Neutrinos that filled the universe a mere second after the big bang make up a third 鈥渄ark鈥� component of the cosmos, alongside dark matter and dark energy. For the first time, cosmologists have detected the influence of these particles on the large-scale structure of galaxies.
Moments after the big bang, our universe was a seething sea of interacting particles, packed together and constantly bouncing off of one another. Among the first to break free from this dense plasma as the universe expanded were neutrinos, which formed what is called the cosmic neutrino background. These neutrinos are everywhere, but it鈥檚 impossible to detect them directly because of their extremely low energies.
However, cosmologists have new indirect evidence of these neutrinos 鈥� a consequence of what happened when the universe was ringing with waves of matter during the first few hundred thousand years after the big bang.
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A big bounce
According to the standard model of cosmology, about 30,000 years after the big bang, random quantum fluctuations led to some regions that had more dark matter than others. Normal matter gravitationally fell towards these pockets of dense dark matter, only to bounce back outward as photons in the compressed plasma pushed back against particles of matter.
So, thin shells of dense normal matter began speeding away from each pocket of dark matter like sound waves from a popped balloon. Around 380,000 years after the big bang, these waves stalled. That鈥檚 because the universe cooled enough for electrons and protons everywhere, including within these shells of matter, to form the first hydrogen atoms. This stopped the interactions between photons and electrons that were driving the waves.
Free photons started streaming across the universe at the same time, and formed what鈥檚 called the cosmic microwave background (CMB), which we can detect all around us today.
Shells of neutrinos would have also sped away from the over-dense regions of dark matter. These shells would have been larger than the shells of normal matter, because neutrinos are lighter and so travel faster. The gravitational influence of the neutrino shells subtly changed the size and shape of the shells of normal matter, which then聽changed the temperature patterns of the CMB photons from those regions. In 2015, cosmologists found these altered patterns in the CMB.
Shell game
Now, of the University of Amsterdam in The Netherlands and his colleagues have found yet more evidence of the cosmic neutrino background in the way galaxies are clustered.
When the waves stalled 380,000 years after the big bang, the shells of normal matter were frozen in time at a certain size and were denser than other regions. These shells became regions where more galaxies eventually formed, relative to other areas of space.
To see the subtle effect of neutrinos on these shells, Baumann and colleagues analysed the data from a survey of about 1.2 million galaxies, out to a distance of about 6 billion light years, carried out by the .
Theory predicts that in such a large volume of space, one should find more pairs of galaxies that are about 500 million light years apart than any other distance, representing overlapping shells and their centres in today鈥檚 expanded universe.
Earlier studies had indeed found more such galaxy pairs. Baumann鈥檚 team showed that the more subtle influence of neutrinos in the early universe can be detected today in the distribution of galaxies. The shells of normal matter that were nearly imperceptibly stretched and distorted by neutrinos have since evolved with the expanding universe. The shape and size of these shells, determined by an excess of galaxies in these regions today, are consistent with our ideas of how they would have been modified by shells of neutrinos in the early universe.
, an astrophysicist at Princeton University in New Jersey is impressed. 鈥淕iven that we are inferring the properties of an unseen component 鈥� neutrinos created in the first few seconds of the universe 鈥� it鈥檚 reassuring to see [the evidence] in many ways,鈥� he says. 鈥淚t鈥檚 yet another successful test of the standard model of cosmology.鈥�
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Read more: Ghostly neutrino could be behind cosmic expansion mystery
