SOLAR SYSTEM

Detailed explanation-1: -The laws of general relativity tell us that for an expanding universe to be flat, its density must be equal to a critical density which is proportional to the square of the Hubble constant. For our universe, the critical density is about 9 x 10-27 kilograms per cubic meter.

Detailed explanation-2: -If it equals exactly critical density, the universe will only stop expanding after infinite time. If it is under critical density, it will expand forever.

Detailed explanation-3: -The ‘critical density’ is the average density of matter required for the Universe to just halt its expansion, but only after an infinite time. A Universe with the critical density is said to be flat.

Detailed explanation-4: -If the density of the universe is less than the critical density, then the universe will expand forever, like the green or blue curves in the graph above.

Detailed explanation-5: -The critical density today depends on the value of the expansion rate today, H0. If the Hubble constant is around 20 kilometers/second per million light-years, the critical density is about 10–26 kg/m3.

Detailed explanation-6: -That means that the critical density was greater at earlier times. It changes by the same factor that the actual density of the universe changes throughout the expansion. So if the universe starts out with a density greater than the critical density, then its density will always be greater than critical density.