The cosmological constant is the worst-ever prediction of quantum field theory, infamously off by 120 orders of magnitude. And as if that wasn’t embarrassing enough, this gives rise to, not one, but three problems: Why is the measured cosmological constant neither 1) huge nor 2) zero, and 3) Why didn’t this occur to us a billion years earlier? With that, you’d think that physicists have their hands full getting zeroes arranged correctly. But Niayesh Afshordi and Elliot Nelson just added to our worries.In a paper that made it third place of this year’s Buchalter Cosmology Prize, Afshordi and Nelson pointed out that the cosmological constant, if it arises from the vacuum energy of matter fields, should be subject to quantum fluctuations. And these fluctuations around the average are still large even if you have managed to get the constant itself to be small.
The cosmological constant, thus, is not actually constant. And since matter curves space-time, the matter fluctuations lead to space-time fluctuations – which can screw with our cosmological models. Afshordi and Nelson dubbed it the “Cosmological non-Constant Problem.”
But there is more to their argument than just adding to our problems because Afshordi and Nelson quantified what it takes to avoid a conflict with observation. They calculate the effect of stress-energy fluctuations on the space-time background, and then analyze what consequences this would have for the gravitational interaction. They introduce as a free parameter an energy scale up to which the fluctuations abound, and then contrast the corrections from this with observations, like for example the CMB power spectrum or the peculiar velocities of galaxy clusters. From these measurements they derive bounds on the scale at which the fluctuations must cease, and thus, where some new physics must come into play.
They find that the scale beyond which we should already have seen the effect of the vacuum fluctuations is about 35 TeV. If their argument is right, this means something must happen either to matter or to gravity before reaching this energy scale; the option the authors advocate in their paper is that physics becomes strongly coupled below this scale (thus invalidating the extrapolation to larger energies, removing the problem).
Unfortunately, the LHC will not be able to reach all the way up to 35 TeV. But a next larger collider – and we all hope there will be one! – almost certainly would be able to test the full range. As Niayesh put it: “It’s not a problem yet” – but it will be a problem if there is no new physics before getting all the way up to 35 TeV.
I find this an interesting new twist on the cosmological constant problem(s). Something about this argument irks me, but I can’t quite put a finger on it. If I have an insight, you’ll hear from me again. Just generally I would caution you to not take the exact numerical value too seriously because in this kind of estimate there are usually various places where factors of order one might come in.
In summary, if Afshordi and Nelson are right, we’ve been missing something really essential about gravity.