Since the James Webb Space Telescope (JWST) began beaming data back to Earth in 2022, it has had a profound impact on astronomy. Among its most revolutionary achievements has been the observation of some of the most distant galaxies ever seen. However, because light does not travel instantaneously, but instead travels at about 300 million meters (985 million feet) per second in a vacuum, we see these galaxies not as they appear today, but as they appeared billions of years ago.
Furthermore, our universe is estimated to be 13.8 billion years old. So we have to assume that the furthest galaxy we could ever see is no more than 13.8 billion light-years away. (A light-year is the distance light travels in a year.) That point would have to be a sort of “cosmological horizon” — beyond which no telescope could see. And since nothing can travel through space faster than c, that means there should be no way for a galaxy to be more than 13.8 billion light-years away, and getting further and further away could affect Earth. Right?
Wrong. If only the universe were that simple.
“A cosmological horizon is a maximum distance from which information can potentially be obtained,” 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 a few different cosmological horizons,” Helton continued, “which have different definitions and depend on different cosmological quantities. The most relevant one here is the cosmological horizon, which is the maximum distance from which light could have traveled to us in the time of the universe. This defines
“the edge of the observable universe.”
Related: James Webb Space Telescope discovers 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 early galaxy ever seen by humanity. The paradox, however, is that JADES-GS-z14-0 is located about 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 the position of JADES-GS-z14-0 at 33.8 billion light-years away mean that we are seeing it as it was 33.8 billion years ago, something that would certainly call into question the estimate of the age of the Universe?
That is not true. Again, this is evidence that the universe has a way of turning sensible and logical conclusions on their heads.
“How can a distant galaxy like JADES-GS-z14-0 ever be observed, given that it is over 13.8 billion light-years away and its light apparently took longer than the age of the universe to reach us?” Helton asked rhetorically. “The answer is the expansion of the universe.”
Seeing a galaxy older than time itself
If the universe were just standing still, light from a galaxy 33.8 billion light-years away would take 33.8 billion years to reach us, and that would be that. But in the early 1900s, Edwin Hubble discovered that distant galaxies seemed to be moving away from each other, and the farther apart they were, the faster they were moving. In other words, the universe is not static; it is expanding.
This became even more complicated in 1998, as the 20th century drew to a close, when two separate teams of astronomers observed that the universe is not only expanding, but that its expansion is accelerating. The force responsible is a mystery, but it has been given the temporary name “dark energy.”
There are two major and distinct periods of expansion in the 13.8 billion year history of the universe. The first is an initial period of rapid cosmic inflation now commonly called the “Big Bang.”
This inflationary era saw the volume of the cosmos expand by a factor of 10^26 (10 followed by 25 zeros). That’s equivalent to your fingernail growing from 1 nanometer per second to suddenly 10.6 light years (62 trillion miles) long. During 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 era that began 47,000 years after the Big Bang. Eventually, universal expansion allowed the cosmos to cool enough for protons to form from quarks and gluons, and then protons to combine with electrons to form the first hydrogen atoms, which formed the first stars and galaxies. During this period, the Big Bang-driven expansion of the universe slowed to a near standstill.
The age of matter dominance 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 accelerated and is still accelerating today. This third major period of the universe is called the age of dark energy dominance. It is the age in which we currently find ourselves.
Thanks to these periods of expansion of the Universe, light from JADES-GS-z14-0 has only traveled to the JWST and Earth for 13.5 billion years, despite the source now being much farther away than 13.5 billion light-years. This means that the JWST sees JADES-GS-z14-0 as it did 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, though it would still experience smaller local motions that could have brought it closer to or farther from nearby galaxies. But such galactic motions 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 of about 46.1 billion light years, a number determined by the expansion of the universe. This is the true 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 measurement scales: a co-moving distance that eliminates the expansion of the universe as a factor, and a proper distance that does include it. This means that the co-moving distance of JADES-GS-z14-0 is 13.5 billion light-years, while the proper distance is 33.8 billion light-years.
However, JADES-GS-z14-0 and other distant and old galaxies will not always be visible.
A lucky era 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 that a signal from JADES-GS-z14-0 reached us in the Milky Way, so a “cause” in this galaxy that existed at the beginning of time could have an “effect” in our galaxy in this modern era of the cosmos.
“Every observable galaxy must be within the particle horizon and must have been causally connected to 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 the other galaxies discovered by JADES are now so far away and being driven away from us so quickly, thanks to dark energy, that no signal sent today could ever reach us again. This is because the photon horizon is receding from us at the speed of light, but Real distant objects, the space between the Milky Way and those galaxies is expanding faster than the speed of light.
This seems unlikely, since Albert Einstein’s theory of special relativity sets the speed of light as a universal speed limit. However, that is a rule for objects with mass that are moving Through space, no rule for the nature of space itself.
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In about 2 trillion years, after Earth and humanity are long gone, the expansion of the universe will mean that whatever intelligent species replaces us in the Milky Way (if that ever happens) will no longer be able to see any galaxy outside our local group, which is about 10 million light-years across.
It’s a sobering thought, and it means that humanity lives at a unique point in the history of the universe when the most distant galaxies are still within our field of vision. We have the potential to know more about the universe and its origins than any intelligent life that follows us. Astronomers, including Helton, plan to use the JWST to take full advantage of this cosmic privilege.
“Working with JWST and the JADES Collaboration has been incredible,” Helton said. “Writing papers on science with JWST, like my recent one on
JADES-GS-z14-0, was the most rewarding and exciting experience
of my research career.”