# Evolution of the Universe - The universe has undergone many different stages of evolution since the Big Bang - As **time increases** from the Big Bang, the **temperature decreases** dramatically - Each stage is characterised by different physical processes and particle formations

Temperature and time are inversely related: as the universe ages, it cools down from extremely hot initial conditions to the current temperature of 2.7 K.

**The cooling of the universe through its major stages:** ![](https://questioncovers.s3.eu-west-2.amazonaws.com/d1705229-4902-4550-ba8d-15a81b890b37) Note: Both axes use logarithmic scales to show the enormous range of values. The universe has cooled from over 10 trillion K to just 2.7 K today. ## Timeline of the Universe The key stages of evolution are: | Stage | Time After Big Bang | Key Events | |-------|---------------------|------------| | **Stage 0** | $t = 0$ | The Big Bang occurs | | **Stage 1** | $10^{-43}$ s → $10^{-35}$ s | Inflation | | **Stage 2** | $10^{-35}$ s → $10^{-6}$ s | Gamma photons dominate | | **Stage 3** | $10^{-6}$ s → $10^{-3}$ s | Quarks and leptons form | | **Stage 4** | $10^{-3}$ s → $10^{2}$ s | Hadrons form | | **Stage 5** | $10^{2}$ s → $10^{5}$ s | Nuclei form | | **Stage 6** | $10^{5}$ s → $10^{8}$ s | Atoms form | | **Stage 7** | $10^{8}$ s → Present | Stars and galaxies form | ## Stage 0: The Big Bang - At time $t = 0$, approximately **13.7 billion years ago**, the Big Bang occurred - At this instant, **both space and time were created** - The universe existed as a **hot singularity** - Infinitely dense - Infinitely hot - Infinitely small

A singularity is a point of infinite density and temperature where the known laws of physics break down

## Stage 1: Inflation **Time after Big Bang:** $10^{-43}$ s to $10^{-35}$ s - The universe undergoes **rapid expansion** - This period is known as **inflation** - During this incredibly brief period, the universe expands exponentially - Space itself stretches at an accelerating rate ## Stage 2: Gamma Photons **Time after Big Bang:** $10^{-35}$ s to $10^{-6}$ s - There is **no matter** present yet - The universe consists entirely of **high-energy gamma photons** - These photons have extremely short wavelengths and very high frequencies - Temperature is still too high for any particles to form and remain stable ## Stage 3: Quarks and Leptons Form **Time after Big Bang:** $10^{-6}$ s to $10^{-3}$ s - **Building block particles** begin to appear: - Quarks - Leptons (including electrons) - Photons - Their corresponding antiparticles - These particles **cannot yet form heavier particles** (like protons and neutrons) - The temperature is still too high for them to bind together - There is **slightly more matter than antimatter** - This small asymmetry is crucial for the future universe - As matter and antimatter **annihilate**, they leave behind a **matter-dominated universe** - Made from particles, not antiparticles ## Stage 4: Hadrons Form **Time after Big Bang:** $10^{-3}$ s to $10^{2}$ s - As the universe cools sufficiently, the **first hadrons** begin to form - **Protons** (made from up-up-down quarks) - **Neutrons** (made from up-down-down quarks) - Quarks combine to form these heavier composite particles - **Matter and antimatter continue to collide and annihilate** - This produces **enormous quantities of high-energy photons** - These photons are continually **absorbed and re-emitted** as they interact with charged particles - The universe is still opaque to radiation at this stage ## Stage 5: Nuclei Form **Time after Big Bang:** $10^{2}$ s to $10^{5}$ s - The universe reaches temperatures **similar to the core of a star** - **Nucleosynthesis** begins: - The process of forming atomic nuclei - The **first nuclei form** through two processes: - **Electron capture**: electrons are absorbed by protons - **Nuclear fusion**: lighter nuclei combine to form heavier ones - **Protons and neutrons fuse** to form light element nuclei: - Deuterium (heavy hydrogen) - Helium - Lithium - Beryllium - Matter exists in **plasma form** - A state where protons and electrons are **not bound** to one another - High temperatures prevent them from combining into atoms - **Rapid expansion** continues until approximately **25% of all matter** is helium nuclei - The remaining 75% consists of hydrogen nuclei (single protons)

