Deceleration parameter

The deceleration parameter ${\displaystyle q}$ in cosmology is a dimensionless measure of the cosmic acceleration of the expansion of space in a Friedmann–Lemaître–Robertson–Walker universe. It is defined by:

where ${\displaystyle a}$ is the scale factor of the universe and the dots indicate derivatives by proper time. The expansion of the universe is said to be "accelerating" if ${\displaystyle {\ddot {a}}>0}$ (recent measurements suggest it is), and in this case the deceleration parameter will be negative. The minus sign and name "deceleration parameter" are historical; at the time of definition ${\displaystyle q}$ was thought to be positive, now it is believed to be negative.

...
Wikipedia