The ‘wildest excuse for cosmic acceleration’ prize so far should go, in my view, to these guys. They argue that the metric form of spacetime is about to flip from (-+++) to (++++), which they say produces the appearance of an accelerated expansion. Here’s a little background on why they’re suggesting this.
Cosmic Acceleration: a very brief background. Around 1998, the majority of physicists accepted that expansion of the Universe is accelerating. Since then, all bets are off as to the large scale structure of the spacetime. The experts simply don’t agree as to what kind of Universe we live in, and it sometimes seems that the wilder the proposal, the better. This makes peering over the fence at the zoo of cosmological theories rather entertaining.
There are a lot of competing theories out there — that exotic matter causing negative vacuum pressure is spread throughout spacetime (this is the “dark energy” that you hear so much hype about), that backreactions due to the inhomogeneities of the early Universe gave the expansion an extra kick, or that the speed of light is not equal to c on cosmic scales (so that we’re not interpreting our data correctly).
But a changing metric signature? As it turns out, this idea isn’t totally unheard of; as of today there seem to be around 50 papers on arXiv dealing with models of quantum gravity in which the signature of spacetime changes. It just wasn’t clear until recently that this could be used to account for anything to do with the accelerated expansion.
(Wait a minute — so why didn’t a treatment of changing signature appear in my general relativity textbook? Well, the metric signature doesn’t change in traditional general relativity. Most derivations of the Einstein Field Equations assume non-degeneracy of the metric, which is sufficient to fix the metric form for all of time. So the approach to cosmic acceleration here falls under the category of — *gasp* — changing the field equations.)
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