Originally Answered: Question about the size of the universe?
Several answers here have stated that the universe is 156 (or 158) billion light years across. Wrong. They even give their source, but haven't read it carefully:
The article says this:
"The universe is at least 156 billion light-years wide."
The phrase "at least" is an important one.
All this article says is that the scale of the universe is at least 78 billion light years. If you think of this as a radius and double it, you get 156 billion light years.
Does this mean that the universe is finite? No. Does it mean it's infinite? No. We don't know whether the universe is finite or infinite in volume, although general relativity appears to allow either possibility.
In your question you say that nothing in the physical world can be infinite. As far as we know, however, the universe may be infinite, and we don't know of any physical laws that prohibit this possibility.
Astronomers sometimes talk about the "observable universe." The farthest galaxies we see formed a few hundred million years after the big bang, so those galaxies are not now quite as far as we can in principle see. We see even farther by looking at the cosmic microwave background; but we don't see anything as far back as the big bang, because the light from the earliest era is scattered like sunlight on a foggy day. When you specify the distance of an object in terms of how far it is now, the edge of the observable universe is 46.5 billion light years away.
What's the difference between these two figures -- 46.5 versus 78 billion light years?
The article referenced above is a dumbed-down version of a published article:
There are really a couple of different aspects involved. First, the following factors limit how far we can see:
- the speed of light
- the age of the universe
- the expansion of the universe (and its history over time)
These things limit us to 46.5 billion light years in the observable universe. There may well be very distant galaxies that we cannot see. (The light from these objects has only been traveling for 13.7 billion years, the age of the universe. Because the universe is expanding, however, those objects now lie much farther from us than 13.7 billion light years.)
On the other hand, space itself could be infinite, or could be of finite volume (and curved like a sphere). If it is finite, consider two possibilities:
1) If it is a large sphere, what we see in one direction is completely different from what we see in the other direction.
2) If it is a small sphere, what we see in one direction may overlap somewhat with what we see in the opposite direction. (It's like standing in the US, and seeing the Indian Ocean in the far distance in two opposite directions.)
The published paper linked above investigated this second possibility, and they found no correlations in patterns in the cosmic microwave background on opposite parts of the sky. They concluded the following:
"For a wide class of models, the non-detection rules out the possibility that we live in a universe with topology scale smaller than 24 Gpc."
(24 Gpc is 78 billion light years.)
Once again, they did not say that this is the size of the universe, but that the universe is probably at least this large.
(Incidentally, the authors of the paper say that the universe has a diameter of at least 78 billion light years. The space.com article misinterprets this number as a radius, and that's where the 156-billion-light-year figure comes from.)
- There are two distinct scales involved -- the size of the observable universe, and the size of the whole universe.
- We know the size of the observable universe. We can see things as far as 46.5 billion light years (or a bit less because of the scattering of light in the early universe).
- We don't know the size of the whole universe, and it may be finite or infinite. The authors in the above paper, however, determined that the size of the whole universe is probably not smaller than the size of the observable universe. (In other words, we're not seeing the same object in opposite parts of the sky.)
Finally, whenever you hear that an object is X light years away, be aware that there are different ways of specifying cosmological distances:
1) The light has been traveling for X years. (This is very commonly used.)
2) The object was X light years away when the light was emitted. (This is rarely used.)
3) The object is now X light years away.
These are all very different things, and it is meaningless to specify a cosmological distance without specifying what you mean by that distance. When you read about distances greater than 13.7 billion light years, however, people are usually referring to method #3.