Could you imagine if pagers made a comeback? Sure, your smartphone would still be your notepad-flashlight-camera-personal assistant-MP3 player-wallet-social media browser. But imagine if pagers made it so easy to do that one important thing – pinging your friends and associates – that we all brought them back into our lives, or adopted them for the first time.

A redemption arc like this is happening in cosmology, the study of the history of our universe. The hero – rather than old technology – is an epoch of cosmological history called Big Bang Nucleosynthesis, or BBN.

Some cosmologists have thought that BBN is too old-school to be as good a tool for cosmology as other probes of the early universe. But BBN is the darling of my research program, where I use cosmology to learn more about particle physics. I have seen firsthand how recent results from this old cosmological epoch can guide new research about our universe.

The smartphone of cosmology

Cosmology’s superstar is the Cosmic Microwave Background, or CMB, which formed some 400,000 years after the Big Bang. It’s an all-sky mosaic of hot and cold patches of light, which appeared while electrons and nuclei combined into the first neutral atoms. Viewed from Earth, each hot or cold patch is small enough for the Moon to block out.

The CMB’s intricate patterning has been the subject of several intense observational campaigns, and for good reason. The key scientific model in cosmology has six major unknown quantities that researchers must measure to understand the universe. All of these quantities can be inferred with good measurements of the CMB.

But the CMB’s much older, less famous cousin is starting to steal some of its limelight.

A game of musical chairs

The universe started with a bang – the Big Bang – and about 10 minutes later, the largest game of musical chairs in history began. By that time, the universe was still dense, and hotter than the Sun. But it had cooled just enough for formerly freewheeling protons and neutrons to fuse into nuclei, which make up the centers of atoms.

The game started, and every proton searched for neutrons to fuse with. A proton lucky enough to find a neutron partner formed deuterium. That couple could snatch either another neutron to make tritium, or another proton to make helium-3. One more proton or neutron each finally made helium-4, which has a remarkably stable nucleus with two protons and two neutrons.

The catch is that the number of neutrons in the universe plummeted during the game, like chairs being yanked out of line. Neutrons are unstable when they’re not bound in nuclei, spontaneously spitting out lighter particles in about 15 minutes to become the slightly lighter proton.

The music stopped for good when the universe became too cold for these particles to continue fusing. By that time – just 20 minutes after the game started – nearly all neutrons had either turned into protons or ended up in helium-4. Any proton that wasn’t lucky enough to partner up probably remained single for millennia, and might even be wandering the universe alone today.

This process is called Big Bang Nucleosynthesis. Astrophysicists can actually measure the quantities of helium-4 and deuterium that formed by the time it ended, referred to as primordial abundances.

But a new measurement of the primordial helium-4 abundance, published in September 2026, has reached stunning precision, providing information about the universe during BBN at a level of detail never before known.

A window to the early universe

Just a few seconds after the Big Bang, neutrons could absorb lighter particles and become protons, setting the stage for Big Bang Nucleosynthesis in processes distinct from the decays that happened toward the end of BBN 15 minutes later. This conversion could occur as long as the universe was very dense and hot. The longer that condition was satisfied, the more neutrons could turn into protons before BBN began.

But the universe expanded quickly after the Big Bang. The stretching of the fabric of the universe was propelled by radiation, where “radiation” refers to hot particles with tiny masses. The rate of expansion – and, by proxy, the amount of radiation – determined how quickly the universe became less dense.

It’s harder for neutrons to absorb particles and convert into protons in a diluted universe, because everybody’s so far away. The quicker the universe expanded, the less time there was to turn neutrons into protons, and so the more neutrons there would be the start of BBN. More neutrons at the beginning of musical chairs means more helium-4 by the end of it.

The amount of radiation therefore determines the final helium-4 abundance. By measuring that abundance, my colleagues and I work backward to find the precise amount of radiation in the early universe.

That’s an important quantity to measure. Among other reasons, particle physicists can use it to learn more about dark matter, a largely invisible substance that constitutes 80% of the matter in the universe.

Many models of dark matter, or other hypothetical physics, call for slightly more or slightly less radiation than what’s expected in standard cosmology. Measuring this quantity precisely helps particle physicists differentiate these predictions from the truth.

Looking ahead

BBN now determines the amount of radiation in the early universe better than the CMB. And its newfound prowess doesn’t stop at helium-4. Measurements of primordial deuterium, taken throughout the 21st century, tell cosmologists the density of particles like protons and neutrons with extreme precision. The precision with which BBN determines this quantity rivals, though does not yet exceed, that of the CMB.

The CMB isn’t quite ready to give up its spotlight. Even more precise measurements of all of the major unknowns in cosmology are already underway, as the Simons Observatory – a new high precision CMB telescope – makes its first observations. But there’s no clear path to improved measurements after the Simons Observatory finishes taking data, since the next-generation experiment CMB-S4 was suspended indefinitely last year.

Maybe it’s good timing, then, for BBN to be making its comeback. Getting even better BBN data doesn’t require dedicated observatories. Instead, observers only need more time on existing or planned telescopes. With near-term improvements to astronomical observations, physicists may get closer than ever to understanding the makeup – and therefore history and future – of our universe.

Cara Giovanetti does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.