$\endgroup$ – pela Apr 21 '16 at 10:13 | … The mass of a given volume of the universe is roughly constant. As you can see, in order to determine the ultimate fate of the universe,we need to know how the average density of the universe compares to thecritical density. the density that determines whether the Universe expands forever or recollapses. expands forever WMAP determined that the universe is flat, from which it follows that the mean energy density in the universe is equal to the critical density (within a 0.5% margin of error). the universal expansion must stop within about twenty billion years. A critical density universe has ``flat'' curvature. Water is much less dense than iron, hence an average is needed for ease of use. Critical density is the value at which the Universe is at balance, and is stopped. The density of matter for a flat Universe, the critical density, is 5 x 10 -27 kilograms per cubic metre or about five atoms per cubic metre! If the density is more than the critical density, the universe is closed and curves like a sphere; if less, it will curve like a saddle. The state of the universe if its total mass and energy density is exactly equal to a specific value, called critical density. If the density is higher than the critical value, it will remain above the critical value for all time (and the Universe will eventually collapse). This is equivalent to a mass density of 9.9 x 10-30 g/cm 3, which is equivalent to only 5.9 protons per cubic meter.

The shape of the universe depends on its density. Schematic illustration of open (Omega 1), closed (Omega > 1) and flat or critical (Omega = 1) Universes. C) the universe is expanding at a rate greater than the escape speed of the universe. The greater the value of the Hubble constant at a given cosmological time, the faster the universe is expanding at that time. This is a hypothesis that is the basis of CURRENT COSMOLOGY. The density parameter equals exactly 1 in a flat universe. WMAP determined that the universe is flat, from which it follows that the mean energy density in the universe is equal to the critical density (within a 0.5% margin of error). The total density of the Universe is actually a special value called the critical density which is used in a lot of astrophysical contexts beyond cosmology, including the studies of … The Universe is not ISOTROPIC AND HOMOGENEOUS. For the critical density case, the scale factor for the Universe goes like the 2/3 power of the time since the Big Bang, so the Universe has grown by a factor of 2 2/3 = 1.59 since the midpoint of the light's trip. Critical Density. If the density of the Universe were greater than the critical density and the Universe contained no dark energy, then the Universe would eventually collapse The Hubble constant is a measure of

Average density of matter in the universe. B) the universe is expanding at a rate less than the escape speed of the universe. The density of the Universe seems to be very close to the critical value, giving the Universe zero curvature How is the "flatness problem" resolved The initial expansion of the Universe proceeded very quickly, so quickly that we cannot see the curvature very well in our little section of space.