The Inconstant Constant
by Philip Lavoy
Did you know our universe is expanding? Right down to the microscopic level space is separating like the skin of a balloon as it blows up. This discovery was made by Edwin Hubble (1889-1953). He not only was the first to discover galaxies outside of our own (We used to think Andromeda was a nebula, not a Galaxy!), he also discovered a relationship between the separations of these galaxies. The farther away a galaxy the faster the degree of separation.
Hubble’s Law as it was called and the resulting Hubble’s Constant (the fixed value which expresses the rate of expansion) have caused nothing but trouble since. Why? Because there are large discrepancies in what the value actually is. The initial rate of expansion was 500 kms/s/Mpc. That’s five-hundred kilometers every second over every megaparsec. A single parsec is about 3.3 lightyears. A megaparsec is a million parsecs. The universe is an astoundingly large place!
The first discrepancy came about in the 1930s, when it was shown that the age of the universe extrapolated from Hubble’s calculations was younger than the age of the Earth as shown by radiocarbon dating. It also showed that the Milky Way was far larger than any Galaxy around, much more so than anticipated by other scientists. Obviously something was wrong!
But what? In the 1950s the problem was corrected. The discovery that Hubble had been using Star Clusters for his calculation instead of standard candles (A standard candle star doesn’t change absolute luminosity over distance). It resolved the issues, for a few years. Physicists continued to refine Edwin Hubble’s constant. They came up with smaller and smaller numbers for the constant. It dropped down to 180 km/s/Mpc and then further in the 1970s to 55 km/s/Mpc. At the same time another pair of astronomers contested their values were around 100 km/s/Mpc. Each of these groups had equations, backed up by science, each saying they were right. But obviously, it couldn’t be different values- so someone somewhere was wrong. But was anyone right?
It’s a good question--the numbers keep changing in part because we’re continually able to observe stars that are farther and farther away. We need to adjust the numbers so as to not create a universe that is younger than the oldest known stars. In 2016 the number was pinned down to 73.4km/s/Mpc, using data from the latest telescopes and observatories. It changed again in 2018 to 67.4 kms/s/Mpc. They may seem close, but in terms of science they aren’t. The error bars don’t overlap- so that’s not good news. One must be right, but what does that mean? If the 2016 estimate is wrong then everything we know about the distance of stars is wrong. If the second is wrong then we need to introduce some exotic physics into the equation to even it out and no one knows exactly what that would mean, besides seeming a bit ad hoc.
There are a number of very precise ways we can measure the Hubble Constant, and we get more certain answers every time – but none of those answers line up. It is, today, one of the greatest mysteries in space. There’s one possible solution. In a recent study, the paper which was published on April 10th, some scientists proposed that we live in a kind of bubble of low density in the universe. That all the easily observable stars in sight are part of that bubble. This is what’s skewing our calculations. They draw their evidence from the CMB (Cosmic Microwave Background). And argue that fluctuations in temperature of the CMB could have given rise to areas of different densities. We could just happen to find ourselves in one of those areas, which would explain the difference in measuring values.
It’s a hypothesis of course, without any direct evidence pointing one way or another. It’s possible to use a galaxy in our local bubble to measure the constant. If this theory is correct, within the bubble both distinct ways of measuring the Hubble Constant should prove to be the same.
Without further testing from a great number of people however, it remains only a hypothesis. Until proven, the Hubble Constant remains one of our greatest mysteries. The inconstant constant.
Philip Lavoy studies stars that are forming deep within molecular clouds in the galaxy. He teaches at a small private college in Illinois. In his spare time, he often dreams of swashbuckling adventures across the universe.
Non-fiction by Philip Lavoy:
"The Inconstant Constant" April 2020