Our entire cosmic history is theoretically well-understood, but the details of the earliest stages, such as how stars and galaxies form and grow up during the first ~1 billion years of cosmic history, are currently being accurately written for the first time thanks to observatories like JWST. (Credit: Nicole Rager Fuller/National Science Foundation)

The scientific story behind the timeline of our Universe

From the earliest stages of the hot Big Bang (and even before) to our dark energy-dominated present, how and when did the Universe grow up?

Today, it’s now 13.8 billion years since the Big Bang occurred. Our observable Universe extends for 46.1 billion light-years in all directions, and is made of:

• 68% dark energy,
• 27% dark matter,
• 4.9% normal (atom-based) matter,
• 0.09% neutrinos,
• and 0.01% radiation,

with no hint of other components like spatial curvature, cosmic strings, domain walls, or any other weird stuff we can imagine.

If we were to run the clock backward, however, we’d find that dark energy wasn’t always dominant. There was a time when matter dominated, and before that, when radiation did. There was a time when there were no stars, no neutral atoms, no atomic nuclei, no protons and neutrons, and even no massive particles.

But how do we know precisely when these events and epochs occurred? That’s what Marshall Randolph wants to know, asking:

“When I read about the epochs of the universe, they are tagged with a specific…