Can we be sure that decay times are constant?
Particle decay is clearly some statistical process. Generally speaking, particles are some stable solutions of some physics (like a field theory) - they are some local/global energy minimums for given constrains like spin or charge.
So from energetic point of view, particle decay should be getting out of some local energy minimum by crossing some energy barrier and finally reaching some lower energy minimum - just like in thermodynamics (?)
Energy required to cross such energy barrier usually comes from thermal noise - in case of particle decay there would be required some temperature of vacuum ...
Generally the universe is built not only of particles, but also can carry different interactions - EM, weak, strong, gravitational. This possibility itself gives vacuum huge amount of degrees of freedom - some fundamental excitations, which not necessarily have nonzero mass like photons ... and if there is some interaction between them, thermodynamics says that they should thermalize - their energy should equilibrate. We can measure thermal noise of EM part of these degrees of freedom - 2.725K microwave background, but degrees of freedom corresponding to the rest of interactions (weak, strong, gravitational) had billions of years to thermalize - should have similar temperature.
The EM part gives about 6*10^-5 of energy of vacuum required to obtain expected cosmological constant, maybe the rest of interactions carries the rest of it ...
Anyway we believe that this microwave background is cooling - so 'the temperature of universe' should so.
Shouldn't it became more difficult for particles to cross the energy barrier to get to a lower energy minimum? It would increase decay times ...
We have experimental evidence that physical constants like e,G are unchanged with time, but is it so with decay times?
Maybe radiometric dated things are a bit younger than expected...
Similar situation is for example for excited electrons ...
.