Astronomers recently announced that they had the discovered the oldest/farthest known galaxy …. it is 13.2 Billion light years away. This is awesome for a number of reasons.
First on the awesome list is how it was detected. To “see” this galaxy, astonomers used a combination of two factors; one related to an interesting characteristic of galaxies and one related to a feature of the expansion of the universe.
Stars inside galaxies can produce Ultraviolet (UV) light that reacts with hydrogen gas. This means that if light from a distant galaxy passes through a cloud of hydrogen gas on its way to us here on earth, then the UV light emitted by the galaxy will be absorbed by the gas. However, other colours (or wavelengths) of light will pass through the gas undisturbed. To see this effect, all you have to do is look at the galaxy through a variety of coloured filters from red to green to blue and on to UV. The galaxy will appear in red, green and blue filters, but will disappear in, or drop out of, the UV filter since the UV light has been absorbed already by the hydrogen cloud. It’s an easy trick to detect galaxies, which scientists have cleverly dubbed the “dropout technique”.
So now that we know it’s a galaxy, how do we know how far away it is? It turns out the expansion of the universe can lend a hand here. As the universe expands, it carries galaxies along with it, and the movement of galaxies away from us causes the wavelength of the light we see from those galaxies to stretch out and become longer, shifting the light towards the red end of the light spectrum (the “rainbow” is the full visible light spectrum) since red light has a longer wavelength than other visible light. This effect is called “redshift”, and it to light what the doppler effect is to sound. The doppler effect causes sound waves to stretch as an object moves away from us, causing a corresponding drop in the frequency of the sound we hear, an effect we hear when emergency response vehicles pass us and we hear their sirens drop in pitch. The further away a galaxy is, the more its light is redshifted, and if Astronomers know how much the redshift is, they can then calculate this distance.
Putting the two pieces together, Astronomers realized that their new galaxy should drop out of UV filters, but also the light would have been redshifted so the dropout would actually occur at some wavelength shifted away from UV and toward red. If they could figure out the wavelength of the dropout, they would know the redshift and could calculate the galaxy’s distance. This turned out to be a really simple thing to do. Just observe the galaxy through different filters and figure out which on results in the dropout effect! After doing just that, Astronomers were able to calculate the distance of the galaxy as 13.2 Billion light years.
Another reason why this is awesome has to do with what we are really seeing. The universe itself is estimated to be about 13.7 billion years old, so by detecting this new galaxy, we are not only looking as far away as we’ve ever seen before, we are also seeing light that is 13.2 billion years old. To put that in reverse, we are seeing light that was forms when when the universe was only half a billion years old. This is extremely young in universe-years, and this early look should help further our understanding of how the universe came to be, and what its nature is.