One of the three scientific justifications to the U.S. Congress for building the Hubble Space Telescope was to use its observing power to give an exact value for the expansion rate of the universe. Prior to Hubble’s 1990 launch, observations from ground-based telescopes yielded huge uncertainties. Depending on the expansion rate, the universe could be anywhere between 10 to 20 billion years old. For the past 34 years Hubble has shrunken this value to an accuracy approaching one percent. This has been accomplished by refining the so-called “cosmic distance ladder” by measuring the gold standard of cosmic milepost markers known as Cepheid variable stars. 

However, the results have puzzled cosmologists for a decade.  The best measurements from Hubble show the universe is now expanding faster than predicted based on observations of how it looked shortly after the big bang. These observations were made by the Planck satellite mapping of the cosmic microwave background radiation – sort of a blueprint for how the universe would evolve structure after it cooled down from the big bang. 

The simple solution to the dilemma is to say that maybe Hubble observations are wrong due to some creeping inaccuracy in its deep-space yardstick. Then along came the James Webb Space Telescope to crosscheck Hubble's results. Webb’s sharp infrared views of Cepheids agreed with Hubble data. Webb confirmed that the Hubble telescope's keen eye was right all along. 

The bottom line is that the so-called “Hubble Tension” between what happens in the nearby universe compared to the early universe’s expansion remains a fascinating puzzle for cosmologists. There may be something woven into the fabric of space that we don’t yet understand.

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