Physicist: If you ever hear a physicist talking about “the edge of the universe”, what they probably mean is “the edge of the visible universe”. The oldest light (from the most distant sources) is around 15 billion years old. Through a detailed and very careful study of cosmic inflation we can estimate that those sources should now be about 45 billion light years away. So if you define the size of the visible universe as the present physical distance (in terms of the “co-moving coordinates” which are stationary with respect to the cosmic microwave background) to the farthest things we can see, then the edge of the visible universe is 45 billion light years away (give or take). However, that “edge” doesn’t mean a lot. It’s essentially a horizon, in that it only determines how far you can see.
Of course, if you wanted to know “how far can we see?” you would have asked that. The picture of the universe that most people have is of a universe enclosed in some kind of bubble. That is, the picture that most people have is of a universe that has an edge. However, there are some big problems with assuming that there’s a boundary out there.
If you decide that space suddenly ends at an edge, then you have to figure out how particles would interact with it. Obviously they can’t keep going, but bouncing off or stopping both violate conservation of momentum, and disappearing violates conservation of mass/energy. Moreover, if you say that spacetime has a definite edge at a definite place then you’re messing with relativistic equivalence (all of physics works the same in all positions and velocities). It may seem easy to just put an asterisk on relativity and say that there’s an exception where the edge of the universe is concerned, but the math isn’t nearly as forgiving.
The nicest theories today suggest that there is no boundary to the universe at all. This leads to several options:
Three possibilities for a homogeneous (same everywhere), edgeless universe.
1) A negatively curved, infinite universe. This option has been ruled out by a study of the distribution of the Cosmic Microwave Background.
2) A flat (non-curved), infinite universe. The measurements so far (devotees may already know how to do these measurements) show that space is flat, or very very nearly flat. However, infinite universes make everyone nervous. An infinite universe will repeat everything in the visible universe an infinite number of times, as well as every possible tiny variation, as well as every vastly different variation. All philosophy aside, what really bothers physicists is that an infinite (roughly homogeneous) universe will contain an infinite amount of matter and energy. Also, the big bang (assuming that the Big Bang happened) would have had to happen everywhere at once. As bad as the mathematical descriptions of the Big Bang traditionally are, an infinitely large Big Bang is much worse.
3) A curved, finite universe. This is the best option. You can think of the universe as being a 3-dimensional space that is the surface of a 4-dimensional ball, in the same way that the surface of a balloon is a 2-dimensional space wrapped around a 3-dimensional ball. Of course, this immediately begs the question “what’s inside the ball?”. Well, keep in mind that what defines a space is how the things inside it relate to each other (the only thing that defines space is rulers). So even if you turned the “balloon” inside-out you’d still have the same space. Or, if you’re not a topologist, then remember that there’s nothing outside of space, and the surface of the 4-d sphere is space. Now, be warned, the “3-d surface of a 4-d ball” description isn’t entirely accurate. Right of the bat, we don’t live in 3 dimensions, we live in 3+1 dimensions (not “space” but “spacetime”), and the metric for that is a little weird. Also, when you talk about “the shape of the universe”, you probably mean “the shape of the universe right now”, and sadly there’s no way to universally agree on what “now” means in a universe with any rotating stuff in it. That being said, the “surface of a sphere” thing is still a good way to talk about the universe.
Since our best measurements show that space is very flat, if the universe has taken the 3rd “curved, finite” path (it probably has), then it must be really really big. This is for the same reason that you can easily show that a ball is curved, but may have some difficulty showing that the Earth is curved.
Also, to answer the original question: the universe doesn’t have an edge.
couldnt be a bubble with stuff in it, with the surface of the bubble (or edge of the universe) expanding at the speed of light? if that were the case wouldnt it remove the problem of ‘how would particles interact with the edge’ since particles could never reach the edge?
It might.
However, it wouldn’t clear up the mathematical difficulties (as much as they count), and that doesn’t seem to be the way the universe is.
I really enjoy your blog; but does your account of a “A curved, finite universe” still allow the universe to be infinite in extent? Most cosmologist I have read, James Bullock (of Irvine) as well as my research on WMAP shows that it is flat, infinite in extent and finite in volume.
Nasa WMAP reads this: “Thus the universe was known to be flat to within about 15% accuracy prior to the WMAP results. WMAP has confirmed this result with very high accuracy and precision. We now know that the universe is flat with only a 0.5% margin of error. This suggests that the Universe is infinite in extent; however, since the Universe has a finite age, we can only observe a finite volume of the Universe. All we can truly conclude is that the Universe is much larger than the volume we can directly observe.”
http://map.gsfc.nasa.gov/universe/uni_shape.html
Any positive curvature necessarily implies a finite universe. What that quote is referring to is the fact that, whether or not the universe is infinite, we can only see a finite part of it.
When you hear about things like the “size of the universe” what you’re almost always hearing about is the size of the visible universe.
Thank you Physicist. So, when it comes to the actual size of the universe (and not just the observable universe), are the most convincing theories positing that the universe is infinite or finite? Is it right for me to assume that, from everything I hear, it is unknown because it is unobservable, but it is “possibly” infinite in actuality?
Also, does the curvature you suggest agree with the WMAP quote? I’m not a physicist, but a poet, so please bear with me.
Not sure what “.. flat to within about 15% accuracy..” means, but it looks like we more or less agree.
The measurements so far show that the universe is extremely flat, so either it is infinite (definite possibility) or it’s so big that its curvature is undetectable (so far). To do that, the universe would need to be so big we’d need a poet to describe it.
So it has no edge but we know it is flat? If that is the case than it would be bigger in the x and y plane than the z plane. So in that respect w
ouldn’t there be an dge in the z plane?
Furthermore, in order to know or even to implied or guess that the universe is flat than you have to have some reason why you think one plane is smaller than the other 2. That could only be found by taking some reading of both edges of one plane and not the others.
Example: if you are in a fish tank and a 1ft cube of water is all you can detect you could say that the water is infinite in every direction. However if on one side you detect the glass of the tank than you know it is not infinite in all directions. Now to think it is flat you would have to detect a barrier on 2 opposite sides of the tank.
Just seems that saying it is flat but there is no edge is a paradox.