Does Universe expands or collapses ?
If the total energy of the Universe = 0 = potential energy + kinetic and mass energy = 0
Than the potential energy has to be negative relatively to kinetic and mass energy. When is this case ?
The equation shows that gravitational potential energy is proportional to both mass and height. For example, raising two similar objects, or raising the same object twice as far, doubles the potential energy.
This assumption is mathematically exactly correct for a spherically symmetrical object (such as, to a reasonable approximation, a planet). It is not generally correct in other cases, though if the dimensions of an object are very small compared to the distance of separation then it is reasonable to consider it as a point mass and ignore the details of its shape.
With this simplifying assumption, integrating force over distance leads to the following general expression for the gravitational potential energy, U(g), of a system of two masses:
U(g)=integral [from h(1) to h(2)]Gm(1)m(2)/r^2 dr
U(g)=Gm(1)m(2)[1/h(1) – 1/h(2)]
If we try to calculate an "absolute" potential energy by setting the reference level at zero then the formula "blows up" with division by zero. In other words, we can only actually use this formula to measure the difference in potential energy between one non-zero separation and another.
In practice it is often convenient to take the reference level at infinite separation (i.e. h(1)=infinity), in which case the formula becomes:
U(g)= - Gm(1)m(2)/r
where r is now the distance between the centres of mass of the two objects (again noting the earlier caveats). For a small object above the surface of the earth, r is the distance from the object to the earth's centre (and similarly for other spherical bodies).
We see the minus in above equation because we calculate potential energy from infinity to a definite distance. The potential energy is related to the potential energy in the infinity.
When we calculate the total energy of the Universe and its potential energy is negative it means that we calculate it relatively to an infinite distance. The negative potential energy is when the matter comes from an infinity and collapses.
In cosmology it might be that a perfect empty space has an infinity small curvature. It started to collapse when the curvature becomes definite.
If it becomes enough sufficient dense the curvature of the gravitational potential energy might be transformed in a localized rest mass energy with its kinetic energy. Kinetic energy is repulsive and the rest mass matter expands then.
We observe a space recession because Vacuum becomes less energetic and less warped (curved). The Big Bang scenario is a consequence of the before collapsed space than.