Planck’s map of the polarisation of the cosmic microwave background (Image:ESA)

The first stars were born nearly 150 million years later than we thought, according to new data from the European Space Agency’s Planck space telescope.

Planck spent three years, between 2009 and 2012, measuring the cosmic microwave background (CMB). This was the first light released in the universe, 380,000 years after the big bang occurred some 13.8 billion years ago. It was released as the hot and dense universe cooled, and is now spread across the entire sky.

At the end of its mission, the Planck team released the highest-resolution map ever of the CMB’s temperature, revealing new measurements of all kinds of cosmological details.


Since then, the Planck team has been analysing readings of the CMB’s polarisation, which provides another way to study this light. “It’s like having an independent experiment to confirm our results,” says project scientist Jan Tauber.

A longer dark age

One of the major findings is that a period called the cosmic dark ages lasted longer than we thought. After the CMB was released, the universe was dominated by a fog of opaque hydrogen gas. It stayed dark for hundreds of millions of years until gravity clumped matter together into the first stars and galaxies, which produced enough radiation to ionise the hydrogen and make it transparent.

Astronomers are still short on details about how this lighter period, known as reionisation, began and ended. The Hubble Space Telescope has spotted very old galaxies from the middle and end of reionisation, thought to be about 1 billion years after the big bang, and Planck’s predecessor, WMAP, pegged the start of the era to about 420 million years after the big bang.

But simulations suggested that wouldn’t give gravity enough time to work its magic and produce stars. “The measurements we had indicated that formation of stars was much earlier than our understanding would allow,” says Tauber.

Cosmic bouncer

Now Planck, like a cosmic bouncer scrutinising a fake ID, has decided the dates don’t add up. It has pushed back the start of reionisation to about 550 million years after the big bang, making the first stars younger by nearly 150 million years. “If the problem had not been resolved, we would have had to think of weird ways to start the formation of stars,” says Tauber.

The polarisation information has also confirmed Planck’s previous measurements of the amount of ordinary matter, dark matter and dark energy in the universe and continued to rule out the possibility of a ghostly particle called a sterile neutrino. “It’s great to confirm we haven’t made any mistakes in 2013,” says Tauber.

But confirming theories also limits where theoretical physicists can go next. “It would be nice to find exotic physics, but we know it is getting harder and harder, at least with the CMB,” says Tauber. “People who believe in that kind of thing will have a harder time making their claims stick.”

Ride the waves

One mystery still remaining is whether the CMB contains signs of primordial gravitational waves. These ripples in space-time, if spotted, would be evidence that the very early universe underwent a massively fast expansion, known as inflation. Earlier this week the Planck team confirmed their polarisation data had ruled out a claimed detection of these waves by another team, BICEP2, last year.

It is possible, but unlikely, that Planck data alone will find primordial gravitational waves, says Tauber. Planck covers the whole sky, meaning its sensitivity to such signals is lower than dedicated ground-based telescopes like BICEP2 and others that scan small patches.

Pooling data from both, as happened with the Planck/BICEP2 work, is likely to be the way forward, he says. “We hope in the future there will be more work like that.”

Reference: http://www.cosmos.esa.int/web/planck/publications#Planck2015