The cosmic microwave background radiation has provided scientists with some of the best data on the earliest days of the universe. Cosmologists may have worked out how to learn even more, involving a painstaking search for the universe's oldest helium.


The cosmic microwave background radiation, or CMBR, is a relic of a time before the advent of stable atoms, where photons bounced between electrically charged protons and electrons, lending the early universe a fog-like glow. Roughly 400,000 years after the Big Bang, the universe had cooled enough for the electrons and protons to bond together into hydrogen atoms, canceling out their electric charges. The photons thus stopped their chaotic zigzagging, instead spreading out in roughly straight lines. These photons are what we now observe as the CMBR.

This phenomenon has allowed cosmologists to gather vital data regarding the formation of hydrogen in the universe. This was a chaotic process of recombination that took millions of years (the 400,000 year figure mentioned above is really just a rough indicator of when the process began) to fully resolve itself, as photons smashed apart many of the emergent hydrogen atoms. It was only as the universe expanded further that the atoms really had enough room to decisively come together, paving the way for a cosmos of matter, stars, and, ultimately, life.


Although the CMBR has been extraordinarily useful in revealing the history of the universe - after all, everything since the 400,000 year mark covers roughly 99.997% of all time - it seemingly blocks much investigation of the time before that and the kinds of processes that might have taken place, including the decay of exotic particles. But now scientists have figured out a way to peer even further back into the ancient cosmos by taking advantage of the other element besides hydrogen that formed in the primordial chaos.

Helium, being the heavier of the two elements, had double the electric charge of hydrogen in their nuclei and thus more quickly attracted the electrons needed to form stable atoms. Cosmologists have pegged the time of first attraction between helium nuclei and electrons at about 15,000 years after the Big Bang, with the second electrons needed to complete the atom being brought at around the 100,000 year mark. Both of these events took place significantly before the release of the CMBR, and the photons that interacted with these helium atoms will look significantly different from those that interacted with hydrogen.

Unfortunately, there are roughly a billion times the photons emitted by hydrogen as there are those emitted by helium, making them exceedingly difficult to find unless specifically looking for them. Still, the helium photons do bunch together at certain frequencies, meaning photon spikes at certain frequencies in the CMBR are a telltale sign. Once located, these helium photons will hopefully provide a window into an even earlier period of the universe's history than previously thought possible.

Searching for these photon spikes is probably years away, however. In order to locate these photon spikes, a satellite investigating the CMBR would have to scan frequencies from a fixed position. The European Space Agency's Planck Satellite, launched this past May, is currently charged with investigating the spatial patterns of the CMBR. This requires precisely the opposite approach, as Planck scans positions from a fixed frequency. As such, the hunt for helium photons will likely have to wait.


[Scientific American]