MadSci Network: Astronomy
Query:

Re: Something about the Big Bang puzzles me.

Area: Astronomy
Posted By: Stephen Murray, Physicist, Lawrence Livermore National Laboratory
Date: Mon Jun 30 14:54:28 1997
Area of science: Astronomy
ID: 866975883.As
Message:

Ah, tricky stuff! You need to account for the fact that, as the light from the distant galaxy is moving towards us, we and the other galaxy are expanding away from each other.

A so-so analogy would be to imagine two airplanes passing each other, traveling at 1/5 the speed of sound, and that you're in one of them. Then imagine that the other plane emits a loud sound when they are one mile past each other. Sound travels one mile in about 5 seconds, and so in 5 seconds, the sound wave will be where you were when the other plane emitted the sound. In the meantime, however, you've moved 1/5 of a mile further along, and so the sound wave has to travel a bit longer to catch up with you. As you move closer to the speed of sound, the sound wave will have to travel for a longer and longer time to reach you.

As I said, though, the analogy isn't perfect. In reality, space itself is what is expanding, and carrying the galaxies "along for the ride" (unless they are so close together that their mutual gravity keeps them from expanding away from each other). Also, unlike sound waves, light has the same speed no matter who measures it. But, the essence of what is happening is the same for the galaxies and the airplanes.

The answer to the second part of your question is yes. If, for example, we see light from a galaxy that took 15 billion years to reach us, then we are seeing that galaxy as it looked 15 billion years ago. In many cases, we see galaxies from very early times in the universe that look remarkably "normal," by which I mean that they seem to have already formed most of their stars and appear much like galaxies today. This has caused trouble for some cosmological theories in which galaxies tend to form at relatively recent times.


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