If the universe oscillates between a Big Bang and a Big crunch, can two particles at opposite ends of it, be considered to be undergoing simple harmonic oscillation? If the potential energy of the oscillator is given by G m1 m2 /r and m1 is the mass of the universe,10^52 kg,r = 10^26 metres - the current size of the universe -then since the PE of a simple harmonic oscillator is given by PE = 1/2 k x^2, the force constant k becomes 10 ^ -37 m2. using frequency of oscillator = ( k / m2 ) ^1/2, frequency = ( 10^ -37m2 / m2 )^ 1/2 = 10^ - 18.5 per second. In other words the universe oscillates every 10 ^ 18.5 seconds - about its current age!! Since the two particles of a simple harmonic oscillator accelerate when potential energy gets converted into kinetic energy, the current acceleration of the expansion of the universe is removing potential energy from the universe.So dark energy removes potential energy from the universe. It is probably doing this by taking energy from gravitons.