Excellent and understandable article by Ethan Siegel about Dark Energy, Fine Tuning, and the Multiverse:
The dark energy puzzle goes back to 1987, when Nobel Laureate Steven Weinberg wrote a now-famous paper showcasing how small and finely-tuned the cosmological constant would need to be to allow stars, galaxies, and other gravitationally bound states. The argument goes like this:
- The value of a cosmological constant in our Universe could, in principle, take on any positive-or-negative value.
- If you try and calculate an estimate based on fundamental constants, you get a (mass)4, where the mass, made out of a combination of the constants G, c, and ħ, is ~10^19 GeV/c2.
- But if the value of dark energy is greater than ±(10^-8 GeV/c2)^4 or so, you get a Universe that either recollapses (for -) or is driven apart (for +) before any stars or galaxies can form.
- Therefore, we must live in a special place, for the Universe to be so finely tuned.
Although this is the commonly-accepted perspective on dark energy for the past 30 years, there are both theoretical and observational reasons to challenge it.
So here we get to this interesting result:
In the new two papers just published, scientists simulating how stars, galaxies, and other structures form in the Universe have demonstrated that even increasing the amount of dark energy by a factor of three, ten, or even fifty will only change the number of stars you form by about 15%. Once the value of dark energy drops below a certain critical threshold, your Universe suddenly becomes very friendly to the same cosmic story that brought human beings into existence.
The whole reason people assume a life-friendly Universe in the Multiverse is rare is because they assume that values of dark energy based on a large, Planck-scale mass (close to 10^19 GeV/c2) is likely, and a finely-tuned version that’s many orders of magnitude lower (like 10^-11 GeV/c2) is rare. But the scientific truth may be far more sobering: we don’t know what causes dark energy to have the value it does. It could be that it varies dramatically from Universe-to-Universe within the Multiverse, or it could be that dark energy has the same values in all iterations of Universes within the Multiverse. It could vary a lot, or it could vary very little, if at all. This depends very strongly on properties of nature that we do not yet understand how to measure.