Preliminary observations made by the Dark Energy Spectroscopic Instrument (DESI) hint that the acceleration of the expansion of the universe has not been constant – in other words, dark energy has changed over the history of the universe.
At the turn of the millennium, astronomers discovered that the universe has been expanding at an ever increasing rate. This came as a shock to most cosmologists who had assumed that the pull of gravity was slowing the expansion of the universe after the Big Bang.
In 1998 and 1999, two independent teams discovered the acceleration by measuring the distances to ancient supernovae and the speeds that they are receding from Earth. Dark energy – a term that was coined in 1998 – was invoked to provide the vast amount of energy required for this constant acceleration. Three leaders of those teams shared the 2011 Nobel Prize for Physics for the discovery, and in the past quarter century a variety of observations have backed the inclusion of dark energy in the Standard Model of cosmology.
Now, another shock could be coming thanks to DESI, which was designed to study the expansion of the universe.
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