  It has become common understanding that the recession of galaxies and the
corresponding redshift of light received from them can only be explained by an
expansion of the space between them and us. In this paper, for the presently
favored case of a universe without spatial curvature, it is shown that this
interpretation is restricted to comoving coordinates. It is proven by
construction that within the framework of general relativity other coordinates
exist in relation to which these phenomena can be explained by a motion of the
cosmic substrate across space, caused by an explosion like big bang or by
inflation preceding an almost big bang. At the place of an observer, this
motion occurs without any spatial expansion. It is shown that in these
"explosion coordinates" the usual redshift comes about by a Doppler shift and a
subsequent gravitational shift. Making use of this interpretation, it can
easily be understood why in comoving coordinates light rays of short spatial
extensions expand and thus constitute an exemption from the rule that small
objects up to the size of the solar system or even galaxies do not participate
in the expansion of the universe. It is also discussed how the two
interpretations can be reconciled with each other.
