A light-year is the distance light travels in a year, about 6 trillion miles (10 trillion kilometers). An object 12.9 billion light-years away is seen as it existed 12.9 billion years ago, and the light is just now arriving.
Thanks for the link, @zoecello. Pretty awesome article, too bad their distance math is wrong. The object was 12.9 billion light years away from us 12.9 billion years ago. The universe is expanding, and that expansion is accelerating (according to the latest calculations of observed phenomena). A common mistake is for people to make the assumption that because the universe is 13.7 billion years old, it must also be 13.7 billion light years across. WRONG. The universe is about 78 billion light years from tip to tail. So this object is now probably 30 to 40 billion light years away from us now. They are correct in that we’re seeing the object as it existed 12.9 billion years ago. A better explanation would read: “The object is seen as and where it was 12.9 billion years ago, but due to the expansion of the universe the object has moved, and of course undergone 12.9 billion years of evolution, and would now be located closer to 40 billion light years from us. 40 billion years from now, we’ll know what it looks like today.”
Unfortunately, we have no way of actually know how far away it is or if it even exists today. We’re not even completely sure that the galaxy even extends as far as we’ve calculated – you’ve played those video games where you go off one side of the screen and appear on the other, such as Joust – if our galaxy was just big enough, but had edges that looped back in on itself, we’d see the same patch of space, but as it existed back farther in time, then when it looped again we’d see it again, but even further back in time, looking much different than the first time. You’d have a hard time discerning a universe that played by those rules from the universe we observe today. Especially if that universe was spherically warped through the geometry of a higher dimension, so everything appears euclidean because we can only detect in the three dimensions, but in reality would be like the unending surface of a ball.
Would we recognize the solar system if we saw it 100,000 years ago? 1,000,000? 100,000,000? 1,000,000,000? Compared to the calculated size of the observable universe and the current life-span, it wouldn’t need to be a very big ball-surface universe to be completely unrecognizable if we looked back at ourselves. And really, we can barely see other galaxies, barely see nearby solar systems. And you couldn’t look for repeating patterns because of the time factor.
Which brings me to another topic. If the universe is expanding, and the universe’s fabric is spacetime, does that mean time is expanding too? Or is it an inverse function, i.e. time is caused by the expansion of the universe? Gravity affects time and is a function of spacetime geometry. Where gravity is less dense, the universe pulls apart faster and thus time appears to run faster? I suppose it’s within the realm of the possible but I haven’t even the foggiest clue as to whom I should ask about this.
Tags: a pedantic nerd, sci-fi, too, yes this is really how I amuse myself