Talk:Hubble's law

Hubble's constant does indeed decrease over time, even in the presence of a non-zero cosmological constant. The cosmological constant causes Hubble's constant to decrease slower (resulting in an accelerating expansion), but a constant or even increasing Hubble's constant is not contained in any of the current models, and would have extremely strange consequences. --AxelBoldt

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You are right, sorry. I forgot that hubble parameter is (da/dt)/a, not just da/dt.--AN

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I removed this:

although some cosmologists have wondered if the expansion of universe is uniform throughout the whole universe. These speculations have been driven in part by the observation that the expansion of the universe seems to be accelerating at least in our section of it.

Have people seriously conjectured that the universe is not homogeneous? Why would the acceleration lead to that conjecture? AxelBoldt

• My understanding is that on large scales, it's been observed to be remarkably homogeneous. (I'm thinking of the cosmic microwave background here.) Fluctuations in the microwave backgrounds are on scales of thousandths (or less!) of the mean value. The distribution of matter is of course not homogenous, but I can't say I've ever run across serious suggestions that the expansion of spacetime might be unequally distributed. Comments on the accelerating universe and cosmological constant seem to cover this. -- April

• I might add that the whole apparatus of cosmology now rest on the cosmological principle: the statement (assumed true for the construction of the theory) that the Universe is isotropic and (at least at large scales), homogeneous. AstroNomer

But this is almost a universal assumption by science, isn't it? That there are no special points in spacetime. Physicists assume that an experiment done on Friday in Berkeley will come out the same if done on Monday in Munich; biologists assume that the laws of chemistry were valid millions of year ago; astronomers assume that the nuclear reactions in distant stars are the same as the ones observed on Earth etc. Of course, this is not just an arbitrary assumption, because many things make eminent sense when interpreted with this assumption. AxelBoldt