Will the Universe expand forever? A frequently asked question in the comments on the website. I touched on this in the past during the cosmology and dark energy topics covered in previous posts but it certainly deserves a post in its own right.
Cosmology deals largely with the universe as a whole considering all the contents within. Cosmologists research the fate of the universe as well as its origin. The entire vacuum of the universe and it’s behaviour is today’s topic. Vacuum pressure, density or the entire vacuum catastrophe won’t be discussed in any great detail as the vacuum itself is getting a Series in its own right.
There is debate regarding the definite shape and geometry of the universe. The Universe has a choice of three geometries or shapes; closed, open and flat. The different potential shapes of the universe will determine wether it will expand forever or not.
1) Closed Universe : in a closed universe the geometry would be the same as that of a sphere. The sum of the angles of a triangle on the overall surface equate to over 180 degrees and there are no parallel lines. All lines eventually meet. In a closed universe, gravity will eventually slow down and reverse the expansion of the universe leading to what physicists and cosmologists call “The Big Crunch”. This would be the Universe stopping and contracting in on itself. The opposite of a Big Bang.
The main issue with this theory is gravity itself. As far as the fundamental forces are concerned gravity is the weakest and the weakest by far. We now know the expansion of the universe is accelerating and gravity isn’t doing much to slow it down as it is.
2) Open Universe: an open Universe would have a saddle shape due to negative curvature. This means the angles of a surface triangle would equate to less than 180 degrees. The observable universe is dominated by Dark Energy which makes up almost 2/3 of all matter in the cosmos. Dark energy is causing the expansion of the universe to accelerate. Even without dark energy the universe would expand forever. Gravity would most likely slow it down but not actually stop it.
3) Flat Universe : in a flat universe the angles of the triangle add up to 180 degrees and parallel lines will never meet. The Wilkinson Microwave Anisotropy Probe has confirmed that the Universe is flat with a 0.4% margin of error. It is reasonably well accepted that the geometry of the Universe is flat. The eternal expansion in a flat universe is the same as an open universe. They both arrive at the same conclusion, the ultimate heat death of the Universe.
Dark energy rules the expansion of the universe. For gravity to slow the expansion of the universe, physical and dark matter would have to accumulate to draw the universe back in on itself. Visible matter like stars and galaxies only accounts for approx 5% of the cosmic inventory. Dark matter is slightly less than 30%. So Physical matter doesn’t stand a chance against dark energy. It is just too powerful.
So the answer is yes, the universe will expand forever.