Scientists still are not entirely sure whether the universe is open (which means

Question

Scientists still are not entirely sure whether the universe is open (which means it will keep expanding forever) or closed (which means it will eventually fall back in on itself in a "Big Crunch"). What determines whether it is open or closed is basically the same thing we calculate to determine escape speeds. Assume that the universe has a uniform density ?. Then the mass contained in some spherical volume of space of radius R is 43?R3?. Model a galaxy located at the edge of this spherical volume as a particle and consider its energy. According to Hubble's law, the speed of this galaxy is HR, where H = 2.2

Explanation / Answer

The fact that the universe is expanding then raises the question "Will it always expand?" Since the action of gravity works against the expansion, then if the density were large enough, the expansion would stop and the universe would collapse in a "big crunch". This is called a closed universe. If the density were small enough, the expansion would continue forever (an open universe). At a certain precise critical density, the universe would asymtotically approach zero expansion rate, but never collapse. Remarkably, all evidence indicates that the universe is very close to that critical density. Discussions about the expansion of the universe often refer to a density parameter ? which is the density divided by the critical density, such that ? = 1 represents the critical density condition.

if the density of the universe is less than the critical density, then there is insufficient cosmic matter to stop the expansion and cause the universe to recollapse. Luminous matter by itself contributes only about one percent of the critical density. When dark matter in galaxies and clusters is taken into account, the figure rises to 20 or 30 percent. The fraction of dark matter on larger scales is uncertain, but it may be even greater than is found in clusters. Most astronomers believe that the present mass density of the universe lies somewhere around 30 or 40 percent of the critical value. Recent observations of distant supernovae suggest that the expansion of the universe may be accelerating, possibly driven by the effects of thecosmological constant, a repulsive force that exists throughout all space. Its physical nature is unknown.

Related Questions

Navigate

• Browse (All)
• Browse S
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.