Elemental origins
The universe didn’t create all of the elements at the same time, though. And each one has multiple pathways to formation. If we rewind to the very first moments of the universe, we find it was dominated by the smallest atomic building blocks — quarks, electrons, and other fundamental particles. Only later, a few millionths of a second after its birth, did the universe form protons (hydrogen) and neutrons as it rapidly expanded and cooled.
Soon after the first hydrogen formed, a couple of heavier elements quickly followed suit. But this process of BBN didn’t really kick off until the universe reached an age of just 10 seconds old. And it only lasted as long as 20 minutes.
Remarkably, the density of the universe at this time was incredibly low, about 100,000 times less dense than liquid water. If that’s the case, though, then why don’t we see nucleosynthesis occurring on Earth, where densities are much higher? The answer is that the temperature at that time of BBN was around 1 billion kelvins (1.8 billion degrees Fahrenheit, or just under 1 billion degrees Celsius). Thus, the earliest hydrogen atoms were zipping around so quickly that they frequently collided with great energy, which allowed them to merge into even heavier atoms like helium.
Within the universe’s first 20 minutes, it created most of the helium that exists today, as well as deuterium (heavy hydrogen) and a small amount of lithium. Over that same period of time, the ambient temperature of the universe dropped from about 1 billion kelvins to roughly 10 million kelvins, which is roughly the temperature found in the cores of stars, where stellar nucleosynthesis still occurs to this day. So, once the universe cooled down enough, BBN ceased producing the earliest and lightest elements.
Nonetheless, this early epoch saw so much helium created that the element ended up accounting for about 25 percent (by mass) of all the matter in the newborn universe. But astronomers want to know precisely how much of each element, particularly helium and deuterium, was produced during BBN. That’s because knowing these exact values is key to astronomers both confirming and better understanding the generally accepted theory for how the cosmos burst into existence: the Big Bang.