Scrabble?! DECELERA

Definitions of DECELERA in various dictionaries:

The deceleration parameter q {\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: q = d e f − a ¨ a a ˙ 2 {\displaystyle q\ {\stackrel {\mathrm {def} }{=}}\ -{\frac {{\ddot {a}}a}{{\dot {a}}^{2}}}} where a {\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 a ¨ > 0 {\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 a ¨ {\displaystyle {\ddot {a}}} was expected to be negative, so a minus sign was inserted in the definition to make q {\displaystyle q} positive in that case. Since the evidence for the accelerating universe in the 1998–2003 era, it is now believed that a ¨ {\displaystyle {\ddot {a}}} is positive therefore the present-day value q 0 {\displaystyle q_{0}} is negative (though q {\displaystyle q} was positive in the past before dark energy became dominant). In general q {\displaystyle q} varies with cosmic time, except in a few special cosmological models; the present-day value is denoted q 0 {\displaystyle q_{0}} . The Friedmann acceleration equation can be written as ...