Expansion of the universe

The metric expansion of space is a feature of many theories of the universe. If accurate, it would explain how the universe expanded in the Big Bang and how it is expanding today. Current experiments and measurements tend to suggest that this model, or something similar to it, is indeed a good model for how the universe really is.

The expansion of space in this way is conceptually different than other kinds of expansions and explosions that are seen in the natural world. Current theories suggest that what we see as "space" and "distance" are not absolutes, but are determined by a metric that can change. Rather than objects in a fixed "space" moving apart into "emptiness", it is the metric of the space those objects are in, which is itself changing.

Theory suggests that in our universe, early in the Big Bang, there was an "inflationary" phase where this metric changed very rapidly for a short time, and it is this changing metric which we see as the Hubble effect, the moving apart of all objects in the universe. The metric continues to expand with passing time, meaning that objects appear to spread apart as a fundamental feature of the spacetime we inhabit. This expansion describes a universe fundamentally different from the static universe Einstein considered when he first developed his gravitational theory.

Because metric expansions of space are not seen on human-scales, the concept has been difficult to grasp for certain laypersons. A number of analogies have been developed to illustrate the ideas associated with the metric expansion. Each analogy has its conceptual benefits and drawbacks and it is important to note that as three-dimensional forms we have some limitations in our conceptual ability to imagine the consequences of metric expansion in such a space.

Overview

The metric expansion of space is a feature of certain solutions to the Einstein field equations of general relativity. In particular, if the cosmological principle is assumed with a time-varying universe the simplest solution allows for the proper distances in space to change with an evolving scale factor. This theoretical explanation allows for a clean explanation of the observed Hubble Law which indicates that galaxies that are more distant from us appear to be receding faster than galaxies that are closer to us.

The expansion of space in our universe proceeds as described by the Big Bang theory. However, the theoretical model which predicted a universe that was dynamical and contained ordinary gravitational matter would contract rather than expand. Einstein anticipated this and added a cosmological constant to balance out the contraction in order to obtain a static universe solution. It wasn't until the observations of Edwin Hubble confirmed a metric expansion of the universe that scientists began to realize that the universe was expanding. Until the 1980s, no one had an explanation for why the universe was expanding, but with the development of models of cosmic inflation, the expansion of the universe became a general feature that resulted from vacuum decay. Accordingly, the question "why is the universe expanding?" can be answered by understanding the details of the inflaton decay process that occurred in the first 10^-30 seconds of the existence of our universe.

Simple models commonly used to explain metric expansion

Ant on a balloon model

The ant on a balloon model is a two-dimensional analog for three-dimensional metric expansion. An ant is imagined to be constrained to move on the surface of a balloon which to the ant's understanding is the total extent of space. At an early stage of the balloon-universe, the ant measures a distances between three separate points on the balloon which serves as a standard by which the scale factor can be measured. The balloon is inflated and then the distance between the same points is measured and determined to be larger by a proportional factor. All three points have appeared to recede from the ant, indeed every point on the surface of the balloon is proportionally farther from the ant than earlier in the life of the balloon universe. This explains how an expanding universe can result in all points receding from each other simultaneously. No points are seen to get closer together.

The third dimension into which the balloon is expanding is not mathematically necessary for such an expansion to occur. THe ant that is confined to the surface of the balloon has no way of determining whether a third dimension exists or not. It may be useful to visualize a third dimension, but the fact of expansion does not theoretically require such a dimension to exist. This is why the question "what is the universe expanding into?" is poorly phrased. Expansion does not have to proceed "into" anything. The universe that we inhabit expands and gets larger, but that does not mean that there is a larger space into which it is expanding.

Raisin bread model

The raisin bread model imagines galaxies as raisins in a raisin bread dough that will "rise" or "expand" when cooked. As the expansion occurs, each of the raisins gets farther from each of the other raisins while the raisins themselves stay the same size. The dough between raisins in this model acts as the space between galaxies while the raisins as "bound objects" are not subject to the expansion. This model is useful for explaining how it is that a standard ruler can be determined for measuring the expansion. In an empty universe, space serves as the only ruler and as rulers expand with space, there would be no way to distinguish between an expanding universe and a static universe. Only in a universe where there are objects which are bound and do not expand so that the rulers are independent of the expansion can the metric expansion be measured.

Like the ant on the balloon model, this model also suffers from the problem that the raisin bread is expanding into the pan. To make the analogy to the universe, it is necessary to imagine a raisin bread that has no observable edge. Expansion would still occur, but the question "what is the raisin bread expanding into?" would be meaningless.