Corey S. Powell provides a history of the Hubble-LeMaitre law and the efforts to pin down a precise value of the Hubble constant that plays a significant role in determining not only the age of the universe but also its ultimate fate. Like the age of the Earth, the value of the Hubble constant, and thus the age of the universe, has shifted considerably over time.
But the latest puzzle is that two different methods of measuring its value have resulted in two fairly precise values that do not overlap with each other, which is of course problematic. Attempts at reconciling these two values have resulted in suggestions that some properties of dark energy may be responsible for the difference.
Following in the tradition of Edwin Hubble, [Adam] Riess and his collaborators are observing stars in neighbouring galaxies to measure the Hubble Constant, with the ambitious goal of pinning down the number to an accuracy of 1 per cent. His research is zeroing in on a value of 73. Today there is another, entirely separate way to measure the Hubble Constant, by analysing subtle patterns etched into the cosmic microwave background, detected by the Planck space telescope. This approach gives an equally precise-looking answer of 67. The disagreement, though tiny by historical standards, is unnerving enough that cosmologists have started calling it the Hubble tension.
Strictly speaking, the two sides are not measuring the same thing. Riess is looking at the expansion of the nearby Universe, at relatively modern times. The Planck telescope measures effects of expansion long ago, shortly after the Big Bang, and then researchers derive a modern value of the Hubble Constant from that measurement. One way to reconcile the two is to suppose that the very early Universe was expanding slightly faster than expected. ‘It could be that there is something funky about dark energy being stronger than we thought,’ Riess says. ‘I don’t think it’s introducing something new to say: “What if dark energy is weird?” because there’s no such thing as it not being not weird.’
This case illustrates why increasing the precision of measurements often results in new problems that in turn require the invention of new theories to resolve them. This happens a lot in science.