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By Root 2003 0

I can imagine your saying, “Well, that’s just the cosmological principle at work: no location in the universe is special when compared with any other, so the temperature at each should be the same.” Fair enough. But remember that the cosmological principle was a simplifying assumption that physicists, including Einstein, invoked to make the mathematical analysis of the universe’s evolution tractable. Since the microwave background radiation is indeed uniform throughout space, it provides convincing observational evidence for the cosmological principle, and it strengthens our confidence in conclusions the principle helped reveal. But the radiation’s astounding uniformity shines a glaring spotlight on the cosmological principle itself. Reasonable though the cosmological principle may sound, what mechanism established the cosmos-wide uniformity that observations confirm?

Faster Than the Speed of Light

We’ve all had the mildly unsettling sensation of shaking someone’s hand and finding it steamy hot (not so bad) or clammy cold (definitely worse). But were you to hold on to that hand, you’d find that the modest temperature differential would quickly subside. When objects are in contact, heat migrates from the hotter to the colder, until their temperatures are equal. You experience this all the time. It’s why coffee left on your desk eventually comes to room temperature.

Similar reasoning would seem to explain the uniformity of the microwave background radiation. As with holding hands and standing coffee, the uniformity presumably reflects the familiar reversion of an environment to an overall common temperature. The sole novelty of the process is that the reversion is supposed to have taken place over cosmic distances.

In the big bang theory, however, the explanation fails.

For places or things to reach a common temperature, an essential condition is mutual contact. It may be direct, as with shaking hands, or, minimally, through an exchange of information so that conditions at distinct locations can become correlated. Only through such mutual influence can a shared, communal environment be achieved. A thermos is designed to prevent such interactions, thwarting the drive to uniformity and preserving temperature differences.

This simple observation highlights the problem with the naïve explanation of the cosmic temperature uniformity. Locations in space that are very far apart—say, one point way off to your right, so deep in the night sky that the first light it ever emitted has only just reached you, and a second, similar point way off to your left—have never interacted. Although you can see both, light from one still has an enormous distance to cover before it reaches the other. Thus, hypothetical observers situated at the distant left and right locations have yet to see each other, and since the speed of light sets the upper limit for how fast anything can travel, they’ve yet to interact in any way. To use the language of the previous chapter, they are beyond each other’s cosmic horizon.

This description makes the mystery manifest. You’d be floored if inhabitants of these distant locations spoke the same language and had libraries filled with the same books. With no contact, how could a common heritage have been established? You should be equally floored to learn that without any apparent contact, these widely separated regions share a common temperature, one that matches to an accuracy of better than four decimal places.

Years ago, when I first learned of this puzzle, I was floored. But on further thought, I became puzzled by the puzzle. How could two objects that were once close together—as we believe all things in the observable universe were at the time of the big bang—have separated so quickly that light emitted by one wouldn’t have time to reach the other? Light sets the cosmic speed limit, so how could the objects achieve a spatial separation greater than what light would have had time to traverse?

The answer highlights a point that’s often not adequately stressed. The speed limit set by light refers solely to