The James Webb Space Telescope (JWST) has made groundbreaking discoveries in astronomy since it began sending data back to Earth in 2022, one of the most remarkable being the observation of some of the most distant galaxies ever seen. These galaxies appear to be older than the universe itself, a paradox that highlights the complexities of our expanding cosmos.
The universe is estimated to be 13.8 billion years old. Considering that light travels at a finite speed, we see distant objects not as they are now, but as they were in the past. Therefore, the farthest galaxy we could theoretically observe should be no more than 13.8 billion light-years away. This theoretical limit is known as the cosmological horizon.
However, the JWST has spotted JADES-GS-z14-0, a galaxy located approximately 33.8 billion light-years away, seemingly defying the age of the universe. How can we see light from an object that appears to have taken longer to reach us than the age of the universe itself? The answer lies in the expansion of the universe.
Edwin Hubble’s observations in the early 1900s revealed that distant galaxies are moving away from each other, with the speed of recession increasing with distance. This means the universe is not static but expanding. In 1998, astronomers discovered that this expansion is accelerating, driven by a mysterious force known as dark energy.
The universe has experienced distinct periods of expansion. The initial rapid inflation known as the Big Bang saw the cosmos expand exponentially. This was followed by a matter-dominated epoch where expansion slowed down. However, when the universe was less than 10 billion years old, it entered a third phase, the dark-energy-dominated epoch, where expansion accelerated once again. This ongoing acceleration is what explains the paradox of observing galaxies like JADES-GS-z14-0.
Due to the expansion of the universe, the light from JADES-GS-z14-0 has been traveling for only 13.5 billion years, despite its current distance of 33.8 billion light-years. This means we are observing this galaxy as it was 300 million years after the Big Bang.
Astronomers use two distance scales: co-moving distance, which ignores the expansion of the universe, and proper distance, which accounts for it. While JADES-GS-z14-0’s co-moving distance is 13.5 billion light-years, its proper distance is 33.8 billion light-years.
The JWST’s ability to see JADES-GS-z14-0 implies that this galaxy was once causally connected to Earth, meaning a signal from it could have reached us. This connection, however, is no longer possible as distant galaxies are moving away from us at a rate exceeding the speed of light.
This seemingly paradoxical situation arises from the fact that the expansion of space itself can exceed the speed of light, a concept that doesn’t contradict Einstein’s theory of special relativity, which applies to objects with mass moving through space.
In the future, the relentless expansion of the universe will eventually render galaxies like JADES-GS-z14-0 beyond our observable horizon. We are currently in a unique window of time where we can observe these ancient galaxies. This cosmic privilege allows us to gather valuable information about the universe’s origins and evolution.
Astronomers, including Jake Helton, a University of Arizona astronomer who is part of the JWST Advanced Deep Extragalactic Survey (JADES) team, are eager to leverage the capabilities of the JWST to fully exploit this extraordinary opportunity for scientific discovery.
