ESA’s Planck satellite reveals to us that the very first stars in the universe are later than previously thought, thanks to a more accurate estimate of reionization.
To understand this new discovery about the past of our Universe, it is necessary to take a “small” leap back. With the multitude of stars populating the skies today, it’s hard to visualize what our 13.8 billion old Universe was like when it was just seconds old. Imagine then a primordial soup densely populated with particles, mainly electrons, protons, neutrinos and photons – the particles of light.
In such a dense environment, the Universe then appeared as a kind of “opaque” fog, the particles of light not being able to travel significant distances before colliding with electrons. As the Universe expanded, it cooled, the gas became rarer and, after around 380,000 years, we could already see a little more clearly. Particle collisions were sporadic and photons could travel freely through the cosmos.
Today, telescopes like Plank can observe these light fossils through the cosmic microwave background; kinds of tiny fluctuations that contain a wealth of information about the history, composition and geometry of the Universe. At that time, electrons and protons came together to form hydrogen atoms; matter then became “electrically neutral” for the first time, and a few hundred million years later, these assemblies of atoms gave rise to the very first generation of stars in the Universe.
As the first stars formed, they began to emit light, which had the effect of splitting neutral atoms: protons and electrons. We then speak of the reionization epoch, which therefore represents the time when a large number of atoms existing in the Universe were ionized by the intense radiation of the probable very first generation of stars to have illuminated the Universe.
Observations of very distant galaxies hosting supermassive black holes tell us that the Universe completely reionized around 900 million years ago. The starting point of this process is however difficult to determine and the subject was until then still hotly debated in the scientific community. This is where this new discovery comes in.
« The cosmic microwave background can tell us when the reionization epoch actually began and, in turn, when the first stars formed in the Universe, » explains Jan Tauberresearcher at ESA.
A first estimate of the reionization epoch made in 2003 suggested that this process may have started early in cosmic history, when the Universe was only 200 million years old. A problematic result, since we have no evidence that stars formed at that time, which would mean the existence of potential exotic sources that could have caused the reionization at that time.
Today, a new analysis of data from other detectors of Plankmore sensitive than any other to date, suggests that reionization began much later: The measurements have clearly demonstrated that reionization was a very rapid process, occurring when the Universe was around 700 million years old. » affirms Jean-Loup Puget, from the Institute of Spatial Astrophysics in Orsay. » These results now help us to model the beginning of the reionization phase » he continues, « and we also confirmed that none of the other agents were necessary, besides the first stars, to start the reionization of the Universe« .
This new study therefore locates the formation of the very first stars much later than previously thought, suggesting that the first generations of galaxies are well within the observation range of future astronomical facilities, and perhaps be even of some current ones.