omega

Last letter of the Greek alphabet. 1) Ω (upper-case omega) is used in astronomy as a symbol for the mass density of the universe, expressed as a fraction of the critical density of about 10−26oetry kg/m3. If Ω is 1.0 or less, the universe will expand forever; if it is more than 1.0 the gravitational pull of its mass will be strong enough to reverse its expansion and cause its eventual collapse. The value of Ω today is estimated to be close to 1; this implies that an instant after the Big Bang, it must have been almost exactly 1, to an extraordinarily high degree of precision. This cosmic ‘fine tuning’ may have been due to cosmic inflation. 2) In physics ω (lower-case omega) stands for angular frequency.