Ethan Siegel in Starts With A Bang:
With all that the Hubble Space Telescope has done — including staring at a blank patch of sky for weeks worth of time — you might think there’s no limit to how far it can see. After all, what appears to be dark, empty space is illuminated by the light from thousands upon thousands of galaxies, leading to the conclusion that there are hundreds of billions of them out there spanning the entire sky.
In fact, some of these galaxies are so faint and distant that Hubble can barelysee them. But what might surprise you is that there are two reasons Hubble’s limited in what you can see, one reason that’s obvious and one reason that’s much more subtle.
- Obviously: Hubble “only” has a 2.4 meter diameter mirror, meaning it can only gather as much light — as many photons — as that mirror can collect. Even over 23 days, the longest exposure of a region ever taken, that only enables it to see very bright galaxies at the greatest distances.
- Subtlely: the farther out we look in the Universe, the redder any object’s light will appear.
For a little while, this second point is actually a good thing!
You see, when it comes to the youngest, hottest, brightest stars, most of their light isn’t what humans perceive as visible: it’s actually ultraviolet. And as the Universe expands, with galaxies getting farther apart, the fabric of space expands along with it.
This means that photons, the individual quanta of light that exist in this spacetime — emitted from distant stars and galaxies en route to our eyes — get redshifted as well, their wavelengths stretched by the expansion of the Universe itself.