Why must I feel like that?
Why must I chase the cat?
Nothin’ but the dog in me. -George Clinton
(And I’ll get it stuck in your head, too, if you like. Have a go at the extended 10-minute version.)
In other words, it might not be as catastrophic as it once was, but the Universe is still just a super hot, dense plasma filled with a huge amount of radiation. Well, what happens now? The surprising answer — for thousands of years — is nothing interesting.
Why not? Well, we’ve got a whole Universe full of heavy, positively charged nuclei and light, negatively charged electrons. The Universe is too cold to blast the nuclei apart, but too hot to form neutral atoms!
I mean, you can form them for an instant, sure, but remember that for every atomic nucleus, you have over a billion photons. If even one of them has enough energy, it’s going to blast that electron not just to a higher energy level (as shown below), but right out of the atom!
So, if we want to do anything else with our Universe, we first need to make some neutral atoms. And the only way to do that? We’ve got to wait. Our Universe needs to expand and cool enough for this to be possible, and unlike nucleosynthesis, where we had to wait a few minutes, we have to wait hundreds of thousands of years!
Why do we have to wait so long? Take a look at the energy spectrum of our photons — the particles of light in the Universe — which was made famous by xkcd.
It takes 13.6 eV of energy to ionize a hydrogen atom. But that doesn’t mean that the average energy of the Universe needs to drop below 13.6 eV. Look at that graph above. Notice how, even though the average photon has a frequency of around 6 in the graph, there are still plenty with a higher frequency (and hence higher energy). In fact, because photons are so numerous (again, outnumbering nuclei by a factor in the billions), we need the average energy to drop down to about 2% of the ionizing energy!
Well, the only way the energy drops is because the Universe cools as it expands. Light — all light — has a wavelength, and the longer the wavelength, the lower the energy, and hence the cooler it gets. As the Universe expands, the wavelength gets stretched out, and so the Universe gets colder!
So now the question becomes: how long do we have to wait for the Universe to cool down this much? And sadly, the answer is 380,000 years! It took 7 parts to just tell some highlights from the first five minutes of the Universe, and then almost nothing of interest happens for hundreds of thousands of years! No stars, no galaxies, nothing. We have to wait this long just to get neutral atoms!
But once you get them, you’re in business! All of a sudden, the light that used to bounce off of protons, electrons, and other nuclei now simply travels in a straight line! All of those billions and billions of photons, once we form neutral atoms, don’t interact with anything anymore.
Until, that is, they run into something. Like, for instance, us. Can we see this leftover radiation from the big bang?
You bet we can! In the 1960s, we discovered the leftover radiation from the big bang, which has now shifted into the microwave! In fact, in the 2000s, we measured the fluctuations in the microwave background, and what did we find?
The fluctuations are exactly the kind we expect to get from inflation! These observations are huge verifications of our picture of how the Universe work. In fact, just this — the story of the formation of neutral atoms and the leftover glow — was to lead to the widespread acceptance of the big bang and the rejection of pretty much every alternative. Why? Because they don’t predict this leftover glow that we see.
So — for those of you who don’t like the big bang — this is your number one challenge when you make a theory of the Universe. Can you explain the observed abundances of the elements? Can you explain the leftover microwave radiation? Can you explain the fluctuations in that microwave radiation? And can you explain the expansion of the Universe?
These first eight parts have put us in a great place, and we’re now set up — with a Universe full of neutral atoms — for things to get both interesting and familiar! See you next time…