10  Origin of the Universe

How do we know the universe began with a Big Bang?

HS-ESS1-2 6–7 Days 🧠 Quiz & Evaluate ↓

💥 How Did Everything Begin?

11 🔥 Engage — The Light From the Past

11.1 Everything Is Moving Away From Us

When astronomers look at distant galaxies, they notice something strange: the light from every galaxy is shifted toward the red end of the spectrum. The farther away a galaxy is, the more its light is redshifted.

This is called the Doppler Effect for light — the same reason an ambulance siren sounds lower-pitched as it drives away from you. If a galaxy’s light is redshifted, it means that galaxy is moving away from us.

And they’re all moving away. In every direction. The farther away, the faster they go.

Plot = require("@observablehq/plot")
d3 = require("d3@7")

11.2 The Doppler Effect for Light

{
  const svg = d3.create("svg")
    .attr("width", 700)
    .attr("height", 220)
    .attr("viewBox", "0 0 700 220");

  svg.append("text").attr("x", 350).attr("y", 20).attr("text-anchor", "middle")
    .attr("font-size", 16).attr("font-weight", "bold").text("Doppler Shift for Light");

  // Spectrum bar
  const gradient = svg.append("defs").append("linearGradient")
    .attr("id", "spectrum").attr("x1", "0%").attr("x2", "100%");
  const colors = ["#8B00FF", "#4B0082", "#0000FF", "#00FF00", "#FFFF00", "#FF7F00", "#FF0000"];
  colors.forEach((c, i) => {
    gradient.append("stop").attr("offset", `${i / (colors.length - 1) * 100}%`).attr("stop-color", c);
  });

  svg.append("rect").attr("x", 50).attr("y", 40).attr("width", 600).attr("height", 30)
    .attr("rx", 5).attr("fill", "url(#spectrum)");
  svg.append("text").attr("x", 80).attr("y", 85).attr("font-size", 11).attr("font-weight", "bold").attr("fill", "#8B00FF").text("Violet (short λ)");
  svg.append("text").attr("x", 560).attr("y", 85).attr("font-size", 11).attr("font-weight", "bold").attr("fill", "#FF0000").text("Red (long λ)");

  // Approaching
  svg.append("rect").attr("x", 50).attr("y", 110).attr("width", 290).attr("height", 90)
    .attr("rx", 10).attr("fill", "#74b9ff").attr("opacity", 0.2);
  svg.append("text").attr("x", 195).attr("y", 135).attr("text-anchor", "middle")
    .attr("font-size", 14).attr("font-weight", "bold").attr("fill", "#0984e3").text("🔵 Blueshift");
  svg.append("text").attr("x", 195).attr("y", 155).attr("text-anchor", "middle")
    .attr("font-size", 12).text("Object moving TOWARD us");
  svg.append("text").attr("x", 195).attr("y", 175).attr("text-anchor", "middle")
    .attr("font-size", 11).attr("fill", "#636e72").text("Light waves compressed → shorter λ");
  svg.append("text").attr("x", 195).attr("y", 195).attr("text-anchor", "middle")
    .attr("font-size", 20).text("← 🌟");

  // Receding
  svg.append("rect").attr("x", 360).attr("y", 110).attr("width", 290).attr("height", 90)
    .attr("rx", 10).attr("fill", "#ff7675").attr("opacity", 0.2);
  svg.append("text").attr("x", 505).attr("y", 135).attr("text-anchor", "middle")
    .attr("font-size", 14).attr("font-weight", "bold").attr("fill", "#d63031").text("🔴 Redshift");
  svg.append("text").attr("x", 505).attr("y", 155).attr("text-anchor", "middle")
    .attr("font-size", 12).text("Object moving AWAY from us");
  svg.append("text").attr("x", 505).attr("y", 175).attr("text-anchor", "middle")
    .attr("font-size", 11).attr("fill", "#636e72").text("Light waves stretched → longer λ");
  svg.append("text").attr("x", 505).attr("y", 195).attr("text-anchor", "middle")
    .attr("font-size", 20).text("🌟 →");

  return svg.node();
}

11.2.1 📝 Engage Activity

1. If every galaxy is moving away from us, does that mean we’re at the center of the universe? Think carefully before answering.
2. What would it look like if the universe were expanding uniformly — like dots on an inflating balloon?
3. If galaxies are moving apart now, what were they doing in the past?

