Can the James Webb Space Telescope see galaxies over the universe's horizon?

The James Webb Space Telescope (JWST) has brought significant advancements to the field of astronomy since it started transmitting data back to Earth in 2022. One of its remarkable achievements is the observation of distant galaxies. However, a fundamental question arises: how can a telescope see a galaxy that is 33.8 billion light-years away in a universe that is only 13.8 billion years old?

The answer lies in the concept of the expansion of the universe and the way light travels through space. Light travels at a finite speed, and since the speed of light is not infinite, we see distant galaxies not as they appear today, but as they were billions of years ago.

According to the estimated age of the universe (13.8 billion years), one might assume that the maximum distance we could detect a galaxy would be 13.8 billion light-years. However, the universe operates in a way that challenges this simplistic assumption.

There are various types of cosmological horizons, with the most relevant for this discussion being the cosmological horizon that defines the maximum distance from which light could have reached us during the age of the universe. This horizon should serve as the boundary of our observable cosmos.

The JWST, in March of 2024, identified JADES-GS-z14-0, the farthest and earliest galaxy ever detected by humanity. Surprisingly, this galaxy is located approximately 33.8 billion light-years away. But how can we receive light from an object that seems to have taken longer than the age of the universe to reach us? The key lies in the expansion of the universe.

Distant galaxies appear to move away from each other as the universe expands. In the early 1900s, Edwin Hubble made this groundbreaking observation. By the end of the 20th century, astronomers discovered that not only is the universe expanding but that its expansion is also accelerating, attributed to a mysterious force called "dark energy."

Over the 13.8 billion-year history of the universe, there have been two major periods of expansion. The first is the cosmic inflation commonly known as the "Big Bang," during which the volume of the cosmos increased by an astounding factor. Subsequently, during the matter-dominated epoch, cooling allowed the formation of hydrogen atoms, stars, and galaxies.

However, the expansion of the universe did not cease during this period; it dramatically accelerated in what we now term the dark-energy-dominated epoch, the current phase. This ongoing expansion accounts for extraordinary observations such as detecting galaxies like JADES-GS-z14-0, which is much farther away than the roughly 13.5 billion light-years we would expect.

The expansion of the universe and the subsequent travels of light make it possible to see galaxies as they existed billions of years ago. JADES-GS-z14-0, even though it is 33.8 billion light-years away, has had its light traveling for 13.5 billion years, arriving at the JWST as a snapshot of the galaxy 300 million years after the Big Bang.

To clarify, there is a cosmological horizon referred to as the "Photon Horizon," marking a sphere around 46.1 billion light-years away. This horizon represents the boundary beyond which we should not be able to observe a galaxy. JADES-GS-z14-0 lies within this horizon.

Clearly, astronomers employ two distance measurement scales: co-moving distance and proper distance. Co-moving distance disregards the universe's expansion, while proper distance accounts for it. JADES-GS-z14-0 has a co-moving distance of 13.5 billion light-years and a proper distance of 33.8 billion light-years.

It's important to note that the visibility of galaxies like JADES-GS-z14-0 won't persist indefinitely. Due to dark energy's influence, these galaxies are moving away from us at a rate that prevents any signals sent today from reaching us. Consequently, astronomers only consider a galaxy observable if it had once been causally connected with our local universe during the history of the universe.

As humbling as it may be, our current epoch grants us the privilege of witnessing galaxies that future civilizations, due to the continued expansion of the universe, will never be able to observe. Humanity resides at a unique point in the universe's timeline, providing us with unparalleled knowledge of its origin and evolution. With the aid of the JWST, astronomers, including Jake Helton of the JADES team, are determined to make the most of this remarkable opportunity.

Working alongside the JWST and the JADES Collaboration gives researchers an exhilarating and fulfilling experience, as seen in Helton's excitement while writing about JADES-GS-z14-0. As the JWST continues to push the boundaries of astronomical exploration, we can truly appreciate our place in the vastness of the cosmos.

Earlier, SSP told that James Webb Space Telescope sees an ancient black hole dance with colliding galaxies.