Lecture.5

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Oct 30, 2023

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Lecture # 5 First Atoms and Dark Ages
Formation of First Atoms Big Bang nucleosynthesis ended at 10 mins after the big bang with 75% of the matter ending up in hydrogen and 25% in Helium. At this point the temperature of the Universe was low enough to forbid formation of elements heavier than berylium and lithium but still high enough to forbid electrons joining the nuclei to form atoms. They would be ejected as soon as they joined the protons. This continued until the Universe became cold enough so that the photons no longer had the energy to kick out the electrons. This happened when the temperature of the Universe reached 3740 degrees Kelvin at 240,000 years after the Big Bang
The Origin of Helium Immediately after the Big Bang, only protons and electrons existed. Shortly after the BB, temperature and density was high enough for deuterium (heavy Hydrogen) to form by fusion. After 100 seconds or so, temperature cooled enough so that deuterium could fuse into helium nuclei. The temperature continued to cool, and fusion stopped after a few minutes. Big Bang theory predicts that around 24% of the matter in the early universe was helium, which matches what we see.
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“To make an apple pie from scratch, you should first invent the Universe.” Carl Sagan
Radiation and First Atoms The Universe at this time was dominated by electrons and radiation (photons) with the photons continuously scattered by the electrons. The rate with which photons scatter competes with the rate of the expansion of the Universe and rapidly decreases as the Universe expands. This continues until the time between subsequent scatter of photons (by electrons) exceeds the age of the Universe at that epoch. At this point photons cease to interact with electrons and therefore, decouple. This epoch is called decoupling .
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History of the Universe
The Universe becomes Transparent After decoupling, photons could move freely and unimpeded by the matter. Therefore, the Universe becomes transparent. The process of decoupling completes when the Universe was 380,000 years old and had a temperature of 3,000 deg K.
Matter-Radiation Decoupling
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Cosmic Background Radiation Before decoupling, matter and radiation were in a state of equilibrium in the Universe, meaning that the temperature was uniform across the Universe. The temperature distribution in the Universe at the time of decoupling (380,000 years after the Big Bang) reflects the matter distribution at that time. The light therefore was no longer impeded by the atoms in the Universe and could escape, making the space transparent. This is the Cosmic Microwave Background (CMB) Radiation. The CMB was in equilibrium with matter in the Universe and is distributed uniformly. Since then, its temperature has dropped to 2.754 degrees Kelvin.
Microwave Background Radiation This is the radiation remaining from the Big Bang. It is homogeneous and isotropic with a temperature of 2.73 K. The CBR was discovered in 1965 by Penzias and Wilson.
CMB has a Cosmological Origin The spectrum of the CMB is very nearly a black body, suggesting the radiation is in thermal equilibrium with matter
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First Structures in the Universe There were fluctuations in the temperature of CMB at 300,000 years after the Big Bang, reflecting clumps in matter distribution at the time the radiation decoupled. These clumps reflect the initial seeds for the formation of structure. These grew to become stars and galaxies and clusters of galaxies.
Seeds of Galaxy Formation There are slight density contrasts in the CMB. These are the initial perturbations which grew into galaxies and clusters of galaxies This is a WMAP-NASA image of the distribution of CMB
Cosmic Background Radiation Distribution of matter in the universe at 380,000 years after the Big Bang The red areas correspond to large matter concentrations which lead to the galaxies we see today
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Dark Ages In the period after the formation of first atoms and before the formation of first stars, the Universe was devoid of any source of light and was mostly dominated by dark matter. The only photons were those of the cosmic background radiation. No stars or galaxies existed. During this time, neutral matter developed an opaque wall, blocking the light from passing through. No information could be obtained from that time as no light could escape. Dark ages started around 300,000 years after the Big Bang and ended around 1 billion years after the Big Bang with the formation of first stars and galaxies.
Rebirth of the Universe By the formation of stars and galaxies, high energy ultra-violet light was produced. This light hit the atoms and removed the electrons (ionized the atoms) making the Universe transparent . This is called re-ionization It took the Universe around 600 million years to gradually come out of dark ages. The clumps of matter at the time of decoupling collapsed under their gravity, forming stars and galaxies. The light generated by stars and galaxies ionized the matter in the Universe, making it possible for the light to escape unimpeded. This is the reason we can see deep to the Universe today.
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Looking at the Re-ionization Epoch
“When we try to look further into the Universe we come to what appears to be the end of space but in fact is the beginning of time” Neil deGrass Tyson

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