The first stars and large-scale structure in our universe formed much later than previously thought, according to the latest maps and data from the European Space Agency’s Planck telescope, which has been scrutinizing the polarized fossil light from the early universe. Planck’s new timeline pinpoints when star formation began in the nascent universe. This signalled the end of the cosmic “dark ages” and knowing when it occurred will help improve our understanding of the earliest epochs of the universe.
Some 380,000 years after the Big Bang, its thermal remnant – known as the cosmic microwave background (CMB) – emerged when neutral atoms first formed and space became transparent to light. While the CMB covers the whole sky at microwave wavelengths, it also includes some detailed information in the form of variations in temperature and polarization. These variations are thought to reveal density fluctuations in the early universe, which were the seeds of the stars and galaxies that we see today.
Indeed, as the universe became neutral, it became nearly unobservable across most of the electromagnetic spectrum, as all of the emitted short-wavelength spectrum was quickly absorbed by the atomic gas. This period is referred to as the “dark ages” and prevailed until some very dense regions began to collapse thanks to gravity. This led to the formation of the first dense structures within the neutral medium.
Lighting up
Gradually, the energetic radiation emitted by these early sources ionized all of the neutral hydrogen in the universe. This is referred to as the “epoch of reionization” and is of great interest to researchers because it tells us how the clumpy, structured universe that we see today evolved from the smoothly distributed matter that existed during the dark ages.
As soon as the bulk of the universe was reionized, light at many wavelengths could travel across the universe from the early sources, revealing the edges of the universe that we see today. Electrons and protons could finally combine and form neutral atoms without being torn apart by incoming photons that would scatter off them. The CMB photons are stamped by their last encounter with dark-age electrons (called the “last scattering surface”), and this is preserved in the CMB’s polarization. As a result, the polarization of the CMB provides crucial information about the ages and locations of first stars and galaxies.
Launched in 2009, Planck mapped the entire sky in nine different frequencies until 2013. Using its Low Frequency Instrument (LFI), which covers the 30–70 GHz range and the High Frequency Instrument (HFI), which spans six frequency bands from 100 to 857 GHz, the Planck collaboration has drawn up the most intricate maps of the of the CMB to date. Apart from Planck, more than 100 experiments have studied the CMB since it was first discovered, including NASA’s Cosmic Background Explorer (COBE) satellite and its Wilkinson Microwave Anisotropy Probe (WMAP). In fact, the CMB spectrum is the most precisely measured black-body spectrum in nature.
“After the CMB was released, the universe was still very different from the one we live in today, and it took a long time until the first stars were able to form,” says Marco Bersanelli of Università degli Studi di Milano, Italy. “Planck’s observations of the CMB polarization now tell us that these ‘dark ages’ ended some 550 million years after the Big Bang – more than 100 million years later than previously thought,” he adds, explaining that, while 100 million years may seem negligible compared to the universe’s age of almost 14 billion years, the timescale has a large impact when it comes to the formation of the first stars.
Later date
Previous measurements of the CMB polarization made by WMAP seemed to suggest that the reionization began some 450 million years after the Big Bang. But this was troublesome, as deep-sky images from the NASA–ESA Hubble Space Telescope provided a census of the earliest known galaxies in the universe, which started forming some 300–400 million years after the Big Bang. However, these galaxies would not have been powerful enough to tip the universe into the reionization epoch and end the dark ages at the 450-million-year mark. Researchers would have had to invoke some more exotic sources of energy to let that happen.
Now though, thanks to Planck, the problem is significantly minimized, as the earliest stars and galaxies alone might have been enough to drive the process. “From our measurements of the most distant galaxies and quasars, we know that the process of reionization was complete by the time that the universe was about 900 million years old,” says George Efstathiou from the University of Cambridge in the UK. “But, at the moment, it is only with the CMB data that we can learn when this process began.”
This later end of the dark ages also implies that it might be easier to detect the very first generation of galaxies with the next generation of telescopes such as the James Webb Space Telescope.
The research is to be published in Astronomy and Astrophysics. Preprints of all the Planck papers are available online.
