“People say sometimes that Beauty is superficial. That may be so. But at least it is not so superficial as Thought is. To me, Beauty is the wonder of wonders. It is only shallow people who do not judge by appearances. The true mystery of the world is the visible, not the invisible.” –Oscar Wilde
Beyond the planet Earth, beyond all the stars in the night sky, and beyond the Milky Way galaxy, there’s literally an entire Universe out there.
The farther away we’re able to look, the more galaxies we’re able to see. As far as our instruments have ever taken us, we’ve always found more and more galaxies filling up the darkest depths we’ve ever been able to peer into. Even the darkest, most devoid-of-light areas we can find, if we look for long enough, will eventually reveal these island Universes to our telescopes.
I can even show you the single most distant galaxy we’ve ever found: its name is UDFj-39546284. Its light has been traveling towards us for 13.4 billion years, it’s currently around 33 billion light-years away, and all the matter in the Universe was a mere 370 million years old — or just 2.6% of its current age — when light was emitted from it.
But that’s just the current record holder. Our list of distance records is constantly changing, because the reality is this: we haven’t yet detected the first stars or galaxies in the Universe. Our instruments — so far, at least — simply haven’t been built for it.
This may seem counterintuitive to you, after all. Should you just be able to point your telescope at a location in the sky, and if you point it for long enough, gathering enough light, shouldn’t you eventually see something if there’s something there?
That is, after all, something we’ve done with the Hubble space telescope, and one of the ways we’ve discovered some of the current cosmic record holders. But there’s an intrinsic limit to what something like the Hubble space telescope is going to see. And unfortunately, it has nothing to do with where the most distant galaxy is.
Telescopes, like any instrument, are limited by the laws of physics. In the particular case of a telescope, it’s limited by the size of the primary mirror — or the telescope’s light-gathering-power — and the wavelengths of light that its instruments can detect. You have to pick a wavelength; if you just “get everything”, you’ll simply be swamped by the strongest signal out there. And as you may have guessed, objects have very, very different appearances and properties when you look at them in different wavelengths.
What does this have to do with distant galaxies? It turns out that galaxies in the very early Universe aren’t expected to be all that different from galaxies today. They’ll still be powered by stars, the vast majority of stars will still be fusing hydrogen into helium in their core, and they’ll still give off light the way the vast majority of stars do today. This should be true whether the Universe is 10%, 1%, or even just 0.01% of its current age!
Except for minute details, stars and galaxies work pretty much the same way at all times and at all distances.
But even though the sources of light aren’t all that different from the ones nearby, they have a hell of a journey to make in order to reach us. For one, the Universe is expanding, meaning that the farther away a galaxy is, the more the light will be stretched, or redshifted, due to the expansion of the Universe. The longer the light has to travel in order to reach our eyes, the more time the expansion of the Universe has to affect the wavelength of all the photons coming from it, as this video from Rob Knop, below, demonstrates.
I’ll tell you so what: so if you’re looking for visible light for an object that’s really, really far away, you’re only going to see the light that was in the ultraviolet when it was emitted! All the visible light you were looking for? That’s been shifted into the infrared. So with Hubble — a telescope that was designed for visible light observations — you’re very limited as far as what you can see.
But things get even worse at very large distances.
You see, the Universe, for the first few million years, didn’t consist of any stars at all. It was simply, once it expanded and cooled enough to form neutral atoms, just full of those boring, neutral atoms. It took many millions of years for gravitational contraction to work well enough for the first stars to form, and when they finally did, their starlight had this terrible problem: any direction it tried to go in, it would simply run into neutral atoms. If that’s the light we’re interested in seeing, we’ve only got two options.
But the fact that many, many redshifts conspire together with neutral hydrogen means that practically all of the light is gone by time it gets to our eyes. The ultraviolet light is gone, the visible light is gone, and even most of the infrared light is gone by time it gets to us. The Universe becomes reionized at only about a redshift of 6, while the first galaxies were probably forming between redshift 20-and-30, and the first stars between redshift 50-and-75.
As a result, all the light you’d expect to see above a certain wavelength is effectively “cut off” by this intervening neutral hydrogen, and this cutoff is known as the Gunn-Peterson trough, which is clearly visible in the highest-redshift quasars shown below.
So, we can build a better-and-better infrared telescope to look for these at higher and higher redshift, and we’ll be able to catch progressively earlier and earlier objects. This is the plan for the James Webb Space Telescope, which will be about 100 times more sensitive than Spitzer, currently the record holder for most sensitive infrared telescope of all time. (For more on James Webb, see here.)
But that won’t be perfect, not by any means. If we wanted to truly reach the first objects, we’d have to use a trick that, technologically, we’re just not ready for.
Option 2: we can take light that starts in the infrared, because infrared light is pretty much immune to neutral hydrogen! So that light can leave our very first stars and galaxies, pass through the intervening neutral gas undisturbed, and eventually reach our eyes, redshifted even farther into the infrared, so far that it’s almost (but not quite) into the microwave region of the spectrum!
It’s good that we’re not in the microwave region of the spectrum, to be honest, because there’s a cosmic microwave background that would make this light totally undetectable.
But, could we see this far-infrared light from the early Universe?
Unfortunately, there is a cosmic infrared background, too, and right now we don’t know how to technically deal with it.
Where does this infrared background come from? Believe it or not, the same neutral hydrogen that’s so transparent to infrared light is also very good at absorbing UV and optical light.
Well, like all matter, it’s going to re-radiate that energy away again, and when it does so — just like the Earth does — it radiates it away in the infrared. So the same neutral hydrogen that’s transparent to infrared light is also a tremendous source of infrared light.
And right now, we don’t know — even in principle — how to extract a signal from the truly first stars against this cosmic infrared background.
So when I hear about the newest big news from the distant Universe, or someone asking how far away the farthest galaxy is, it’s great that we’re taking another step forward in our understanding. But let’s not pretend we’ve got the very first one of anything; we’ve still got a long way to go to get there! The most distant galaxy in the Universe is out there, but we’re going to have to really make an investment if we want to find it!