While the light of the Big Bang started to fade, the early Universe became darker and colder. There were no stars, just gas – mostly hydrogen, some helium, traces of lithium and beryllium – from which, the first stars would be eventually formed. Nobody knows for sure how long this “dark age” took, but at some point during the first hundreds of millions of years, some stars were condensed from this gas and started to shine.
According to theory, these first stars had a larger mass and were brighter than those we see nowadays. They lived for only a million years before they exploded spectacularly, shooting the chemical elements accumulated inside their cores into space.
Even the most powerful telescopes that currently exist are not able to detect light coming from individual stars belonging to this first generation. Space telescopes in the future will be technically capable of registering the huge light issued by the explosion of this type of stars, but the opportunities to do so – even once – during the life span of an observatory are rare.
Paradoxically, our best hope of detecting the era of the first stars lies in one of the weakest elements in the Universe: dust. Among the material ejected into space by these stars, we find dust, formed by the thermonuclear fusion of lighter elements contained in the stars. Thus, the first appearance of dust is our best evidence of the life and death of the first stars.
ALMA is designed to detect dust in the early Universe. While probing space in depth – let us bear in mind that the further we look, the more we go back in time – ALMA detects the glow of tepid dust present in the most distant galaxies, i.e., the earliest ones, with better resolution than could be possible in the deepest observations using visible or infrared light.