Once we discovered that the Universe was expanding, the next scientific step was to determine what the rate of expansion was. Despite the fact that it’s been more than 80 years, we still don’t have agreement on how fast that rate actually is. By looking at the largest cosmic scales and the oldest signals - the leftover radiation from the Big Bang and the largest-scale galaxy correlations - we get one number for the rate: 67 km/s/Mpc. But if we look at individual stars, galaxies, supernovae and other direct indicators, we get another number: 74 km/s/Mpc. The uncertainties are very small: ±1 on the first number and ±2 on the second; statistically, there’s less than a 0.1% chance these numbers will
Watch: stars orbit the supermassive black hole in the center of our galaxy. No astronomer has even seen a black hole through a telescope. One way we know: Last year, physicists “heard” for the first time the gravitational waves produced from two black holes colliding.