The Big Bang: How It All Began

The universe wasn't born in silence — it began with a rapid expansion called the Big Bang. At this moment, space and time emerged from a singularity. From nothing came everything: particles, energy, space, and laws of physics. Let's dive deep into its story, timeline, and math!

If you're just joining us, catch up on earlier posts: What is the Universe? | Galaxies – Cosmic Cities of Stars | Spiral vs Elliptical Galaxies

🧨 What Was the Big Bang?

The Big Bang was the beginning of the universe approximately 13.8 billion years ago. At \( t = 0 \), all matter was concentrated in a point of infinite density and temperature: a singularity.

As time progressed, space itself expanded. This is described by the Friedmann Equation derived from Einstein's General Relativity:

\[ \left( \frac{\dot{a}}{a} \right)^2 = \frac{8\pi G}{3} \rho - \frac{k}{a^2} + \frac{\Lambda}{3} \]

Where:

• \( a(t) \): Scale factor (describes expansion)
• \( \rho \): Energy density
• \( k \): Curvature constant
• \( \Lambda \): Cosmological constant (dark energy)

🕒 Timeline of the Early Universe

• \( t = 10^{-43} \, \text{s} \): Planck Era — quantum gravity effects
• \( t = 10^{-36} \, \text{s} \): Inflation causes exponential expansion
• \( t = 10^{-6} \, \text{s} \): Quarks combine to form protons and neutrons
• \( t = 3 \, \text{min} \): Nucleosynthesis — helium and deuterium form
• \( t = 3.8 \times 10^5 \, \text{years} \): Atoms form, light decouples (CMB)

🌡️ Temperature and Expansion

The temperature of the universe decreased as it expanded. This follows the relation:

\[ T(t) \propto \frac{1}{a(t)} \]

Where \( T \) is the temperature and \( a(t) \) is the scale factor. As the universe grew, the energy density dropped and matter could form.

🔭 Fun Fact: At 1 second after the Big Bang, the temperature was over 10 billion Kelvin!

🔍 Observational Evidence

• Redshift: Hubble observed galaxies moving away: \( v = H_0 d \)
• Cosmic Microwave Background: Planck satellite measured it as \( T_{\text{CMB}} = 2.725\,K \)
• Primordial Elements: Abundance of helium: ~24% by mass predicted by Big Bang Nucleosynthesis (BBN)

🔭 Hubble’s Law

The expansion of the universe can be seen with Hubble’s Law:

\[ v = H_0 d \]

Where:

• \( v \): Recession velocity of a galaxy
• \( H_0 \): Hubble constant (currently ~70 km/s/Mpc)
• \( d \): Distance to the galaxy

🌟 Conclusion: The Big Bang was not an explosion but the beginning of space-time and everything in it. With equations like the Friedmann equation and evidence like the CMB, the story of the cosmos becomes a scientific adventure!

📚 Learn More

• 🧠 Friedmann Equations (Wikipedia)
• 🔙 Previous: Spiral vs. Elliptical Galaxies
• ⏭️ Coming Soon: Dark Matter: The Hidden Skeleton of the Cosmos