The Universe Is Expanding...Eternally?
 
    There are two important factors in determining whether the universe will expand forever or will slow down.  These two factors are the Hubble Constant and the cosmological constant.  The Hubble Constant (H) is the expansion rate of the universe.  The cosmological constant is a force that acts against gravity, also know as anti-gravity.  If the cosmological constant does not exist then the universe will be expanding at an increasingly slower rate.  Which means that the universe was expanding faster in the beginning than it is now (Pogge).
    If there is no cosmological constant then the amount of deceleration depends on the density of matter in it, Omega.  Other things to consider when deciding the fate of the universe (still considering there is no cosmological constant) is what type of universe is it.  If it is an empty universe then the universe will expand at a constant rate.  If it is open then the universe will expand forever at a decreasing rate.  If it is closed then the universe will eventually stop expanding and re-collapse into itself known as the Big Crunch (Pogge).
    Now if the cosmological constant actually does exist then the fate of the universe will be a little different.  In the case of the open universe, if the cosmological constant is small then the acceleration of the universe will slow down over time.  However, if the cosmological constant is large then the acceleration of the universe will speed up over time (Pogge).
    To find out the fate of the universe astronomers observe the redshifts of distant Type Ia supernovas.  These redshifts tell the astronomer how much the universe has expanded over the time it took the light to reach us here on Earth.  If the total amount of expansion is larger than the current expansion rate the universe must be decelerating (Supernova).
 

 

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