How do we observe galaxies further than 13.7 billion light years away?

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.

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10 Responses to How do we observe galaxies further than 13.7 billion light years away?

  1. Michael Taylor says:

    If light took 13.7 billion years to get to us from the remote galaxy, that was supposedly far enough away/back in time to be just after the big bang, how did we get HERE from the site of the big bang, in time to observe the big bang light arriving. Our galaxy is only travelling at 300 miles per second, not the 186,001 miles per second needed to stay ahead of the light we have now observed!
    So the light from 13 billion light years away can have nothing to do with the big bang. The light that was connected to the big bang passed us or rather the matter that we coalesced from, within a second of the big bang taking place. That light is far away by now and gone forever, heading outwards.

  2. I am very much an amateur who is interested in this subject, however, to my understanding, it would be incorrect to conceptualise the big bang as originating from a “site”. Theoretically, the universe emerged everywhere and inflated. There wasn’t a site in existence beforehand!

    • Tea says:

      If the big bang emerged “everywhere” and expanded. Isn’t that like saying the universe was already “everywhere” and must be expanding into itself for it was already everywhere. How can the universe expand if it was already everywhere. Everywhere implies that there is nowhere to expand for the universe is already everywhere.

  3. Why 8 billion years ago the speed of the expansion of the Universe was less than it is now? According to the laws of attraction the speed of the expansion of the Universe was supposed to be less now than 8 billion years ago… but the speed of expansion is accelerating as if our “Dwarf Universe” is attracted and cannibalized by other Giant Universes … I’d say from the Local group of Universes from vicinity in the Multiverse! The fact that at “the Edge of the Universe” were found many “adult” galaxies, well formed, instead of young undeveloped galaxies as they were supposed to be in the Early Universe, makes me believe that those galaxies are adult “alien” galaxies from other Universes from vicinity much older than our Universe (that’s why we see those galaxies very well formed), and that is a proof that our Universe is merging with those giant Universes, like Milky Way will merge with Andromeda in the far future… not because the space between galaxies is expanding like the space between those dots from the example with the balloon that it is inflated … It has more sense! It has more sense to think about this than to say that the space in the Local Galaxies does not expand so the galaxies could merge ?!?!?! I also do not agree with the concept that the Universe (and of course the Multiverse) evolved “Out of Nothing” and though Mr Hawking does not believe in God, it is common sense to think that if something appears and evolves Out of Nothing then that is a Miracle, and it is therefore considered to be the work of a divine agency. I strongly believe that the Multiverse is a projection of a hyper dimension of One Super Dimension of Space onto the Space Time Continuum… actually the scientists can’t explain the existence of our 3D Universe without embedding into equation more than 4 dimensions (at least seven), therefore I consider the evolution Top-Down not Down-Up as we are inclined to believe. If most of the people call One Super Dimension of Space (infinite dimensions) God, then that it’s Ok with me… as long as they do not portrait God as the one from the ceiling of the Sistine Chapel!

    • I’m not a physicist but I think that because we already know the Mass of our Universe (that includes the Dark Matter), the speed of expansion and knowing the attraction force that makes our Universe expand we could find the mass of the Universes from vicinity using the Relativistic Generalization of the Newtonian Universal Gravity formula F = G (m1m2) / r2) where m1 is the mass of our Universe, and m2 are the masses of the surrounding Universes that attract and merge with our Universe. I do believe that the projection of the Hyperverse onto Space-Time Continuum created the Multiverse so if you want to name the Hyperverse “God” so be it, I agree with it because the Multiverse is an intrinsic part of the Hyperverse. But if you try to convince people that God and the Universe are two different entities that belong to a Bigger Space then you diminish God, and you lose all your credibility…. like Shirley MacLaine who said in her very successful biography book “Out on a Limb” that she enjoyed the sightseeing of a huge Spiral Galaxy just behind the Moon (though the rest of us could not see from Earth huge spiral galaxies with the naked eyes), and she still does not agree that her experience was nothing more than a banal lucid dream while her mind was wide awake?!?!! There is no difference between Mr. Hawking’s statement that the Universe was created “Out of Nothing” and MacLaine’s vision of the huge spiral galaxy she say just behind the Moon I strongly believe that the Hyperverse is far more important than the Multiverse because without its “Top-Down” projection onto 3D Subspace the Multiverse would not exist. I think from a Hyperverse point of view the beginning, the end and everything in between of the Multiverse …including our thougths, could be perceived at once. I guess in order to get from the Multiverse to Hyperverse it is needed Infinite Energy… like a Square needs Infinite Energy to get the Height in order to become a Cube.

    • HaHa! says:

      @Octavian Peagu .. You people are hilarious, great entertainment… “Local group of universes” and “multiverse”.. LOL.. There is no “loal group of universes” and no multiverse.. There is absolutely not one single minuscule thread of evidence to suggest there’s any other universe other than the one we live in.. Even if, “if”, there was, nobody in our universe would ever know it.. We will never be capable of observing the entirety of this universe, much less anything outside this universe.. The multiverse hypothesis is an idea from science fiction and fantasy in which the evolutionist hopes to find a reason for maintaining the belief in the chance argument as an explanation for the existence of the fine-tuned universe.. It’s pseudoscience and you say “multiverse” like it’s a fact or something.. lmao.. F’ing idiot..

      “Multiple universe theory represents the height of irrationality.”~Richard Swinburne

    • Logan says:

      I just have to point out that agreeing or disagreeing with existence of god is in no way logical. If the universe could not have come from nothing or had to be made, then god could not have come from nothing and thus god had to be made… in this order, there had to be a beginning where there was no god or space/matter or a god or the universe did, in fact, either come from nothing or always existed. At no point is god THE logical conclusion.

  4. AJAY.PANDE says:


  5. Paul J Hamilton says:

    Since we have almost no direct knowledge of anything outside of our solar system and 99% of that is within our own planetary system is it possible that most of the assumptions are either slightly or grossly erroneous? I can talk about something that I have never experienced and can sound quite convincing, but until I’ve been there, it’s all conjecture.

  6. fds says:

    So this means that one of the “flaws” with creation has been resolved, as the universe doesn’t necessarily have to be 13.8 or whatever billion years old for us to be able to see that far.

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