Big Bang Cosmology

Problems

The standard Big Bang cosmology has three major problems that are known as

    1) the horizon problem,
    2) the smoothness problem and
    3) the flatness problem.

The Horizon Problem: The microwave background radiation is observed to be isotropic to a very high precision.

We believe that the only plausible way to get such a high degree of isotropy in the radiation is to suppose that the radiation from every part of the universe was at some time in mutual interaction. The interaction continued until the radiation achieved a uniform temperature throughout the universe.

The problem, however, is that there has not been enough time since the beginning of the universe for radiation to have traversed the space between points on opposite sides of the universe, not even once. So it is not clear how the isotropy could have arisen.

The Smoothness Problem: We observe galaxies, clusters of galaxies, and clusters of clusters of galaxies.

These structures must have arisen from tiny variations in the energy density in the early universe. Where the densities were greatest is, presumably, where gravity caused matter to collapse into the structures we see today.

The problem is that to explain these structures seems to require a universe that was created in an incredibly smooth non-chaotic manner. This seems extremely unlikely.

The Flatness Problem: Why is the geometry of the universe so flat, or equivalently, why is the observed energy density of the universe so close to the critical density

rc = 3H²/8p G

where H is Hubble's constant and G is Newton's gravitational constant?

Let the actual density of the universe be r. We can define a parameter W = r /rc that measures how close the actual density is to the critical density. The value of W determines the global geometry of space. In particular, for W = 1 the universe will have a spatial geometry that is flat.

Current measurements suggest that 0.1 < W < 2.

The problem with this value of W is this: to get an W in that range at the present epoch requires a value of W that differs from unity by less than one part in a trillion when the universe was no more than one second old! The Big Bang cosmology cannot explain why W in the past is so close, but not exactly equal, to one.

These three problems deal with the initial conditions of the universe's history.

One solution is simply to assert that the universe just started with these highly unusual conditions. But most cosmologists do not find this satisfactory and want to find a deeper explanation. They want to know if the observable universe is inevitable or if is it merely an extraordinarily wonderful accident.