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View Full Version : Big Bang - having a problem understanding something.



Albion
2010-Dec-21, 10:11 PM
I was listening to the Star Stuff podcast today and the guest kept talking about the initial blast of light at the beginning then darkness until the first clouds of hydrogen gas settled and then formed into the first stars. I don't understand how the hydrogen gas came before the first stars. There had to be a point in the expansion of the universe where the stuff within the universe was at a pressure and temperature high enough to cause fusion. Wouldn't that have been the first "star"? Then when the volume of the universe was large enough, due to expansion, to create empty spaces between all the stuff, why didn't it just clump into one giant star/black hole due to gravity? Did gravity exist at that time? How did all the stuff, that should have been fusing when the volume of the universe was small enough, stop fusing and diffuse into clouds of hydrogen gas? To only then start falling into gravity wells and then form the first stars? I picture the first stars "dripping" out of that primordial fusion soup into the empty spaces created by the expansion of the universe. I don't understand, can someone help me?

Ken G
2010-Dec-21, 11:03 PM
I was listening to the Star Stuff podcast today and the guest kept talking about the initial blast of light at the beginning then darkness until the first clouds of hydrogen gas settled and then formed into the first stars. I hate that term the "dark" ages, it makes no sense. That period would have been spectacularly bright if you went there. They just mean "dark" in the sense of "shrouded from our view", it's a pretty nonsensical choice of language.

I don't understand how the hydrogen gas came before the first stars. There had to be a point in the expansion of the universe where the stuff within the universe was at a pressure and temperature high enough to cause fusion. Wouldn't that have been the first "star"? That was in the first few minutes, and it wasn't considered a "star", because it was spread out homogeneously. A star is a local concentration of density and light, against a dark vacuous background. That's just what the term has come to mean.


Then when the volume of the universe was large enough, due to expansion, to create empty spaces between all the stuff, why didn't it just clump into one giant star/black hole due to gravity?The rate of expansion prevented that. But as the universe expanded, it also cooled, and this ended up increasing the gravitational instability. Gravitational instability caused the creation of galaxies and stars, and that happens when the infall time due to the instabilities gets shorter than the current age of the universe. When the universe was young, the infall would have taken too long, and the instability wasn't strong enough.

How did all the stuff, that should have been fusing when the volume of the universe was small enough, stop fusing and diffuse into clouds of hydrogen gas?It stopped fusing because it cooled down-- expansion does that. Then for a long time you have evenly spread H gas that was not fusing, which ultimately started to clump and make stars and reignite the fusion in localized spots. You are right that this seems a bit counterintuitive-- the universe achieved localized fusion sites by expanding away its prevailing fusion that was happening everywhere at an earlier age. Interestingly, the widespread fusion was happening at too low a density to get the triple-alpha fusion needed to make carbon-- so we only got carbon (and everything heavier) in those isolated high-density fusion sites in stars. We are "stardust."

forrest noble
2010-Dec-22, 05:34 AM
I was listening to the Star Stuff podcast today and the guest kept talking about the initial blast of light at the beginning then darkness until the first clouds of hydrogen gas settled and then formed into the first stars. I don't understand how the hydrogen gas came before the first stars. There had to be a point in the expansion of the universe where the stuff within the universe was at a pressure and temperature high enough to cause fusion. Wouldn't that have been the first "star"? Then when the volume of the universe was large enough, due to expansion, to create empty spaces between all the stuff, why didn't it just clump into one giant star/black hole due to gravity? Did gravity exist at that time? How did all the stuff, that should have been fusing when the volume of the universe was small enough, stop fusing and diffuse into clouds of hydrogen gas? To only then start falling into gravity wells and then form the first stars? I picture the first stars "dripping" out of that primordial fusion soup into the empty spaces created by the expansion of the universe. I don't understand, can someone help me?

Hydrogen gas according to the big bang scenario didn't happen right away. Accordingly the first creation of matter included primarily protons, electrons, and light element nucleons, anti-particles and neutrinos. After the supposed Inflation period (you need to look that up) matter started to form atoms. A Hydrogen atom is one proton and one electron, and hydrogen gas one of two atoms forming a molecule. Accordingly, only after this initial matter which was created and had time to cool down could gravity wells begin to form which accordingly could have led to the first stars. The BB itself was supposedly a fusion process involving nucleo-synthesis of the light elements but following that, without cooling, no stars could form.