FIGURE 3. A compilation of density fluctuations on different scales from various observations.

Shown are data from a galaxy survey, deep radio surveys, the X-ray background (XRB) and cosmic microwave background (CMB) experiments. The measurements are compared with two popular cold dark matter (CDM) models. The figure shows mean-square density fluctuations (/)². The solid and dashed lines correspond respectively to the standard CDM power spectrum (with shape parameter = 0.5) and a 'low-density' CDM power spectrum (with = 0.2). Both models are normalized such that the r.m.s. fluctuation within 8 h^-1 Mpc spheres is _8,M = 1. The open squares at small scales are estimates of the power spectrum from three-dimensional inversion of the angular APM galaxy catalogue¹²,¹³. The elongated 'boxes' at large scales represent the COBE 4-yr (refs 57, 90, 91) (on the right) and Tenerife⁹² (on the left) CMB measurements. The filled triangles represent constraints from the quadrupole moment of the distribution of radio sources³². This quadrupole measurement probes fluctuations on scale _* 600 h^-1 Mpc. The top and bottom filled triangles are upper limits of the amplitude of the power spectrum at _*, assuming CDM power spectra with shape parameters = 0.2 and 0.5, respectively, and an Einstein-de Sitter universe. The crosses represent constraints from the XRB HEAO1 quadrupole³⁸,⁴². Assuming evolution, clustering and epoch-dependent biasing prescriptions, this XRB quadrupole measurement probes fluctuations on scale _* 600 h^-1 Mpc, very similar to the scale probed by the radio sources. The top and bottom crosses are estimates of the amplitude of the power spectrum at _*, assuming CDM power spectra with shape parameters = 0.2 and 0.5 respectively, and an Einstein-de Sitter universe. The fractional error on the XRB amplitudes (due to the shot noise of the X-ray sources) is 30%.