Abstract

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 counts^2,3) and that there are about 10⁹¹ photons per baryon⁴. Penrose and others^5,6 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 conserved^7,8 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.