![]() ![]() ( blue) curve shows an open, low density universe whose expansion is also slowing down, but notĪs much as the critical density universe because the pull of gravity is not as strong. time: the bottom ( green) curve represents a flat, critical density universe in which theĮxpansion rate is continually slowing down (the curves becomes ever more horizontal). The picture above shows a number of possible scenarios for the relative size of the universe (accelerating), raising the possibility that the evolution of the universe is now dominated by aīizarre form of matter which has a negative pressure. Raised the possibility that the expansion of the universe might in fact be speeding up Universe is whether or not the pull of gravity is strong enough to ultimately reverse theĮxpansion and cause the universe to collapse back on itself. Since due to the gravitational pull of the matter on itself. For the most part this rate of expansion has been slowing down (decelerating) ever Started from a very small volume, an event later dubbed the Big Bang, with an initial expansion (force it exerts per unit area), but the generic picture that emerges is that the universe The details depend on some further informationĪbout the matter in the universe, namely its density (mass per unit volume) and its pressure How space and the matter in it evolves with time. Universe? While the answer is not yet known for certain, it appears to be tantalizingly close toĪbout how the matter is distributed, the next step is to work out the dynamics of the universe. Scientific questions in cosmology today is: what is the average density of matter in our The value of the critical density is very small: it corresponds to roughly 6 hydrogenĪtoms per cubic meter, an astonishingly good vacuum by terrestrial standards! One of the key If the density just equals the critical density, the universe is flat, but still presumably If the density is greater than the critical density the universe is closed and finite. If theĭensity of matter is less than the so-called critical density, the universe is open and Uniquely determines the geometry of the universe (up to the limitations noted above). ![]() ![]() It turns out that the average density of matter ![]() Matter plays a central role in cosmology. There may be some ways to test this idea, but most of the following Portrayed here, while still having the same local curvature. It is possible that the universe has a more complicated global topology than that which is.Horizon - space-time could well be quite different out there. Theīig Bang Model does not attempt to describe that region of space significantly beyond our Starting point in an infinite universe, you would never return.īefore we discuss which of these three pictures describe our universe (if any) we must make aĪge (~13.77 billion years) we can only see a finite distance out into space: ~13.77īillion light years. Start a journey off in one direction and, if allowed enough time, ultimately return to your Limitation of the picture shown here is that we can only portray the curvature of aĢ-dimensional plane of an actual 3-dimensional space! Note that in a closed universe you could "flat" and infinite in extent - our "ordinary" conception of space. "negatively" curved like a saddle and infinite in extent or it can be It canīe "positively" curved like the surface of a ball and finite in extent it can be Matter) can only have one of three forms, as shown schematically in the picture at left. That the corresponding distortion of space-time (due to the gravitational effects of this Matter in the universe is homogeneous and isotropic (The Cosmological Principle) it can be shown Since gravity is a property of space-time in General Relativity, this isĮquivalent to computing the dynamics of space-time itself. Largest scales, one can use General Relativity to compute the corresponding gravitationalĮffects of that matter. By assuming that the matter in the universe is distributed uniformly on the The Big Bang model of cosmology rests on two key ideas that date back to the early 20thĬosmological Principle. ![]()
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