Light from distant galaxies arrives shifted toward red. The farther the galaxy, the bigger the shift.
For centuries, people thought the universe never changed. In the 1920s, Edwin Hubble studied the light from distant galaxies. Its colors were shifted toward red, and the farther the galaxy, the bigger the shift. Space itself is stretching. Run the clock back 13.8 billion years and everything was hot and dense. This page shows three pieces of evidence for the Big Bang.
Core Claims
- Redshift. Light from distant galaxies is shifted toward red. The farther the galaxy, the bigger the shift. Space is expanding.
- No center. Galaxies do not fly through still space. Space itself stretches, and it stretches the light on its way to us.
- Hydrogen and helium. In the first minutes, fusion made about 75% hydrogen and 25% helium by mass. Old gas clouds still show this ratio.
- The microwave glow. A faint glow of microwaves fills the sky. It is the leftover light of the early universe, cooled by expansion.
Three pieces of evidence
Retrieval Checklist
- Say what redshift is and what it shows about space.
- Explain why moving galaxies do not put us at the center.
- Say why about 75% of ordinary matter is hydrogen and 25% is helium.
- Say what the cosmic microwave background is and where it came from.
Light from distant galaxies is stretched toward red.
Each element absorbs light at its own set of colors, like a barcode. Hydrogen absorbs red light at 656.3 nm.
Hubble found the same hydrogen barcode in distant galaxies. But every line sat at a longer, redder wavelength than in the laboratory. This shift is called redshift.
Space is expanding. It stretches light on its way to us, so the light arrives redder than it left. The farther the galaxy, the more space has stretched the light.
Picture raisin bread rising in an oven. Every raisin moves away from every other raisin, and no raisin is the center. Galaxies move apart the same way.
Thinking galaxies fly outward from a center, like pieces of an explosion. Space itself is stretching, so every galaxy sees the others moving away. The universe has no center.
About 3/4 of ordinary matter is hydrogen, and 1/4 is helium.
Astronomers measure gas clouds that have never formed stars. These clouds hold the same ingredients everywhere.
| Ingredient | Share |
|---|---|
| Hydrogen-1 | 74% to 75% |
| Helium-4 | 24% to 25% |
| Deuterium and lithium-7 | under 0.01% |
Elements heavier than helium make up only about 2% of ordinary matter. Stars made those. So where did a full quarter of helium come from?
It came from the Big Bang itself. For the first 3 to 20 minutes, the whole universe was hot and dense enough for fusion. Calculations predict one helium nucleus for every 12 hydrogen nuclei. That gives 75% hydrogen and 25% helium by mass, and telescopes find exactly that.
Thinking stars made all the helium. Stars made only a little. Most helium formed in the first minutes of the universe, before any star existed.
A faint microwave glow comes from every direction.
The early universe was hot and dense, and it glowed. As space expanded, the universe cooled. Expansion also stretched that old light until it became microwaves.
We still detect this light from every direction. It is the cosmic microwave background. It is cold: about 2.7 K, only a few degrees above absolute zero.
In 1965, Arno Penzias and Robert Wilson found it by accident. Their antenna at Bell Labs in New Jersey picked up a steady hiss that came from the whole sky. They could not make it go away.
Thinking the glow comes from one place, like a distant star. It comes from every direction, because the whole early universe was hot.
EnrichmentHubble’s law, the early glow and the 2.725 K spectrum
Extra depth. The rest of the lesson works without it.
Measuring the shift
Astronomers write the shift as a number, z:
Hubble’s law
Hubble found that the speed v of a galaxy moving away is proportional to its distance d. A galaxy three times farther moves away three times faster. Today’s value is H0 ≈ 70 km/s per Mpc, where 1 Mpc ≈ 3.26 million light-years.
The early glow
About 380,000 years after the Big Bang, the universe cooled to about 3,000 K. Protons captured electrons and made the first neutral hydrogen atoms. Light could then travel freely. This event is called recombination.
Expansion has since stretched that light by about 1,100 times. An orange glow at 3,000 K became microwaves at 2.725 K.
A perfect spectrum
Satellites named COBE, WMAP and Planck measured the glow. It matches the spectrum of a perfect glowing object at 2.725 K, with its peak near 160 GHz (Figure 5.3). Each cubic centimeter of space holds about 411 of these photons. Tiny ripples of ±0.00003 K became the seeds of galaxies.
