A map of the Universe’s cosmic microwave background radiation, and its polarization, measured by the Planck satellite.Credit: ESA/Planck Collaboration

A transformative era in cosmological science ended this week when the European Space Agency’s Planck telescope released its final maps of the early Universe. Planck was the last in a line of three major space telescopes to study the cosmic microwave background (CMB), the faint afterglow of the Big Bang, resulting in the most precise measurements yet of the age, geometry and composition of the cosmos. With space agencies in Europe and in the United States hesitant to fund a follow-up CMB-focused satellite, Planck looks set to be the last space telescope to study the CMB for many years — marking a big change for cosmologists.

“There’s a whole generation of young scientists who grew up with Planck,” says cosmologist Jan Tauber, the mission’s project scientist with the European Space Agency (ESA) in Noordwijk, the Netherlands.

For more than two decades, scores of ground-based and balloon-borne experiments, alongside the space telescopes, have also studied the CMB. They have largely focused on mapping minute variations in the CMB’s temperature across the sky to create charts of the Universe that have become the gold standard of cosmology. Planck, which collected data from 2009 to 2013, did this with more precision than ever before: its data helped researchers to pin down the age of the Universe (about 13.8 billion years), its geometry (essentially flat) and its composition (95% dark matter and dark energy). In particular, the latest release solidifies an earlier prediction based on Planck data that the Universe should be expanding 9% slower than is currently observed.

The temperature maps and the science they produced have been a “great achievement” but they don’t have much more to give, says Peter Coles, a theoretical cosmologist at Maynooth University in Ireland who is not part of the collaboration.

Planck diaspora

Many scientists who worked on the mission have already moved on to other projects. Silvia Galli became part of Planck in 2013 after her PhD and is one of the few dozen scientists left on the mission, and helped to lead the latest study. Now, she says she will probably join many of her colleagues who are working on Euclid, a major European mission that will map the Universe’s galaxies on an unprecedented scale and is preparing for launch in 2021. Euclid is an old-fashioned optical telescope, not a microwave detector, which makes it a technically different kind of mission requiring different skills. On a personal level, it is exciting to move on to new endeavours, she says.

But the lack of a major CMB mission in the pipeline worries many researchers. “Scientifically, it would be a disaster,” says Galli, who is at the Institute of Astrophysics in Paris (IAP). “There is a risk that a lot of know-how and expertise that had been accumulated will be lost.”

The field’s main focus is now to make detailed measurements of other CMB parameters, including its polarization: a slight tendency for the microwaves’ electromagnetic fields to align in particular directions. In this, researchers hope to find a signature of inflation, the brief period at the beginning of the Big Bang during which the Universe would have expanded exponentially. Cosmologists could also measure matter’s distribution across the Universe by studying how large-scale structures such as galaxy clusters curve space-time and deform the CMB’s temperature and polarization maps, says Karim Benabed, a senior Planck researcher at the IAP. This effect is called gravitational lensing.

Planck mapped polarization — as did a NASA telescope in the 2000s — but with limited sensitivity. “Only 10% of the information in the polarization has been exploited,” says Jacques Delabrouille, an astrophysicist at the University of Paris–Diderot who helped to design Planck. “The CMB still has plenty of secrets to yield,” he says.

The polarization problem

NASA and ESA have so far declined to fund big new satellites to study the CMB — although several US groups are working on ground and balloon instruments to measure polarization. Julien Carron, a Planck postdoc at the University of Sussex in Brighton, UK, has already joined one of these projects, the Simons Observatory in Chile. He says that one of the attractions is the possibility of using gravitational lensing to address another fundamental physics problem: estimating the masses of elementary particles called neutrinos.

One reason for space agencies’ reticence to fund big CMB projects is that experiments have yet to find a polarization signature of inflation. An experiment called BICEP2 claimed to have detected that signature in 2014, but Planck data later showed that it was just dust in the Milky Way. In the latest studies, the Planck collaboration also looked for an inflation signature with the help of BICEP2 data, but didn’t detect one. However, “the fact that we haven’t seen it yet doesn’t mean that it’s not there”, says cosmologist Richard Gott of Princeton University in New Jersey.

Many of the US teams are now joining forces and seeking funding to build a US$400-million, next-generation network of ground telescopes called CMB-S4, which will be much more sensitive to the inflation signature than anything that has come before. That’s the next big thing on the horizon, Tauber says.

And many CMB researchers continue to advocate for space missions. Planck cosmologist Erminia Calabrese of Cardiff University, UK, is pushing for Europe to join LiteBIRD, a proposed Japanese probe that would look for the signature of cosmic inflation while keeping costs down. “The idea is to build a smaller, more focused satellite,” Calabrese says. But from space, it would still have the advantage of seeing the entire sky — something that cannot be done from the ground. Others hope that ESA will team up with India on a proposal for another polarization mission, called CMB-Bharat.

Other cosmologists are trying to jump-start CMB-polarization research in Europe. Davide Maino, a senior Planck member at the University of Milan, Italy, is collaborating on an Italian-led polarization experiment that will be based on the Spanish island of Tenerife. If ground-based experiments see even a hint of inflation, that will encourage space agencies to fund major missions, researchers say. “Of course there will be money,” Benabed says.