MASS FLUCTUATIONS ON 600 H(-1) MPC - A RESULT FROM CLUSTERS OF GALAXIES

Recently, many experimental results have shown that the nearby universe is highly structured, and quite far from being very smooth, as both experiment (cosmic background anisotropy measures) and theory (Friedmann-Robertson-Walker models) suggest it to be on large scales. Hence emphasis is put on what happens on the largest scales, where few astronomical data are available today. We present here a result on such very large scales, obtained by considering an all-sky, volume-limited sample of Abell clusters with a diameter of 600 h-1 Mpc. Through an estimate of mass fluctuations in the distant shell of depth 200-300 h-1 Mpc, we obtain relatively low values for mass fluctuations on scales of wavelength gamma almost-equal-to 800 h-1 Mpc, and an amplitude for expected cosmic microwave background (CMB) anisotropies consistent with present upper limits. The main result is that the spectrum of density fluctuations must have a turnover from that observed on small scales. Taking the figures at face value, we derive an estimate of the fluctuation level which yields sigma(300 h-1 Mpc) congruent-to 1 x 10(-2)/b(gp) in the scale-invariant spectrum case. The standard cold dark matter model has less power on these scales than is observed, by roughly a factor of 2, but it would be in agreement for a low value of the cosmic mass density, h-OMEGA-0 approximately 0.2. This value is in agreement with recent suggestions, from scales smaller than those considered here, by Efstathiou and coworkers, and which in this scenario leads us to consider the presence of a cosmological contrast to avoid conflicts with CMB anisotropies. Conversely, still keeping OMEGA-0 = 1, if the spectrum of density fluctuations, which on small scales has an observed index v congruent-to -1.2, is phenomenologically assumed to retain such a slope up to medium scales and then to become scale-invariant (n = 1) on the largest scales, we expect the break to take place at a wavelength gamma less than or similar to 300 h-1 M .