The
deceleration parameter ![](http://info.babylon.com/onlinebox.cgi?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=1726)
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:
![](http://info.babylon.com/onlinebox.cgi?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=1664)
where
![](http://info.babylon.com/onlinebox.cgi?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=350)
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
![](http://info.babylon.com/onlinebox.cgi?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=1323)
is positive (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
![](http://info.babylon.com/onlinebox.cgi?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=1726)
was thought to be positive, now it is believed to be negative.