We still don’t know how fast the Universe is expanding
“As it stands today, we know the Hubble expansion rate better than we ever have, and yet our two different method of arriving at it seem to give irreconcilable values. There are a myriad of different measurements going on right now attempting to find out which camp is right, which camp is wrong, and exactly where the errors lie. If history has taught us anything, we can say for certain that two things will come of this: we’re going to learn something additional and wonderful about the nature of our Universe when this gets resolved, and that this current controversy won’t be the last one concerning how the Universe expands.”
It seems like the simplest, most fundamental quantitative question about the expanding Universe of all: how fast is it expanding? Even though it’s been more than 80 years since Hubble’s most career-defining discovery, we still don’t know the answer. In fact, the two main methods we have of measuring it give incompatible results. The largest-scale observations, like the cosmic microwave background and baryon acoustic oscillations, give a result that’s on the low side: 67 km/s/Mpc. On the other hand, distance ladder measurements, relying on individual stars, galaxies and supernovae, give a higher result: 74 km/s/Mpc. But these are known well enough that they’re inconsistent with one another.
It seems like every couples spend a wonderful day together. They go out, eat in expensive restaurants, give each other something that makes the other one happy and show their infinite love to the world. Nothing could be more perfect, right?