In 2009, the Planck spacecraft began its observations of the cosmos, and the program continued until 2013. Planck was designed to look for anisotropy, or non-uniformity, in the cosmic microwave background (CMB). On very large scales, the Universe looks pretty much the same in all directions, like a carpet that’s just as green and fluffy whichever corner of the room you’re standing in.

But the Universe is not like a carpet. There are small variations in the CMB, and by studying them, researchers can gain insight into the very early Universe. After all, the CMB is an “afterglow” of the Big Bang itself.

The Planck team has been reporting its results, which have big implications for cosmology. By and large, the results have confirmed the standard cosmological picture, but there were some surprises. For one thing, the new results show that the first stars were born over 100 million years later than previously estimated. For another thing, they put new constraints on inflation in the early Universe. As if that weren’t enough, they also constrain the density of dark energy in the early Universe.

Dark Ages

The Big Bang is often compared to the biblical phrase “Let there be light,” and it’s often imagined that light expanded out with the rest of the Universe, and from that day forward the place was illuminated. Photons were indeed produced in the Big Bang, but that isn’t the light that we see. Most of our (non-artificial) light comes from the Sun, and at night we see light from other stars.

After the Big Bang, stars didn’t come into existence for hundreds of millions of years. During that time, the Universe went through a phase where any photons would interact with the free electrons and protons present. Once it had expanded and cooled somewhat, however, those things combined to form hydrogen atoms, releasing photons of one specific wavelength in the process (which we now see as the CMB). For millions of years, there was no new light being produced—all the Universe had was the photons produced by the formation of hydrogen atoms.

This era was called “the dark ages,” and it lasted far longer, and was far darker, than the one experienced by Europe.

First Light

The dark ages only came to an end when the first stars appeared. These first stars were behemoths, with some being as large as 300 times the mass of the Sun (though most were only tens of solar masses). These giants would have ionized the hydrogen gas around them, exciting it enough that the electrons were torn off their atoms again. With the electrons stripped, the gas has an electrical charge. And when light from the CMB passed through that charged gas, it became polarized.

By examining these polarizations in fine detail, the researchers were able to pin down when this re-ionization occurred, and hence the birth of the first stars and the end of the dark ages: 560 million years after the Big Bang. The previous best estimate, made using data from the WMAP satellite in the 2000s, held that the re-ionization occurred just 420 million years after the Big Bang—140 million years sooner than the new estimate.

"This difference of 140 million years might not seem that significant in the context of the 13.8-billion-year history of the cosmos, but proportionately it's actually a very big change in our understanding of how certain key events progressed at the earliest epochs," explained Professor George Efstathiou, one of the leaders of the Planck Science Collaboration.

The WMAP estimate conflicted with observations from Hubble, which suggested there were not enough stars and galaxies to have ionized the gas at the time WMAP suggested. As a result, scientists have been trying to come up with elaborate models to explain the discrepancy, suggesting that perhaps there was an even earlier population of stars which could have ignited re-ionization, or even energetic black holes. But with the new data, such solutions are no longer necessary. Planck’s results agree with Hubble’s observations.

"We had two groups of astronomers who were basically working on different sides of the problem,” explained Prof Richard McMahon of Cambridge University. “The Planck people came at it from the Big Bang side, while those of us who work on galaxies came at it from the 'now side.'"

"It's like a bridge being built over a river. The two sides do now join where previously we had a gap," he told BBC News. Among other things, it paves the way for future observations. The James Webb space telescope, Hubble’s successor, will be able to look right through the epoch of re-ionization with confidence.

Inflation

As important and exciting as the discovery that the first stars were born later than we thought is, the Planck results have implications for inflation as well. Inflation, currently the leading explanation for the evolution of the Universe, posits that the Universe underwent a period of extremely rapid expansion after the Big Bang. If so, it would have created gravitational waves, which in turn could have polarized the light passing through them.

You might recall a report from last year that the BICEP2 telescope had measured light polarization from these gravitational waves, providing conclusive evidence of inflation. Since that time, it’s become apparent that their signal was probably caused by dust rather than primordial gravitational waves.

While some may have taken this to mean that inflation isn’t correct, it is still very much possible. The BICEP2 results were thought to have reported that the signal had a ratio of gravitational waves to ordinary density perturbations of about 0.2. This was the highest value it could have had, the absolute best case scenario. Now that those results seem to have fallen apart, we know the value is lower than that, which we sort of expected it would be anyway.

The Planck team, working with the BICEP2 and Keck teams, has now pushed that value down to 0.15. This number is still very much consistent with the idea that inflation occurred—but it’s also possible that it didn’t. Astrophysicist Sean Carroll has suggested that this leaves an opening for alternative hypotheses.

https://twitter.com/seanmcarroll/status/561001294572560385

It’s hoped that future measurements will be able to further constrain the value down very close to zero within the next few years. If inflation happened, we’ll most likely be able to find out then—and if not, we’ll know that as well.

Dark energy

The Planck results also constrain the dark energy density in the early Universe. Not to be confused with dark matter, dark energy is... something... that seems to permeate the Universe and is responsible for its accelerating expansion. Dark energy can’t be observed directly and little is known about it, though it can be estimated that it comprises most of the "stuff" in the Universe, far exceeding every other kind of energy and matter put together.

The new results show, with 95 percent accuracy, that the dark energy density in the early Universe had to be below two percent of its critical density.

For the Universe to be flat—meaning that it isn’t curved or doesn’t curve back on itself like the surface of the Earth—dark energy would have to be equal to its critical density. Otherwise, if it is curved, to paraphrase a student who was in my undergrad astrophysics course, the Universe would be like a Pac-Man game: if you go far enough in any one direction, you “come out the other side” and end up where you started.

Currently, observations suggest that the Universe is flat, though the observations are also compatible with other shapes. It might be that it is curved but that the curvature is over such a large scale that it’s hard to notice, just as the surface of the Earth appears flat to someone standing on it.

If the dark energy density is greater than the critical density, we’ll have a “spherical” Universe, which would act like this Pac-Man board. If it’s exactly the critical density, we get a flat Universe, while if it’s less than the critical density, we get a hyperbolic Universe, one that has the shape of a saddle.

The two percent constraint is well below critical density. But bear in mind that this represents the early Universe and not the present one.

And more

We’ve highlighted some important results from the Planck data, but there’s a lot more. The results of this massive endeavor are published in a series of papers that contain quite a few interesting results both for general astrophysics and cosmology. To name some additional ones, the data puts constraints on the kinds of alternative gravity models (some of which hope to do away with dark matter); the papers also provide some data on primordial magnetic fields, on non-Gaussianity in the CMB, and more.

These papers, which are the culmination of a massive effort with over 200 scientists and technicians in total, are all available on arXiv.