Can scientists estimate the size of the entire universe based on the rate of expansion since the Big Bang?

Simple answer: No.

There is the observable part of the universe (those things we can see right now), and there is an unobservable part of the universe. We know the size of the observable universe. Light from the most distant objects has traveled nearly 14 billion light years. If we could get a bird's-eye view of the universe right now, we'd see that those objects are currently roughly 47 billion light years away (more or less).

The question is what else is out there that we can't see? Although the observable universe is finite, cosmologists don't know whether the entire universe (observable plus unobservable) is finite or infinite. The fact that the total energy/mass content of the universe (ordinary matter, dark matter, and dark energy) appears to be near the critical density tends to imply that the universe is flat and infinite; but whether it really is infinite is an open question.

The notion of observable and unobservable is an interesting one. See the article in a recent Scientific American, "The End of Cosmology":

http://www.sciam.com/article.cfm?id=the-end-of-cos...

As the acceleration of the expansion of the universe continues, galaxies that are observable now will become unobservable. In about 100 billion years, it is expected that all galaxies beyond the local group will be unobservable. They will get redder and dimmer to the point of non-detectability. An observer at that time would think that there's nothing beyond the local group (which may have merged into one big galaxy by then), and would have no concept of the vast number of galaxies that he's not seeing. By the same token, we may be unable to see a vast number (or perhaps even an infinite number) of galaxies, and we have no way of estimating how many we're missing.

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You ask about whether the universe can be infinite. Obviously, I don't know whether or not it's infinite, but I do know that cosmologists are undecided on this issue. The best I can do is point you to what others say on this matter.

One thing I should point out, however, is that when astronomers make calculations about the Big Bang (such as early nucleosynthesis), they start from a point of extremely high density and temperature. They do not start from a time of infinite density, because they don't know how to deal with that. If you think about the universe when it had an extremely high density, there is no paradox at all. Either space had a finite volume then and still has a finite volume today, or else space had an infinite volume then and still has an infinite volume today. (There seems to be no debate, however, that the observable part of the universe is finite.)

Astronomer Ned Wright has a web page entitled "How can the Universe be infinite if it was all concentrated into a point at the Big Bang?", exactly the question you ask. He says the universe was not concentrated into a point, but the observable universe was.

http://www.astro.ucla.edu/~wright/infpoint.html

Joe Silk is a prominent cosmologist, and the link below has an interview from 2001. He's asked whether the universe is finite or infinite, and he says "We don't know." See the link for a bit more information.

http://www.esa.int/esaSC/SEMR53T1VED_people_0_iv.h...

The site below has a good summary of the Big Bang model. It says that if space is infinite now, then it was born infinite. If it is finite now, then it was born with zero volume.

http://map.gsfc.nasa.gov/universe/bb_concepts.html

I think part of the problem is that one often sees the Big Bang described as starting from a point. This might be an oversimplification for the popular press. This issue is dealt with in the web site below, which says this:

"Although space may have been concentrated into a single point at the Big Bang, it is equally possible that space was infinite at the Big Bang. In both scenarios the space was completely filled with matter which began to expand. "

http://www.atlasoftheuniverse.com/bigbang.html

there are three answers that i really like (i'm not sure if they are right or not, and they do seem to contradict slightly, AND confirm each other) ...and 2 of the users i cannot spell their names.

we can estimate. Ballpark figures? maybe. the truth is, we really don't actually know. We have observational data of the expansion of the universe, and the acceleration of that expansion. we have no way to observe how constant that acceleration has been.

we also have no way to know, really, how far away the farthest "thing" is, let along space, which is not composed of matter... well, in principal.

if the 13-14 billion year range is accurate for the time period when the big bang occurred, and if the expansion of the universe had a mathematical consistency, be it a constant, an exact acceleration, or whatever, then, possibly, we could determine the radius of the entire universe... but that also depends on which model of the universe we exist in...

open

closed

other

also, the "end of the universe" cosmology could have something to do with it.

will the universe "end" in one of the following ways:

the big crunch

the big chill

the big rip

(other)

we have not determined all of the constants in the universe, nor all of the variables. this means, any estimate we get, would be only dependent on what we can observe,(which, to steal an analogy from another user) is somewhere between the dog and the canary.

Estimated Size Of The Universe

Not really - the observable universe is that we can see given the 14B or so since the big bang. We are at the center of an expanding sphere of

'visible' universe - with time we see more. Nothing we can see suggests the 'edge' of the whole universe...

Conceptualize this:

You are 3 meters away from a dog. And 9 meters away from a cat. There might be a canary somewhere out there, but you cannot see it, let alone measure its distance.

After 1 second, you are 9 meters away from the dog. You are 81 meters away from the cat.

How far away would the canary be, given this rate of expansion, if the expansion began 13.7 seconds ago?

<EDIT>

This also assumes that the "dog, cat, canary univese" was *always* expanding at a constant rate. Which is not likely the case.

Yes, sort of

http://www.space.com/scienceastronomy/mystery_mond...

Quoted from the article:

"Imagine the universe just a million years after it was born, Cornish suggests. A batch of light travels for a year, covering one light-year. "At that time, the universe was about 1,000 times smaller than it is today," he said. "Thus, that one light-year has now stretched to become 1,000 light-years."

All the pieces add up to 78 billion-light-years. The light has not traveled that far, but "the starting point of a photon reaching us today after travelling for 13.7 billion years is now 78 billion light-years away," Cornish said. That would be the radius of the universe, and twice that -- 156 billion light-years -- is the diameter. That's based on a view going 90 percent of the way back in time, so it might be slightly larger."

The observable universe is quite little in perspective to what is really out there. We only see a fraction of the universe, a grain of sand in a never ending beach. Of course there are those who will try and count those grains and "estimate" whats out there and how far it stretches but they wont even come close. he universe holds to many secrets and far to much dimension and holds no time. Its is constantly expanding as nebula's create new stars and black holes destroy them. We can only guess how big it is....in my eyes its infinite and incomprehensible to the human mind.

Yes. And they have done so. The Universe is apparently about 96 billion light years in diameter.

They can always ESTIMATE... In fact, they estimate that the universe, because of the changed rate of expansion, is actually a whole lot bigger than the part we can see.

I read somewhere that somebody stared for awhile at the WMAP blotches and said that the patterns indicated that the ENTIRE universe, as opposed to observable, had a diameter of at least 7,000 billion light years.

If that were the case, and the entire universe originated with a point-source big bang, and it was a sphere with a radius presently at 3,500 billion light years, then some extraordinary numbers are involved. If the sphere started at zero volume and expanded at a constant rate of acceleration for 13.77 billion years, reaching a present radius of 3,500 billion light years, the constant rate of acceleration would be a minuscule 1.84e-7 m/s^2. However, due to the time it is accelerating, its outer rim would reach a velocity of 261 c relative to its starting velocity 13.77 billion years ago.

Do you agree? If not, where have I gone awry?