13 DARK AGES

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The dark ages is a period in the history of the universe that spans from around 380,000 years after the Big Bang to about 150 million years after the Big Bang. It is called the dark ages because during this time, the universe was largely devoid of light-emitting sources such as stars, galaxies, and quasars. Here is a step-by-step overview of what happened during the dark ages:

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13.1 Decoupling / Recombination

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Decoupling refers to a period in the development of the universe when different types of particles fall out of thermal equilibrium with each other. This occurs as a result of the expansion of the universe, as their interaction rates decrease (and mean free paths increase) up to this critical point. The two verified instances of decoupling since the Big Bang which are most often discussed are photon decoupling and neutrino decoupling, as these led to the cosmic microwave background and cosmic neutrino background, respectively.

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During this time, electrons combined with protons to form hydrogen atoms, resulting in a sudden drop in free electron density. Decoupling occurred abruptly when the rate of Compton scattering of photons Γ was approximately equal to the rate of expansion of the universe H, or alternatively when the mean free path of the photons λ was approximately equal to the horizon size of the universe H⁻¹. After this photons were able to stream freely, producing the cosmic microwave background as we know it, and the universe became transparent.  [2]

The interaction rate of the photons is given by

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where nₑ  is the electron number density, σₑ is the electron cross sectional area, and c is the speed of light. In the matter-dominated era (when recombination takes place),

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where a is the cosmic scale factor. Γ also decreases as a more complicated function of a, at a faster rate than H. [3] By working out the precise dependence of H  and Γ on the scale factor and equating Γ=H, it is possible to show that photon decoupling occurred approximately 380,000 years after the Big Bang, at a redshift of z=1100  [4]  when the universe was at a temperature around 3000 K.

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Photon decoupling is closely related to recombination, which occurred about 378,000 years after the Big Bang (at a redshift of z = 1100), when the universe was a hot opaque ("foggy") plasma. During recombination, free electrons became bound to protons (hydrogen nuclei) to form neutral hydrogen atoms. Because direct recombinations to the ground state (lowest energy) of hydrogen are very inefficient, these hydrogen atoms generally form with the electrons in a high energy state, and the electrons quickly transition to their low energy state by emitting photons. Because the neutral hydrogen that formed was transparent to light, those photons which were not captured by other hydrogen atoms were able, for the first time in the history of the universe, to travel long distances. They can still be detected today, although they now appear as radio waves, and form the cosmic microwave background ("CMB"). They reveal crucial clues about how the universe formed.

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13.2 The universe becomes transparent

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With the formation of neutral atoms, the universe became transparent to radiation, meaning that light was no longer scattered by free electrons.

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​13.3 Dark matter dominates

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Dark matter is a form of matter that does not interact with the electromagnetic force, meaning it does not emit, absorb, or reflect light. It is, however, affected by gravity, and as a result, it plays a crucial role in the formation of structures in the universe, such as galaxies and galaxy clusters. Although we cannot directly observe dark matter, its presence is inferred through its gravitational effects on visible matter and the large-scale structure of the universe.

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In the early universe, both dark matter and normal (baryonic) matter were present, but dark matter dominated the gravitational interactions for several reasons:

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• Abundance: Dark matter is estimated to make up about 85% of the total matter content of the universe, with baryonic matter accounting for the remaining 15%. This greater abundance of dark matter means it has a more substantial gravitational influence on the large-scale structure formation in the universe.

• Weak interaction: Dark matter particles only interact through gravity and possibly the weak nuclear force. Baryonic matter, on the other hand, interacts through all four fundamental forces: gravity, electromagnetism, strong nuclear, and weak nuclear. In the early universe, baryonic matter was a hot plasma of charged particles (ions and electrons), which interacted strongly with photons via the electromagnetic force. This interaction caused the baryonic matter to be tightly coupled with the photons and limited its ability to clump together under gravity. Since dark matter doesn't interact electromagnetically, it was not affected by this coupling and could begin to collapse under gravity earlier.

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As the universe expanded and cooled, the baryonic matter eventually decoupled from the photons during the recombination epoch, allowing it to fall into the gravitational potential wells created by dark matter. This process led to the formation of the cosmic web, with dark matter providing the scaffolding for the formation of galaxies and galaxy clusters.

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​13.4 The first stars and galaxies form

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• Dark matter halos: As the universe continued to expand, dark matter started to collapse under its own gravity, forming structures called dark matter halos. These halos created gravitational potential wells that attracted baryonic (ordinary) matter, primarily consisting of neutral hydrogen gas.

• Gas cooling and fragmentation: The hydrogen gas that fell into the dark matter halos started to cool down and contract under gravity. As the gas cooled, it fragmented into smaller clumps, eventually forming dense regions called "protostars." This process took several hundred million years after the Big Bang.

• Nuclear fusion and star formation: When the density and temperature in the cores of these protostars became high enough, nuclear fusion ignited, converting hydrogen into helium and releasing energy in the form of light and heat. This marked the birth of the first stars, known as Population III stars. These stars were massive, hot, and short-lived, and they emitted intense ultraviolet (UV) radiation.

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13.5 The universe becomes reionized

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The UV radiation from the first stars ionized the surrounding neutral hydrogen gas, creating pockets of ionized gas around the stars. This process, called reionization, gradually made the universe transparent to UV light, allowing it to travel further and ionize more hydrogen gas.

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As more stars formed and grouped together under gravity, they eventually created the first galaxies. These early galaxies were smaller and more irregular than the ones we observe today. They continued to grow in size and complexity through processes like mergers and the accretion of more gas and dark matter.

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Overall, the dark ages was a period of transition in the early universe, marking the end of the era of radiation domination and the beginning of the era of cosmic structure formation. During this time, the universe was dominated by dark matter, which provided the gravitational potential for the first stars and galaxies to form. The end of the dark ages with the onset of reionization marked the beginning of the era of the universe as we know it today.

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13.6 Recent Developments :  Lunar Surface Electromagnetics Experiment [1]

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The Lunar Surface Electromagnetics Experiment (LuSEE-Night) is a planned robotic radio telescope observatory designed to land and function on the far side of 11]Earth's Moon. [6] [7] The project is under development by the U.S. Department of Energy and the National Aeronautics and Space Administration. [8] If successfully deployed and activated, LuSEE-Night will attempt measurements of an early period of the history of the Universe that occurred relatively soon after the Big Bang, referred to as the Dark Ages of the Universe, which predates the formation of luminous stars and galaxies. [9] The instrument is planned to be landed on the lunar far side as soon as 2026 aboard the Blue ghost lunar lander. [10] [ LuSEE-Night, not to be confused with a companion lander planned for lunar landing in 2024 named LuSEE-Lite, is to be delivered to the lunar far side by Commercial Lunar Payload Services (CLPS). [12]

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