Specific gravity is the ratio of this quantity for a substance to this quantity for a reference.
For the universe, the "critical" form of this quantity equals three times the square of the Hubble constant divided by 8 times pi times Newton's gravitational constant. This quantity names the waves in a theory proposed by Lin and Shu to explain galactic spiral structures. This value is the same for isopycnic surfaces, and this quantity for a barotropic fluid depends only on the pressure. Specific gravity is the ratio of this quantity for a substance to this quantity for a reference. The lowercase Greek letter rho symbolizes this quantity, which is often measured in kilograms per cubic meter. For 10 points, name this quantity equal to an object's mass per unit volume.
For the universe, the "critical" form of this quantity equals three times the square of the Hubble constant divided by 8 times pi times Newton's gravitational constant.
density