Re: Why will we _always_ be seeing the light (CBR) from the Big Bang?

Andrew,

You ask a very good question, and I think you've almost answered it yourself! Let's walk through the following thought experiment together...

Let's say that we use a high-powered telescope to observe the birth of a galaxy that occurred 1,000,000 years ago. As you pointed out, the instant we look at it, we're seeing light from 1,000,000 light years away. Now let's say we put our telescope away for 100 years and then come back and look at the same galaxy 100 years later. What would we expect to see?

The answer is that we would see a galaxy that is approximately 100 years old (it's a *little* less if you include the expansion of the universe ). So how "old" is the light we're seeing?

This answer is a little tricky. The *light* will still be approximately 1,000,000 years old, because the galaxy is still 1,000,000 light years away. With current values for the Hubble constant, I calculate that you would have to wait about 5000 years before the galaxy would move 1 light year further away. At that point, the light coming from the galaxy would be 1,000,001 years old, but the galaxy would be 5000 years older. Thus, once you've observed it, you can't go "back in time" to see it in the past (at least not with the same telescope).

And that's where the final catch lies: if you get a telescope that can observe the galaxy at a "redder" or longer wavelength, and at a higher power than your first telescope, you may be able to see an "older" version of the galaxy. This is because as the galaxy expands, everything becomes red-shifted away from us and the light actually changes wavelength as it travels towards us. This is why astronomers use microwaves to see the cosmic microwave background radiation (remenants of the Big Bang), because microwaves are at very long wavelengths. But in order to see this "redder" light, you also have to be able to see further back in time, which means your telescope needs more and more power. Astronomers are still pushing the limits there as we speak.

Hope that helps,
Amber

[Moderator's Note: Another possible problem you may be having is that, because of the time changes that Amber mentioned, 1000 years from now we will be seeing the CBR coming from a different point in space. We think of these kind of events as happening in a particular place, but in the case of "recombination" (the event that produced the CBR), the event actually occurred everywhere. So although we see the recombination event as it occurred at a slightly different place, we still observe the event anytime. Check out the link above to learn more.]