THE temperature of the cosmic microwave background – the radiation bathing all of space – is remarkably uniform. It varies by less than 0.001 degrees from a chilly 2.725 kelvin.
But while that might seem natural enough, this consistency is a real puzzle. For two widely separated areas of the cosmos to reach thermal equilibrium, heat needs enough time to travel from one to the other. Even if this happens at the speed of light, the universe is just too young for this to have happened.
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Cosmologists try to explain this uniformity using the hypothesis known as inflation. It replaces the simple idea of a big bang with one in which there was also a moment of exponential expansion. This sudden, faster-than-light increase in the size of the universe allows it to have started off smaller than an atom, when it would have had plenty of time to equalise its temperature.
“On the face of it, inflation is a totally bonkers idea – it replaces a coincidence with a completely nonsensical vision of what the early universe was like,” says Andrew Pontzen at University College London.
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Yet it seems to work. The sudden expansion would also help explain why the observable universe appears flat (see “The universe is flat as a pancake. Coincidence?“). Much as the curvature of Earth only becomes visible when viewed on a large scale, space-time’s complex geometry might appear flattened to observers only viewing a fraction of the whole.
What’s more, it even explains how galaxies sprang up, as tiny quantum fluctuations in space blew up to form their gravitational seeds.
But not everyone is a fan. “Although the inflationary paradigm is in good agreement with data, it suffers from some conceptual problems,” says Robert Brandenberger at McGill University in Montreal, Canada.
The chief snag is that no one can find a mechanism in established physics that could trigger inflation. “There are many toy models of inflation,” says Brandenberger, but it is unclear whether any of them have a solid theoretical basis.
To kick-start inflation, something needs to put a tremendous amount of energy into the vacuum. The culprit may be a process known as baryogenesis, which was responsible for bringing more matter than antimatter into being – but as no one knows exactly how it took place, this remains little more than a guess. And even if we can identify the trigger, working out why inflation didn’t carry on forever has also proved beyond us.
To Pontzen, however, inflation is worth sticking with because it fits the data. “That’s about the best criteria we have for judging a theory,” he says.
But there’s another contender that cosmologists have been considering: a variable speed of light. If we assume that light travelled much faster in the past, then heat could have been transferred between remote reaches of the universe in time to equalise their temperatures. The trouble is that, since we measure light’s speed as a constant today, its value must either be decreasing at an ever slower rate, or have ceased to change some time ago. But as with inflation, no one can find a theoretical reason for such perplexing behaviour.
This article appeared in print under the headline “It’s all at the same temperature”
