I am not a scientist, much less a physicist. But, I will offer a summarized account of what involves the calculation of zero energy density in quantum field theory (as I understand it), which ends up predicting the vacuum energy density of space with an error of 120 (10^120) magnitudes from actual measurements.

I do this to try to spell out what are the assumptions used for such calculation, that could, perhaps, be found to be shaky.

Physicists feel free to correct, add, and clarify for us (thanks in advance).

1. Quantum field theory posits that in every point in space there is a harmonic oscillator of every possible frequency

2. They can be excited in a different way, and, a certain excitation appears to us as particles.

3. Quantum field theory postulates such view because it could be said that everything in physics is formulated in terms of harmonic oscillators.

4. Essentially any perturbative quantum field theory that is Lorentz invariant is going to be described to first approximation as a quadratic Lagrangian with up to 2 derivatives. This is the heart of it all.

5. This is why harmonic oscillators show up everywhere in physics. It would appear that the structure of physics is almost always approximated by a harmonic oscillator.

6. The equation of the Harmonic oscillator is roughly of the following form

H = ( n + 1/2) h Wn n=0,1,2,3,4...

7. And what is interesting and important is that even at energy level n=0, its lowest state,its ground state (zero point energy ) H is not zero (this is due to Heisenberg´s uncertainty principle when applied to energy)

H= (1/2) h * W0

8. This, as far as I know, is the heart of quantum field approaches, as explained above, this is motivated by what appears to be a pervasive fact of nature.

Then.

9. If we wanted to compute the ground zero energy density in a given volume of space, we would add up all the energies of all the oscillators of that volume of space (for n=0).

10. This means adding all of the ground zero energy-momentum of all the harmonic oscillators in the given volume.

11. This addition is done up to around Planck scale, where we would expect Quantum field theory to break down.

12. And this yields an energy density 10^120 greater than what has been measured to be the energy density in vacuum space.