Chris Peterson wrote:FloridaMike wrote:Chris Peterson wrote: ... By the time the Universe was a picosecond old, it was already a substantial fraction of its current size....

In the old days, the inflationary period brought the size of the universe from microscopic to "grapefruit" sized. To the best of my recollection anyway. When did that change?

The increase from its original size to grapefruit sized is many, many orders of magnitude larger than from grapefruit sized to the present size.

Hmmmmm, let me riff on that thought, to use a "beatism".

If by "its original size" you mean zero, well yes, you'd be right. Indeed you'd be right to say the universe's original size was an infinite number of orders of magnitude smaller than a grapefruit. But that gets us into math that involves dividing by infinity, and that way lies insanity, so let's not go there. Let's instead pick a size of about 1 picometre (1 trillionth of a metre) just to get started. (I pick that size only because it's a good deal smaller than any atom.) So, to restate your contention as a question, how close is a grapefruit to the midpoint of the range between 1 picometre and the current size of the universe?

The universe is thought to be about 156 Bly (billion light years) in size now. Let's keep the math simple and call it 100 Bly. A light year is about 10 trillion km. A (smallish) grapefruit is about 1/10th of a metre in size. Putting all that together we can say that the universe is about 1 x 10

^{25} grapefruits in size (unless I've misplaced a zero somewhere, which is almost certain.)

So we need to see if a picometre is more or less than 1/(1 x 10

^{25}) of a grapefruit in size. Well, it's a trillionth of a metre and a grapefruit is a tenth of a metre, so a picometre is about one 100-billionth of a metre, or 1/(1 x 10

^{11}). (When you start writing out "one 100-billionth of a metre", you realize why we use scientific notation instead.)

So a picometre, my arbitrary starting point, is still 1 x 10

^{14} times larger than it would be if a grapefruit were at the midpoint of the range. So we can go a lot smaller still for the starting size of the universe (as indeed we should, of course) before we even get to that equilibrium point, let alone get anywhere near "many, many orders of magnitude" smaller than a grapefruit.

I think you may have overstated your case Chris -- unless, as I mentioned above, you're starting with a size of zero, in which case any quantification is essentially meaningless.

Rob (who is avoiding work)