Question

I have a general question that my physics professor was not sure on. I am looking for any resources that may be helpful. My current understanding is that: energy cannot be created or destroyed; the universe is expanding, but at less than the speed of light in a vacuum; light in space is traveling at the speed of light in a vacuum; and light is energy. This leads me to believe that there is energy(light) "escaping" from our universe. Would this not imply either a destruction of energy or that the "edge of the known universe" is only a human concept and there is no boundary?

Answer 1

And where is the edge of universe??

Answer 2

If there is light anywhere that area will be considered a part of universe.. Universe doesn't only mean area up to where baryonic matter is found.. It even includes area where energy is found in any form..

Answer 3

I am not the one defining the edge of the universe.
So you are saying that the universe has no limits then? Where can I read more on theories that claim that?

Answer 4

May be universe has limits.. but if there's energy, that will lie within the so called "edge of universe"..

Answer 5

but for all of time, the light will propagate outward? I am going to read Feynman's QED book over the summer, as recommended by a professor. I am looking for any other sources that may help me to have a better grasp on this. Thank you, for responding. Any other information/knowledge you have would be much appreciated.

Answer 6

I found this question really interesting...

Answer 7

Thank you again.

Answer 8

First of all, the universe does not have a single velocity of expansion. The velocity with which different parts of it recede from each other is quite different, and generally increases with the distance between them, which is Hubble's Law. For example, if they were not gravitationally attrracted to each other, the Moon would be receding from the Earth by an incredibly tiny 8.7 x 10^-10 m/s, or 2.7 meters per century. On the other hand, ignoring gravity again, the Andromeda galaxy, 2.6 million light years away, would be receding at the substantial clip of 56 km/s. Objects further away are receding still faster.

You can show that objects more than about 14 billion light-years apart should be receding from each other at more than the speed of light. This doesn't violate relativity, since the velocity isn't "real" in the sense that the objects weren't accelerated to that velocity -- it's the space between them that expanded, and is expanding. Do such objects exist? We'll never know, because by definition we can't see them -- the light from them will never reach us. They could be there, and some theories (e.g. the inflationary early universe) say they must be there, but no direct observation will ever show they are. We can regard the distance at which the recession velocity reaches the speed of light to be the edge of our observable universe, since anything outside that distance will never have any effect on us.

But, secondly, your question presupposes that light and the universe can be separated, that they are two distinct things. That they can't be. The universe is, by definition, that which exhibits the phenomenon of light (among other things), so anywhere light is, the universe is, too, necessarily. Light can't escape the universe any more than you can escape your own skin, or thoughts. Where your body and thoughts are, you are, and there's no sense in which 'you" can be somewhere your body and thoughts aren't.

Finally, you are implicity imagining a finite universe embedded in something larger, a "background" of some kind, so that you can be "outside" the universe, looking back at it. But that's a logical problem, because, again, the universe is defined to be everything that exhibits the phenomena (such as light) that allow you to observe. If there is a "background" that would be part of the universe, too.

This is not to say there are not ways in which the universe may be finite. For one thing, it could be that the universe is finite but without boundaries, the way the surface of a sphere is a finite area without boundaries. You can go as long as you like in any direction and never reach an edge -- but the total area is finite. In the same sense, the universe could curve back on itself (obviously in a 4th dimension) so that it has no edge, but still has a finite volume. It is strange to imagine a universe of finite volume -- one is compelled to ask what is "outside" -- but then again, it is equally strange, if you think about it, to imagine a universe that is truly infinite.

Other possibilities are that our observable universe is embedded in some kind of parent universe, which has properties that may or may not differ from ours. These ideas are at the very edge of cosmology, if not philosophy, and unfortunately we have very little experimental guidance in thinking about them.

Answer 9

Thank you very much. I am a physics student, working on my B.S., and am looking for some books or papers to read over the summer. I find all of the information fascinating, but am still reserving judgement until I have more knowledge on these topics. Thank you again, and if you would recommend any good reads, it would be appreciated as well.