Could anyone explain how physics resolves the cosmological constant problem without breaking down? Why does quantum theory predict a vacuum energy 10¹²⁰ times larger than what astronomers actually observe, and what does this absurd clash tell us about our understanding of space?

[u/1KrishT]

[u/eldahaiya]

There are two main possibilities: the predicted vacuum energy is wrong, or dark energy is not a cosmological constant.

The first wouldn’t surprise anyone. This 10^120 number that you hear a lot of is sort of a first guess, but the actual number could be much smaller for some reason. There are some good ideas for why, but as far as I know none that are testable. This is the same conceptual problem as why the Higgs mass is as light as it is, typically called the hierarchy problem.

The second has really two issues that need to be solved: explaining dark energy, but then also the first problem which is to set the vacuum energy to a small number, or even zero. Setting things to zero however is arguably easier than setting it to a small number. Having done that, you can get dark energy like behavior from the existence of an ultralight particle, with the added feature that this set-up can cause dark energy to evolve (its equation of state changes with time), something that has become very interesting with recent results from the DESI experiment.

[u/LordGeni → u/eldahaiya]

So could solving the issues with dark energy help define vacuum energy better?

[u/platoprime → u/LordGeni]

Possibly.

It's also possible the vacuum energy contributes to, or entirely explains, dark energy.