# Dark Energy and Fine Tuning

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:

1. The value of a cosmological constant in our Universe could, in principle, take on any positive-or-negative value.
2. 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 Ä§, is ~10^19 GeV/c2.
3. But if the value of dark energy is greater than Â±(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.
4. 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 thatâ€™s many orders of magnitude lower (like 10^-11 GeV/c2) is rare. But the scientific truth may be far more sobering: we donâ€™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.

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We donâ€™t even know for sure if its real. In this universe, never mind others that we may speculate exist. Nor can I think of a good way to test for its value in whatever other universes may exist out there.

Is this not also an example of a question which is beyond science? I mean science is a method of truth discovery which relies on testing for natural causes in the natural universe. Does that even apply here?

PS- I was disappointed E. Siegel got away from his blog to a FORBES columnist that is somewhat harder to communicate with, but I guess a guy has to make a living.

@anon46279830

There are 2 commonly cited â€ślimitsâ€ť to science:

1. Religious or miraculous processes;

2. what happened before the Big Bang.

I would be surprised if Dark Energy was something that science couldnâ€™t access in some wayâ€¦ but I suppose if Dark Energy operated or occupied some other dimensional spaceâ€¦ that might move it beyond our graspâ€¦

There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable.

There is another theory which states that this has already happened.

â€• Douglas Adams, The Restaurant at the End of the Universe

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My comment was in regard to discerning the possible values of â€śDark Energyâ€ť if any, in other universes. I donâ€™t think its a question for science as we have been defining it.

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