Since it began sending data back to Earth in 2022, the James Webb Space Telescope (JWST) has had a major impact on astronomy, with one of its most revolutionary achievements being observing some of the most distant galaxies ever observed. However, because light doesn’t travel instantly—it moves at about 300 million meters (985 million feet) per second in a vacuum—we don’t see those galaxies as they are today, but as they were billions of years ago.
Furthermore, the universe is estimated to be about 13.8 billion years old. So we should assume that the most distant galaxy we can 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 kind of “cosmic horizon”—no telescope should be able to see beyond it. Since nothing can travel through space faster than the speed of light, this means that no galaxy should be farther than 13.8 billion light-years away, and that its distance from us over time would affect Earth. Right?
Wrong. If only the universe were that simple.
“The cosmic horizon is the farthest distance from which information can be retrieved,” Jake Hilton, an astronomer at the University of Arizona who is part of the JWST Advanced Deep Extragalactic Survey (JADES) team, told Space.com.
“There are a few different cosmological horizons,” Hilton continued, “which have different definitions and depend on different cosmological quantities. The most important one here is the cosmological horizon, which represents the furthest distance light can travel to us in the lifetime of the universe. This determines
“The edge of the visible universe.”
Related: James Webb Space Telescope Observes Most Distant Galaxy Ever Observed (Image)
In March 2024, JADES scientists revealed that the powerful telescope had discovered JADES-GS-z14-0, the most distant and oldest galaxy humans have ever seen. But the irony 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 age of the universe does not allow it to reach us? Doesn’t JADES-GS-z14-0’s location at 33.8 billion light-years away mean we’re seeing it as it was 33.8 billion years ago, something that would surely challenge estimates of the age of the universe?
Not so. Again, this is evidence that the universe has a way of turning even reasonable, logical conclusions upside down.
“How is it possible to observe a galaxy as distant as JADES-GS-z14-0, more than 13.8 billion light-years away, whose light appears to have taken longer than the age of the universe to reach us?” Hilton asked rhetorically. “The answer is the expansion of the universe.”
Seeing a Galaxy Larger Than Time Itself
If the universe were static, 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 found that distant galaxies appear to be moving away from each other, and the farther apart they are, the faster they are moving. In other words, the universe isn’t static; it’s expanding.
The matter became more complicated in 1998, as the twentieth century drew to a close, when two separate teams of astronomers observed that the universe was not only expanding, but that its expansion was accelerating. The force responsible for this remained a mystery, but it was dubbed “dark energy.”
There are two main and distinct periods of expansion over 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 universe grow in size 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 nanometers). trillion The universe is 1.2 million miles long. At this time, the universe was dominated by energy, and this period is known as the energy dominance era.
This was followed by a matter-dominated era that began 47,000 years after the Big Bang. Eventually, the expansion of the universe cooled enough for protons to form from quarks and gluons, and the protons then combined with electrons to form the first hydrogen atoms, which formed the first stars and galaxies. During this period, the expansion of the universe caused by the Big Bang slowed almost to a halt.
The era of matter dominance ended abruptly when the universe was less than 10 billion years old. At this time, the universe suddenly began to expand rapidly again. Moreover, this expansion became faster and faster and even continues to accelerate today. This third important period of the universe is called the era of dark energy dominance. It is the era we are currently living in.
Thanks to these periods of expansion, the light from JADES-GS-z14-0 has only traveled to the James Webb Telescope and Earth for about 13.5 billion years, even though its source is now much farther away than 13.5 billion light-years. This means that the James Webb Telescope 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 smaller, local motions that would have brought it closer to or farther away from nearby galaxies. But such galactic motion is nowhere near the kind caused by the expansion of the universe.
According to Hilton, the cosmological horizon, or “photon horizon,” is a sphere with a boundary about 46.1 billion light-years away, a number determined by the expansion of the universe. This is the actual horizon beyond which we should not “see” a galaxy. The galaxy JADES-GS-z14-0 is actually inside this horizon.
To avoid confusion, astronomers actually use two distance scales: a common motion distance that excludes 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, the distant and ancient galaxies JADES-GS-z14-0 and others will not always be visible.
Lucky age to have the James Webb Space Telescope
The fact that the James Webb Telescope can see the galaxy JS-Z14-0 means that it was once “causally connected” to Earth and the local universe. In other words, a signal from JS-Z14-0 in the Milky Way could have reached us, and so a “cause” in this galaxy that existed at the dawn of time could have had an “effect” on our galaxy in this modern era of the universe.
“Any observable galaxy must be within the particle horizon, and must be causally connected to us at some point in the history of the universe,” Hilton said.
But that’s no longer the case. Galaxies like JADES-GS-z14-0 and others discovered by JADES are now so far away and moving away from us so fast, thanks to dark energy, that no signal from them could reach us today. That’s because the photon horizon is moving away from us at the speed of light, but for JADES, the photon horizon is moving away from us at the speed of light. truly Distant objects, and the space between the Milky Way and those galaxies is expanding. faster From the speed of light.
This may seem counterintuitive, 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. during Space, not the basis of the fabric of space itself.
About two trillion years after Earth and humanity disappear, the expansion of the universe means that any intelligent species that replaces us in the Milky Way (if it ever does) will not be able to see any galaxies outside our Local Group — which is about 10 million light-years in diameter.
It’s a shocking idea, 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 sight. We are able to learn more about the universe and its origins than any intelligent life that might follow us. Astronomers, including Hilton, intend to use the James Webb Telescope to make the most of this cosmic privilege.
“Working with JWST and the JADES Collaboration has been amazing,” Hilton said. “Writing science papers with JWST, like my recent paper on
The JADES-GS-z14-0 has been a most exciting and rewarding experience.
“From my research career.”
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