12 🔍 Explore — Hubble’s Discovery

12.1 The Universe Is Expanding

In 1929, Edwin Hubble made one of the most important discoveries in science: the velocity at which a galaxy moves away from us is proportional to its distance. This is now called Hubble’s Law.

\[v = H_0 \times d\]

where \(v\) is recession velocity, \(H_0\) is Hubble’s constant (~70 km/s/Mpc), and \(d\) is distance.

galaxyData = [
  {name: "LMC", distance: 0.05, velocity: 278, color: "#2ecc71"},
  {name: "Andromeda", distance: 0.77, velocity: -301, color: "#e74c3c"},
  {name: "NGC 300", distance: 6.1, velocity: 144, color: "#3498db"},
  {name: "M81", distance: 11.8, velocity: 300, color: "#9b59b6"},
  {name: "NGC 4258", distance: 23.5, velocity: 448, color: "#f39c12"},
  {name: "Virgo Cluster", distance: 54, velocity: 1150, color: "#e17055"},
  {name: "Coma Cluster", distance: 99, velocity: 6900, color: "#00cec9"},
  {name: "Abell 2029", distance: 310, velocity: 23000, color: "#d63031"},
  {name: "CL 0024+17", distance: 1700, velocity: 120000, color: "#636e72"}
]

Plot.plot({
  title: "Hubble's Law: Galaxy Distance vs. Recession Velocity",
  subtitle: "Farther galaxies move away faster — the universe is expanding uniformly",
  width: 700,
  height: 400,
  grid: true,
  x: {label: "Distance (Mpc)", type: "log"},
  y: {label: "Recession velocity (km/s)", type: "log"},
  marks: [
    Plot.dot(galaxyData.filter(d => d.velocity > 0), {
      x: "distance",
      y: "velocity",
      fill: "color",
      r: 8,
      stroke: "white",
      strokeWidth: 1
    ,
        tip: true}),
    Plot.text(galaxyData.filter(d => d.velocity > 0), {
      x: "distance",
      y: "velocity",
      text: "name",
      dy: -15,
      fontSize: 10,
      fontWeight: "bold"
    }),
    // Hubble's law line
    Plot.line(
      [{distance: 1, velocity: 70,
        tip: true}, {distance: 2000, velocity: 140000}],
      {x: "distance", y: "velocity", stroke: "#00cec9", strokeWidth: 2, strokeDasharray: "8,4"}
    )
  ]
})

12.1.1 💡 Hubble’s Key Insight

Notice that Andromeda has a negative velocity — it’s actually moving toward us! This is because Andromeda is so close that its gravitational attraction to the Milky Way overcomes the expansion. But every galaxy beyond our local group is moving away, and the trend is perfectly linear on a log-log scale.

Running the movie backwards: If everything is moving apart now, then in the past everything was closer together. Go far enough back, and everything was in the same point. That’s the Big Bang.

13 💡 Explain — Evidence for the Big Bang

13.1 Three Pillars of Evidence

The Big Bang theory isn’t just an idea — it’s supported by three independent lines of evidence that all point to the same conclusion.

13.2 Evidence 1: Universal Expansion (Redshift)

✅ Every distant galaxy is redshifted → the universe is expanding → it was once much smaller and denser.

13.3 Evidence 2: Cosmic Microwave Background (CMB)

In 1965, Arno Penzias and Robert Wilson accidentally discovered a faint microwave signal coming from every direction in the sky. This is the afterglow of the Big Bang — light released when the universe cooled enough for atoms to form, about 380,000 years after the Big Bang.

