now will laugh themselves silly at the idea that we really believed something so ridiculous.


... but the BBT can probably predict the size of the universe.

using the BBT age estimate, it was done and published a few years back.

http://arxiv.org/abs/astro-ph/0604616

Extending the WMAP Bound on the Size of the Universe
Authors: Joey Shapiro Key, Neil J. Cornish, David N. Spergel, Glenn D. Starkman
(Submitted on 28 Apr 2006)

... A full sky search of the CMB, mapped extremely accurately by NASA's WMAP satellite ... placed a lower bound on the size of the Universe at 24 Gpc. This lower bound can be extended by optimally filtering the WMAP power spectrum...

Journal reference: Phys.Rev.D75:084034,2007
DOI: 10.1103/PhysRevD.75.084034
Cite as: arXiv:astro-ph/0604616v1
Submission history
From: Neil J. Cornish [view email]
[v1] Fri, 28 Apr 2006 18:19:11 GMT (618kb)

I'm told that translates to 78 billion lightyears (minimum) ...

The universe is bigger than the biggest thing you can imagine....and then some!(Douglas Adams)

Originally Posted by bradford28

The universe is bigger than the biggest thing you can imagine....and then some!(Douglas Adams)

I agree 100% with bradford.

the universe is soooooooooooo big and there is no proofn it ends at 27 billion lightyears because it never ends. There Is Absolutely No End!!!

We cannot know how big the universe is. If you take it that we can see over 13 billion light years in opposite directions, that gives a diameter of over 26 billion light years, but it is unlikely that we are in the centre of the universe.

According to the big bang, the universe has an expansion rate of something like 15 mps over a million light years which means the bigger it gets, the faster it expands. If you scale this back, and inflation ended when the size of a cricket ball then after 13.7 billion years of expansion, the universe would now be the size of a football.

its still expanding, so we cant really tell with exact precision, but ill say it would be a scale we cant imagine (not literally)
of course the idea of the expanding universe has been scientifically confirmed by the famous Hubble's analysis of redshift.

Originally Posted by gib65

Originally Posted by kojax

Big Bang theorist who are able to estimate the time when the Bang occurred, and its rate of expansion since, would also be able to predict how far it has expanded.

The going model has it that the BB began in an accelerating expansion, then started decelerating, and only recently started accelerating again. They would have to know not only the current rate of expansion or the average rate of expansion throughout its history, but the exact rates of acceleration/deceleration and the points at which the expansion switched from one to the other.

Don't you just love a theory that can be dialed in to any data set, just by changing the rates of expansion arbitrarily to fit? I think the BB theory of expansion is just an attempt to bypass an apparent violation to the principle of conservation of energy, by proposing a scenario that, if true, .....would also violate the conservation of energy, just in a different way.

Is anyone comfortable with the idea that maybe the universe is infinity big?

Another Douglas Adams quote. (The it in the quote is the Hitchhiker's Guide)
"Space," it says, "is big. Really big. You just won't believe how vastly hugely mindbogglingly 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,........