A three-minute creation
Deuterium, or heavy hydrogen, is an isotope of hydrogen whose nucleus contains one proton and one neutron. Nearly every single deuterium atom in existence was forged in a very short period starting about one minute after the Big Bang. This process, known as Big Bang Nucleosynthesis (BBN), lasted only a few minutes, ending when the universe became too cool for nuclear fusion to continue. In this brief window, protons and neutrons collided to form deuterium. Most of this deuterium then fused into helium-4, with trace amounts of lithium also being produced.
The abundance of deuterium left over is extremely sensitive to the physical conditions of that era. Specifically, it depends on the density of ordinary matter, what physicists call baryons. A higher baryon density would have caused the fusion reactions to proceed more efficiently, converting more deuterium into helium and leaving less behind. A lower baryon density would have resulted in more leftover deuterium. This sensitivity makes the primordial deuterium abundance a "baryometer," a tool for measuring the density of ordinary matter in the early universe.
Unlike other light elements, deuterium is not produced in any significant quantity by stars. In fact, stars do the opposite: they destroy it. The relatively low temperatures needed to fuse deuterium mean it is quickly consumed inside stellar interiors, a process known as astration. Because of this one-way process, the amount of deuterium in the universe can only decrease over time. This makes the deuterium found in the most ancient gas clouds a record of the Big Bang's first few minutes.
The ultimate cosmic yardstick
To measure the primordial deuterium abundance, astronomers use distant quasars as cosmic lighthouses. Quasars are incredibly bright, and their light travels for billions of years to reach Earth. On its journey, the light passes through intergalactic clouds of gas that have not been significantly processed by stars. Atoms in these clouds absorb light at specific, characteristic wavelengths, leaving dark absorption lines in the quasar's spectrum. Because the deuterium nucleus is twice as heavy as a proton, its electron absorbs light at a slightly different wavelength than ordinary hydrogen. By measuring the relative strength of these absorption lines, astronomers can determine the deuterium-to-hydrogen (D/H) ratio with high precision.
Decades of these difficult observations, using powerful instruments like the Keck Telescope's HIRES spectrograph, have converged on a remarkably precise value. The current best measurement of the primordial abundance from quasar absorption systems is about 25.27 deuterium atoms for every million hydrogen atoms, with an uncertainty of just 0.03.
This observational result agrees with a completely independent prediction from the Planck satellite, which mapped the cosmic microwave background (CMB) — the afterglow of the Big Bang. By analyzing the temperature fluctuations in the CMB, cosmologists can calculate the universe's baryon density. Standard BBN theory then predicts what the D/H ratio should be based on that density. The Planck data predicts a D/H ratio of (2.535 ± 0.05) × 10⁻⁵, a near-perfect match with the quasar observations. This powerful agreement between theory and observation, from two completely different epochs of the universe's history, is part of the standard cosmological model.