How big was the universe immediately after the big bang?

I’ve been Googling this for hours and can’t come up with an answer. I’ve found dozens of pages which indicate that the universe is believed to have been about the size of a grapefruit at the end of the inflationary epoch, but scant information about how fast it grew after that. There’s also no shortage of information about what happened at the subatomic level during roughly the first second of the universe’s existence, but I want to know how big it got, and how fast. I think I read in my college astronomy text that the universe grew to the size of the local group essentially instantaneously. Is that true, and if so, what happened after that? When did the expansion start slowing down, and how big and how old was the universe when this happened?

I don’t think it has.

If the Universe is infinite, as current evidence seems to suggest, then it has always been infinite, at all times after the Bang (how big it was at the moment of the Bang itself is not well-defined). One can talk about the size of the observable Universe, but then you have to be careful about how you define distance, since there are several different ways of defining distance on cosmological scales.

So far as we know, the expansion has never slowed down: It’s still getting faster and faster with time. The rate at which it’s getting faster, however, decreased sharply at the end of the inflationary epoch, somewhere in the vicinity of 10[sup]-32[/sup] seconds after the Beginning.

But if it’s infinite ( I assume you mean in a spatial sense), how can it be expanding? I suspect I’m misunderstanding what you mean by “expansion”.

My limited understanding is that it is not infinite and what it’s expanding “into” simply wasn’t there before.

ETA: I think this means that, at every moment since the Big Bang, the Universe was exactly as big as the Universe. :eek:

Well, if everything keeps getting further apart from everything else(*), then in that sense it’s expanding even if there’s no “edge of the universe”.

(*) At the largest scales, anyway. As we know from the film Annie Hall, Brooklyn is not expanding.

It was this big.


For 20 picoseconds.

Not quite true. The expansion slowed down for the first 8 billion years or so. Then Dark Energy started to overcome the slowing and the rate began increasing again.

Oh, space itself is expanding, yes, forgot about that. Still hard to imagine no ultimate boundary, but it is meaningless to speak of a boundary between “universe” and “nothingness”, I guess.

Arthur Eddington was right: “Something unknown is doing we don’t know what.”

I am repeating information I learned from Brian Greene’s The Fabric of the Cosmos, 2004. I really like this book-even though it has made me really mad at my college physics teachers. Even way back then they left so much out!

The universe is believed to have gone through 3 phases:
Inflationary era lasting for 10e-35 sec. Not very long at all
Standard Big Bang era: up until 7 billion years ago
Accelerated expansion: from 7 billion years up through the present and on into the future.

as for the amount of expansion, it is interesting to note that virtually all the expansion of the universe occurred during the Inflationary era. The universe expanded billions of times more during this era than the subsequent eras. Of course since it started out so small, at the end of the Inflationary era it was still about the size of a grapefruit, but even that was tens of billions of time larger than it’s initial size. As a multiple of the the size of the universe just after the end of the inflationary era, the present universe hasn’t grown very much.

Oh, anyone know how much the universe weighed at the beginning? about 20 lbs.
Explaining that brings in Higgs fields and vaporizing a vacuum and lots of things I don’t understand.

Dr. Greene’s book is fascinating.

Also, at the risk of highjacking this thread, Dr. Greene explains the first three years of the Wilkinson Microwave Anisotropy Probe (launched in 2001) results. These results show amazingly good agreement between the inflationary model of the universe and the present condition of the universe. Wrinkles imposed on the universe by quantum jitters during the inflationary era are now written across the entire universe and can be measured by WMAP and compared to theory. The match is extraordinary.
It is so good, I don’t see how anyone can doubt the inflationary model.

I’m still confused. Let me see if I got this straight. The universe went from essentially a zero-dimensional point to a twenty-pound grapefruit during the inflationary epoch. Then right after that, it went POOF to something at least a few billion LY across? Just to put some nice, round numbers on it, one second after the big bang the universe was probably larger than the observable universe we see right now? The observable universe is 28 billion LY in diameter, and the universe itself is believed to be 78 billion LY in diameter, right? (Can’t remember where I read that, so forgive me if I appear to be pulling things out of my ass here.)

Well, it didn’t all expand from the same point. (double eek)

Okay, what we have here is a failure to communicate, and I ain’t helping much.

  1. The Big Bang Classic expanded from a single point: This resulted in lots of people, such as you and I, to say, “But to reach the current size of the universe you need to project back from its current expansion rate to a rate that significantly exceeded C.”

  2. The Big Bang had multiple expansion points: This resulted in many of us throwing up our hands and switching to theology and homeopathy because they made more sense.

  3. String Theory, which caused some of us to hope that the Wicked Witch was Dead, but which piled crazy upon mindbending crazy. I think it currently holds the crown, but Physics is dead competitive and no hypothesis is guaranteed a place in the Pantheon.

I’ll be in my BA bunk.

Forget it—my head hurts. I just became a YEC theist. I’m going to go take some Oscillococcinum and go to bed. :frowning:

That’s what I did when I was faced with anything past basic Relativity. My wife’s reaction to, “your left front tire is low” gets the response, “That’s why I got married.” Me, I have some idea how full my tires are, but I get an answer that, at best, tells me, “You are too stupid to understand it,” or, “With the proper training you still couldn’t understand it,” and ending with an explanation I could never understand, even with Arthur Godfrey cooing the answers in my ear.

Before the Big Bang, the universe was a singularity, very tiny. After it was no longer a singularity, it went all the way across and still does. Of course, this is bullshit.

On a more pointed point, how do we know that everything isn’t shrinking really fast like zillion flames extinguishing leaving more space between them? I do seem to recall one pill makes you smaller.

And it’s what causes the brains of scientists to turn inside out.

Me, I have enough trouble with a woman I remember from high school turning my brain inside out, except I have chums who are working on it and I generally can’t even get them to come out on Fridays.

Not quite right. The 20 lbs was when it was a zero-dimension point. By the time it was done inflating, it’d put on all the extra pounds we see around us today. Think of it this way: At one time there was nothing but a vacuum. But the vacuum was not just nothingness. It was a froth of virtual particles, randomly coming and going. Eventually about 10 kg of virtual particles all happened to occur at a single point and presto! a universe was born.

However, don’t take this as gospel. It’s just one theory and I’m not sure how well supported it is.

Nowhere near that large, although I don’t know the actual 1-second size off-hand.

When astronomers report that some distant galaxy or quasar is N billion LY away, what they are really reporting is how long the light took to go from that object to us. But remember, we are looking into the past. We see the object as it was N billion years ago. In the N billion years since then, the object has continued to expand away from us, so its current distance is a lot more than N billion LY. The 28 billion is 13.7 * 2 (the universe is about 13.7 billion years old; they probably were using an earlier estimate). The 78 billion is (I’m guessing) the size of the observable universe as it is right now.