The Expansion of the Universe

The idea that such an episode of inflation might have occurred was first proposed in 1980.

It was based on considerations that go beyond Einstein’s General Relativity and take into account aspects of quantum theory.

We do not have a complete quantum theory of gravity as the details are still being worked out. Physicists aren’t sure exactly how inflation happened.

But according to the theory, the expansion caused by inflation would not be completely uniform, as predicted by the traditional big bang picture.

These irregularities would produce minuscule variations in the temperature of the CMBR in different directions. The variations are too small to have been observed in the 1960s, but they were first discovered in 1992 by NASA’s COBE satellite, and later measured by its successor, the WMAP satellite, launched in 2001.

As a result, we are now confident that inflation really did happen.

Ironically, though tiny variations in the CMBR are evidence for inflation, one reason inflation is an important concept is the nearly perfect uniformity of the temperature of the CMBR. If you make one part of an object hotter than its surroundings and then wait, the hot spot will grow cooler and its surroundings warmer until the temperature of the object is uniform.

Similarly, one would expect the universe to eventually have a uniform temperature. But this process takes time, and if inflation hadn’t occurred, there wouldn’t have been enough time in the history of the universe for heat at widely separated regions to equalize, assuming that the speed of such heat transfer is limited by the speed of light.

A period of very rapid expansion (much faster than light speed) remedies that because there would have been enough time for the equalization to happen in the extremely tiny preinflationary early universe.

Inflation explains the bang in the big bang, at least in the sense that the expansion it represents was far more extreme than the expansion predicted by the traditional big bang theory of general relativity during the time interval in which inflation occurred. The problem is, for our theoretical models of inflation to work, the initial state of the universe had to be set up in a very special and highly improbable way.

Thus traditional inflation theory resolves one set of issues but creates another—the need for a very special initial state. That time-zero issue is eliminated in the theory of the creation of the universe we are about to describe.

Since we cannot describe creation employing Einstein’s theory of general relativity, if we want to describe the origin of the universe, general relativity has to be replaced by a more complete theory.

One would expect to need a more complete theory even if general relativity did not break down, because general relativity does not take into account the small-scale structure of matter, which is governed by quantum theory.

We mentioned in Chapter 4 that for most practical purposes quantum theory does not hold much relevance for the study of the large-scale structure of the universe because quantum theory applies to the description of nature on microscopic scales.

But if you go far enough back in time, the universe was as small as the Planck size, a billion-trillion-trillionth of a centimeter, which is the scale at which quantum theory does have to be taken into account.

So though we don’t yet have a complete quantum theory of gravity, we do know that the origin of the universe was a quantum event. As a result, just as we combined quantum theory and general relativity—at least provisionally—to derive the theory of inflation, if we want to go back even further and understand the origin of the universe, we must combine what we know about general relativity with quantum theory.

To see how this works, we need to understand the principle that gravity warps space and time. Warpage of space is easier to visualize than warpage of time. Imagine that the universe is the surface of a flat billiard table. The table’s surface is a flat space, at least in two dimensions.

If you roll a ball on the table it will travel in a straight line. But if the table becomes warped or dented in places, as in the illustration below, then the ball will curve.

It is easy to see how the billiard table is warped in this example because it is curving into an outside third dimension, which we can see. Since we can’t step outside our own space-time to view its warpage, the space-time warpage in our universe is harder to imagine. But curvature can be detected even if you cannot step out and view it from the perspective of a larger space.

It can be detected from within the space itself. Imagine a micro-ant confined to the surface of the table. Even without the ability to leave the table, the ant could detect the warpage by carefully charting distances.

For example, the distance around a circle in flat space is always a bit more than three times the distance across its diameter (the actual multiple is π). But if the ant cut across a circle encompassing the well in the table pictured above, it would find that the distance across is greater than expected, greater than one-third the distance around it.

In fact, if the well were deep enough, the ant would find that the distance around the circle is shorter than the distance across it. The same is true of warpage in our universe—it stretches or compresses the distance between points of space, changing its geometry, or shape, in a way that is measurable from within the universe. Warpage of time stretches or compresses time intervals in an analogous manner.

Armed with these ideas, let’s return to the issue of the beginning of the universe. We can speak separately of space and time, as we have in this discussion, in situations involving low speeds and weak gravity.

In general, however, time and space can become intertwined, and so their stretching and compressing also involve a certain amount of mixing. This mixing is important in the early universe and the key to understanding the beginning of time.

The issue of the beginning of time is a bit like the issue of the edge of the world. When people thought the world was flat, one might have wondered whether the sea poured over its edge.

This has been tested experimentally: One can go around the world and not fall off. The problem of what happens at the edge of the world was solved when people realized that the world was not a flat plate, but a curved surface. Time, however, seemed to be like a model railway track.

If it had a beginning, there would have to have been someone (i.e., God) to set the trains going. Although Einstein’s general theory of relativity unified time and space as space-time and involved a certain mixing of space and time, time was still different from space, and either had a beginning and an end or else went on forever.

However, once we add the effects of quantum theory to Gemeral Relativity, in extreme cases warpage can occur to such a great extent that time behaves like another dimension of space.