z8_GND_5296. Credit: V. Tilvi (Texas A&M), S. Finkelstein (UT Austin), the CANDELS team, and HST/NASA
Sorry for such a lengthy title, but the subject came up because of a widely circulated announcement of the discovery of “The Most Distant Galaxy Yet Seen,” a title that’s being constantly revived (Matt Francis over at Universe Today has a nice discussion of why this galaxy really is a big deal).
The galaxy, which for now goes by z8_GND_5296 was discovered by the CANDELS collaboration at a distance of about 30 billion light-years.
“But wait!” my friends said on facebook and twitter, “how can we even see a galaxy 30 billion light years away when the universe is only 13.7 billion years old? Isn’t light the ultimate speed limit in the universe?”
Yes. It is. But the universe was smaller in the past.
What does “size of the universe” mean?
To begin with, the CANDELS team didn’t actually find that the galaxy was 30 billion light-years away directly. Rather, they found that it had a “redshift” of about 7.5. Or, to put it another way, the universe was about
the size it is now, and we measure that fact by noting that light that left this distant galaxy has grown by a factor of 8.5 in the time it takes to reach us. That’s what redshift is all about. Long wavelengths of light are “redder” than blue ones.
Now for the misconception. Naively you might suppose that if the universe were 11.7% the size it is now at some point in the past, that must mean that the bounds of the universe were 11.7% smaller than they are now. But the universe has no bounds!
Instead, the standard picture is that the universe is much like a balloon, and as it inflates, the distances between galaxies becomes larger by a fixed rate. “Doubling in size” is shorthand for “galaxies getting twice as far apart from one another,” as well as “gas and dark matter becoming eight times as diffuse.” (8 times, because the universe increases in each of 3 dimensions). Here’s a particularly crude version of the whole shebang that Jeff drew for the User’s Guide:
Light and Space
Imagine that, rather than beaming light to us directly, our pal z8_GND_5296 sent us a signal by means of post-stations or whisper down the lane. It sent a signal to a galaxy a few million light years away from it. Galaxy 1 sent a signal to galaxy 2, and so on, until we got the message. Further, imagine that each of those galaxies are currently 10 million light-years apart from one another. This, by the way, is known as their “comoving distance” if you want the technical term.
But the first signal took much less than 10 million years to transmit. Because the universe was so much smaller then than now, it took about 1.1 million years. The next 10 million light-years of comoving distance might be traversed by a light beam in 1.3 million years, and so on, so that after 13 billion years of whispering down the lane, the total distance now between us and z8_GND_5296 is about 30 billion light years.
I warn you, though, even this 30 billion light year number isn’t terribly useful. After all, if I say that it’s 2 miles to the drug store, the implication is that you’ll have to traverse 2 miles to get there. On the other hand, a galaxy 30 billion light years away will be significantly farther by the time you try to reach it. Indeed, there are galaxies out there that, even traveling at the speed of light, we couldn’t ever reach. That’s just a consequence of living in an expanding universe.
What is the maximum distance?
Though we can see to a distance of more than 13.8 Billion light-years, we can’t see infinitely far. The ultimate limit is what’s known as the particle horizon, which for us is a (comoving) distance of about 45 billion light-years. Anything further than that and we have no hope of seeing it.
On the other hand, the universe also has an “event horizon” (yes, just like a black hole), the maximum distance we ever could reach. Because our universe is accelerating, the limit maxes out at a certain point, and there are regions of space that are forever inaccessible to us. Those are systems more than about 60 billion light years from here.
Beyond that is anyone’s guess.
ps If there’s interest, I may do a followup on this as to why it is that cosmologists claim there must be cosmic inflation. It’ll even use Zeno’s paradox to talk about how the smallness of the early universe was trumped by its youngness.