Here’s a bit of trivia to stump even your most science-savvy friends: What is the strongest possible acid, so reactive you cannot even measure it on the pH scale? Or try this one: What is the very first molecule that formed in the universe, before water, before even molecular hydrogen? While your victim is scouring his or her brain, you could offer a taunting clue: The two compounds are one and the same.
The unexpected answer is … helium hydride, HeH+. Jerome Loreau of the Free University of Brussels calls it the mystery molecule (or rather the “mystery ion,” since it carries an electric charge), and for good reason. Your chemistry teacher probably taught you that helium, a noble gas, never reacts with anything, but that turns out to be entirely untrue — at least, under certain highly exotic conditions. Helium hydride is so obscure that many astronomers have never heard of it, even though it marked a key turning point in the evolution of the universe. It was step one in the birth of chemistry and the emergence of stars, planets and life itself.
Which brings me to the last and most mysterious thing about helium hydride. “We can’t observe it,” Loreau says sheepishly. “It just seems to be somehow invisible in space.”
It’s not just helium hydride that researchers can’t see; the other first-generation molecules are invisible, too. The missing pieces make up an entire torn-out chapter of cosmic history, an important era known, fittingly, as the Dark Ages.
At first, immediately after the Big Bang, space was a hot tangle of intertwined matter and radiation. As the universe expanded, it steadily cooled. Once it reached a temperature of about 4000 kelvins, 380,000 years after the Big Bang, protons could combine with electrons to form hydrogen atoms. Unlike the particle soup that came before, hydrogen is transparent, allowing radiation to start streaming freely through the now dark universe.
The radiation from that time is still easily detectable today as the cosmic microwave background,
Helium hydride is so obscure that many astronomers have never heard of it, even though it marked a key turning point in the universe."
the all-pervasive glow that provided clinching evidence in the 1960s for the Big Bang. The matter, on the other hand, disappeared from view. The next thing astronomers can see is a population of rather well-developed proto-galaxies a few hundred million years later.
What happened during the long stretch in between? Right. Dark Ages.
Even though we cannot see the Dark Ages, we have some idea what was happening then. Models of the Big Bang give a precise account of the initial makeup of the cosmos: hydrogen, helium, a little deuterium (a heavy form of hydrogen) and trace amounts of lithium. That’s it. As for what they were doing, once the universe was cool enough to form hydrogen atoms, it was also cool enough for those elements to start interacting with each other and combining into molecules. In other words, the day the cosmos went dark is also the day chemistry began.