1.2. Big Bang: Guilty of Not Having an Explanation

"...the standard big bang theory says nothing about what banged, why it banged, or what happened before it banged. The inflationary universe is a theory of the "bang" of the big bang." - Alan Guth (1997).

Although the standard big bang model can explain much about the evolution of the Universe, there are a few things it cannot explain:

• The Universe is clumpy. Astronomers, stars, galaxies, clusters of galaxies and even larger structures are sprinkled about. The standard big bang model cannot explain where this hierarchy of clumps came from- it cannot explain the origin of structure. We call this the structure problem.
• In opposite sides of the sky, the most distant regions of the Universe are at almost the same temperature. But in the standard big bang model they have never been in causal contact - they are outside each other's causal horizons. Thus, the standard model cannot explain why such remote regions have the same temperature. We call this the horizon problem.
• As far as we can tell, the geometry of the Universe is flat - the interior angles of large triangles add up to 180°. If the Universe had started out with a tiny deviation from flatness, the standard big bang model would have quickly generated a measurable degree of non-flatness. The standard big bang model cannot explain why the Universe started out so flat. We call this the flatness problem.
• Distant galaxies are redshifted. The Universe is expanding. Why is it expanding? The standard big bang model cannot explain the expansion. We call this the expansion problem.

Thus the big bang model is guilty of not having explanations for structure, homogeneous temperatures, flatness or expansion. It tries - but its explanations are really only wimpy excuses called initial conditions. These initial conditions are

• the Universe started out with small seeds of structure
• the Universe started out with the same temperature everywhere
• the Universe started out with a perfectly flat geometry
• the Universe started out expanding

Until inflation was invented in the early 1980's, these initial conditions were tacked onto the front end of the big bang. With these initial conditions, the evolution of the Universe proceeds according to general relativity and can produce the Universe we see around us today. Is there anything wrong with invoking these initial conditions? How else should the Universe have started? The central question of cosmology is: How did the Universe get to be the way it is? Scientists have made a niche in the world by not answering this question with "That's just the way it is." And yet, that was the nature of the explanations offered by the big bang model without inflation.

"The horizon problem is not a failure of the standard big bang theory in the strict sense, since it is neither an internal contradiction nor an inconsistency between observation and theory. The uniformity of the observed universe is built into the theory by postulating that the Universe began in a state of uniformity. As long as the uniformity is present at the start, the evolution of the Universe will preserve it. The problem, instead, is one of predictive power. One of the most salient features of the observed universe - its large scale uniformity - cannot be explained by the standard big bang theory; instead it must be assumed as an initial condition."
- Alan Guth (1997)

The big bang model without inflation has special initial conditions tacked on to it in the first picosecond. With inflation, the big bang doesn't need special initial conditions. It can do with inflationary expansion and a new unspecial (and more remote) arbitrary set of initial conditions - sometimes called chaotic initial conditions - sometimes less articulately described as `anything'. The question that still haunts inflation (and science in general) is: Are arbitrary initial conditions a more realistic ansatz? Are theories that can use them as inputs more predictive? Quantum cosmology seems to suggest that they are. We discuss this issue more in Section 6.