September 21, 2018

• 10 mysteries of the universe: How did it all begin?

  A faint afterglow in the sky tells of a universe that exploded into being 13.8 billion years ago. But we haven't got the full story of the big bang nailed yet

  NASA/WMAP Science Team

  Mystery: How did it all begin?  By Joshua Howgego

  WHEN the curtain came up on the universe, the lights stayed down for a moment. For about the first 380,000 years, a mere instant on the cosmic stage, charged particles buffeted light around the early universe as if in an opaque fog, and not a glimmer escaped. Then things cooled enough for atoms to form, scattering ceased – and light was liberated.

  Remarkably, we can still see that light. We know from observations of galactic motions that space has been expanding since the cosmic beginning, and that this light has expanded and cooled with it. Now it suffuses all of space, a bath of low-frequency microwaves with a temperature of 2.7 kelvin.

  Read more: 10 mysteries of the universe

  From dark matter and energy to our own enigmatic existence, here’s our pick of the greatest cosmic conundrums – told through the bizarre objects embodying them

  Since its discovery in 1964, we have made incredibly precise maps of this cosmic microwave background all across the sky. The best, the Planck satellite’s four-year scan completed in 2014, caused some head-scratching. The big bang afterglow seemed to cast doubt on whether the bang was so big at all.

  The problem has to do with inflation, a theory devised by cosmologist Alan Guth and others in the 1980s to explain why stuff in the universe seems so uniformly distributed in all directions. In a plain-vanilla big bang, quantum fluctuations should have produced differences in the density of matter that grew as the universe expanded. Guth explained cosmic sameyness by proposing the existence of an “inflaton” field that filled space-time at the big bang, forcing it apart at faster than light speed. This would mean everything we see originated from a tiny, uniform region of original space.

  Inflation quickly became gospel. But the more energy the inflaton field had, the more space-time would have been shaken by tiny gravitational waves at the beginning of time. And yet we see no traces of gravitational wave effects in the Planck map.

  It is not impossible to square this with inflation, but it is difficult, says Anna Ijjas, a cosmologist at Columbia University in New York. “What we learned from the Planck is that the simplest models are out,” she says.

  That leaves inflationary theory out of sorts. “We can try to fix it, or we can find something better,” says Ijjas. The Planck map is prodding her and others, including one of inflation’s key architects, Paul Steinhardt at Princeton University, towards a different view of the start. It wasn’t a bang, they say, but a bounce.

  Models describing “cyclic” universes that expand, contract and then expand again have been around for a while, and recently other circumstantial evidence has built up in support of them (see “10 mysteries of the universe: What came before the big bang?“). Their attraction is that although they squeeze the universe down very small, it would never have been at the tiny sizes where the most poorly understood quantum effects come into play. The uniformity arises naturally from the squeeze.

  The right answer is still anyone’s guess, but Ijjas says she expects to be able to make predictions based on bouncing models within a couple of years, and compare them with observations of the cosmic microwave background. The start of the cosmos may have been dark, but we may soon see it in a new light.

  This article appeared in print under the headline “Object: Cosmic microwave background”

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