An observatory records the hydrogen line Hα from Galaxy GN-X7. In a laboratory, Hα is at 656.3 nm. In the light from GN-X7, it is at 853.2 nm. Use z = (λobs − λrest) / λrest and v ≈ z · c. Here c = 3.0 × 105 km/s.
(a) Calculate the redshift z and the speed v of GN-X7. [2 marks]
(b) Use H0 ≈ 70 km/s per Mpc. Calculate the distance to GN-X7 in Mpc. [2 marks]
- Redshift (1 mark): You find z = (853.2 − 656.3) / 656.3 = 0.300.
- Speed (1 mark): You find v = 0.300 × 300,000 = 90,000 km/s.
- Method (1 mark): You divide the speed by H0.
- Distance (1 mark): You find d = 90,000 ÷ 70 ≈ 1,290 Mpc.
Move a galaxy farther away and watch its hydrogen lines.
The top bar is the hydrogen barcode in the laboratory. The bottom bar is the same barcode in the light of a galaxy. Change the distance and compare the two.
How does the light from a galaxy change with its distance?
The explanation appears after you predict.
- A dark line where hydrogen absorbs light (Hδ, Hγ, Hβ, Hα, from left to right)
- How far each line has moved
- Look at a nearer galaxy. Load 1 billion light-years and press Record. Look for: the Hα line moves from 656 nm to 702 nm, a shift of 46 nm.
- Look at a farther galaxy. Load 3 billion light-years and press Record. Look for: the Hα line is at 794 nm, past the red end of the bar. It is now infrared light.
- Look at a far galaxy. Load 5 billion light-years and press Record. Look for: the Hα line is at 886 nm, a shift of 230 nm. The other three lines have moved toward red too.
Use your evidence
Fill the blanks from memory.
Type each blank from memory, then press Check. Press Show answers after your first check if you are stuck.
1. Light from distant galaxies is shifted toward red. This shift is called .
2. The shift happens because itself is stretching the light.
3. In the first minutes, fusion made about 75% hydrogen and % helium by mass.
4. The leftover glow of the early universe is the cosmic background.
5. Space expands everywhere at once, so the universe has no .
A student says: “The Big Bang was an explosion. The galaxies fly outward from the place where it went off.”
Explain what is wrong with this picture. Use the redshift evidence.
Think about where the center would be, and what an observer in another galaxy would see. Write your answer first, then compare it with the model answer.
The student is wrong. The Big Bang was not an explosion into empty space. Space itself expanded everywhere at once, so there is no center.
An observer in any galaxy sees the other galaxies moving away. Space stretches the light on its way, which is why distant galaxies show a redshift. It looks the same from every place, like raisins in rising bread.
Write your answer first. Then grade yourself.
Give yourself 1 mark for each idea you wrote down.
An observatory studies the light from Galaxy GN-X7. In a laboratory, the hydrogen line Hα is at 656.3 nm. In the light from GN-X7, the same line is at 853.2 nm. Astronomers also find that old gas clouds are about 75% hydrogen and 25% helium by mass.
(a) Say what the change from 656.3 nm to 853.2 nm shows about space. [1 mark]
(b) A student says: “GN-X7 is moving away from us, so we are at the center of the universe.” Explain why the student is wrong. [1 mark]
(c) Name the two other pieces of evidence for the Big Bang. Say what each one shows. [2 marks]
- Redshift (1 mark): You say the light is stretched to a longer wavelength, so space is expanding.
- No center (1 mark): You say space stretches everywhere, so every galaxy sees the others moving away.
- Hydrogen and helium (1 mark): You say fusion in the first minutes made 75% hydrogen and 25% helium.
- Microwave glow (1 mark): You say the glow is leftover light from the early universe, cooled by expansion.
The James Webb Space Telescope, launched in 2021, orbits 1.5 million kilometers from Earth. Light from galaxies that formed 13.5 billion years ago has been stretched far into the infrared. Optical telescopes cannot see it. Webb’s gold-plated mirrors are chilled to −233 °C, so it can see the first stars light up.
Adaptive Competency Practice: HS-ESS1-2
Test your understanding of the Big Bang one question at a time. The adaptive engine covers redshift, the amounts of hydrogen and helium, and the cosmic microwave background.