What’s at the edge of the universe and how far can we see?

Dr Katie Mack is a theoretical astrophysicist at the University of Melbourne. Her research explores new ways to learn about the early universe and fundamental physics using astronomical observations. She helps us understand the edges of the universe, which means getting our heads around not just vast space but deep time. Hold onto your helmet, it’s going to be a wild ride. 

It takes eight minutes for sunlight from the sun to reach the earth, and when we look at the stars at night, which are in fact other far away suns, it takes a lot longer for that light to reach the earth. In some cases, we are looking at the stars in their positions and seeing them as they were thousands of years ago. Even though the light from these stars has travelled at the speed of light, the distances are so vast it takes thousands of years for that light to get here. So the photographs through telescopes we take today show how the cosmos was positioned long ago. Some of these stars may have already burnt out thousands of years before the light is apparent to us.

Cosmologists use light waves, radio waves, radiation and microwaves to see our universe. But how far or long ago can we ‘see’?

“The farther away objects are from us, the farther into the past we can see,” Dr Mack explains. “If we look far enough away, we see things as they were in the very distant past, even as far back as the beginning of the universe. That defines what we call the observable universe – we can’t see anything beyond that because light would have taken longer than the age of the universe to get here. We can’t ‘see’ the edges of the observable universe as they are ‘now’ because distances are so vast that any light produced today will take billions of years to get to us. On top of this, the universe is expanding, and some parts of the universe are moving away from us faster than light can travel, so their light can never catch up with the expansion to reach us.

“Even though we have no way of seeing the edges as they are today, we can see signatures of the edges as they were in the past, a very long time ago. The European Space Agency’s Planck satellite orbits our sun and ‘sees’ differences in the intensity of microwave light. This telescope captures the oldest light of the universe, when it was just 380,000 years old (compared with its current age of 13.8 billion years). 

“When the universe was very young it was dense and glowing with heat. As it expanded and cooled, the leftover light from the ‘primordial fireball’ began travelling freely through space, being stretched out by the expansion. This light is known as the cosmic microwave background.”

This begs the question: if we paused the expansion of the universe, could we calculate its distances?

“If you could pause the expansion of the universe and measure distances, the ‘observable’ universe would be about 46 billion light-years in diameter. Given the universe being only about 13.8 billion years old, you would think light just reaching us from the Big Bang would be from a point 13.8 billion light years away. But in reality, the expansion of the universe means the origin of that light is much farther away now,” Dr Mack says. 

“The most distant objects we have seen are galaxies, which can be seen through infrared detectors on telescopes like Hubble. These galaxies are around 31 billion lights years distant so we see them as they were when the universe was less than 500 million years old, remembering that seeing distance in the cosmos is seeing a point of time in the past.”

www.physics.unimelb.edu.au