marcus said:
does anyone want to explain a little?
What I do not understand at the moment is how one can have an ellipsoidal SoLS.
they say that a weak magnetic field throughout the relevant portion of space could be one thing that might cause this eccentricity---would you like to say in elementary fashion how a magnetic field would do that?
Let me see if I can write something that complements what Garth wrote.
Friedmann-Robertson-Walker models of the universe assume spatial homogeneity and isotropy. I guess they are popular for a number of reasons: they exhibit a sort of Copernican cosmological principle; the symmetries make them easy to analyze; they model observations fairly well.
But nowhere is it written in stone that the universe has to be so simple.
This paper models an ansitropic universe that has two scale factors, a and b, and that has spacetime metric
ds^2 = dt^2 - a^2 \left( t \right) \left( dx^2 + dy^2 \right) -b^2 \left( t \right) dz^2.
Energy-monemtum tensors that give rise to this solution to Einstein's equation have an an isotropic part, from stuff like dark energy and normal matter (galaxies), and an anisotropic part. Uniform magnetic fields can give rise to suitable anistropic energy-momentum tensors.
The paper assumes that at the present instant in cosmic time, a and b are equal, so that the spatial geometry is presently spherical. If the scale factors evolved at different rates, then in the past, in particular at the time of last scattering, a and b were different, and the spatial geometry of the universe was ellipsoidal.
The paper also looks at the physical reasonableness of a magnetic field that does the job. The paper says that if the magnitude of the magnetic field evolves in time as the inverse of the square of the scale factors, then, to within an order of magnitude, the cosmic magnetic field presently observed seems to be appropriate.
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