Astronomers agree that the universe is expanding in all directions, a notion now called the “Hubble expansion” to refer discoverer Mr. Edwin Hubble.
A useful analogy to understand the Hubble expansion is to draw dots onto a balloon to represent galaxies in the universe. As you blow up the balloon the dots expand away from each other. While there seems to be no way around a universal Hubble expansion, now there is controversy brewing regarding the exact value for this rate of expansion. What is at stake may be a tiny misunderstanding in how we make the measurements, or may be a signal of new physics.
Oh, we all agree now on the approximate answer, that the space between galaxies grows such that for every 3.3 million light years a galaxy moves in distance away from us, the velocity of that distant galaxy becomes 70 km/s faster. Equivalently, in astronomer’s jargon we say that the rate of expansion (H0) equals 70 km/s/Mpc, with the symbol H0 used again to recognize the work of Mr. Hubble.
Although this law was first established in 1929, astronomers are still working on getting its exact value nearly 100 years later. To recap the 21st century part of the story, in 2015 one research group measured H0 by studying distant exploding stars, or supernovae. This group, led by Nobel Laureate Dr. Adam Reiss, reported a value for H0 from their high quality data set in the range of 71.2 – 74.8 km/s/Mpc.
Somewhat surprisingly, a different group working with also with high quality data this time from the space satellite “Planck” compute a value for H0 of 67 km/s/Mpc with quoted error bars that make it incompatible with the supernova-based measurement.
This mismatch of the value for H0 between the two groups may sound a bit like scientists being nitpicky. Afterall, we could very well find out later that one or more of the measurements that went into the computations mentioned above have larger uncertainties than originally projected. Then again, if the numbers do hold, then this could be a gentle but persistent beacon that queues us into new physics.