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On the isotropy and homogeniety of the Universe


It is generally believed that the Universe began from a hot big bang. Among the clues to its very early history are the present day observations that the Universe is isotropic and homogeneous (as shown by the 3 K microwave background1 and galaxy counts2,3) and that there are about 109±1 photons per baryon4. Penrose and others5,6 argue that these two observations imply that the Universe was initially very nearly isotropic and homogeneous. Their arguments rely on absolute baryon conservation so that the baryon number of the Universe can be used as a fiducial to which the number of photons can be compared. In recently proposed grand unified theories of particle interactions, baryon number is not absolutely conserved7,8 and thus the baryon number of the Universe is no longer a constant. It is argued here that in the absence of this fiducial the photonJbaryon ratio is not a ‘footprint’ of the initial conditions, and therefore the initial singularity could well have been highly irregular as Misner suggested9.

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Turner, M. On the isotropy and homogeniety of the Universe. Nature 281, 549–550 (1979).

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