What are the three types?
#-> " Open Universe"#
#-> " Closed Universe"#
#-> " Flat Universe"#
What is critical density, and how does it determine universe type?
#->#The critical density is the minimum density that ensures that the Universe could not expand forever, but will not collapse back on itself either. A universe with the critical density is said to be flat. The critical density of the universe is used to determine the matter density parameter #Omega_(c)#
#Omega_(c)<1# For an open universe
#Omega_(c)=1# For an flat universe
#Omega_(c)>1# For an closed universe
What is the fate of each universe type?
#-># Closed Universe : Density of the universe is less than the critical density so the universe will stop expanding and gravity will cause it to crunch, causing a Big Crunch (exact opposite of Big Bang).
#-># Open Universe : Density of the universe is greater than the critical density so the gravity in the universe is too weak to stop expansion, therefore the universe will continue expanding forever. The fate of this universe can be a "Big Rip" caused by acceleration caused by dark matter overwhelming the four fundamental forces, and this in my opinion would be the most undesirable end as it would result in every star, planet, person, and eventually molecule ripping apart until atoms themselves rip apart into quarks and the universe theoretically ends.
#-># Flat Universe : Density of the universe is equivalent to the critical density so the universe will only start to contract after an infinite amount of time (makes no sense, I know).It will continue to expand at an increasing rate, but the acceleration of this expansion will be proportional to the amount of dark energy between masses instead of proportional to the square root of that amount of dark energy (like in the Open Universe Model).
Though this means that the universe will at some point experience a "Big Rip" scenario, since the acceleration is much slower the stars in the sky will run out of fuel before it happens.