What Happened as the Universe’s Temperature Cooled Down after the Big Bang?

From the Lecture Series: The Big Bang and Beyond: Exploring the Early Universe

By Gary FelderSmith College

Two important changes both took place coincidentally at about the one-second mark after the big bang, when the universe’s temperature had cooled to about 10 billion degrees. That’s 100 times cooler than the temperature at which quarks combined, but still hundreds of times hotter than, say, what the core of the sun is today.

Image of the cosmic microwave background radiation
The radiation emitted just before atoms formed still persists in the universe. (Image: NASA/Public domain)

Antimatter’s Annihilation

Antimatter isn’t a single type of particle but a whole category of particles. Every type of matter particle has an antimatter counterpart with the same mass as the matter particle but an opposite charge. For example, the antielectron particle is called a positron. It has the same mass as an electron, but it has a positive charge that’s equal and opposite to the electron’s negative charge.

When a matter particle collides with its antimatter counterpart, the two can annihilate and turn all their energy into electromagnetic radiation. When quarks combined into protons and neutrons, antiquarks also combined into antiprotons and antineutrons. So the universe, after a hundred-thousandth of a second, contained protons, neutrons, and electrons, and also antiprotons, antineutrons, and antielectrons (aka positrons).

Those matter and antimatter particles collided and mutually annihilated, and after about a second, all of the antimatter was gone. But for reasons we don’t know, a tiny bit more matter than antimatter was created after inflation, so after all the antimatter was gone, there was still matter left over. That leftover matter went on to form all the stars, galaxies, and planets in the universe today.

Neutrinos: Loners of the Universe

Neutrinos are all around us, constantly passing through our bodies. We don’t notice them because in the universe today, they almost never interact with other particles or with each other.

Illustration of photons, atoms, and neutrinos
As the universe’s temperature dropped, neutrinos no longer interacted with the environment and still don’t interact with anything as they move through space. (Image: Kakteen/Shutterstock)

But the universe in the first second after the big bang was so dense that even neutrinos were constantly colliding with other particles. At one second, at a temperature of about 10 billion degrees, the density dropped low enough that the universe became transparent to neutrinos. We say that neutrinos “froze out” at that time, which is a fancy way of saying they stopped interacting with other particles. 

Virtually all of the neutrinos that were present at that time are still freely streaming through the universe today, passing unaffected through stars, planets, and us. In 2015, that cosmic neutrino background from the early universe was measured, providing direct confirmation of our story of the universe back to one second after the big bang.

So at that one second mark, the universe was full of protons, neutrons, and electrons, constantly colliding with each other. Neutrinos moved freely through space with virtually no interactions, just as they do today. And intense electromagnetic radiation suffused all of space, left over from the annihilation of all of the antimatter with almost all of the matter.

This article comes directly from content in the video series The Big Bang and Beyond: Exploring the Early UniverseWatch it now, on Wondrium.

The Formation of Nuclei

After the first few seconds, the next big event was when some of the protons and neutrons began to fuse into nuclei. Nuclei could only form once the universe dropped to about 1 billion degrees, which happened at the one-minute mark.

The formation of these first nuclei is called big bang nucleosynthesis. It was mostly over by three minutes, although a few rare fusion events probably continued to occur for another 10 to 15 minutes more.  At the end of that process, the nuclei were (by mass) about 75% hydrogen and 25% helium, the two lightest elements. 

The next lightest element, lithium, accounted for less than one in a billion of the resulting nuclei, and essentially no elements heavier than lithium were produced. After 20 minutes, the universe had the mix of elements that it would continue to have for tens of millions of years until fusion began again in the cores of stars.

Formation of Atoms

The next major cooling milestone happened 370,000 years later: the universe cooled down to 4,000, the temperature at which electrons and nuclei stick together and form atoms. The hot material that filled the universe before 370,000 years glowed, meaning it was constantly emitting radiation. But after the nuclei and electrons combined into neutral atoms, the gas no longer glowed. 

Nonetheless, the radiation emitted just before atoms formed was still there, filling all of space. Even today, that early radiation persists in the form of microwaves and is called cosmic microwave background radiation. The measurement of that radiation was one of the most important confirmations of the big bang model.

After atoms formed, the universe entered a long, relatively uneventful period that lasted many millions of years. That period is called the dark ages because no new light was emitted during that time, although the leftover glow from the moment of atom formation continued to fill all of space.

Gas in the dark
The universe’s dark ages began with space filled with an almost perfectly uniform gas. (Image: Social Media Hub/Shutterstock)

The Universe’s Dark Ages

At the beginning of the dark ages, space was filled with an almost perfectly uniform gas. But throughout the dark ages, that gas gradually became less uniform as gravity slowly, but inexorably, pulled matter together into ever denser clumps. 

The average density and temperature of the universe continued to decrease as the clumps moved farther apart from each other, and the space in between became empty and cold. But within those clumps, the density and temperature began to increase.

Within those protogalaxies, hydrogen and helium formed ever smaller and denser clumps. After tens of millions of years, some of those balls of gas eventually got so hot and dense that they began fusion reactions in their cores, making the first stars.

Common Questions about What Happened as the Universe’s Temperature Cooled Down after the Big Bang

Q: What happened to antimatter particles after the big bang?

Approximately one second after the big bang, when the universe’s temperature had somewhat cooled down, particles of matter and antimatter collided and were mutually annihilated, though some matter was left for unknown reasons.

Q: How do neutrinos interact with the environment?

When the universe’s temperature was high enough, neutrinos constantly collided with other particles, but as it cooled down, neutrinos no longer interacted with other particles or even other neutrinos, and they still don’t.

Q: What were the first elements to come into existence?

After the universe’s temperature reached a point where nuclei could be created, the first elements that came into existence were hydrogen and helium and very little lithium.

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