The Universe is expanding, this we know for sure, but for decades Physicists have been struggling to understand and work out the rate of this expansion – this is often thought of as one of the greatest mysteries in science.
The Hubble Constant, named after the famous American Astrophysicist Edwin Hubble, is the value given to the rate of the expansion of the Universe. Though it seems the Universe is expanding much faster than current theories of Physics allow. One of the main methods used looks to the early universe by studying Cosmic Microwave Background (CMB) radiation – leftover radiation from the Big Bang – using the Planck satellite. This method puts the rate of expansion at around 67 km/s/megaparsec – if this value is correct then it would mean scientists have been inaccurately measuring distances to distant astronomical objects for many years.
Meanwhile, a team led by astrophysicist, Wendy Freedman had been using the Hubble telescope to look at a specific type of star, called a Cepheid variable. This star, which periodically gets dimmer and brighter, gave a value of around 72 km/s/megaparsec. The same team redid the experiment, now focusing on another type of star, Red Giants, which gave them a value of 70 km/s/megaparsec. If either of these values are correct, then this introduces significant cracks in the standard model of the Universe, and therefore our understanding of the evolution of the Universe.
These discrepancies in the value of the Hubble constant could simply be due to mismeasurement, but scientists are also well-aware that the evidence seems to be pointing more towards there being errors in the current standard cosmological model, and thus are not ruling out that the fundamental physics of the Universe needs to be rewritten.