Plasma is a state of matter where nuclei and electrons exist separately due to high temperatures preventing them from combining into neutral atoms

## Stage 6: Atoms Form **Time after Big Bang:** $10^{5}$ s to $10^{8}$ s - The universe continues cooling - **Electrons combine with nuclei** to form the first atoms - Primarily **hydrogen atoms** (one proton, one electron) - Also **helium atoms** (two protons, two neutrons, two electrons) - **Decoupling** occurs: - More and more electrons become **attached to protons** - **Radiation and matter separate** from each other - Previously, photons constantly interacted with charged particles - **Photons now travel freely** through space - They are no longer constantly scattered by charged particles - The universe becomes **transparent** to radiation - These ancient photons still exist today - They have been stretched by the expansion of the universe - They now appear as **microwave background radiation** - Currently detected at a temperature of approximately **2.7 K** ## Stage 7: Stars and Galaxies Form **Time after Big Bang:** $10^{8}$ s to Present ### Formation of the First Stars (~ 30 million years) - After approximately **30 million years**, the **first stars** begin to form - Gravitational forces pull together clouds of hydrogen gas - As these clouds collapse, temperatures and pressures increase - Nuclear fusion ignites in the cores of these protostars ### Galaxy Formation (~ 200 million years) - **Galaxies begin to form** from tiny density fluctuations in the early universe - **Gravitational forces** pull together: - Clouds of hydrogen gas - Existing stars - Dark matter halos - Think of it like cities forming: just as people cluster together in populated areas, stars and gas cluster together in galaxies ### Heavy Element Formation (billions of years later) - **Heavy elements form** through the gravitational collapse and explosion of stars - During supernova explosions, elements heavier than iron are created - These elements are scattered into space, enriching the interstellar medium ### Formation of the Solar System (~ 9 billion years after Big Bang) - Approximately **9 billion years** after the Big Bang, our **solar system forms** - It originates from a **supernova nebula**: - A cloud of gas and dust left behind by an exploding star - **The Sun forms** at the centre of this rotating nebula - Remaining material coalesces to form planets and other objects ### Formation of Earth (~ 10 billion years after Big Bang) - **Earth forms** almost **1 billion years** after the Sun - It is created from the accumulation of rocky material in the inner solar system - Early Earth is molten and hostile to life ### Life Begins (~ 11 billion years after Big Bang) - Approximately **11 billion years** after the Big Bang, **primitive life begins on Earth** - Simple single-celled organisms emerge in Earth's oceans - This marks the beginning of biological evolution on our planet ### Modern Humans (~ 13.7 billion years after Big Bang) - **13.7 billion years** after the Big Bang, the **first modern humans evolve** - Homo sapiens develop approximately 200,000-300,000 years ago - This is remarkably recent in cosmic terms: like the last few seconds of a 24-hour day ### Present Day - The **current temperature** of the universe is **2.7 K** - The universe continues to expand and cool - Galaxies continue to move away from each other - Stars continue to form in gas clouds, though at a slower rate than in the early universe

The percentages of matter formed (75% hydrogen, 25% helium) during nucleosynthesis remain approximately the same today throughout the universe.

## Summary Table: Evolution Timeline | Time Period | Event | Significance | |-------------|-------|--------------| | $10^{-43}$ s | Inflation begins | Universe expands exponentially | | $10^{-6}$ s | Quarks and leptons appear | First fundamental particles form | | 1 s | Hadrons form | Protons and neutrons emerge | | 100 s | Nucleosynthesis | Light element nuclei form | | 380,000 years | Atoms form | Universe becomes transparent | | 30 million years | First stars ignite | Nuclear fusion begins in stellar cores | | 9 billion years | Solar system forms | Our Sun and planets emerge | | 13.7 billion years | Present day | Universe at 2.7 K, continuing to expand |