| MadSci Network: Astronomy |
The statement "The galaxy is 15 billion light years away" does not mean that the galaxy is physically 15 billion light years away from us now, and it does not mean that the galaxy was 15 billion light years away from us when it emitted the light. The statement is really saying how far the light has journeyed: that the light has travelled for 15 billion years to reach us. If the Universe was not expanding, the galaxy would indeed physically be 15 billion light years away. Think of the light like a runner on a race track moving from the starting line to the finish line while at the same time the race track itself is being stretched a bit during the racer's trip. Back when the light was emitted, when the Universe was 5 billion years old, the distance between us and the galaxy was much smaller, because the Universe was smaller. The galaxy's ACTUAL position today could be more like 30 billion light years away. So to address your concern, let's look at a different version of your question, that is, how can a galaxy actually be 30 billion light years away from us when the Universe is only 20 billion years old? (Note that 20 billion years is just an example, it is actually a pretty high estimate for the age of the Universe.) In 20 billion years, light that was emitted at time zero can travel 20 billion light years. But, the crucial point is that the Universe is expanding that entire time. Just like the race track and the runner, space is expanding and the light has had to travel much further than 20 billion light years. In other words the distance has grown since time zero (the Big Bang), and the actual distance the light has travelled can be much farther than the age of the Universe times the speed of light. For a worked example that will help you understand this, look at http://www.astro.ucla.edu/~wright/cosmology_faq.html#ct2