Evidence for the Big Bang

By the end of this lesson, you will be able to describe the scientific data that scientists collect and analyze as evidence for how the universe began.

Think about standing near a highway in Texas when an ambulance races past. As it comes toward you, the siren sounds high; as it speeds away, the sound drops lower. The sound waves get stretched out as the ambulance moves away from you. Light behaves in a similar way. When astronomers study the light from faraway galaxies, they see redshift, which means the light waves are stretched toward the red end of the color spectrum because those galaxies are moving away from us. Almost every galaxy shows redshift, and the more distant ones are moving away fastest.

That data supports the Big Bang theory, the leading scientific explanation that the universe began about 13.8 billion years ago as an extremely hot, dense point and has been expanding ever since. Notice that a scientific theory is not a guess; it is a well-tested explanation supported by many separate lines of evidence.

A second piece of evidence is the cosmic microwave background, a faint glow of microwave energy that comes from every direction in space. Scientists measured it with telescopes and satellites, and it matches what you would expect from leftover heat of a young, hot universe that has been cooling for billions of years.

A third piece of evidence comes from analyzing which elements the universe contains. Measurements show that ordinary matter in space is roughly three-quarters hydrogen and one-quarter helium, with only traces of heavier elements. That is exactly the recipe predicted for matter formed in the first few minutes after the Big Bang.

Try it: if a galaxy's light shows a large redshift, is that galaxy moving toward Earth or away from Earth, and what does that tell you about the universe?