The Dark Ages of the Cosmos | Crash Course Pods: The Universe #3

Introduction (00:00:00)

• The universe was initially in a hot and dense state and then began to expand.
• The rules of the universe were different when things were very hot and very dense.
• Over the first few minutes, things cooled down and spread out, resulting in the laws of physics we know today.
• The strong and weak nuclear forces, the Higgs field, gravity, and electromagnetism emerged.
• Two episodes into the podcast, they have only covered a minute and a half of the universe's history.
• After the earliest moments of the universe, the connection between the hot, dense early universe and today is incredibly strange yet logical.
• The mathematical possibilities led to the formation of photons, stars, and eventually humans.
• The trajectory of the universe's evolution can be observed.
• The existence of a multiverse is a possibility.

CMB & The Cosmic Web (00:02:09)

• The cosmic microwave background (CMB) is the leftover radiation from the Big Bang, the event that created the universe.
• The CMB shows that the early universe was not perfectly uniform, but had small variations in temperature.
• These variations correspond to the distribution of galaxies in the universe, with denser regions of the CMB corresponding to regions with more galaxies.
• This means that the CMB can be used to study the early universe and how galaxies formed through the process of gravitational collapse.

The Possibility of a Multiverse (00:09:39)

• Cosmic inflation is a theory that suggests the Universe underwent rapid expansion in its early stages.
• Cosmic inflation explains the uniformity of the cosmic microwave background.
• Cosmic inflation implies that the observable universe is a small part of a much larger universe or Multiverse.
• The larger space beyond the observable universe may be constantly inflating.
• Pocket universes within the larger space may have their own inflationary phases and matter phases.
• There is a hypothesis that pocket universes could collide, but observational evidence for this is lacking.
• After the Big Bang, the Universe was filled with hot, dense plasma.
• As the Universe expanded and cooled, protons and neutrons formed.
• The first atoms formed when protons and electrons combined.
• The Universe was opaque due to the presence of neutral hydrogen atoms.
• The Dark Ages lasted for about 400,000 years.
• The first stars and galaxies formed at the end of the Dark Ages.

The Inflaton Field (00:14:20)

• Inflation, driven by the inflaton field, caused the rapid expansion of the universe, which then stopped, leading to the decay of the inflaton field and the beginning of the hot phase of the early universe.
• Quantum fluctuations in the inflaton field created density fluctuations, which can be traced back to the wiggling of the inflaton field due to quantum uncertainty.
• Inflation with quantum fluctuations explains the cosmic microwave background radiation and the large-scale structure of the universe.
• The universe evolved from protons and gluons to the complex structures we see today, including humans.
• The "Dark Ages of the Cosmos," a period of darkness in the early universe, lasted from about 380,000 years to a billion years after the Big Bang.
• A thick fog of neutral hydrogen gas blocked out most of the light from the first stars and galaxies during the Dark Ages.
• The first stars and galaxies formed when the universe expanded and cooled enough for the hydrogen gas to condense and collapse under its own gravity, marking the end of the Dark Ages and the beginning of the cosmic dawn.

Viewing Early Galaxies (00:20:20)

• We can observe the evolution of the universe by looking into the past with astronomy.
• The cosmic microwave background allows us to directly observe the universe 380,000 years after its beginning.
• Through theory and experiment, we can infer events down to picoseconds after the beginning of the universe.
• We can observe the timeline after cosmic inflation through astronomy.
• The James Webb Space Telescope (JWST) allows us to compare early galaxies to more recent ones.
• Distant galaxies appear larger because the universe was smaller when their light left them.
• The universe was smaller when distant galaxies emitted their light, making them appear larger to us now.

The Surface of Last Scattering (00:24:22)

• The early universe was a hot, dense plasma similar to the center of the Sun.
• As the universe expanded and cooled, photons were able to move more freely, leading to the release of the cosmic microwave background radiation.
• The cosmic microwave background is a picture of the "surface of last scattering," the point at which photons were able to escape the dense plasma of the early universe.
• After the surface of last scattering, the universe became less dense and cooler, allowing electrons and protons to find each other and bond.

The Dark Ages of the Cosmos (00:30:35)

• Recombination era begins when the first neutral atoms form.
• The Dark Ages is named as such because there are no stars yet, just cooling hydrogen gas with some helium.
• The Dark Ages lasted for millions of years.
• The physics of the Dark Ages is simple and can be described by equations for gas cooling.
• Variations in density in the gas clouds led to some clouds becoming denser than others.
• Without these variations in density, there would never have been stars or galaxies in the universe.
• The structure in the universe is a result of these early quantum fluctuations that occurred during inflation.

Dark Matter & Cosmic Dawn (00:34:25)

• Dark matter, which comprises 85% of the universe's matter, is invisible and intangible as it doesn't interact with light or other matter. It plays a crucial role in the formation of the first stars and galaxies by providing gravitational pull to clump together cold gas.
• The early Universe was hot and dense, making star formation difficult. The first stars, likely hundreds or thousands of times more massive than present-day stars, formed within the first few hundred million years of the Universe's existence.
• These early stars produced heavier elements through nuclear reactions and scattered them into the intergalactic medium. Subsequent generations of stars formed from this enriched gas, leading to the creation of planets and the universe we see today.
• The process of star formation and enrichment took several million to billions of years. Fully evolved galaxies existed within the first 400 million years, or possibly even 200 million years, of the universe's existence.
• Each generation of stars becomes slightly easier to form due to the presence of heavier elements that aid in the cooling process necessary for star formation.

Feeling Awe (00:47:01)

• The hosts express their awe and amazement at the vastness and complexity of the cosmos.
• They discuss the concept of the "numinous" and how it relates to the feeling of awe.
• They also mention the feeling of being overwhelmed by the beauty and power of the universe.
• The hosts wrap up the episode by discussing how cosmologists need to be flexible with time scales.
• They express their excitement about continuing their conversation and learning more about the universe.
• They also mention the thrill of being able to conceptualize the early universe and connect the dots to star formation millions of years later.
• The hosts acknowledge that the vastness and power of the universe can also raise serious questions about free will and determinism.