ee, 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!