{
  const svg = d3.create("svg")
    .attr("width", 700)
    .attr("height", 280)
    .attr("viewBox", "0 0 700 280");

  svg.append("text").attr("x", 350).attr("y", 25).attr("text-anchor", "middle")
    .attr("font-size", 16).attr("font-weight", "bold").text("The Cosmic Microwave Background (CMB)");

  // CMB representation (simplified)
  const cmbColors = ["#0984e3", "#00cec9", "#2ecc71", "#f39c12", "#e74c3c", "#d63031"];
  for (let i = 0; i < 60; i++) {
    for (let j = 0; j < 15; j++) {
      const color = cmbColors[Math.floor(Math.random() * cmbColors.length)];
      svg.append("rect")
        .attr("x", 100 + i * 8.5)
        .attr("y", 40 + j * 8)
        .attr("width", 8)
        .attr("height", 7.5)
        .attr("fill", color)
        .attr("opacity", 0.5 + Math.random() * 0.5);
    }
  }
  svg.append("rect").attr("x", 100).attr("y", 40).attr("width", 510).attr("height", 120)
    .attr("fill", "none").attr("stroke", "white").attr("stroke-width", 2).attr("rx", 10);

  svg.append("text").attr("x", 355).attr("y", 110).attr("text-anchor", "middle")
    .attr("font-size", 14).attr("font-weight", "bold").attr("fill", "white")
    .attr("stroke", "black").attr("stroke-width", 0.5)
    .text("CMB: Temperature ≈ 2.725 K everywhere");

  // Temperature info
  svg.append("text").attr("x", 350).attr("y", 195).attr("text-anchor", "middle")
    .attr("font-size", 13).text("Tiny variations (±0.0002 K) show where matter was slightly denser");
  svg.append("text").attr("x", 350).attr("y", 215).attr("text-anchor", "middle")
    .attr("font-size", 13).text("Those dense spots became galaxy clusters billions of years later");
  
  svg.append("rect").attr("x", 150).attr("y", 230).attr("width", 400).attr("height", 35)
    .attr("rx", 17).attr("fill", "#6c5ce7");
  svg.append("text").attr("x", 350).attr("y", 253).attr("text-anchor", "middle")
    .attr("font-size", 13).attr("font-weight", "bold").attr("fill", "white")
    .text("The CMB is literally the oldest light in the universe");

  return svg.node();
}

13.4 Evidence 3: Hydrogen & Helium Ratio

The Big Bang model predicts the early universe should have produced about 75% hydrogen and 25% helium (by mass), with tiny traces of lithium. This is almost exactly what we observe:

elementData = [
  {element: "Hydrogen (H)", predicted: 75, observed: 73.9, color: "#74b9ff"},
  {element: "Helium (He)", predicted: 25, observed: 24.0, color: "#fdcb6e"},
  {element: "Lithium (Li)", predicted: 0.00001, observed: 0.000006, color: "#ff7675"},
  {element: "Everything else", predicted: 0, observed: 2.1, color: "#a29bfe"}
]

Plot.plot({
  title: "Predicted vs. Observed Element Abundances (% by mass)",
  subtitle: "Big Bang nucleosynthesis predictions match observations almost perfectly",
  width: 650,
  height: 250,
  marginLeft: 140,
  x: {label: "Abundance (% by mass)", grid: true, domain: [0, 80]},
  y: {label: null},
  color: {legend: true, domain: ["Predicted (Big Bang model)", "Observed"], range: ["#0984e3", "#e17055"]},
  marks: [
    Plot.barX(
      elementData.filter(d => d.predicted > 0.01).flatMap(d => [
        {element: d.element, value: d.predicted, type: "Predicted (Big Bang model)"},
        {element: d.element, value: d.observed, type: "Observed"}
      ]),
      {x: "value", y: "element", fill: "type", dx: 0, fx: null}
    )
  ]
})

13.4.1 💡 Three Independent Lines = Strong Theory

Evidence	What It Shows
Redshift	Universe is expanding → was once smaller
CMB	Afterglow of a hot, dense beginning
H/He ratio	Element creation matches Big Bang predictions

All three lines of evidence converge on the same conclusion: the universe began about 13.8 billion years ago from an incredibly hot, dense state.

🧠 When you tune an old TV to static, about 1% of that “snow” is actually the Cosmic Microwave Background — the afterglow of the Big Bang. You’re literally watching the birth of the universe.

