So let's go back to the beginning!
This is a busy graphic, which we'll show again at the end of this presentation, but from right-to-left it shows key events in reverse chronological order, from now to 14 billion years ago; and from left-to-right these events in proper chronological order from 14 billion years ago to today.
First, the first stars.
Running the clock backwards from the present day--that is, looking out further and further away--we can see galaxies with less and less metallicity, and eventually a time/place where there were no galaxies. This is a time when the first-generation stars were born out of primeval hydrogen. (Video link: "WMAP’s Portrait of the Early Universe.")
Observations of these hyper-massive stars forming from raw hydrogen show them exciting the hydrogen atoms around them (as discussed in a previous presentation), creating gigantic emission nebulae. The process of exciting hydrogen electrons causes the electrons to be stripped off, ionizing the hydrogen.
Second, the first photons.
Looking further out away from the first-generation stars means looking further back in time, to before the the first-generation stars were born, where there is only hydrogen, and no stars--this is the dark age of the universe. (Video link: "WMAP’s Portrait of the Early Universe.")
Looking even further away and further back in time, we run into a figurative wall of light in every direction: this is the cosmic microwave background, and while this radiation is not the first photons ever created, it is the first and oldest photons that we are allowed to see. (Video link: "WMAP’s Portrait of the Early Universe.")
These intense photons are detectable today as a faint cosmic microwave background, which can be detected with special radio telescopes, and also televisions with old standard-definition rabbit ears--on an unused channel, a portion of the static "snow" that you can see is produced by the CMB signal.
So what would suddenly release this wall of photons, while making older photons that existed before this time be unobservable to us today?
This is a much hotter and denser universe than the dark ages, so hot that positively charged protons and negatively electrons are separated from each other, and are too energetic to be allowed to join up to make neutral atoms. Think of a mosh pit of protons and electrons constantly bashing into each other. Any photons in this hot, dense universe would be continuously kicked around and scattered, so light from this time and earlier could not have survived intact be observable today. (Video link: "Journey to the Centre of the Sun.")
But as the universe gradually cools and expands, protons and electrons calm down such that they can join up with each other, and as soon as they are allowed to do so, they all do so. This event where all protons and electrons suddenly form neutral atoms is recombination. Back to our mosh pit analogy, when the protons and electrons calm down, pairing off to slow dance with each other, then space on the floor drastically opens up. Photons at this point are now free to travel without being kicked around and scattered, and the cosmic microwave background are the first, oldest photons that started out from recombination. (Video link: "Journey to the Centre of the Sun.")
Third, the first fusion...even before the first-generation stars.
Going back in time even before recombination to a even hotter and denser time, the entire universe resembles the core of a star, where temperatures and pressures are high enough for protons to collide with each other and fuse. This is the nucleosynthesis phase of the early big bang.
For a brief time in this early universe, hydrogen fused into helium as in the cores of main-sequence stars, but can also fuse into deuterium (an unstable form of hydrogen) as well as lithium. What makes this early fusion in the universe unique is that deuterium and lithium cannot survive the high temperatures in the core of main-sequence stars--only helium survives--but the expansion and cooling of the universe shut off fusion such that deuterium and lithium from nucleosynthesis did survive to the present day. All the deuterium (found as "heavy water" in certain springs on Earth) and all the lithium (used in batteries for hybrid cars, computers and smartphones) that is found today can only have come from this early universe fusion.
Fourth, the first matter...stuff that makes up you, me, and the rest of the universe.
For this process of pair production, if energy creates a proton, it must also create the proton's "evil twin," an antiproton as well. Annihilation is the process where if a proton and its antiproton meet up with each other, they will be destroyed and converted back into energy. Everything that makes up matter has an antimatter "evil twin," and the very early universe continuously converted energy into equal amounts of matter and antimatter, and vice versa.
This presents a puzzle, as everything that we observe in the universe today is made of matter, but from pair production matter must be created with an equal amount of antimatter--so perhaps, somewhere else in the universe there is an equal amount of antimatter waiting to partner up with and subsequently annihilate us. Have you ever seen Seven Brides for Seven Brothers (MGM, 1954)?
What would happen if there was a sequel to this move...titled Seven Brides for Eight Brothers?
This is the hypothesis for why the universe is only made of matter today. Somehow instead of being produced in equal amounts, if there was slightly more matter than antimatter, then there would be something left over after all the antimatter produced is annihilated with an equal amount of matter. We today would then be the eighth unmatched brother, which makes us either fortunate or forlorn, depending on your perspective. In particle colliders equal amounts of matter and antimatter always seem to generated from pair production, but experiments being carried out today are searching for indications that there are certain processes that produce just a smidgen more matter than antimatter.
We've run the clock backwards, watching the first stars, first photons, first fusion, and first matter stages of the early big bang occur in reverse chronological order. We haven't gone all the way back to time zero, and there are some interesting events observed and conjecture that occurred even before these four stages that we will not cover in this course.
So concentrate on being able to describe what happened in each of these four stages, and the evidence that is either observed or recreated for each of these four stages.