The Hubble constant describes the current rate of the universe’s expansion. It has been found to have two values based on different calculations. This is weird because constants are supposed to have a single, fixed value.
Several thousand years ago, a star some 160,000 light-years away from us exploded as a Type Ia supernova. The aftermath of this energetic detonation is shown in this image from the NASA/ESA Hubble Space Telescope. The remnant has been designated DEM L71. Credit: gsfc/Flickr, CC BY 2.0
The famous astronomer Allan Sandage condensed cosmology – the study of the universe and its origins – to two numbers. One is the Hubble constant, H0 (pronounced “H nought”), which describes the current rate of the universe’s expansion. The other number is the rate at which this expansion is decelerating, called q0 (“q nought”). While these two numbers adequately sum up the work of several cosmologists, there have been, and continue to be, extraordinary efforts to measure H0 and q0 more precisely.
Recent interest in q0 has been fuelled by the discovery in the late 1990s that the universe’s expansion is accelerating. That is, q0 is negative. Where is space getting the energy from that is stretching ...