Universe Expansion and Limits ```Name: Peter Status: other Grade: other Location: NV Country: N/A Date: N/A ``` Question: Dear Scientists! Last night I came up with a problem. The universe is supposed to be infinite. It is also supposed to be expanding. It has been likened to a piece of dough with raisins in it, one of the raisins being us as the observers. As the dough expands, the raisins move away from each other, at a velocity, "the farther away the faster". Which leaves me with the problem that, as the velocity of light is the uppermost speed limit for anything huge and material like stars, there must be a local end to the universe. Otherwise there must be stars traveling with speed of light or faster. Where is the fault in my logic? Replies: Peter, I am not a theoretical astrophysicist, so I am not up to par on all of the latest and greatest theories of the universe, but I will answer your question to the best of my ability. Your idea of a local universe is accurate! Our known universe is only as large as it is old. This is because we can only see the things that have had enough time to have the light from that object reach Earth. Let us say the universe was only 1 year old. This means that we would only be able to see light from objects 1 light year away or closer. This includes our solar system and a few comets etc. The nearest star is about 4.2 light years away, so after 4.2 years on Earth had passed, then we would be able to see Proxima Centuari and so on. Now, let us say that there is also another observer on Proxima Centuari. They would also be able to see us after the same time had passed, but from their view point, they can see things that we cannot. While the radius of both universes are the same, the point of origin (center point of the sphere) is different. Think of this like a Venn diagram (http://en.wikipedia.org/wiki/Venn_diagram) if you have trouble visualizing this. So, yes, we do have a local universe. The other point you bring up, about the supposed problem of star systems approaching the speed of light, is not really a problem at all. The detailed explanation of exactly what is going on here is difficult to explain without having a good understanding of special relativity. (http://en.wikipedia.org/wiki/Special_relativity). Be assured that no massive objects will ever get to the speed of light. What will happen at the very edges of our visible universe can be explained via special relativity. As speeds increase to near the speed of light for massive objects, observers will note changes in time (time dilatation) and length (Lorentz contractions). These are musts in order to maintain the speed of light. But what an observer sees depends on his inertial frame of reference. i.e. are you on Earth viewing a distant star system, that distant star system looking back at Earth, or on a planet somewhere in between? Since the only place that light cannot escape from is a black hole, you will see the light from the object, but it will be red shifted (http://en.wikipedia.org/wiki/Red_shift) due to the Doppler Effect. If you review these concepts, you should start to understand what is going on, though by no means is this an easy topic to fully comprehend! Matt Voss Click here to return to the Astronomy Archives

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