According to this paper, fine tuning of the cosmological constant is not needed. The universe will expand whatever the cosmological constant is as a result of quantum fluctuations alone.

Iâ€™m curious to see what @PdotdQ thinks of this; I am not well enough versed in GR / quantum gravity approximations to follow the derivation.

First of, what @Patrick said:

This is not the fine-tuning of the cosmological constant problem. The fine-tuned cosmological constant problem refers to the fact that the observed cosmological constant is 10^120 times smaller than a particular theoretical prediction. It is a problem about the **acceleration** of the universal expansion, and not on whether the universe expands or not.

This team at the University of British Columbia has claimed that the quantum vacuum can solve the cosmological constant problem since a few years back. If true, this is a big deal! I have a few concerns:

- The way they calculate how quantum fluctuations affect the spacetime is based on â€śstochastic semiclassical gravityâ€ť, which is an approximation (or perhaps the better word is a â€śguessâ€ť) of how quantum mechanics affect the GR spacetime. It is an approximation and thus does not take into account the full quantum nature of the Universe (e.g. the metric is not taken to be quantum). The limitation of this approximation is unclear, but taking it all the way to spacetime singularities (which they did in this paper) seems suspect.
- If you look at the left figure of Figure 1., the scale factor, â€śaâ€ť which roughly determine the cosmological expansion is equal to 0 at many points. At a=0, the spacetime is singular and General Relativity breaks down. This means that there is no reason to think that their prediction past the first a=0 point is correct. They try to justify this by assuming that gravity IRL obeys a particular modification to general relativity that does not have this problem - but I think modifying GR to solve this problem might open a whole other can of worms.
- Their spacetime is not inflating. @Patrick, as someone who is really into inflation, I am surprised that you take so kindly to this model! It might be possible to extend their calculation to an inflating spacetime, but this has not been done yet.

For fine-tuning arguments in general, especially for the kinds that are of interest to people in this forum:

- I still think that most fine-tuning arguments are ill-defined and should not be used in philosophy or theology
- If you disagree with 1), then this work only solves one of the many fine-tuning problems

Thatâ€™s my view as well. I liked Steven Weinbergâ€™s example of Earthprime which was exactly like our Earth except that it was perpetually covered in clouds. On Earthprime they had all of the same physical constants we have, but they had an additional constant of 1.99 calories per minute per cubic centimeter. They remark on how finely tuned this constant is for life. Change it a little bit up or down and life wouldnâ€™t exist. This was the amount of energy coming into Earthprime, and they didnâ€™t know that it was coming from the sun. Later on, they develop spaceflight and get above the clouds. They are finally able to see all of these other planets and realize their fine tuning arguments werenâ€™t as solid as they once thought.