What is the cosmological constant problem trying to tell us? Is it, as Weinberg suggested, analogous to the ultraviolet catastrophe which led to the development of quantum theory, or is it merely a fine-tuning problem?

It has to be telling us something… about interesting new physics that will come  out of somewhere in trying to answer it towards a theory of QG.

Its in our nature to have a reason for everything in our physical theories. By trying to understand it we might make breakthroughs in other areas.

We are not happy with a theory that simply correctly “describes” nature, we want a theory that understands and explains why this is the case, and tells us something more fundamental about physics, reality etc.

However, it should be noted that we also post-select on successful theories.

The ultraviolet catastrophe that lead to the development of quantum theory required a revision of the known physics at the time and it turned out that a new theory was needed. For the case of the cosmological constant problem, this may no be the case. Although the problem is still unsolved, it is unlikely that this problem will cause a revolution as the ultraviolet catastrophe did. Experimental evidence shows that the universe is expanding as if there was a non-zero cosmological constant whose value has not been predicted by any known theory.  To this problem there are several other questions that can be added like “why is the vacuum energy so large?”, “why does the vacuum background doesn't seem to gravitate as predicted by General Relativity?”.  It seems that the problem of the cosmological constant might be related to the applicability of General Relativity at different scales. Some theories like supersymmetric string theory claim to solve the problem, however, the vacuum seems to be not supersymmetric, and hence there is no conclusive solution to the problem. In this sense it looks like present theories do not solve such a problem and hence we will need a new theory, but there is scepticism about the necessity of a more fundamental theory than the ones we have.

It was pointed out that question 3 and 4 are quite similar.

A good part of the discussion focused on understanding what the fine tuning problem actually is and whether it is really a problem. Different opinions were voiced, as this also depends on how much explanatory power one wants to attribute to physics. It was said that without addressing fine tuning problems one is left to anthropic reasoning which brings us outside of science.

Several historical examples were cited: In the competition between Ptolemaic and Copernican models it is actually the Copernican models which requires more fine tuning parameters. Only with Kepler (allowing ellipses) these got drastically improved.

On the other hand the question why our universe is so much fine-tuned to be almost flat motivated the theory of inflation.

Unfortunately, cosmological constant problem doesn't tell us the direction to go to solve the problem.

The problem could be that coupling quantum field theory to gravity is non-trivial and it causes the problem.

Missing something deep and important – like UV catastrophe

Actually an IR problem

What’s problem – not actually inconsistent

Is it non-local?

Where is the line between what constitutes fine tuning and not?

This doesn’t seem to be a big concern for most people; not many sleepless nights are lost thinking about it. At low energies, we have an effective field theory that we trust, but the cosmological constant problem appears to signal the breakdown of the effective field theory, making its predictions invalid. Anthropic arguments, where the incredibly small value of the cosmological constant is justified by claiming that if it wasn’t this small, humans would not have existed to discuss its value in the first place, are seen by many as useless. They only serve to justify the fine tuning, but do not actually give us any insight as to how the more fundamental theory should work.

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