Everything but the Unicorn

David Kaiser

The Cosmic Microwave Background – as seen by Planck. Credit: ESA and the Planck Collaboration

Last month an international team of physicists and astronomers working with the Planck satellite released a remarkable set of baby photos: images of the universe taken with light emitted when it was a mere 378,000 years old, less than 0.003 per cent of its present age.

The light is known as the cosmic microwave background radiation, or CMB. Using data collected by the Planck team, as well as measurements by several other scientific collaborations, we know more than ever before about the fundamental forces that govern the universe – right down to the quantum level.

Nearly a century ago, while working out his general theory of relativity, Einstein concluded that space and time were as wobbly as a trampoline: spacetime could warp or distend in the presence of matter and energy. Around the same time, Max Planck helped usher in the quantum revolution with his studies of blackbody radiation: the characteristic glow emitted by objects that have been heated up. What ultimately struck Planck was the universal nature of this radiation: embers heated in a fireplace would approximate his newfound pattern, as would stars glowing in the sky. For a given temperature, his equation predicted the intensity of light that should be emitted at various wavelengths, accounting for why smoldering embers emit more yellow and orange light than blue or green.

Data from the Planck satellite confirm with breathtaking precision that the universe began in an incredibly hot and dense state. The remnant radiation from that hot big bang – the CMB – is the best approximation to a true blackbody ever observed. (A true blackbody is one that radiates in exactly the pattern laid out in Planck's equation. Most substances depart from that pattern at least a little bit at some part of the frequency range; none match the equation so well across the entire frequency range as the CMB.) Detectors on the Planck satellite also measured minuscule deviations from the familiar blackbody pattern: fluctuations around the average temperature of the sky of one part in ten thousand.

Those tiny fluctuations, much like the blackbody pattern itself, almost certainly arise from quantum effects. According to Heisenberg’s uncertainty principle, matter is subject to an unavoidable jitteriness, like a small child who can’t sit still. We rarely notice such quantum jitters in everyday life, though they can be measured precisely in a laboratory, subtly affecting the behaviour of atoms or elementary particles.

Whatever type of matter dominated the universe during its earliest moments, it would by necessity have suffered quantum fluctuations: there would have been slightly more matter or energy in some regions of space than others. That mass of jiggling matter, in turn, would have induced little ripples in spacetime. Since the 1980s, physicists have made increasingly precise calculations of the pattern of fluctuations that these quantum jitters should have left in the CMB.

The ripples’ wavelength would have started out tiny, much less than the size of an atom. But according to physicists’ reigning account of the early universe, the fluctuations would have stretched to astrophysical scales incredibly quickly during an early, lightning-fast period of expansion of the universe known as inflation.

Stretched to enormous wavelengths, those minute imbalances in the distribution of matter and energy would seed galaxy formation. Regions of the sky with a tiny excess of mass would attract additional matter over time, clumping into dense conglomerations. The CMB captures a portrait of those primordial wiggles early in their evolution.

To make a specific, quantitative prediction about the pattern of the primordial wiggles, one has to know – or guess – what types of matter filled the universe during its earliest moments. And that’s where the other big scientific news from last month comes in.

Independent of the Planck satellite collaboration, two large groups of physicists working with the Large Hadron Collider (LHC) at CERN confirmed that the new particle they had discovered last summer looks remarkably like the long-sought Higgs particle, which theorists had predicted fifty years ago. Last July the experimenters had talked of a Higgs-like particle, or a particle with Higgsian properties, careful locutions that reminded me of ‘the artist formerly known as Prince’. With trillions more datapoints in hand, the teams announced a few weeks ago that the new particle is almost certainly a genuine Higgs particle.

The confirmation of the Higgs-particle detection is remarkably felicitous timing. When Alan Guth introduced his first model of cosmic inflation more than thirty years ago, he assumed that the early universe had been filled with Higgs particles – a speculative leap at the time. Only matter like the Higgs could account for inflation and for the types of quantum fluctuation needed to seed galaxy formation. Reasoning about the behaviour of Higgs particles now, in light of the news from CERN, is on firmer footing than ever.

No competing explanation for the large-scale structure in our universe has ever come close to the incredible match between predictions of cosmic inflation and the observed fluctuations in the CMB. In particular, the Planck satellite data confirm a key prediction of inflation to unprecedented accuracy. Inflationary models predict that longer-wavelength jiggles should have a slightly larger amplitude than shorter-wavelength ones, with a special ‘tilt’ parameter between 0.03 and 0.05. The Planck team measured a tilt of 0.0397 ± 0.0073. Given the precision of that measurement, the odds that the real wiggles in the sky are tilt-free, in contradistinction to inflationary predictions, are nearly one in a hundred million.

Some of my colleagues have expressed disappointment, even concern, that both the LHC and Planck satellite data are consistent with our simplest conventional theories about matter at its most fundamental and spacetime at its most grand. There are powerful arguments suggesting that our account of wobbly spacetime and jittery matter should break down at some point, when applied to (perhaps) even more extreme conditions than those that have now been tested so well.

I can understand the disappointment, though it also seems a bit like a child rushing downstairs on the morning of her birthday to find every exotic present she had ever wished for, apart from a pet unicorn. For now, I am content to wonder at the gifts we have firmly in hand. Many of our once wildest dreams – speculations about warping spacetime, the awesome stretching of cosmic inflation, or Higgs particles executing a particular quantum dance – may indeed have come true.