The word observable used in this sense does not depend on whether modern technology actually permits detection of radiation from an object in this region (or indeed on whether there is any radiation to detect). It simply indicates that it is possible in principle for light or other signals from the object to reach an observer on Earth. In practice, we can see light only from as far back as the time of photon decoupling in the recombination epoch. That is when particles were first able to emit photons that were not quickly re-absorbed by other particles. Before then, the universe was filled with a plasma that was opaque to photons.
as other have mentioned already, the force of gravity does not propagate faster than the speed of light c. while Newtons laws of motion/mechanics provide an excellent approximation of Einstein's field equations in our non-relativistic setting here on earth and around the solar system in general, he was actually incorrect to assume that gravity was "instantaneous." according to Newton, if the sun were to suddenly vanish, the earth would instantaneously trail off tangentially. but this is not what would happen in reality - in reality, the earth would continue to orbit the location where the sun was for approx. 8.5 minutes, and then it would trail off tangentially. why? b/c it takes ~8.5 minutes for the sun's gravitational affects to reach earth at the speed of light from a distance of 93 million miles.SO I was thinking about how gravity is faster than the speed of light (black holes) then it got me thinking of the universe expanding and all that various jazz. But my real question is, what happens to light when it reaches the border of the universe?
you're statement only holds true in a closed universe, or one with positive curvature. in this scenario, the approximate shape of the universe is spherical, which is why your analogy to the earth works. the problem w/ this is that its only an assumption - we don't know if the universe is closed (has positive curvature), flat (has zero curvature), or open (has negative curvature, in which case the approximate shape of the universe would be hyperbolic, or saddle-like). in a flat or open universe, one would never end up back where he/she started by following a geodesic.Let's assume that you could travel infinitely fast... This means that you COULD go fast enough to reach the edge of the universe. You would eventually wind up back where you started. It is similar of you leave your house and go due north. Eventually, you would hit the north pole and start heading south - to the south pole, and then right back where you started.
SO I was thinking about how gravity is faster than the speed of light (black holes) then it got me thinking of the universe expanding and all that various jazz. But my real question is, what happens to light when it reaches the border of the universe?
That it simply loops back on itself, doesn't ring true to me. As we look outward we're not seeing backwards, we see different objects. When the Webb telescope goes up, it too will take remarkable pictures showing even greater distances, but it won't show the end either. No end, no middle... hard to imagine.
Why isn't gravity instantaneous? If we assume gravity is a wave or particle, then it's not difficult to jump to the conclusion that it's held by the same speed limits of all other radiation...C. But, if it's simply the warping affect that mass has on space-time, why can it be instantaneous. The instant the mass exists, the warpage of space-time exists. It's part and parcel of mass. We really don't have a good grip on gravity at all. It defies the inverse square law and forces cosmologists to devise all kinds of weird contrivances such as multi-verses, membranes where gravitons can pass out of this universe into others where electromagnetic radiation cannot. The near impossibility of finding "gravitational waves" isn't helping matters either.
I am simply not intelligent enough to conceive of a Big Bang expanding basically nothing into an infinite something. My son guided me to this forum after I posed the question about light rays extending outward from the farthest things we can observe in this universe. The Hubble Deep Sky images showed that the farther we see the more aged are the galaxies and quasars. They become more amorphous and clearly less developed. The light from these objects extends out it all directions including directly away from our point of view. My question was "where's it going?" I don't know... that's why I asked it. It's nice to see that others ask the same question.
That it simply loops back on itself, doesn't ring true to me. As we look outward we're not seeing backwards, we see different objects. When the Webb telescope goes up, it too will take remarkable pictures showing even greater distances, but it won't show the end either. No end, no middle... hard to imagine.
There are number of correct things already stated in this thread, but a worrying number of incorrect or inaccurate statements as well. Allow me to try to collect some of the "correct as we know it" statements together:
1) Sunny129 has the way of it with the positive/flat/negative curvature statement. With a simple caveat, we think we know the answer. COBE, WMAP and recently Planck have confirmed that the universe is flat to very high precision. This means that space doesn't wrap around or anything like that.
2) Space is expanding at a rate which is faster than the speed of light. We can see this by observing distant galaxies "moving" away from us faster and faster. For clarification on what I mean by "moving" see the post about the expanding universe.
As long as we're correcting things, I know you didn't mean to say: "Space is expanding at a rate which is faster than the speed of light." but rather: Space beyond the edge of our observable universe is expanding at >c.
As long as we're correcting things, I know you didn't mean to say: "Space is expanding at a rate which is faster than the speed of light." but rather: Space beyond the edge of our observable universe is expanding at >c.
Let's assume that you could travel infinitely fast... This means that you COULD go fast enough to reach the edge of the universe. You would eventually wind up back where you started. It is similar of you leave your house and go due north. Eventually, you would hit the north pole and start heading south - to the south pole, and then right back where you started.
The universe is believed to be "isotropic" which means that it looks pretty much the same everywhere. No center, no edge.
Yes, it hurts your head if you think about it too hard, but you eventually get over it and just accept it.
All it requires is that the universe be infinite in size. If you go in any direction for a certain distance (and they calculate the distance) you will HAVE to encounter a (subset of the) universe that is identical to this one.
They start quite simple. Imagine a universe in which there are only four atoms: A,B,C,D and only four locations for those atoms to exist: 1,2,3,4.
There are only a certain number of ways you could lay out this grid before you had to start repeating a pattern. This would happen in any direction you chose to go. You can derive a formula that says "with X atoms and Y spaces you can go only Z distance in any direction before you are forced to repeat the pattern of 4 atoms in 4 spaces".
(I'm not sure how they get around the obvious flaw in the argument: Who says the rest of that infinite universe has to have any atoms in it at all? Then you could repeat hard vacuum indefinitely without ever duplicating a pattern of atoms. I think there's some condition about large-scale homogeniety.)
Anyway, when you scale it up to universe size, the numbers (while very large) are not infinite. I think the number was something like 10^120 metres.
This means that, given the premise of a universe of infinite size, you could travel (let's say) 10^120 metres in any direction of your choosing, and land your spaceship next to an identical copy of yourself.
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Our life is nothing but a spark in comparison to the age of our Solar System and insignificant within Space and Time - Unless we save this planet and ourselves, we can not even begin.