I’ve looked all over for an answer to this but can’t find anything. Seeing as the SDMB has some of the smartest (read: cleverest/wisest/most opinionated/self assured, etc) people around, I hope someone can help me.
Every time I read a report about the size of the universe they will say that the most distant object is over 15 billion light years away (I can’t remember if that is the actual record holder, I’m just giving an example). This gives the impression that the know universe is over 15 billion light years across. In another report they will say that the universe is over 15 billion years old since the Big Bang (BB). Combined this gives the impression that the farthest objects are the oldest at over 15 billion years.
Now imagine that the universe started as a singular point in the bull’s-eye of a target. At the BB it expanded in every direction. When we are looking at the most distant objects, we have to look past the bullseye. If it is 7.5 billion LY to the bullseye and another 7.5 billion LY to the object, that will give the total distance of 15 billion LY, but the actual age from the bullseye is 7.5 billion LY. That means the universe is not as old as they keep stating.
Furthermore, this assumes that the Milky Way is on the outermost ring of the target. But we know that the Milky Way is not on the outermost ring, there are objects that are moving in the same direction as us but are further away from the bull’s-eye. So when they talk about the size of the universe shouldn’t they take into account the distance from those objects to us as well?
OK, what it really comes down to is: what is the real size (diameter) of the known universe and what is its age? I don’t think the way it has been presented before gives an accurate answer.
WAG From one end to the other?
It has no beginning nor end in the conventional sense as space is curved and possibly reentrant.
Here is another answer, more to what you are looking for:
Some other questions semi-related to this: What, if anything, is at that bull’s-eye now? I mean everything left that point in one hell of a hurry 15 billion+ years ago, but did anything get left behind? Or has anything moved into that spot? When we look at the bull’s-eye, would we see the cosmic equivalent of a crater?
You’re thinking of this in a way that is very common (i.e. we’ve had a million threads like this already) but completely wrong.
Space itself expanded when the universe inflated shortly after it came into existence. There is no center to the universe, no bulls-eye. If there were, that would imply a special reference point, something that is prohibited by the Theory of Relativity. Every point in the universe today is equally privileged, equally far from the farthest point that light could have traveled from the beginning.
All scientists need to do is to figure out which points are receding from us at the fastest rate. That is the radius of the visible universe. Double that and you get the diameter - the size - of the universe.
This would be true no matter where in the universe you were. Counter-intuitive, yes, but that’s the way the universe works.
Or, if you prefer, we and every other point in the universe are in the bullseye. It’s not just matter and energy that came into being in the Big Bang. It’s space and time itself - we’re not moving through space, we’re being carried along by space, if you will. All of the space in the universe now is the product of that original kablooie.
One metaphor that’s useful is to imagine space like a loaf of raisin bread. The raisins - galaxies - end up further apart by the time it’s done rising, but they aren’t moving through the bread dough, they’re moving with it. And it doesn’t make much sense to ask which part of the loaf of raisin bread is the “bullseye” from which the dough originally expanded - the dough started out much smaller than it ended up, but every part of the loaf of raisin bread was originally part of the much smaller glob of dough.
There are two different entities discussed here: The Universe and the known universe. The Universe does not have a center. Or, at least, if it does have a center, the center is not part of the Universe itself.
The known universe, by contrast, is limited by distance from us. It’s just the part of the Universe which we know about (or, alternately, that we can know about). So we are at the center of the known universe. This does not imply anything special about us; another civilization living ten billion lightyears away would have a different known universe, partly overlapping with ours, but mostly different, and with them in the center.
Thanks, but I can’t claim credit. It was just a subtle homage to Bill Waterson. Calvin (of Calvin and Hobbes, of course) once proposed the name “The Horrendous Space Kablooie” to replace “The Big Bang”.
The two “sizes” of the known universe I know of is determined by the speed of light and the beginning of time (i.e. 14.5 billion lightyear radius)
and
with the Hubble Constant: the farther away an object, the faster it is receeding from us, thus, when the receeding velocity equals lightspeed, that is the “edge”…though how far away THAT is escapes me.