Main elements of the model are easily listed: in the large-scale average the universe is close to homogeneous, and has expanded in a near homogeneous way from a denser hotter state when the 3 K cosmic background radiation was thermalized.

The standard cosmology assumes conventional physics, including
general relativity theory. This yields a successful account of
the origin of the light elements, at expansion factor
*z* ~ 10^{10}. Light element formation tests the relativistic
relation between expansion rate and mass density, but this
is not a very searching probe. The
cosmological tests discussed in Section 3 could
considerably improve the tests of general relativity.

The model for the light elements seems to require that the mass density in baryons is less than that needed to account for the peculiar motions of the galaxies. It is usually assumed that the remainder is nonbaryonic (or acts that way). Our reliance on hypothetical dark matter is an embarrassment; a laboratory detection would be exceedingly welcome.

In the past decade many discussions assumed the Einstein-de Sitter case, in which there are negligibly small values for the curvature of sections of constant world time and Einstein's cosmological constant (or a term in the stress-energy tensor that acts like one). This is what most of would have chosen if we were ordering. But the evidence from the relative velocities of the galaxies has long been that the mass density is less than the Einstein-de Sitter value [3], and other more recent observations, notably the curvature of the redshift-magnitude relation ([1], [2]), point in the same direction. Now there is increasing interest in the idea that we live in a universe in which the dominant term in the stress-energy tensor acts like a decaying cosmological constant ([4] - [10]). This is not part of the standard model, of course, but as discussed in Section 3 the observations seem to be getting close to useful constraints on space curvature and .

We have good reason to think structure formation on the scale of galaxies and larger was a result of the gravitational growth of small primeval departures from homogeneity, as described by general relativity in linear perturbation theory. The adiabatic cold dark matter (ACDM) model gives a fairly definite and strikingly successful prescription for the initial conditions for this gravitational instability picture, and the ACDM model accordingly is widely used in analyses of structure formation. But we cannot count it as part of the standard model because there is at least one viable alternative, the isocurvature model mentioned in Section 3.3. Observations in progress likely will eliminate at least one, perhaps establish the other as a good approximation to how the galaxies formed, or perhaps lead us to something better.

The observational basis for this stripped-down standard model is reviewed in references [11] and [12]. Here I comment on some issues now under discussion.