14 🔬 Elaborate — From Big Bang to Solar System

14.1 The Cosmic Timeline

After the Big Bang, it took billions of years for the universe to produce the conditions needed for planets and life. Here’s the sequence:

viewof timeSlider = Inputs.range([0, 13.8], {
  label: "Time after Big Bang (billion years):",
  step: 0.1,
  value: 0
})

{
  const events = [
    {start: 0, end: 0.0000003, title: "🔥 Big Bang & Inflation", desc: "All matter and energy created. Universe expands from a point. Only H, He, Li form.", color: "#d63031"},
    {start: 0.0004, end: 0.0004, title: "✨ First Light (CMB)", desc: "Universe cools enough for atoms to form. Photons stream freely — this becomes the CMB.", color: "#fdcb6e"},
    {start: 0.2, end: 0.5, title: "⭐ First Stars Form", desc: "Gravity pulls H & He together. First generation of massive stars ignite. They fuse heavier elements.", color: "#74b9ff"},
    {start: 0.5, end: 1.0, title: "🌌 First Galaxies Form", desc: "Stars cluster into early galaxies. Supernovae spread heavy elements into space.", color: "#6c5ce7"},
    {start: 1.0, end: 9.0, title: "🌟 Stellar Recycling", desc: "Generations of stars live, die, and seed space with carbon, oxygen, iron — the stuff of planets and people.", color: "#00cec9"},
    {start: 9.2, end: 9.3, title: "☀️ Our Solar System Forms", desc: "A cloud of gas and dust (containing recycled stardust) collapses. Sun ignites. Planets form. Age: 4.6 Gyr ago.", color: "#f39c12"},
    {start: 9.7, end: 10.0, title: "🦠 First Life on Earth", desc: "Simple bacteria appear within 700 million years of Earth's formation. Life begins!", color: "#2ecc71"},
    {start: 13.5, end: 13.8, title: "🧠 Humans Arrive", desc: "After 13.8 billion years of cosmic evolution, a species evolves that can understand its own origin.", color: "#e17055"}
  ];

  const current = events.filter(e => timeSlider >= e.start);
  const latest = current.length > 0 ? current[current.length - 1] : events[0];

  const div = d3.create("div")
    .style("padding", "20px")
    .style("border-radius", "15px")
    .style("background", `linear-gradient(135deg, ${latest.color}33, ${latest.color}11)`)
    .style("border-left", `6px solid ${latest.color}`);

  div.append("h3").style("color", latest.color).style("font-family", "Space Grotesk, sans-serif")
    .text(`${timeSlider.toFixed(1)} billion years after the Big Bang`);
  div.append("h2").style("margin-top", "5px").text(latest.title);
  div.append("p").style("font-size", "1.15em").text(latest.desc);
  
  if (timeSlider >= 9.2) {
    div.append("p").style("font-style", "italic").style("color", "#636e72")
      .text(`That's ${(13.8 - timeSlider).toFixed(1)} billion years ago from today.`);
  }

  return div.node();
}

14.1.1 💡 You Are Made of Stardust

Every atom in your body heavier than helium was forged inside a star that exploded billions of years before our solar system formed. The calcium in your bones, the iron in your blood, the oxygen you breathe — all created by nuclear fusion in stars, then scattered by supernovae, then gathered by gravity into our solar system.

We are the universe’s way of understanding itself.

15 ✅ Evaluate — Constructing the Argument

15.1 Putting the Evidence Together

15.1.1 🧪 Evaluate Questions

1. Explain Hubble’s Law in your own words. What does it tell us about the universe’s past?

2. Compare the three lines of evidence for the Big Bang. Which do you find most convincing and why?

3. Construct a timeline from the Big Bang to the formation of our solar system. At each stage, explain what had to happen before the next stage could begin.

4. Argue: Could a universe that produced only hydrogen and helium ever develop life? Why or why not?

5. Calculate: If Hubble’s constant is 70 km/s/Mpc and a galaxy is 100 Mpc away, how fast is it receding? If a galaxy is receding at 35,000 km/s, how far away is it?

15.1.2 📝 Model Update

Add to your model of “probability of life in the universe”:

• The universe needed multiple generations of stars to create the heavy elements necessary for rocky planets and life
• Our solar system formed 9.2 billion years after the Big Bang — why did it take so long?
• Does the size and age of the universe make life elsewhere more or less likely?