Chapter 26: Problem 25

(a) Was there ever an era when the universe was radiationdominated and matter and radiation were at the same temperature? If so, approximately when was this, and were there atoms during that era? If not, explain why not. (b) Was there ever an era when the universe was radiation-dominated and matter and radiation were not at the same temperature? If so, approximately when was this, and were there atoms during that era? If not, explain why not.

Short Answer

Expert verified

Yes, there was an era when the universe was radiation-dominated and matter and radiation were at the same temperature- the era of matter-radiation equality, around 37,000 years after the Big Bang. Atoms did not exist during this time. Also, there was an era when the universe was radiation-dominated and matter and radiation were not at temperature equilibrium- this was early in the universe's history. Atoms also did not exist during this time due to extremely high temperatures.

Step by step solution

Radiation Dominance and Equal Temperature Era

To begin answering the first question, it's necessary to understand a period in the early universe around 37,000 years after the Big Bang, known as 'the era of matter-radiation equality'. In this era, the universe was populated with both matter and radiation but was becoming increasingly matter-dominated. Critically, during this time, matter and radiation were at the same temperature due to frequent interactions that allowed them to exchange energy.

Existence of Atoms during the era

The second part of the question asks about the existence of atoms during this era. It's important to note that at this stage, the universe was still too hot for atoms to form. Atoms didn't start to form until around 380,000 years after the Big Bang, in an era known as 'recombination' or 'decoupling'. Before this time, the universe was filled with a plasma of free electrons and atomic nuclei, which kept matter and radiation at the same temperature.

Radiation-dominance and Unequal Temperature Era

The third question asks about an era where the universe was radiation-dominated and matter and radiation did not share the same temperature. This could be early in the universe's history, shortly after the Big Bang but before matter-radiation equality. In this era, radiation was dominant, and due to the high energy environment, matter and radiation were not at the same temperature.

Existence of Atoms in this era

The fourth question asks if any atoms existed during the radiation-dominated, unequal temperature era. Similar to the first situation, the universe was far too hot for stable atoms to exist yet. During this period, which we can refer to as the 'very early universe', it was dominated by a hot quark-gluon plasma, disrupted by the high energy radiation.

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Radiation Dominance and Equal Temperature Era

Existence of Atoms during the era

Radiation-dominance and Unequal Temperature Era

Existence of Atoms in this era

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