You can define the size of the Universe in different ways, because the Universe has kept expanding since its formation. A light ray from the formation of the Universe that has just now reached us has taken 14 thousand million years (the current age of the Universe) to reach us, but the point from where that ray came has in the meantime gotten much farther away from us because the Universe has kept growing, so that place of origin is now much farther away from us than 14 thousand million lighyears. And if we were to send a ray of light back to that point today, then that ray would have to travel even further still, because during its journey the Universe would expand yet more. Also, there may be much more Universe beyond the boundaries to which we have been able to see so far, but we don't know anything about those regions, and in particular we don't know how big they are. Which distance you can use best for the size of the Universe depends on what you want to use that distance for.
Cosmologists (astronomers who study the structure of the Universe as a whole) usually measure the size of the Universe in a relative fashion, with a scale factor that indicates the size of the Universe compared to its size at a reference time, for which usually today is chosen. They don't say "when the Universe was 7 thousand million lightyears in size", but "when the Universe was half as large as today".
Light coming from the furthest galaxies that we have detected took so long to get to us that it must have started travelling when the Universe was much younger and smaller than it is today. Even though the Universe was much smaller then, the light has still needed all of this time to get to us, because the Universe has kept expanding the whole time. Imagine an ant walking over a balloon that is being inflated. While the ant is walking, the distance to its target increases, so it has to walk a long way even though the target was not so far away in the beginning. The same happens to light rays traveling between galaxies in the Universe.
A source emits a signal in our direction at a certain moment. If the distance between the source and us decreases while the signal travels, then the signal reaches us sooner than if the distance stayed constant. If the distance increases while the signal travels, but not faster than the speed of the signal compared to the source, then the signal reaches us later than if the distance stayed the same. And if we travel faster away from the source than the signal does, then the signal will never reach us. It does not matter what the signal is; it could also be an object.
This holds whether we're talking of someone who tosses an apple to us from a moving train, or of a galaxy that sends a ray of light to us from a great distance in an expanding Universe.
So, if you were to ask how for 14 thousand million years we can stay ahead of a ray of light that was sent in our direction when the Universe was much smaller and we were only 2 thousand million lightyears from the source, then the answer is that the distance between the source and us has been increasing with nearly the speed oflight, so that the ray of light takes a very long time to catch up with us.
Now the question shifts to why the distance between the source and us increases with nearly the speed of light. The answer is that the source is so very far away. Hubble's Law says that the speed at which the distance between two points in the Universe increases is proportional to the starting distance between those points, so you can find any speed of separation that you want if you begin at a sufficiently large separation.
A large speed of separation does not mean that the source moves at that speed compared to its surroundings, but only that the distance between the source and us increases that fast, just like the distance between two marks on a balloon increases when you inflate the balloon, even though the marks do not move compared to their surroundings on the balloon.
The "edge" of the visible Universe is at those locations from where light has taken exactly the current age of the Universe to reach us now. The simplest models give galaxies on that edge a speed of recession (from us) equal to the speed of light, so then the edge travels with those galaxies. A galaxy that was on that edge a long time ago will then still be on the edge today, so then even people from the far future won't be able to receive information from galaxies that are today just beyond the edge. More complicated models (with acceleration or deceleration of the expansion) allow movement of the edge compared to the local galaxies, so then there would be a chance that people from the very far future might receive information about galaxies that are today still beyond the edge.
The simple model fits with the "ant-on-a-balloon" model. It works best if you increase the radius of the balloon at about 1/3 of the speed of light on the balloon. Then the "other side of the balloon" corresponds to the edge of the visible Universe. Faster is OK, too, and then the visible Universe covers a smaller part of the balloon.
The end conclusion is that a source of which light took (for example) 99 % of the age of the Universe to get to us now must lie at 99 % of the distance to the edge of the visible Universe.
More Universe Wallpapers in FullHD
Niciun comentariu:
Trimiteți un comentariu