Since it began sending data back to Earth in 2022, the James Webb Space Telescope (JWST) has had a huge impact on astronomy, and one of its most revolutionary achievements is observing some of the most distant galaxies ever observed. However, because light doesn’t travel instantaneously (but rather at about 300 million meters (985 million feet) per second in a vacuum), we don’t see these galaxies as they are today, but as they were billions of years ago.
Furthermore, the age of our universe is estimated to be 13.8 billion years. So we should assume that the most distant galaxy we could hope to see is no more than 13.8 billion light-years away (a light-year is the distance light travels in one year). This point should be a sort of “cosmological horizon” beyond which no telescope should be able to see. And since nothing can travel through space faster than c, this means that a galaxy that is more than 13.8 billion light-years away and moving away should have no impact on Earth. Right?
False. If only the universe were that simple.
“A cosmological horizon is a maximum distance from which we can possibly retrieve information,” Jake Helton, an astronomer at the University of Arizona who is also part of the JWST Advanced Deep Extragalactic Survey (JADES) team, told Space.com.
“There are several different cosmological horizons,” Helton continued, “that have different definitions and depend on various cosmological quantities. The most relevant here is the cosmological horizon, which is the maximum distance from which light could have traveled to us at the time of the universe. This defines
“the edge of the observable cosmos.”
Related: James Webb Space Telescope spots most distant galaxy ever observed (image)
In March 2024, JADES scientists revealed that the powerful telescope had spotted JADES-GS-z14-0, the most distant and oldest galaxy ever observed by humanity. The paradox, however, is that JADES-GS-z14-0 is approximately 33.8 billion light-years away.
How can we see light from an object so far away that the universe is not old enough for it to reach us? Doesn’t JADES-GS-z14-0’s location at 33.8 billion light-years mean that we are seeing it as it was 33.8 billion years ago, which would surely call into question the estimate of the age of the universe?
This is not the case. Again, this proves that the universe has a way of reversing reasonable and logical conclusions.
“How can a distant galaxy like JADES-GS-z14-0 be observed, when it is more than 13.8 billion light-years away and its light would have taken longer than the age of the universe to reach us?” Helton asked rhetorically. “The answer is the expansion of the universe.”
See a galaxy older than time itself
If the universe stood still, light from a galaxy 33.8 billion light-years away would take 33.8 billion years to reach us, and that would be it. But in the early 1900s, Edwin Hubble discovered that distant galaxies appeared to be moving away from each other, and that the farther away they were, the faster they were moving. In other words, the universe is not static; it is expanding.
The situation became even more complicated in 1998, at the end of the 20th century, when two separate teams of astronomers observed that not only was the universe expanding, it was also accelerating. The force responsible for this phenomenon remains a mystery, but it has been given the name “dark energy.”
Over the 13.8 billion year history of the Universe, there have been two major periods of expansion. The first is an initial period of rapid cosmic inflation, commonly referred to as the “Big Bang.”
This inflationary epoch saw the volume of the cosmos increase by a factor of 10^26 (10 followed by 25 zeros). This is equivalent to your fingernail going from growing at 1 nanometer per second to suddenly growing 10.6 light-years (62 thousand billion At that time, the universe was dominated by energy, and this period is known as the Energy-Dominated Epoch.
This period was followed by a matter-dominated period, which began 47,000 years after the Big Bang. Eventually, the universal expansion allowed the cosmos to cool enough for protons to form from quarks and gluons, and then for protons to bind with electrons to form the first hydrogen atoms, which gave rise to the first stars and galaxies. During this period, the expansion of the universe caused by the Big Bang slowed to a near-stop.
The matter-dominated era came to a surprising end when the universe was just under 10 billion years old. At that point, the universe suddenly began to expand rapidly again. Moreover, this expansion became increasingly rapid and is even continuing to accelerate today. This third major period of the universe is called the dark energy-dominated era. This is the epoch we are currently in.
Thanks to these periods of expansion of the universe, light from JADES-GS-z14-0 has only actually been traveling toward JWST and Earth for 13.5 billion years, even though its source is now much farther away than 13.5 billion light-years. This means that JWST sees JADES-GS-z14-0 as it was 300 million years after the Big Bang. Without the expansion of the universe, JADES-GS-z14-0 would still be about 13.5 billion light-years away, although it would still have undergone small local motions that could have moved it closer to or further from nearby galaxies. But such galactic motion would not have been comparable to that caused by the expansion of the universe.
According to Helton, the cosmological horizon, or “photon horizon,” is a sphere whose boundary is about 46.1 billion light-years away, a number dictated by the expansion of the universe. This is the actual horizon beyond which we should not be able to “see” a galaxy. The galaxy JADES-GS-z14-0 is indeed within this horizon.
To avoid confusion, astronomers actually use two scales of distance measurement: a co-movement distance that eliminates the expansion of the universe as a factor, and a proper distance that includes it. This means that the co-movement distance of JADES-GS-z14-0 is 13.5 billion light-years, while its proper distance is 33.8 billion light-years.
JADES-GS-z14-0 and other distant and ancient galaxies will not always be visible, however.
A Lucky Time to Have the James Webb Space Telescope
The fact that the JWST can see JADES-GS-z14-0 means that it was once “causally connected” to Earth and our local universe. In other words, it was possible for a signal from JADES-GS-z14-0 to reach us in the Milky Way, so a “cause” in that galaxy that existed at the dawn of time could have an “effect” in our galaxy in this modern epoch of the cosmos.
“Any observable galaxy must be within the particle horizon and must have been causally related to us at some point in the history of the universe,” Helton said.
This is no longer the case today. Galaxies like JADES-GS-z14-0 and the other galaxies discovered by JADES are now so far away and moving away from us so quickly, thanks to dark energy, that no signal sent by them today could ever reach us. This is because the photon horizon is moving away from us at the speed of light, but for Really distant objects, the space between the Milky Way and these galaxies is widening faster than the speed of light.
This may seem implausible, since Albert Einstein’s theory of special relativity sets the speed of light as the universal speed limit. However, this is a rule for objects with mass in motion. through space, not a rule for the structure of space itself.
In about 2 trillion years, after Earth and humanity are gone, the expansion of the universe means that whatever intelligent species replaces us in the Milky Way (if it ever does), it will be unable to see the galaxies that exist beyond our Local Group, which is about 10 million light-years across.
It’s a sobering thought, and it means that humanity is living at a unique moment in the history of the universe when the most distant galaxies are still within our reach. We have the potential to know more about the universe and its origins than any intelligent life that might come after us. Astronomers, including Helton, intend to use JWST to take full advantage of this cosmic privilege.
“Working with JWST and the JADES collaboration has been incredible,” Helton said. “Writing science articles with JWST, like my recent article on
JADES-GS-z14-0, has been the most rewarding and exciting experience
“of my career as a researcher.”