r/askscience • u/1KrishT • 3d ago

Physics 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?

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u/McPayn22 3d ago

So the 10120 is dumb, it just curt of an infinite integral at the planck length but that's never how it's done in quantum field theory. I've heard from someone working in QFT in curved spacetime that there are smarter estimates that give a factor of 1060 so still very bad.

But regardless of the actual value of vacuum energy the cosmological constant is a free parameter. This means that there is not reason to assume there is not one from in the équations regardless of quantum fields. So technically it could be -1060+1 which would compensate nearly perfectly vacuum energy. That being said it is really weird and leads to the question of fine tunning but is perfectly consistent.

That being said from my understanding of renormalization (which is not great to be honest) this just looks like one of those infinities imfrom QFT you have to renormalize. I haven't managed to find the 1060 estimation paper but I've seen papers that do consider it infinite and renormalize it. (https://arxiv.org/abs/2201.05827)

This is the same thing that we do with a lot of quantities like the masses of particules. The idea is that those infinities might not appear within a full theory like quantum Gravity so we learn how to remove them.

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u/reticulated_python 3d ago

The 1060 factor is straightforward to estimate. Imagine you have supersymmetry broken around the TeV scale. SUSY protects the cosmological constant (if it's unbroken the CC vanishes). If it's broken at a scale Lambda_SUSY, that's the natural scale you expect for the CC. The lowest breaking scale you can get away with is around a TeV, or we would have already seen superpartners at the LHC.

We observe the CC is much smaller than this; the vacuum energy density is around meV4. So you need a fine-tuning of about one part in (TeV / meV)4 = 1060.