[sillysaurusx]

But if it was dense and infinite in all directions, why do so many people use language like “it was the size of an apple”? This is the interesting contradiction. An apple is only a certain number of atoms, far fewer than the 10^80 we have today.

[ben_w]

I think that's supposed to refer to the currently observable bit being the size of an apple; though given how bad newspaper analogies are, this may all derive from misunderstandings one hungry physicist 60 years ago and subsequently all quoting each other.

[aziaziazi]

Here’s my take:

- today’s observable universe is roughly 13.8B years. By the time you travel to the edge of that universe, others stuff has travel from that edge to even further position at various speed including faster than you do. Therefore you never get to that edge, making it infinite.

- before/at big bang universe is a single point. You are within this single point but can’t go outside because time does not exist and traveling need time.

- when the big band happened, it grew in size very, very fast, close to speed of light. During that enlargement phase it grew to the size of an Apple and if you were within it and try to look in any direction, the further think you can see is the Apple skin. That’s the observable universe. But to see the skin you need to go there or wait for particule to come to you (eg photon). By that time the universe has already dilated and grown a lot wider. Some particules from the skin are already far away. You’ll never be able to see them : by the time needed for information to come to you, other information goes in the opposite direction.

[orra]

> today’s observable universe is roughly 13.8B years. By the time you travel to the edge of that universe

Careful. That's the age of the observable universe, not the size. The radius of the observable universe is 47B light years, which is larger because of inflation.

> Therefore you never get to that edge, making it infinite.

FWIW, I disagree with the conclusion. Finite but expanding fast is not infinite (disclaimer: mathematician not a physicist).

[consilient]

> today’s observable universe is roughly 13.8B years. By the time you travel to the edge of that universe, others stuff has travel from that edge to even further position at various speed including faster than you do.

This is true but it's not what astronomers mean when they say the universe may be infinite. They're talking about the whole universe, not the just the observable part, and they really do mean actually infinite in extent.

> before/at big bang universe is a single point.

Unlikely. We know that what is now the observable universe was very very very very very dense. Predictions of a literal singular point are the result of extrapolating past the point where we know for a fact that our best physical theories break down.