Expert: Courtney Seligman - 11/4/2010

Question
Hi Courtney. When scientists talk of the big bang they say expansion started from a size 'smaller than a spec of sand' or similar statements. Isnt this a nonsensical statement? Size is relative. If the big bang contained everything, there is nothing to compare its size with? 'We' only know the size of the observable universe, not its absolute size? Is it really irrelevant for us to talk of size?  Thankyou very much.

Answer
You are correct in saying that we only know the size of the observable Universe, not its absolute size. It is undoubtedly much larger than the portion we can see, but how much larger depends upon theoretical calculations which cannot be directly verified. Those calculations make predictions about the appearance of the early Universe which seem to be correct, but if we somehow learned the correct answer, I wouldn't be at all surprised if it were "off" by a factor of a thousand. Still, with most estimates of the actual size of the Universe running millions or billions (or even millions of billions) of times larger than what we can see, even a factor of a thousand would still make the actual Universe much larger than the observable Universe.

(I might note that although the expansion of the Universe is making both the actual and observable Universe larger, the fraction that we can see is steadily shrinking. As parts of the Universe now visible to us move away from us faster and faster, those regions now moving away at nearly the speed of light will eventually be carried away at more than the speed of light, making their past observable but their future unobservable.)

As far as the actual size of the early Universe, however, size isn't entirely relative. Even if the early Universe was "everywhere", the fact that the Universe is expanding means that "everywhere" wasn't as large in the past as it is now, and to at least a certain extent, the early Universe must have been very small.

How small depends once again on theoretical calculations. If the most popular theory of the origin of the Universe -- that it arose from a single point of empty space-time in some earlier (but now completely separate) Universe -- is correct, then the starting size would have to be smaller than the "Planck size", which is a decimal point, then more than 40 zeros before you get to anything other than a zero. This is not only smaller than a grain of sand. It is over 20 zeros smaller than an atom.

Whether that theory is correct could be debated, but it does make certain predictions about the appearance of the early Universe that are, as far as we can tell, absolutely correct; so I feel certain that no matter how the Universe started off, it was much smaller than an atom, and the part we can see (being thousands, millions or even billions of times smaller than the actual Universe) was smaller yet. The details of how things got from there to here may prove to be wrong (hence news reports about new observations "surprising" astronomers -- meaning that the accepted theories, though reasonably convincing, aren't as perfect as their proponents would like to suppose), but the overall picture is closer to being proven right than anyone would have thought possible as recently as twenty or thirty years ago.