Next Contents Previous

3.4. Problems with nu DM

A number of potential problems with the neutrino dominated universe had emerged by about 1983, however.

These problems, while serious, would perhaps not have been fatal for the hot DM scheme. But an even more serious problem for HDM arose from the low amplitude of the CMB fluctuations detected by the COBE satellite, (DeltaT / T)rms = (1.1 ± 0.2) x 10-5 smoothed on an angular scale of about 10° [55]. Although HDM and CDM both have the Zel'dovich spectrum shape (P(k) propto k) in the long-wavelength limit, because of the free-streaming cutoff the amplitude of the HDM spectrum must be considerably higher in order to form any structure by the present. With the COBE normalization, the HDM spectrum is only beginning to reach nonlinearity at the present epoch.

Thus the evidence against standard hot DM is convincing. At very least, it indicates that structure formation in a neutrino-dominated universe must be rather more complicated than in the standard inflationary picture.

The main alternative that has been considered is cosmic strings plus hot dark matter. Because the strings would continue to seed structure up until the present, and because these seeds are in the nature of rather localized fluctuations, hot DM would probably work better with string seeds than cold DM. However, strings and other cosmic defect models are now essentially ruled out [56, 57] because they predict that the cosmic microwave background would have an angular power spectrum without the pronounced (doppler/acoustic/Sakharov) peak at angular wavenumber l ~ 220 that now appears to be clearly indicated by the data, along with secondary peaks at higher l.

Next Contents Previous