Since it began sending data back to Earth in 2022, the James Webb Space Telescope (JWST) has greatly influenced astronomy, and one of its most revolutionary achievements is the observation of some of the most distant galaxies ever seen. However, because light does not travel instantaneously – but moves at about 300 million meters (985 million feet) per second in a vacuum – we do not see those galaxies as they are today, but as they were billions of years ago.
Furthermore, our universe is estimated to be 13.8 billion years old. So we have to assume that the most distant galaxy we can ever hope to see is no more than 13.8 billion light years away. (A light year is the distance light travels in one year). That point should be a sort of “cosmological horizon”—beyond which no telescope should be able to see. And, because nothing can travel through space faster than c, that means there should be no way a galaxy farther than 13.8 billion light-years away, and moving away all the time can affect Earth. Right?
Wrong. If only the universe were that simple.
“A cosmological horizon is a maximum distance from which information can be obtained,” Jake Helton, a University of Arizona astronomer who is also part of the JWST Advanced Deep Extragalactic Survey (JADES) team, told Space.com.
“There are several different cosmological horizons,” Helton continued, “which have different definitions and depend on different cosmological quantities. The most important here is the cosmological horizon which is the maximum distance from which light could have traveled to us in the epoch of the universe this defines
the edge of the observable cosmos.”
Connected: James Webb Space Telescope discovers most distant galaxy ever seen (image)
In March 2024, JADES scientists discovered that the powerful telescope had spotted JADES-GS-z14-0, the most distant and earliest galaxy that humanity has ever seen. However, the paradox is that JADES-GS-z14-0 is located about 33.8 billion light-years away.
How can we see light from an object so distant that the universe is not old enough to have allowed it to reach us? Doesn’t JADES-GS-z14-0’s position 33.8 billion light-years away mean we see it as it was 33.8 billion years ago, something that would surely challenge the estimate of the age of the universe?
Not like that. Again, this is proof that the universe has a way of turning reasonable and logical conclusions on their heads.
“How could a distant galaxy like JADES-GS-z14-0 ever be observed, since it is more than 13.8 billion light-years away and its light would apparently have taken longer than the age of the universe to reach us?” Helton asked rhetorically. “The answer is the expansion of the universe.”
Looking at a galaxy older than time itself
If the universe stood still, then 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 the farther away they were, the faster they were going. In other words, the universe is not static; is expanding.
This was further complicated in 1998, as the 20th century drew to a close when two separate teams of astronomers observed that not only was the universe expanding, but that the expansion was also accelerating. The force responsible is a mystery, but it has been given the placeholder name of “dark energy”.
There are two main and distinct periods of expansion during the 13.8 billion year history of the universe. The first is an initial period of rapid cosmic inflation now 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 growing from 1 nanometer per second to a sudden increase of 10.6 light years (62 trillion miles) long. At this time, the universe was dominated by energy, and this period is known as the energy-dominated era.
This was followed by a matter-dominated epoch that began 47,000 years after the Big Bang. Eventually, the universal expansion allowed the cosmos to cool enough to allow protons to form from quarks and gluons, and then protons to bond with electrons to form the first hydrogen atoms, which formed the first stars and galaxies. During this period, the expansion of the universe driven by the Big Bang slowed to a near standstill.
The era dominated by matter came to a startling end when the universe was just under 10 billion years old. At this time, the universe suddenly began to expand rapidly again. Plus, this expansion became ever faster and even continues to accelerate today. This third important period of the universe is called the era dominated by dark energy. It’s the era we’re in right now.
Thanks to these periods of the universe’s expansion, light from JADES-GS-z14-0 has actually been traveling toward JWST and Earth for 13.5 billion years, despite its source now being much more distant than 13.5 billion light-years away. . 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 experienced smaller local motions that could have moved it closer to or further away from nearby galaxies. But such galactic motion would have been nowhere near the kind caused by the expansion of the universe.
According to Helton, the cosmological horizon, or “Photon Horizon,” is a sphere with a boundary about 46.1 billion light-years away, a figure 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 that horizon.
To avoid confusion, astronomers actually use two distance scales: a common distance that eliminates the expansion of the universe as a factor, and a proper distance that includes it. This means that JADES-GS-z14-0’s common motion distance is 13.5 billion light years, while its proper distance is 33.8 billion light years.
However, JADES-GS-z14-0 and other distant and ancient galaxies will not always be visible.
A lucky era to have the James Webb Space Telescope
The fact that 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 this galaxy that existed at the dawn of time could have an “effect” in our galaxy. in this modern age of the cosmos.
“Every observable galaxy must be within the particle horizon and must have been associated with us at some point in the history of the universe,” Helton said.
However, this is no longer the case. Galaxies like JADES-GS-z14-0 and other galaxies discovered by JADES are now so distant and moving away from us so fast, thanks to dark energy, that no signal from them today could ever reach us. This is because the photon horizon moves away from us at the speed of light, but for really distant objects, the space between the Milky Way and those galaxies is expanding faster than the speed of light.
This may seem incredible, since Albert Einstein’s theory of special relativity places the speed of light as a universal speed limit. However, this is a rule for objects with mass that move through space, not a rule for the structure of space itself.
In about 2 trillion years after Earth and humanity are long gone, the expansion of the universe means that whatever intelligent species replaces us in the Milky Way (if it ever does) won’t be able to see any galaxies that exists beyond our local group. – which has a diameter of about 10 million light years.
It’s a sobering thought, and it means that humanity lives at a unique point in the history of the universe, in which the most distant galaxies are still within our view. We are able to know more about the universe and its origins than any intelligent life that might follow us. Astronomers, including Helton, aim to use JWST to take full advantage of this cosmic privilege.
“Working with JWST and the JADES Collaboration has been incredible,” said Helton. “Writing papers about science with JWST, like the last one in
JADES-GS-z14-0, has been the most rewarding and exciting experience
of my research career.”