The absorption lines at a variety of redshifts show that the fundamental physics and sizes of atoms have not changed throughout the Universe, even as the light has redshifted due to its expansion. The absorption features imprinted onto the distant quasar light helps reveal the relative abundances of the light elements, teaching us about the nuclear reactions and the early composition of our young Universe. (NASA, ESA, AND AND A. FEILD (STSCI))

How Scientists Use Hydrogen Gas, In Space And On Earth, To Measure The Big Bang

Even 13.8 billion years after the Big Bang, we can reconstruct the first 3 minutes.

About 100 years ago, we began to truly understand the nature of the Universe for the very first time. The grand spirals and ellipticals in the sky were determined to be enormous, distant collections of stars well outside of the Milky Way: galaxies unto themselves. They were receding away from us, with more distant galaxies exhibiting faster recession speeds: evidence that the Universe was expanding. And if space is expanding today, that means the Universe was smaller, denser, and even hotter in the past. Extrapolate back far enough, and you’ll predict that the Universe began a finite amount of time ago in an event known as the hot Big Bang.

If the Universe was hotter and denser in the past, but cooled, that means there was once a time where neutral atoms couldn’t form, because things were too hot, but then did as the Universe cooled. That leads to a prediction of a now-cold, but mostly uniform background of radiation: this was discovered in the 1960s, validating the picture of the hot Big Bang and ruling out many alternatives. But there’s an entirely independent way to validate the hot Big Bang: by the nuclear reactions that must have occurred when…