How Far Away is the Farthest Galaxy?

“Master looks after us now, we don’t need you anymore. Leave now and never come back!” –Smeagol, LOTR

You all know how to find the farthest galaxy ever, right? You take the most powerful telescope in the world, put it into space, and have it stare into the darkness for days on end. What do you find?

Image credit: Hubble Ultra Deep Field.

Galaxies! In just a tiny area like this, only about a fiftieth of a single degree on a side, over ten thousand galaxies are visible.

And, as you’ll notice if you click through, and zoom in on a small section of these, some of these galaxies are much dimmer and redder than others.

Sure, you might notice the brighter, closer galaxies at first. But take a look at the left-hand-side of this image. Notice that red “dot” about mid-way down? That — like every other light source in this image — is a galaxy. But it’s very dim and red, not because it’s a dim, red galaxy.

It’s dim because it’s super far away, and it’s red because the Universe is expanding!

How so? Let’s take a look at (roughly) the same field-of-view, but instead of using a visible light telescope, like Hubble, let’s use an infrared space telescope: the ESA’s Herschel!

It’s pretty spectacular, and almost just as good a resolution, too. (Check out an amazing showcase here.) We can zoom in on the same area of this image as we did on the earlier one, for comparison. What do we find?

Look at that! That galaxy — once you find it — which was so dim, red, and point-like with Hubble’s eyes, is now just as visible as the other galaxies around it.

Having difficulty finding it? Let me do you the favor of zooming in, incredibly close, to the area I’m talking about on both images, and presenting them side-by-side.

That’s Herschel’s image on the left, and Hubble’s on the right. (Pretty amazing, isn’t it?) Notice how the dim, red galaxy, as seen by Hubble, appears just as bright and vibrant as all the others through Herschel’s eyes.

Now, what you’re looking at is a galaxy whose light is “only” about 10 billion years old.

But our Universe is about 13.7 billion years old, and we can do much better. (For a fantastic example of how you can do even better, check out the brilliant work that reader Dave W. has done!)

Thanks to the Hubble Ultra Deep Field team, they’ve gone deeper into the Universe than ever before, and managed to pull out a galaxy whose light has been traveling to us for 13.2 billion years. Want to see?

Image credit: NASA, ESA, and the HUDF09 team, Illingsworth et al.

Now, like I said earlier, there’s a big reason why a galaxy like this is invisible to your visible-light eyes: the Universe is expanding.

Let’s provide you with a visual of how this works.

Image credit: Einstein-online.info.

So, that’s the Universe expanding. As you’ll notice, the farther away something is from you, the faster away from you it appears to move. The tricky part, of course, is that it isn’t actually moving!

What’s really going on — according to General Relativity — is that the space between these galaxies is expanding. As light travels through this space, the wavelength of that light gets longer, causing it to shift towards the red end of the spectrum. (What we colloquially call redshift.)

Have a look at this beautiful illustration by Rob Knop.

So we need to look with longer and longer wavelength light, and to be sensitive to ever-fainter light sources, to be able to see it!

I can hear the wheels turning in your mind, wondering about this whole 13.2 billion light years thing. What does that actually mean? I recently got this question from Scott Welty, a high school teacher:

Light from a galaxy 13.2 billion light years away is coming from where? That galaxy is no longer where it was, right? Does the galaxy even exist any longer if we’re seeing it as it was 13.2 billion years ago? I have this problem in general with deep sky pictures. You can’t really take a snap shot of the universe because nothing is standing still and farther away is longer ago!

You can find plenty of illustrations on the web that get it wrong, like this guy. (Sorry, opencourse.info.)

If you learn General Relativity, you can figure out how the separation distance between any two points evolves over time. For this galaxy in particular, let’s take a look at what would happen. (Images stolen from Rob Knop’s above video.)

Back when the light from this galaxy — the light we are seeing now — was emitted, it was “only” about 1.4 billion light years away from us. This is still a tremendous distance, and it’s made all the more impressive by the fact that the Universe is only about 500 million years old at the time!

This should really underscore to you the difference between “this thing is moving away from us faster than the speed of light,” which isn’t happening, and “the space between things is expanding,” which is what’s happening. Let’s take a look at what happens as the light travels towards us, to better illustrate this.

As the light leaves the distant galaxy, the space between that galaxy and us continues to expand! The light is always getting closer to us; it’s never farther than the initial 1.4 billion light years away. (Update: After working it out, galaxies farther away than about 400 million light years, at the time, do have their light get farther away from us before it gets closer to us.) But because of how that space expands, it takes far longer than the expected 1.4 billion years for that light to reach us.

In fact, it takes 13.2 billion years for that light to reach us! When you see figures quoted like “this thing is 13.2 billion light years away,” that’s actually deceptive. What they usually mean is that the light has been traveling for that long. But by time we come to now…

…that distant galaxy is way farther away than 13.2 billion light years! In this example, the galaxy is more like 32 billion light years away, and the light it’s emitting right now will never reach us! (Thanks a lot, dark energy!)

You might ask, then, what’s the greatest distance away a galaxy could be from us, in principle, and still be visible today?

Image credit: xkcd.

The answer is about 46 billion light years, although we know that the Universe doesn’t have galaxies in it that early on. We are about to go hunting for the new record-breakers with the James Webb Space Telescope; we expect to be able to push that record back until we’re possibly seeing the very first galaxies that form in the Universe!

But it’s important to look sooner rather than later, at least on a cosmic timescale. This “redding-out” of distant galaxies that I just described to you is real, and it means that our distant future Universe will look like this,

where the actual Universe is in blue, but the part visible to us is in red. The red sphere is growing ever larger, but the Universe is expanding faster than that. Admittedly, we’re fine for billions of years longer, but that is our eventual fate, as far as we know it. Lucky for us that we live now, in a Universe full of galaxies, near and far!