"Until the 1990s, there were few reliable observations about movement at the scale of the entire universe, which is the only scale dark energy effects. So dark energy could not be seen until we could measure things very, very far away." -Adam Riess
Just thirty years ago, scientists argued over the value of the Hubble expansion rate, and what it meant for the age, history and fate of the Universe. Was the Universe expanding slowly (~55 km/s/Mpc), was it very old, and would it coast to infinity? Or was it expanding rapidly (~100 km/s/Mpc), was it young, and would it eventually recollapse?
Since the Hubble space telescope was launched in 1990, those questions have been definitively answered, with a 13.8 billion year old Universe composed mostly of dark energy, with an expansion rate of around 70 km/s/Mpc. But recent measurements from one method give a value of 73 km/s/Mpc, while other method give 67-68 km/s/Mpc, each with an uncertainty of just 1 or 2. Could this tension lead to a catastrophe?