Since it began sending data back to Earth in 2022, the James Webb Space Telescope (JWST) has had a major impact on 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 rather travels at about 300 million meters (985 million feet) per second in a vacuum—we see these galaxies not as they are today, but as they were billions of years ago.
In addition, the age of our universe is estimated at 13.8 billion years. So we should assume that the most distant galaxy we could ever hope to see is no more than 13.8 billion light years away. (One light year is the distance light travels in a 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, that means there shouldn’t be any galaxy more than 13.8 billion light-years away, and an ever-increasing distance could affect Earth. Right?
Poorly. If only the universe were that simple.
“The cosmological horizon is the maximum distance from which information can 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 several different cosmological horizons,” Helton continued, “which have different definitions and depend on different cosmological quantities. The most relevant here is the cosmological horizon, which is the maximum distance that light could travel to us in the age of the universe that defines this
the edge of the observable cosmos.”
Related: James Webb Space Telescope Captures Farthest Galaxy Ever Seen (Image)
In March 2024, JADES scientists revealed that the powerful telescope spotted JADES-GS-z14-0, the most distant and earliest galaxy ever seen by mankind. But 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 far away that the universe is not old enough to have reached us? Doesn’t the position of JADES-GS-z14-0 at a distance of 33.8 billion light years mean that we are seeing it as it was 33.8 billion years ago, something that would surely challenge the estimate of the age of the universe?
It is not so. Again, proof that the universe has a way of turning reasonable and logical conclusions on their head.
“How can one even observe a distant galaxy like JADES-GS-z14-0 when it is more than 13.8 billion light-years away and its light would appear to take longer than the age of the universe to reach us?” Helton asked rhetorically. “The answer is the expansion of space.
Seeing a galaxy older than time itself
If the universe were just sitting 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 20th century, Edwin Hubble discovered that distant galaxies were moving away from each other, and the farther apart they were, the faster they were going. In other words, the universe is not static; 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 was not only expanding, but that the expansion was also accelerating. The force responsible is a mystery, but has been given the nickname “dark energy”.
During the 13.8 billion year history of the universe, there are two major and distinct periods of expansion. The first is the initial period of rapid cosmic inflation commonly known today 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 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 epoch.
This was followed by a matter-dominated epoch that began 47,000 years after the Big Bang. Cosmic expansion eventually allowed the universe to cool enough to allow protons to form from quarks and gluons, and then protons to combine with electrons to form the first hydrogen atoms that formed the first stars and galaxies. During this period, the expansion of the universe driven by the Big Bang slowed to a near halt.
The epoch dominated by matter surprisingly ended when the universe was less than 10 billion years old. At this time, the universe suddenly began to rapidly expand again. Moreover, this expansion has accelerated and accelerated even today. This third major period of the universe is called the epoch dominated by dark energy. It’s the era we’re in right now.
Thanks to these periods of cosmic expansion, the light from JADES-GS-z14-0 actually took only 13.5 billion years to travel to JWST and Earth, even though its source is now much more distant 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 experience minor local motions that could move it closer to itself or further away from nearby galaxies. But such galactic motion would be nowhere near that 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 true horizon beyond which we should not be able to “see” the galaxy. The galaxy JADES-GS-z14-0 is indeed on this horizon.
To avoid confusion, astronomers actually use two scales to measure distance: the comoving distance, which eliminates the expansion of the universe as a factor, and the proper distance, which includes it. This means that the common motion distance of JADES-GS-z14-0 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 happy era to have the James Webb Space Telescope
The fact that JADES-GS-z14-0 is seen by JWST means that it was once “causally linked” to Earth and our local universe. In other words, it was possible for the 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 in 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 other galaxies discovered by JADES are now so far away and driven away from us by dark energy so fast that no signal from them sent to us 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 expanding faster than the speed of light.
This might seem unlikely, since Albert Einstein’s theory of special relativity sets the speed of light as the universal speed limit. However, this is the rule for mass movement objects over space, is not a rule for the structure of the 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 one ever does) will not be able to see any galaxies that exist outside of our local group. — which is about 10 million light-years across.
This is a sobering thought, and it means that humanity lives at a unique point in the history of the universe, where the most distant galaxies are still within our view. We are capable of knowing more about the universe and its origins than any intelligent life that may succeed 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 mine recently
JADES-GS-z14-0, was a most rewarding and exciting experience
of my research career.”