Beginning to End

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Lecture 7: Beginning to End

Lecture 7 Beginning to EndDownload

🌌 Overview

This lecture explores the Big Bang Theory, the expansion of the universe, and how our understanding of the universe’s past gives insight into its future. It begins with a seemingly simple question: Why is the night sky dark? — and uses it to unpack the very structure and timeline of the cosmos.

🌑 Olbers’ Paradox: Why is the Night Sky Dark?

  • If the universe were infinite in space and time and filled with infinite stars, then the night sky should be blindingly bright.
  • But it’s not — and this leads to the paradox:

“In an infinite and eternal universe, every line of sight should end on a star.”

Implication:

This contradiction suggests:

  • The universe is not infinitely old
  • Or it’s not infinite in size
  • Or it doesn’t have infinite stars

This opens the door to a finite universe that had a beginning.

🌀 The Expanding Universe

Doppler Shift & Spectroscopy

  • Light from distant galaxies is redshifted, indicating they are moving away from us.
  • This is not because we’re the center of the universe — rather, space itself is expanding.

Edwin Hubble’s Discovery:

  • Distant galaxies move faster → Velocity ∝ Distance
  • This relation is called Hubble’s Law:
    v = H₀ × d

Where:

  • v = recessional velocity
  • d = distance
  • H₀ = Hubble constant (~70 km/s/Mpc)

Consequence:

  • The universe is expanding
  • If we rewind time, all matter and energy condense to a single point

💥 The Big Bang

  • Rewinding the expansion leads to a beginning: the Big Bang
  • Approximate age of the universe:
    t ≈ 1 / H₀ ≈ 14 billion years

This is not the origin of all existence, but rather the start of the observable universe and space-time as we know it.

🧪 Evidence for the Big Bang

1. Cosmic Microwave Background (CMB)

  • Residual heat from ~400,000 years after the Big Bang.
  • Universe cooled enough for atoms to form, allowing light to travel freely.

2. Primordial Nucleosynthesis

  • Within the first 3 minutes, the universe formed:
  • Hydrogen (most common)
  • Helium
  • Trace amounts of Lithium, Beryllium
  • These ratios match predictions and are found in stars and our own bodies.

“You’re not just star stuff — you’re Big Bang stuff.”

🔭 Henrietta Leavitt & Measuring Distance

  • Cepheid variable stars used as standard candles.
  • Leavitt’s Law (Period–Luminosity relationship) enabled measurements of galaxies beyond the Milky Way.

Edwin Hubble applied this:

  • Discovered Andromeda is outside the Milky Way.
  • Cemented the existence of other galaxies and the expansion of the universe.

🕳️ Black Holes and the Far Future

  • The Sun will die ~5 billion years from now.
  • Eventually, all stars will die: universe filled with white dwarfs, neutron stars, black holes.
  • After 10¹⁰⁰ years, even black holes may evaporate (via Hawking Radiation).

Final stages:

  • The universe cools and dims toward heat death
  • Possibility of quantum fluctuations leading to new universes

🧬 What Comes Next?

  • We are now observing galaxies as they were, not as they are.
  • The cosmic horizon limits what we can see: ~45 billion light-years away.

Open questions:

  • Was there a universe before the Big Bang?
  • Are there other universes?
  • Are we alone?

💡 Final Thoughts

This lecture offers a sweeping cosmic narrative:

  • From a paradox about darkness…
  • To the realization of a universe with a finite beginning
  • Supported by multiple lines of evidence
  • And leading to profound questions about our origins, existence, and destiny
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