What does it mean to say that the universe is a certain age or a certain size?

Question

I've heard people say that the universe is _ years old or _ light years across. What does that mean? What are they measuring with respect to? Does it make any sense? Is there some external measuring system they are using? Its not like there is an "outside" of the universe that you can step into. There is no external time or space. Even within the universe, space and time depend on your frame of reference. Are people kicking Einstein in the crotch when they say stuff like this? :D

Answer 1

"...What are they measuring with respect to?..."
First of all, few if any astronomers and cosmologists speak about the "size" of the universe. To assume the universe has a definite size requires it to have an edge or boundary. There's absolutely not a single shred of evidence that it does. Another reason pros don't talk about the size of the universe is that doing so might imply its size is being compared to some "thing" else that's even bigger. Can't do that either because, again, there's no hard, scientific evidence that such is the case.

The age of the universe is another matter. To begin with, you've got to understand the meaning of the word 'isotropic.' This two-bit word simply means "...the same in all directions." In the case of our universe it's isotropic, meaning that no matter where in it you might be, it looks the same in all directions. For example, we can observe some far distant galaxy and find that it's receding from our galaxy at 5,000 meters/second. But, because the universe is isotropic, if we were in that distant galaxy looking back at the Milky Way we'd measure its recessional velocity as 5,000 meters/second. An astronomer named Edwin Hubble discovered this isotropic feature of the universe in the 1920's

So, the bottom line is that no matter where you are in the universe, and if your observations are precise enough, you'll measure the age of the universe as being about 13.7-billion years.

Yes, it ain't easy stuff to comprehend, but that's the way it all works.

Answer 2

The current best cosmological model of the universe says that the universe is 13.7+-0.2 billion years old. (Billion = 10^9) This is counting the time from the big bang that started the current expanding universe to now. The big bang started with a VERY rapid expansion of space that has slowed down some by now, but the expansion is starting to accelerate again because of dark energy - but that's a different topic. Anyway, that is what is meant by the age of the universe.

Now, although the universe is probably infinite (or at least VERY big), the size of the observable universe is roughly a sphere centered on YOU with a radius of 13.7 billion light years. The reason for this is that any light photon you see could only have been traveling for 13.7 billion years. Now the atom that emitted the photon 13.7 billion years ago that is just hitting your eye now, has gotten further away during the last 13.7 billion light years because the universe is continuing to expand. So, I think, right now it would be about 40 or 50 billion light years away from us. But when we see the light from it, it was only 13.7 billion light years away. Clear?

Answer 3

Interesting question, but the universe does in fact have dimensions that can be measured (although it is in a constant state of change.)

Furthermore, there was a point in time that the universe didn't exist.

Answer 4

It means it had an origin at a certain time in the past and has epanded outward in 4-space to its present size in 3-space

Answer 5

Good point. I guess our only choice is to measure in terms of our own frame of reference.

Answer 6

it means that it originated once ,because it was not there before