And the way you do that is to form enough stars throughout the Universe that you - for all intents and purposes - reionize the vast majority (99%+) of the neutral atoms in it. So what we'd need, if we even want to see the light coming from these stars (or any light source) in the first place, is to get rid of these neutral atoms. Image credit: Bok Globule Barnard 68, courtesy of the ESO. The first problem is, when we create these first stars, the neutral atoms block the light from them, just as a thick cloud of interstellar gas can block the starlight from behind it. Video created as a release for the Spitzer Space Telescope, retrieved here.
Video credit: Johannes Hidding, using the Weinberg & Gunn (1990) algorithm.Īfter enough gravitational collapse happens, the Universe becomes dense enough, in spots, to finally form stars for the first time! The regions that become densest the fastest form stars first - as soon at 50-150 million years after the Big Bang - while other regions remain neutral, devoid of stars, and pristine for longer. The problem, of course, is that these "dark age" atoms are too cold themselves to emit those emission lines, and the radiation coming from behind them is too low in energy to induce these absorption lines! So again, we have to wait for gravitation to work its magic on these atoms, and to gravitationally attract enough of them into one place so that we can get to work on making something energetic enough to induce these atomic absorption features! Image credit: Terry Herter, Cornell University.
And at this time, it's still hot and dense enough that the protons and neutrons attempt to undergo nuclear fusion, into deuterium, the first heavy isotope of hydrogen!
(See here for some quantitative notes.) By time the Universe is three seconds old and the interconversions have mostly stopped, the Universe is more like 85% protons and 15% neutrons. Initially, these reactions proceed at about the same speed, giving a Universe whose normal matter is made up of 50% protons and 50% neutrons.īut due to the fact that protons are lighter than neutrons, it becomes more energetically favorable to have more protons than neutrons in the Universe. Every time a proton collides with an energetic-enough electron it produces a neutron (and a neutrino), while every time a neutron collides with an energetic-enough positron, it produces a proton (and an anti-neutrino). However, the Universe is also filled with electrons and anti-electrons, better known as positrons. Remember that the Universe is expanding and cooling now, which means it was hotter and denser in the distant past! Sure, when the Universe was less than 380,000 years old, it was too hot to have neutral atoms, but what if we go to even earlier times?Īt some point it was too hot and dense to even have nuclei, and at some even earlier point than that, the Universe was too energetic to even have individual protons and neutrons! Back when the Universe was a tiny fraction of a second old, all we had was a sea of quarks, gluons, leptons, antileptons and ultra-hot radiation, swirling around in the primordial soup of the Early Universe!
How so? Let's take you back to the earliest times we can speak of where we still have near-100% confidence in our physics. The Big Bang not only tells us when we should form atoms for the first time, it tells us what types of atoms we expect there to be. But there is a theoretical prediction of the Big Bang that comes from even earlier times it is perhaps the earliest testable prediction we have about the Universe!
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