The age of the universe
||How can we figure out when the universe began? Image: Sandbox Studio with Ana Kova|
Looking out from our planet at the vast array of stars, humans have always asked questions central to our origin: How did all of this come to be? Has it always existed? If not, how and when did it begin?
How can we determine the history of something so complex when we were not around to witness its birth?
Scientists have used several methods: checking the age of the oldest objects in the universe, determining the expansion rate of the universe to trace backward in time, and using measurements of the cosmic microwave background to figure out the initial conditions of the universe and its evolution.
Hubble and an expanding universe
In the early 1900s, there was no such concept of the age of the universe, says Stanford University associate professor Chao-Lin Kuo of SLAC National Accelerator Laboratory. "Philosophers and physicists thought the universe had no beginning and no end."
Then in the 1920s, mathematician Alexander Friedmann predicted an expanding universe. Edwin Hubble confirmed this when he discovered that many galaxies were moving away from our own at high speeds. Hubble measured several of these galaxies and in 1929 published a paper stating the universe is getting bigger.
Scientists then realized that they could wind this expansion back in time to a point when it all began. "So it was not until Friedmann and Hubble that the concept of a birth of the universe started," Kuo says.
Tracing the expansion of the universe back in time is called finding its "dynamical age," says Nobel Laureate Adam Riess, professor of astronomy and physics at Johns Hopkins University.
"We know the universe is expanding, and we think we understand the expansion history," he says. "So like a movie, you can run it backwards until everything is on top of everything in the big bang."
The expansion rate of the universe is known as the Hubble constant.
—Amelia Williamson Smith