It's not easy to figure out how far away other galaxies are. Astronomers have to be exceptionally clever to calculate distances to objects that are billions and billions of light years away, and they might now have a brand new measuring tool that involves supermassive black holes and radiating gas clouds. Sexy.
There's really only one way to measure the distance to an object that's astronomically far away, and that's by using the only information about other stars and galaxies that makes it all the way to Earth: light. Far away objects are dimmer than close objects, so if you know how bright something is, you can get a sense of how far away it is. This only works, however, if you already know what the intrinsic brightness of the object is. Otherwise, there's no way to tell whether you're looking at something very dim and very close, or very bright and very far away.
There are a few different astronomical phenomena with known brightnesses, and if we spot one of them, the ratio of its intrinsic brightness (or luminosity, in astro-speak) to its apparent brightness can be used to figure out how far away it is. Astronomers call these phenomena "standard candles," and they're incredibly important to cosmologists. One such candle is a Cephid variable star, which has a measurable rotational period that's directly related to its luminosity. Edwin Hubble used distance measurements from Cephid variables to prove that the universe is expanding, so that was kinda important. Another type of standard candle is the type 1a supernova, caused by white dwarf stars which always explode in the same way. Type 1a supernovae have been used to prove that the rate of expansion of the universe is accelerating. Again, kinda important.
So, it's a pretty big deal when astronomers find a new type of standard candle, especially if it lets them measure even greater distances. Researchers at the Dark Cosmology Centre at the University of Copenhagen have discovered a new relationship between supermassive black holes that live at the center of galaxies and the brightness of the gas clouds surrounding them that may be able to be used as a standard candle that can measure distances out to, well, I'll explain how freakin' far away it is in just a second here.
Anyway, these black holes are absolute monsters, and they emit huge amounts of radiation, which we can measure. This radiation causes the clouds of gas around the black hole to light up, which we can also measure. There's a tight relationship between the luminosity of the gas cloud and its distance from the black hole, and using a fancy spectral technique called reverberation mapping, we can figure out this distance. The distance gives us the intrinsic brightness of the cloud, and bam, we can use its apparent brightness to solve for distance.
The black holes and gas clouds that we're dealing with here are known as active galactic nuclei (ANG), and they're some of the brightest objects in the entire universe. This means that they can be seen from really, really, really far away. Until now, the farthest objects we could reliably measure (using type 1a supernovae) were about 11 billion light years away. Using this ANG method, it may be possible to measure distances (and look back in time) out to just 750 million years after the birth of the universe, which could suggest new theories of gravity and possibly even explain dark energy. Each time a new standard candle has allowed us to look back into the past farther and more accurately, it's meant a fundamental shift in what we know about the structure of our universe, and (hopefully) this time will be no exception.