The First Molecule in the Universe

To discover the universe's first molecule, we first need to find the first atom. Molecules, by definition, are combinations of several atoms. In the immediate aftermath of the Big Bang, the universe was so hot that the formation of atoms was not possible. Not even protons or neutrons could have been formed. All that existed were elementary particles such as quarks, gluons, neutrinos, and electrons. These particles were formed within the first microsecond after the Big Bang.

In this short duration, the temperature fell to 10 trillion Kelvin. This temperature was incredibly hot, but ideal for creating the first protons and neutrons. Protons and neutrons were created by the fusion of three quark particles each.

With the discovery of the first proton, the basic building block of an atom was in place. The identity of an atom is defined by the number of protons. One proton is hydrogen, two protons is helium, and three protons is lithium. The key distinction between the 118 elements that have been discovered so far is the number of protons they possess. Hydrogen has the fewest with a single proton, while Oganesson has the most with 118 protons.

About one microsecond after the Big Bang, the first atom was formed. While a lone proton can be referred to as hydrogen, it is not considered a neutral atom. A neutral atom requires that a proton be paired with an electron, so that their positive and negative charges cancel each other out. However, at 10 trillion Kelvin, the environment was far too intense for a proton to pair up with an electron. Therefore, calling it a hydrogen nucleus rather than a hydrogen atom is more accurate.

From about one microsecond to one second after the Big Bang, the formation of all protons and neutrons was complete. The period that followed this event was a significant cooling phase, lasting about 100 seconds, that cooled the temperature to one billion Kelvin. At this point, the universe cooled down sufficiently for neutrons and protons to stick together. This formed the nucleus of an alternate hydrogen isotope called deuterium. Typically, a hydrogen nucleus contains a single proton; in the case of deuterium, it contains one proton and one neutron. Chemically speaking, the two isotopes are virtually identical, except under specific well-defined conditions. Thus, at this point, the universe was almost entirely hydrogen.

After the hydrogen nuclei formed, heavier combinations of neutrons and protons produced helium nuclei. A helium nucleus has two protons and one or two neutrons. Likewise, very small traces of lithium nuclei formed in the same way. Lithium has three protons in its nucleus.

These are the only ones in any appreciable quantities out of the primordial soup: hydrogen, helium, and lithium. Heavier elements were formed within stars.

The process of converting nuclei into neutral atoms was very slow. It took the universe about 380,000 years after the Big Bang for the temperature to cool down to 10,000 Kelvin. Now it was possible for an electron to join with a helium nucleus. One would think that forming neutral hydrogen by combining a proton with an electron would be an even simpler process. The conditions in this case were very different.

The energy required to remove an electron from a hydrogen orbital—more accurately called an orbital rather than an orbit—is much less than that needed to remove an electron from helium. In a hot, dynamic universe, any electron that managed to pair up with a hydrogen nucleus was easily knocked off by thermal energy. If, however, a helium nucleus picked up an electron, it held onto that electron with much greater stability, even under these high-temperature conditions.

Thus, helium nuclei progressively acquired two electrons each to become neutral helium atoms. Chemists write these neutral helium atoms simply as He.

In terms of numerical abundance, hydrogen nuclei dominated the early universe, making up more than 90%, with helium making up about 8%. However, when considered by mass, helium made up about one-quarter of the total mix, with hydrogen (or protons) making up the other three-quarters.

Even though neutral helium atoms could form, the universe had not yet cooled enough for protons to be able to capture electrons and become neutral hydrogen atoms. In contrast, protons fused with helium atoms to form helium hydride, also known as helonium. This process marked the formation of the first chemical bond in the universe—the combination of neutral helium and a hydrogen nucleus. And so, the first molecule in the universe came into existence. Chemists denote helium hydride as HeH⁺, where the superscript plus sign indicates that one electron is missing from the hydrogen component.

From that, helium hydride reacted with more electrons to form neutral molecules, which later broke apart to create neutral hydrogen atoms and molecules.

Today, helium is recognized as the least reactive of the 118 elements currently known. Its inert nature makes it rarely able to form molecules. But ironically, helium played a crucial role in creating the universe's first molecule.

The formation of neutral atoms finally made the universe transparent. Photons from that era, having lost energy as the universe expanded, are what constitute the cosmic microwave background radiation—a story for another time.

The theoretical groundwork for helium hydride as the first molecule was laid many years ago, but discovery of the molecule in space had eluded scientists until now. Though helium hydride was discovered in a lab back in 1925, it was never found in space.

In the late 1970s, scientists theorized that helium hydride might exist in plasma-rich regions of the universe. Since its spectroscopic signature was already known from 1925, researchers were optimistic that it would be detectable in light coming from such plasmas.

It wasn't until forty years later that confirmation of its existence was made. The spectral signature of helium hydride was finally detected in 2019 by a team of researchers led by Rolf Güsten at the Max Planck Institute for Radio Astronomy in the planetary nebula known as NGC 7027.

The discovery was made possible by a unique observatory—a highly modified Boeing 747SP aircraft known as SOFIA (Stratospheric Observatory for Infrared Astronomy). This flying telescope was specially designed to sense infrared light while flying at altitudes of about 40,000 feet, above most of Earth's atmospheric disturbances.

In 2016, aboard one of SOFIA's flights, helium hydride was observed outside of the Earth. But this helium hydride wasn't of Big Bang origin—it formed in the planetary nebula. Even so, being able to see astrochemistry beyond the laboratory was historic.

To date, scientists have discovered nearly 290 molecules in space, 73 of which have been observed outside the Milky Way. Over the past five decades, astrochemistry has grown into a vast and flourishing field of research.