  When considering the statistical properties of a bundle of cosmic microwave
background (CMB) photons propagating through space, the effect of `mixing
geodesics' appears with a distinct signature that depends on the geometry of
space. In a Universe with negative curvature this effect is expected to produce
elongated anisotropy spots on CMB maps. We used COBE-DMR data to look for such
effect. Based on the analysis of a measure of eccentricity of hot spots it
appears that there is a clear indication of an excess eccentricity of hot spots
with respect to that expected from noise alone. This result must be interpreted
with caution as this effect can be due in part to galactic emission. If the
detected eccentricity of anisotropy spots can be attributed to the effect of
mixing it implies the negative curvature of the Universe and a value of $\Omega
< 1$.

Astrophysics