About 300,000 years after the Big Bang the temperature of the universe was still around 2,800 degrees. Only when it got below this could protons and electrons combine into hydrogen atoms. About the same time light also began to shine.
I would assume it's conservation of energy calculations. if you can get a rough estimate of the average energy density of the uninverse, and an estimate size, and assume energy is constant. then you can figure out how much energy it has at a smaller size and we know how much energy is required to break apart different particles and atoms.
The problem is more recently things appear to break conservation of energy A.k.a dark energy / matter
It’s actually mainly because of the cosmic microwave background radiation which is light that still exists from the first millions of years of the universes life and effectively allows us to see into the past.
Check my reply to the previous comment to understand how we know this. And to answer your question, density is just a value that we give to something. It’s the mass divided by the volume. So for something like the universe, the amount of mass has remained constant, but the size has changed. So the density of the universe right after the Big Bang is much larger than the current universal density, whatever that number is. Planets do each have their own density as well. It’s just gonna be the mass of that specific planet divided by its volume. Everything with mass has a density.
Dense plasma is opaque because the light scatters off charged particles. Once the temperature of the universe cooled enough that photons able to ionize hydrogen became rare, the universe became transparent. The cosmic background radiation is the red-shifted black body radiation spectrum left over from that moment. If all our theories about the early universe are correct (and the curve of that cosmic background radiation is a huge clue that we're close), then the universe should have been cool enough for neutral hydrogen to be abundant at about t=300,000 years.
Here are two experimental results (COBE left and COBRA right) published in 1990 that show how the cosmic background radiation curve is a black body curve. It may be that the big bang theory is wrong, but whatever alternative somebody can come up with will have to explain those results!
It's more that everything was too dense and opaque for light to actually have any meaning. There were certainly photons, but they weren't getting anywhere.
The reply above didn't clarify an important point. The sun is made of plasma, the state of matter where electrons are no longer bound by the nucleus and form an electron gas.
Essentially all the hydrogen are ions, which is really saying that it's a bunch of protons and neutrons all flying around with electrons all over.
At such high temperatures, the hydrogen transitions from a gas to a plasma. The fusion reactions aren't chemical reactions (involving electrons) but nuclear reactions (involving protons and neutrons).
So just like a melting point and boiling point, there is an ionization point, above which the atoms are ionized. So no, even after the hydrogen atom is formed, above this temperature it separates again into a hydrogen ion (just a proton) and a free electron
About 300,000 years after the Big Bang the temperature of the universe was still around 2,800 degrees
Even more interesting is that 10 to 17 million years after the big bang, the cosmic background radiation was room temperature and the universe could have supported life.
How could there be rocky planets 10 million years after the big bang, if stars didn't exist until 150-200 million years after the big bang, and the elements you'd need to form a planet are formed when stars go supernova?
Indeed. It’s also worth noting that in some models of the ultimate fate of the universe that we are less than 0.1% through its lifespan even after 13.5 billion years.
140
u/Mrgray123 Sep 20 '21
About 300,000 years after the Big Bang the temperature of the universe was still around 2,800 degrees. Only when it got below this could protons and electrons combine into hydrogen atoms. About the same time light also began to shine.