There are some suggestions that the universe is infinite in extent. If this is granted, what happens to the thesis that it’s 14B +/- years old?
Calculations of the universe’s age seem to be based on “running the film backward”, i.e., visualizing the current (expanding) universe as contracting instead of expanding so as to figure out how long it would take for everything to be scrunched into a singularity and thus extracting its age. However, if the universe extends infinitely then it could never scrunch and, by extension, the Big Bang appears to have occurred infinitely long ago.
The 14b year age for the universe is arrived at starting at the big bang, then modelling the expansion of the universe, gallaxy formation, stella life cycle, etc. It agrees very well with observation, background microwave, redshift, distribution of elements, etc. If the big bang occurred infinitely long old, you have to come up with a new models that match our observations.
Current theory posits that the Universe is “Finite, but unbounded,” a seeming contradiction. As with most things dealing with the limits of our understanding, wrapping your mind around it can be a bit tricky. An effective illustration is to consider the concept of “Space,” as we understand it, “expanding.” The instinctive thought is, “Expanding into what?”
That way lies confusion. Think, instead, of the surface of a sphere, expanding in 3-Dimensional Space. At any given point in time, the Sphere has a known, finite area and radius, but no boundary. You can never see what is “beyond” the expanding sphere, because you are constrained in its two-dimensional “surface.”
Now extrapolate to a 3-Dimensional Space expanding into 4-Dimensional Space-Time. Since it is Space, Itself, which is doing the expanding, there is nothing “beyond” the edge into which it expands. Current theory holds that, like the surface of a sphere, space is curved, and is therefore unbounded. You may travel infinitely far, and never reach the “edge,” because there is no edge to the extent of the universe. Since we are “embedded” in the 3-Dimensional Space, we are much like the entity constrained in the 2-Dimensional surface of the sphere, unable to detect a “limit” to the size of the universe. We can, however, measure the speed of recession of the other galaxies, and determine, with some accuracy, the time at which they must have been in extreme proximity—the “Big Bang.”
Contrary to how it is often used, “singularity” doesn’t mean “point”. It means “domain where the rules break down”.
It’s more correct to think of the Big Bang as a rapid transition from dense to less more dense than a transition from small to big. It’s true that the entire VISIBLE universe was packed into a very small space, but the universe as a whole is much larger than what we can see, and we don’t know how big the universe as a whole was at the moment of creation. Maybe it was infinite in size then and it’s still infinite in size now, only much, much less dense.
There is the Ekpyrotic universe (or colliding branes) theory of the universe that postulates the universe could be infinite in extent. The big bang was a time 17 bn years ago when two “nearby” branes collided. The energy of this collision gave rise to the stuff in the visible universe as we now see it.
There is no conflict between the Universe being infinite and it having a finite age. The finite age does imply that the observable universe is finite, but we have every reason to believe that there’s more of it out beyond the horizon.
If the resident of the surface of a sphere traveled in any arbitrary direction in what he perceives to be a straight line, then he will travel in a Great Circle and eventually arrive at the point he started. This would also imply that a person having sufficient telescope power and long enough to wait for the photons to circumnavigate, he could see the back of his own head. Are you implying that a similar thing could happen in our universe? Granted, it would take a long damn time.
I believe that in some string theory models, where there are extra dimensions beyond the four of space-time, that it is possible to circumnavigate some or all of the additional dimensions.
And further, that’s what is meant when they say “wrapped up small”: that the extra dimensions are geometrically closed and tiny to the point of being sub-atomic in extent.
That doesn’t sound right, since nothing (according to current theories) can travel faster than the speed of light.
Here’s my take on it: The universe is about 13.75 billion years old. The oldest thing we have seen was about 13 billion years old. Presumably, where that object was still exists, and from there an intelligent observer can see things 13 billion years old – but we (or our descendants) will never see them, because they will take more than 13 billion more years for the light to get to our spatial position in the universe.
Isn’t this dependent upon the universe being open or closed? Closed would be analogous the balloon analogy and open is hyperbolic. At least that’s the way it’s been explained to me. I need to watch Krauss’ Universe from Nothing again.
Actually, the expansion of spacetime is not limited to c. Only velocity through spacetime is limited in this way.
In fact, one hypothetical means of working around the c “speed limit” is to ride a bubble of expanding space (link). Most theorists do not believe that the Alcubierre drive could really work, even in theory, but it’s interesting nonetheless.
Yes, but an expansion of space itself is not limited by the speed of light. Space can - and seemingly does - expand, and “points in space” - like galaxy clusters - that are far enough apart can appear to be moving apart faster than the speed of light - the further away they are from one another, the faster - simply because the space between them is expanding.
Yeah, I thought the 13 billion years old thing applied to the observable Universe. All of these other posts are fascinating for me to try and fail to grok, but I think the OP is answered by pointing out the difference between the observable Universe and anything beyond that.
The age applies to the whole shebang, which is plausibly infinite in size. Because the age is finite, though, the portion of the Universe which is observable is finite in size.
Pretend the fabric of spacetime is a moving conveyor belt and a photon from a distant galaxy is an airplane sitting on top of it with its engines running at the speed of light. As the belt accelerates faster and faster, the airplane gets redder and redder until the bearings on its wheels give out and the universe ends.
