Size of the Universe

One of the follow-on letters from here suggested you could work out the diameter of the universe by multiplying the time since the Big Bang by the speed of light.

Alas, as I understand it the theory is that space-time itself expanded, rather than just stuff inside space-time. The expansion of space-time is not, as far as I am aware, limited to the speed of light.

This hypothesis explains why stars (at least those that are furthest away from us) are all moving away from us, and also why the cosmic background radiation seems pretty uniform rather than all coming from one direction - the afterglow of the big bang is all around us.

That’s right. There are many different ways of calculating the radius of the Universe; one particularly useful measure, the co-moving distance, gives a radius of 46 billion light years.
However this increase in size wouldn’t affect the calculations in the letter referred to by very much.

In the 1970s cosmologist Alan Guth discovered that some problems arising from Big Bang theory
were solved with adoption of the hypothesis that shortly after the BB the universe underwent a
brief period of expansion much faster than the speed of light. Guth named this phenomenon “Inflation”,
and it remains part of standard BB theory today.

Well, the UNIVERSE expanding isn’t matter or light expanding. It is space-time. THAT can expand at any rate, the energy universe being stuck at the somewhat pedestrian velocity of the velocity of light.
Of course, different theories describe space-time in different terms, so we have no math that has consensus for either space-time or what expansion rate is acceptable currently. But, the root is, something without mass or energy (electromagnetic or gravitational) has no real limit in our current understanding of physics.

Matter and energy expanded along with spacetime:

The Cosmological Horizon Problem

(from cite):

During inflation the electroweak and strong nuclear forces were still combined;
gravity had separated from the others earlier.

Well, I managed to misread my own cite here: separation of the SF from EW triggered inflation.

There’s nothing in the big bang theory that states the universe was tiny in size. It’s just that the current known dimensions were contained in a tiny space. The universe could have been much larger, even infinite in size when the big bang occurred.

The question is one of the curvature of the universe. If the universe isn’t infinite in size, it has to curve back upon itself. Therefore, it might be possible that we could observe the same part of the universe while looking in different locations. Imagine a telescope seeing a particular spot in the universe to the west, and then the same spot in the east. There are people looking at the background cosmic radiation looking for such spots.

By the way, there’s also a question on how constant are universal constants. It has been stated that if some of these universal constants were a bit different, matter as we know it couldn’t form which brings us to the problem of the just right universe. To get around this issue, some cosmologists have speculated on parallel universes with different sets of these constants, and we just happen to be in the one with the right constants because if we weren’t, we wouldn’t be here.

However, if the Universe is infinite, it could be possible that various parts of the universe have different sets of constants. Imagine if these values could vary over vast distances in an infinite space – say over tens of billions or even 100s of billions of light years. It could be possible that we live in a part of the universe with the right constants while most of the universe has the wrong values. There has been recent speculatin on the fine structure constant alpha, and whether we actually see differences between one side of the universe to the other.

I remember, back in the 60s, speculation that quasars were round-the-universe views of the Big Bang.

I don’t think the calculation by Dr. Neil Basescu at the end of the article is correct.

He uses 20 billion light years as a “reasonable value for the radius of the [visible] universe”. This is not correct because the universe is expanding. The actual radius of the visible universe is thought to be about 46 billion light years.

So 46 billion light years = 4.35184307e+36 angstroms. Completing the calculation: 0.5 / (2 * 4.35184307e+36) = 5.744692444e-38 hence the 39 places of pi from the original quote.

Mike Musson, Centreville, VA

How can something be both “contained in a tiny space” and “infinite in size”?

And according to this cite you are mistaken about what the theory predicts about initial conditions:

NASA: The Big Bang

(from link):

I remember reading a popular science book which claimed that there was a possibility that the conditions immediately preceding the big bang (if such language is permitted*) were a field of infinite singularities. I don’t recall the reasoning though.

  • Since language is mired in references to space and time, which may be properties of the universe.

Huh… I trust NASA, but I thought the standard scientific spiel was that there is NO point in time where all of space was in one spot, because physics completely breaks down and none of it (including the concepts of space and time) has any meaning until one planck time “after” the Big Bang. Which is rather hard to get your head around, but I know around here we’ve had some physicists mention quite a few times that things like “at the instant of the Big Bang” and such phrases are meaningless.

According to this cite when Inflation began the universe was ~10^-32 seconds in age (pretty young) and ~10^-25 meters in size (pretty small).

During Inflation the universe went from being smaller than an atom to more than a billion light-years in diameter.

Your NASA link does not speculate on the size of the point the universe sprang from. It says it was a “single point”, but, absent any frame of reference, to characterize it as tiny would be incorrect. It is just as correct to say the universe sprang from a point as big as the universe, and just as meaningless.

The size of the universe is 42.

The real issue is that cosmological science has yet to determine whether it’s 42 regular, long, short, portly, or athletic.

Anyone who suggests that in this context that a “point” might be as big
as the universe is a sophist, not interested in reaching the truth, but only
in spinning meaningless rhetorical points.

Apparently the popularizers, including NASA, assume a more well-versed
readership than you, because specifics took some googling to find. Here is
what astrophysicist John Gribbin informs us:

Inflation for Beginners

(from link):