Deceleration parameter

Full-sky image derived from nine years' WMAP data

The deceleration parameter $\! q$ 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 $\! a$ 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 $\ddot{a}$ 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 $\! q$ was thought to be positive, now it is believed to be negative.

单词	Acceleration parameter
释义	Acceleration parameter 英语百科 Deceleration parameter The deceleration parameter $\! q$ 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 $\! a$ 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 $\ddot{a}$ 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 $\! q$ was thought to be positive, now it is believed to be negative.
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Acceleration parameter

Deceleration parameter