CMB Anisotropies and Large-Scale Structure in the Universe

By comparing CMB anisotropies with spatial distribution of galaxies, we can learn how fluctuations have evolved from the time of last scattering when the Universe was about 300,000 years old to the present day. The evolution of irregularities depends on the cosmological parameters, principally $\Omega_0$, $\Lambda$, and also on the matter and radiation content of the Universe, i.e. the precise mix of cold dark matter, baryonic material, massive neutrinos and relativistic components.

At present, however, large errors in both the CMB measurements and in the values of the cosmological parameters, lead to poor constraints on the form and evolution of the matter power spectrum. The situation will be improved dramatically by PLANCK as illustrated in Figure 1.13. The points in the Figure show constraints on the matter power spectrum from various galaxy redshift surveys, assuming that galaxies trace the matter fluctuations. The boxes in the left hand panel show constraints on the matter power-spectrum at the present day inferred from various CMB experiments, assuming that the Universe has a critical density ($\Omega=1$); COBE provides constraints on fluctuations with physical scales $\lambda \lower.5ex\hbox{$\; \buildrel \gt \over \sim \;$}1000 \,\hbox{\rm h}^{-1}\,\hbox{\rm Mpc}$, i.e. about ten times larger than the largest structures that have been observed in galaxy surveys. The boxes in the wavenumber range k = 0.01-$0.1 h {\rm Mpc}^{-1}$, sampling scales 100-$1000 \,\hbox{\rm h}^{-1}\,\hbox{\rm Mpc}$, show results from a number of balloon and ground based experiments with angular resolution of $\sim 1^\circ$(see also Observations of the CMB); these suggest a positive signal on such scales but with an uncertainty in the inferred matter power spectrum P(k) of nearly two orders of magnitude (since the matter power spectrum is proportional to $(\Delta T/T)^2$). The panel to the right shows the enormous improvement in accuracy that will be achieved by PLANCK, especially on physical scales of 100-$1000 \,\hbox{\rm h}^{-1}\,\hbox{\rm Mpc}$. Furthermore, since PLANCK will constrain the cosmological parameters to unprecedented precision, it will be possible to extrapolate the spectrum of irregularities to the present day modulo small residual uncertainties concerning the nature of the dark matter.

Over the next decade, we can expect dramatic improvements in our knowledge of the large-scale distribution of galaxies. At present, the largest redshift survey contains about 30,000 galaxies (Schectman et al. 1995). However, two large galaxy surveys, the Sloan Digital Sky Survey (Gunn and Weinberg 1995) and the Anglo-Australian 2-degree field survey (Efstathiou 1996), are about to begin which aim to measure redshifts of more than 10⁶ galaxies over the next few years. By combining the results of these new surveys with PLANCK it will be possible to establish a consistent theory of the formation of cosmic structure and so elucidate the nature of the dark matter that dominates the present Universe.

Figure 1.13: The boxes in the left hand panel show constraints on the power spectrum P(k) of the matter distribution in an $\Omega=1$ universe implied by observations of the microwave background anisotropies (adapted from White et al. 1994). The points show the power spectrum of the galaxy distribution determined from various galaxy surveys (see Efstathiou 1996). The right hand panel illustrates the accuracy with which PLANCK will be able to determine the power spectrum. The solid curve shows the matter power spectrum expected in an inflationary cold dark matter (CDM) universe. The dotted curve shows a theoretical prediction for a `mixed dark matter' (MDM) universe consisting of a mixture of CDM (60%), massive neutrinos (30%) and baryons (10%).

[last update: 1 August 1999 by P. Fosalba]