A. Key factors

There are several important factors to support our current view of cosmic structure formation:

• The discovery by Hubble in 1928 of the linear velocity-distance relationship for galaxies (Hubble, 1929). This relationship was soon interpreted by Robertson (1928) as being due to the expansion of the Universe in the manner described by the Friedman-Lemaitre cosmological solutions of the Einstein Field equations for gravitation. These solutions described a homogeneous and isotropic Universe emerging from a singular state of infinite density: the Big Bang. Later on, Bondi and Gold (1948) and Hoyle (1948) provided an alternative homogeneous and isotropic expanding model that avoided the initial singularity: the Steady State Theory.

• The discovery in 1965 of the Cosmic Microwave Background Radiation tells us the cosmological framework within which we have to work. Our Universe is, in the large, homogeneous and isotropic; it was initially hot enough to synthesize the element Helium. This is the Hot Big Bang theory promoted early on by Gamow. This discovery signaled the end of the Steady State Theory.

• The observation in 1992 by the COBE satellite of the large-scale structure of the Universe at very early times provides us with precise information about the initial conditions for structure formation. This is ongoing research that will lead to detailed knowledge of the fundamental parameters of our Standard Model and to detailed knowledge of the initial conditions in the Big Bang that resulted in the currently observed structure.

We know a great deal about our Universe. Studies of cosmic structure must fall within the precepts set by our Standard Model or they will simply be dismissed at best as being academic curiosities or at worst as being totally irrelevant.