How big is the universe?

How big is the universe? It's one of the fundamental questions of astronomy. By looking for the farthest observable point from Earth (and by extension, the oldest given the speed of light) we can estimate a diameter. 

Thanks to evolving technology, astronomers are able to look back in time to the moments just after the Big Bang. This might seem to imply that the entire universe lies within our view. But the size of the universe depends on a number of things, including its shape and expansion. 

As a result, while we can make estimates as to the size of the universe scientists can't put a number on it. 

In 2013, the European Space Agency's Planck space mission released the most accurate and detailed map ever made of the universe's oldest light. The map revealed that the universe is 13.8 billion years old. Planck calculated the age by studying the cosmic microwave background.

"The cosmic microwave background light is a traveler from far away and long ago," said Charles Lawrence, the U.S. project scientist for the mission at NASA's Jet Propulsion Laboratory in Pasadena, California, in a statement. "When it arrives, it tells us about the whole history of our universe."

Because of the connection between distance and the speed of light, this means scientists can look at a region of space that lies 13.8 billion light-years away. Like a ship in the empty ocean, astronomers on Earth can turn their telescopes to peer 13.8 billion light-years in every direction, which puts Earth inside of an observable sphere with a radius of 13.8 billion light-years. The word "observable" is key; the sphere limits what scientists can see but not what is there.

But though the sphere appears almost 28 billion light-years in diameter, it is far larger. Scientists know that the universe is expanding. Thus, while scientists might see a spot that lay 13.8 billion light-years from Earth at the time of the Big Bang, the universe has continued to expand over its lifetime. If inflation occurred at a constant rate through the life of the universe, that same spot is 46 billion light-years away today according to Ethan Siegel, writing for Forbes, making the diameter of the observable universe a sphere around 92 billion light-years. 

These estimations are further complicated by the possibility that the universe is not expanding in an even manner. ESA reported on a 2020 study using data from ESA’s XMM-Newton, NASA’s Chandra Space Telescope and Rosat X-ray observatories suggests that the universe is not expanding at the same rate in all directions. The study measured the X-ray temperatures of hundreds of galaxy clusters and compared that against their brightness. Some clusters appeared less bright than expected, suggesting they were not moving at the same rate. "This possibly uneven effect on cosmic expansion might be caused by the mysterious dark energy," ESA stated.

Centering a sphere on Earth's location in space might seem to put humans in the center of the universe. However, like that same ship in the ocean, we cannot tell where we lie in the enormous span of the universe. Just because we cannot see land does not mean we are in the center of the ocean; just because we cannot see the edge of the uni