Travel from the Big Bang to today. Explore cosmic history, redshift, the Cosmic Microwave Background, and the telescopes — Hubble and James Webb — that let us see back in time.
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About 13.8 billion years ago, all the matter and energy in the universe was packed into a single point that was unimaginably hot and dense. We call this beginning the Big Bang. The Big Bang was not an explosion in space — it was the rapid expansion of space itself. Every galaxy, every star, and every atom you can see today traces back to that moment.
In the first tiny fraction of a second, the universe went through a period called cosmic inflation, where space stretched faster than the speed of light. Within the first three minutes, the universe cooled enough for protons and neutrons to fuse into the simplest elements: hydrogen, helium, and a tiny amount of lithium. This stage is called Big Bang nucleosynthesis. Even today, about 75% of the ordinary matter in the universe is hydrogen and 25% is helium, exactly what the Big Bang model predicts.
For the next 380,000 years the universe was a hot fog of charged particles. Light could not travel freely. Then, as the universe cooled to about 3,000 K, electrons combined with nuclei to form the first neutral atoms — a moment called recombination. Light was finally free to stream across space. That ancient light is still arriving at Earth today as the Cosmic Microwave Background (CMB). Discovered accidentally in 1964 by Penzias and Wilson, the CMB is one of the strongest pieces of evidence for the Big Bang.
The universe has been expanding ever since. In 1929, Edwin Hubble noticed that galaxies are moving away from us, and the farther away a galaxy is, the faster it appears to recede. The light from those galaxies is stretched to longer wavelengths — a phenomenon called redshift. This relationship, called Hubble's Law, tells us the universe is still growing.
Inside our own Milky Way, a much later chapter began about 4.6 billion years ago: a cloud of gas and dust collapsed to form our Sun, the planets, and eventually the rocks that make up Long Island. Every atom heavier than helium in your body — the calcium in your bones, the iron in your blood — was forged inside an exploding star. As Carl Sagan said, we are made of star-stuff.
Below is a simplified diagram of the major eras of the universe, from the Big Bang to today.
Figure 1. Major eras in the evolution of the universe.
When you look at a star, you are looking back in time. Light travels at 300,000 kilometers per second, but space is so vast that light from distant objects takes years, even billions of years, to reach our eyes. The unit astronomers use is the light-year, defined as the distance light travels in one year — about 9.46 trillion kilometers. Looking at a galaxy 10 million light-years away means seeing it as it was 10 million years ago.
The Hubble Space Telescope (HST), launched in 1990, orbits Earth above the distorting effects of the atmosphere. Its 2.4-meter mirror collects mostly visible and ultraviolet light. The famous Hubble Deep Field images showed thousands of galaxies in a tiny patch of sky and pushed our view back more than 12 billion years.
The James Webb Space Telescope (JWST), launched in December 2021, is the most powerful telescope ever built. Its 6.5-meter gold-coated mirror is more than six times larger in collecting area than Hubble's. JWST observes in the infrared, which is essential for studying the early universe. As space expands, light from very distant objects is stretched (redshifted) out of the visible range and into the infrared. JWST can therefore see galaxies that formed only 200–400 million years after the Big Bang — galaxies Hubble could never detect.
Other telescopes round out the picture. Chandra observes in X-rays to study black holes and supernova remnants. The Planck space mission mapped the Cosmic Microwave Background in stunning detail, confirming the universe's age. Together, these instruments build a complete portrait of cosmic history.
Telescopes are time machines. Every photon they collect is a message from the past — sometimes from a moment shortly after the universe began.
Figure 2. Major space telescopes and the wavelengths they observe.
Use what you read to complete the activities below. Each correct answer is worth 1 point.
The universe began about billion years ago in an event called the . The leftover light from this early universe is still detectable today as the . The most abundant element produced shortly after the Big Bang was , and the stretching of light from distant galaxies, called , shows that the universe is still expanding.
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"Hubble launched."
Expand using: When? and Why?
"Galaxies move away."
Expand using: Where? and How? (How do we know?)
"The CMB is leftover light."
Expand using: When? (when was it released?) and Why? (why is it important?)
Press 🎬 Big Bang ▶ to start the simulation. The timeline will automatically advance through each stage in cosmic history, pausing at each step so you can read the description. You can pause, change speed, or click any marker on the timeline to jump to that stage. After watching the simulation, fill in the data table below — it is worth 4 points.
Each stage of cosmic history will appear here as the simulation reaches it. Read each description carefully — you'll need this information for the data table.
Use what you learned from the timeline to fill in this table.
| Epoch | When (Time After Big Bang) | Key Event |
|---|---|---|
| Big Bang | ||
| Nucleosynthesis | ||
| Recombination | ||
| First Stars | ||
| Solar System Forms |
Click each telescope card to flip it and learn its specifications. Then complete the data table below — it is worth 4 points.
| Telescope | Launch Year | Wavelength Type | What it best observes |
|---|---|---|---|
| Hubble Space Telescope | |||
| James Webb Space Telescope | |||
| Chandra X-ray Observatory | |||
| Planck Space Telescope |
In 1929, Edwin Hubble discovered that distant galaxies are moving away from us, and the farther a galaxy is, the faster it recedes. This is summarized by Hubble's Law: v = H₀ × d, where H₀ ≈ 70 km/s per Megaparsec (Mpc). Drag the slider below to push a galaxy farther from Earth and watch how its velocity, color (redshift), and light waves change in real time.
Use Hubble's Law (v = 70 × d) to calculate the recessional velocity at each distance. Slide the simulation to verify your answers when you're done!
| Distance (Mpc) | Calculation | Velocity (km/s) |
|---|---|---|
| 50 Mpc | 70 × 50 = | |
| 200 Mpc | 70 × 200 = | |
| 500 Mpc | 70 × 500 = | |
| 1000 Mpc | 70 × 1000 = |
1. What relationship between a galaxy's distance and its recessional velocity does Hubble's Law describe?
2. A galaxy located 100 Mpc from Earth has a recessional velocity of about 7,000 km/s. Using Hubble's Law, what is the recessional velocity of a galaxy located 300 Mpc from Earth?
3. Why does the galaxy in the simulation appear redder as you slide it farther from Earth?
Three real-data graphs about the universe. Use each graph to answer the dropdown question below it. Each correct answer is worth 1 point.
This graph shows how the speed at which a galaxy moves away from us increases with its distance from Earth.
8.Based on the graph, what happens to a galaxy's recessional velocity as its distance from Earth increases?
This graph shows the percentage of ordinary (baryonic) matter made of each element produced shortly after the Big Bang.
9.According to the graph, the most abundant element in the universe is:
The diameter of a telescope's primary mirror determines how much light it can gather. A bigger mirror = ability to see fainter, more distant objects.
10.Based on the graph, which telescope has the largest primary mirror?
11.A telescope with a larger mirror can see:
5 questions drawn from a bank of 20. You need 60% (3 of 5 correct) to pass. If you don't pass, you'll get fresh questions on the next attempt. Each correct answer is worth 1 point.
Your final grade is calculated from every part of the lab. Use the print button below to save a PDF of all your work — including data tables, graph answers, written responses, and quiz results.