We don't need no stinkin dark energy II
David Wiltshire has opened a new front in the dark energy wars with gr-qc/0503099. He has an allegedly simple and exact model using the Kolb,
Matarrese, Notari and Riotto (KMNR) idea of super-Hubble density variations to explain apparent cosmic acceleration without dark energy.
I have been trying to understand his "spirit of Mach's principle" assumption. My current guess goes like this: At the end of inflation, the universe was nearly flat and uniform in density, so that FRW "comoving" coordinates closely approximated the actual geodesics. As the density variations amplified with time, these coordinates increasingly deviate from geodesics. Here is what I consider the money quote: "...an observer who would measures
no dipole anisotopy even in the observable universe, must
be referred to the time, t..." I think that means he is assuming that the observers measuring no dipole anisotropy are the ones moving along those (non-geodesic) FRW t coordinates. Meanwhile, back in the underdense bubble, our geodesics are departing from these, and are more conveniently described by a different set of FRW coordinates applying only to our bubble and which are boosted wrt the large-scale FRW coordinates. It would seem to me that this implies that observers in our local bubble who look FRW, presumeably including us, should see a dipole anisotropy. I don't know if that can be reconciled with the small dipole anisotropy we do see.
Or maybe I'm just confused. Oh wait, on that point I can give a definite yes.
I don't know if his assumption is correct, wrong, or even nonsensical, but if one of the latter, I should hope one of the smart guys could soon point out the specific error.
Matarrese, Notari and Riotto (KMNR) idea of super-Hubble density variations to explain apparent cosmic acceleration without dark energy.
I have been trying to understand his "spirit of Mach's principle" assumption. My current guess goes like this: At the end of inflation, the universe was nearly flat and uniform in density, so that FRW "comoving" coordinates closely approximated the actual geodesics. As the density variations amplified with time, these coordinates increasingly deviate from geodesics. Here is what I consider the money quote: "...an observer who would measures
no dipole anisotopy even in the observable universe, must
be referred to the time, t..." I think that means he is assuming that the observers measuring no dipole anisotropy are the ones moving along those (non-geodesic) FRW t coordinates. Meanwhile, back in the underdense bubble, our geodesics are departing from these, and are more conveniently described by a different set of FRW coordinates applying only to our bubble and which are boosted wrt the large-scale FRW coordinates. It would seem to me that this implies that observers in our local bubble who look FRW, presumeably including us, should see a dipole anisotropy. I don't know if that can be reconciled with the small dipole anisotropy we do see.
Or maybe I'm just confused. Oh wait, on that point I can give a definite yes.
I don't know if his assumption is correct, wrong, or even nonsensical, but if one of the latter, I should hope one of the smart guys could soon point out the specific error.
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