In Activity 5.2.1, you ran the expansion backward: galaxies crowd closer and closer until, about 13.8 billion years ago, everything was packed together, extremely hot and dense. That starting state is the Big Bang. The expansion is only the first of three major pieces of evidence for it:
| Evidence | What we observe | Why it points to a Big Bang |
|---|---|---|
| Expansion | Galaxies recede faster the farther away they are (Hubble's law) | Run backward, everything was once packed together |
| Cosmic microwave background | A faint glow of microwaves coming from every direction, at about 2.7 K | It's leftover light from when the whole universe was hot, now stretched by expansion |
| Light elements | The oldest stars and gas are about 3/4 hydrogen and 1/4 helium | That's the exact mix predicted for nuclear fusion in the universe's first few minutes |
The cosmic microwave background(CMB) was found by accident in 1965, when Arno Penzias and Robert Wilson couldn't get rid of a faint hiss in their radio antenna — no matter which direction they pointed it. Space missions since (COBE, WMAP, and Planck) have mapped it in exquisite detail. Its tiny ripples, about one part in 100,000, are the seeds that later grew into galaxies.
The early universe changed astonishingly fast. As it expanded, it cooled, and each drop in temperature let something new happen:
| Time after the Big Bang | What happened |
|---|---|
| < 10⁻³² s | Inflation — a split-second burst of extremely rapid expansion |
| ~ 10⁻⁶ s | Cooling lets quarks bind into protons and neutrons |
| ~ 3–20 minutes | Protons and neutrons fuse into helium nuclei — the whole universe acts like a star core, briefly |
| ~ 380,000 years | At ~3,000 K, atoms form, space turns transparent, and the CMB light is released |
Notice how lopsided that list is: three major events in the first few minutes, then a wait of 380,000 years for the next. That's the central challenge of your project. On a timeline drawn to true proportions, all four of these events land on the same point.
After the CMB was released, the universe went dark. There were atoms of hydrogen and helium, and dark matter, but no stars yet — a stretch called the cosmic dark ages. Slowly, dark matter's gravity pulled gas into denser and denser clumps (those ripples in the CMB, growing). Somewhere around 100 to 200 million years after the Big Bang, the densest clumps ignited as the first stars.
Because light takes time to travel, looking at distant galaxies means looking back in time. This is called lookback time. The Andromeda Galaxy's light left it 2.5 million years ago; light from the most distant galaxies seen by the James Webb Space Telescope left them less than 300 million years after the Big Bang. Telescopes are, quite literally, time machines, which is how we can see the history on your timeline instead of only guessing at it.
Small galaxies merged into bigger ones (Activity 5.1.2), and within its first billion years or so the Milky Way began to take shape — its oldest halo stars date back almost that far. About 5 billion years ago, as matter spread thinner, dark energy began to dominate and the expansion started speeding up (Activity 5.2.2). A little later, 4.6 billion years ago, a cloud enriched by earlier generations of stars collapsed into the Sun and Earth. Life appeared on Earth within about a billion years, complex animals about 540 million years ago, and modern humans only about 300,000 years ago.
Thirteen milestones on one line. Switch scales and watch where they bunch up. Click any numbered marker.
On a true-proportion line, events 1–4 all land on the very first point, and everything human is squeezed into the last pixel.
A timeline has to lie a little — the question is how. Every design choice squeezes some eras and stretches others. Here are the main options:
| Scale | How it works | Good for | What it distorts |
|---|---|---|---|
| Linear | Equal distances = equal times | Showing how long the later eras really are | Crushes the first 380,000 years — and all of human history — into specks |
| Linear + zoom boxes | One true-scale line, with magnified insets for crowded moments | Honest proportions plus readable detail | Each inset has its own scale, which must be labeled |
| Logarithmic | Each step is 10× (or 10¹⁰×) longer than the last | Spreading out the first seconds, minutes, and years | Makes the last billions of years look tiny |
| Cosmic calendar | All 13.8 billion years squeezed into one calendar year | Making deep time feel real | Still crowds the early universe into January 1 and humans into the last minutes |
The cosmic calendar was made famous by astronomer Carl Sagan. On it, the Sun and Earth don't form until the start of September, the dinosaurs die out on December 30, and modern humans show up only about 11 minutes before midnight on New Year's Eve.
Squeeze all 13.8 billion years into one calendar year, with the Big Bang at midnight on January 1 and today at the stroke of midnight on December 31. Pick an event, or type in your own "years ago."
On this calendar, one day ≈ 38 million years, and one second ≈ 438 years.