When you look up at the stars, you are actually looking back in time. Because light takes time to travel, the starlight hitting your eyes tonight actually left those cosmic bodies years, centuries, or even millennia ago. This incredible time-traveling property of light is exactly how astronomers answered one of humanity's oldest questions: How old is the universe?
The Age of Everything
Our current, most accurate measurement states that the universe is approximately 13.8 billion years old. But how can anyone possibly know that?
The Expanding Balloon
In the 1920s, astronomer Edwin Hubble made a groundbreaking discovery: the universe is expanding. Almost every galaxy we look at is moving rapidly away from us. Think of baking a loaf of bread with raisins in it. As the dough expands in the oven, every raisin moves farther apart from the others.
scientists realized that if the universe is expanding like a balloon today, then if you rewind the clock, everything in the universe must have been closer together in the past. If you "hit rewind" all the way to the beginning, all matter and energy in the universe must have been compressed into a single, infinitely hot, infinitely dense point.
The Big Bang
Exactly 13.8 billion years ago, that incredibly dense point rapidly expanded. This rapid inflation is known as the Big Bang. It wasn't an explosion in space, it was an explosion of space itself. Energy cooled to form matter, matter clumped together through gravity to form stars, and stars grouped together to form galaxies.
The static you used to hear on old, untuned analog television sets was partially caused by the "Cosmic Microwave Background"—the fading echo of radiation left over directly from the Big Bang!
How Are We Sure?
Astronomers calculate this 13.8-billion-year age using two main methods:
- Observing the oldest stars: By finding the oldest globular star clusters in the Milky Way, we can set a minimum age (the universe must be older than the stars within it).
- Measuring the expansion rate: By calculating exactly how fast galaxies are flying apart today (the Hubble Constant), physicists can calculate precisely how long it took for them to get from a single point to where they are now.
