Physics Blog Number 6 - December 5, 2011

The Big Bang

Edward Harrison has a nice paper (Astrophysical Journal, 403 28-31 1993) discussing the fact that Hubble's law, while originally formulated as a relationship between redshift and distance, is more correctly a linear relationship between recession velocity and distance.  For small redshifts, both relationships are linear, but the velocity-distance relationship is linear for all distances.

Most interestingly, he has a nice description of the Big Bang, nicely explaining the 'expanding space paradigm,' in which space expands, rather than the universe expanding into space:

"From a purist point of view one cannot but deplore the expression 'big bang,' loaded with inappropriate connotations, which conjures up a false picture of a bounded universe exploding from a center in space.  In modern cosmology, the universe does not expand in space, but consists of expanding space* ... "

*The theory of general relativity brought the insight that space and time are not merely the stage on which the piece is produced, but are themselves actors playing an essential part in the plot.  --- de Sitter, 1931

and:

"The expanding space paradigm emerged during the formative stages of modern cosmology amidst the controversy concerning the physical meaning of the extragalactic redshifts... Milne revolted against attributing to space ... the properties of curvature and expansion, and in protest developed a theory--kinematic relativity--that rejected general relativity.  Of the expanding space paradigm, he said, 'This concept though mathematically significant has by itself no physical content; it is merely the choice of a particular mathematical apparatus for describing and analyzing phenomena. An alternative procedure is to choose a static space, as in ordinary physics, and analyse the expansion phenomena as actual motions in this space.' Rejection of the expanding space paradigm in favor of Milne's picture of expansion in fixed Euclidean space contradicts general relativity and leads to the conclusion that the universe possesses a center and an edge.  In numerous popular but misleading treatments, Milne's center is associated with the big bang."

For more information, read the Lineweaver and Davis article in Scientific American, March 2005.