IN-DEPTH ANALYSIS: Why the James Webb Space Telescope Sees into the Universe’s Past

Galaxies Too Mature for Their Age

One of the biggest surprises from the James Webb Space Telescope’s first years of observation is the discovery of surprisingly massive and well-structured galaxies in the early universe. According to standard cosmological models, the first galaxies were expected to be small, chaotic, and rudimentary. What the JWST has found challenges these models: galaxies with masses comparable to those of modern galaxies, existing just 500 to 700 million years after the Big Bang. Some even appear to have a regular disk-like shape —a structure that was thought impossible to form so early in cosmic history.

These discoveries do not “shatter cosmology,” as some sensational headlines might have suggested—they refine and challenge it. Scientists are adjusting their models of galaxy formation, exploring new avenues regarding the rate at which the first stars formed and the role of primordial supermassive black holes in the rapid growth of these early cosmic structures. James Webb does not destroy our understanding of the universe; it refines it with a precision that Hubble could never have achieved in the far-infrared.

CO₂ in Exoplanetary Atmospheres and the Birth of Stars

James Webb doesn’t just look into the distant cosmological past. Its instruments also allow us to analyze the chemical composition of the atmospheres of planets outside our solar system— exoplanets. In 2022, it detected CO2 in the atmosphere of a gas giant exoplanet, WASP-39 b —a first. In 2023, it observed silicate dust clouds on another planet. These spectroscopic analyses pave the way for the search for chemical signatures of life in the atmospheres of rocky planets in habitable zones.

At the same time, the James Webb Space Telescope is probing nearby nebulae —those “star nurseries” where new stars and planetary systems form—with unprecedented resolution. Its iconic image of the Carina Nebula shows hundreds of young stars and jets of matter invisible in Hubble images, revealing details of star formation never before observed. The telescope thus spans the entire spectrum of time: from the most distant past to the star formations currently taking place in our galactic neighborhood.

What impresses me about the James Webb is that it doesn’t just do one thing. It looks back 13.8 billion years and also analyzes clouds in the atmospheres of planets just a few hundred light-years away. The same instrument, the same gold mirror, for questions on such radically different scales. It’s an optical encyclopedia.

The observable universe has a boundary

The universe is 13.8 billion years old. This does not mean that the universe has a radius of 13.8 billion light-years. Over that time, space itself has expanded, so that the farthest regions that light has been able to reach since the beginning are now about 46 billion light-years away from us. This is what is known asthe observable universe —the sphere of space from which light has had time to reach us since the Big Bang. Beyond that, there is likely more of the universe, but its light has not yet had time to reach us—and may never do so, if the expansion continues to accelerate indefinitely.

James Webb is getting closer and closer to this limit. The galaxy MoM-z14, observed at a redshift of 14.44 in 2026, existed when the universe was barely 2% of its current age. It was not yet very far away in space at that time—but expansion has since pushed that region of the universe to a distance of about 33 billion light-years. The boundary of the observable universe lies at a point in time known as the last scattering surface —approximately 380,000 years after the Big Bang, before which the universe was so dense and hot that light could not travel freely. This barrier from the past is the cosmic microwave background, and the James Webb Space Telescope cannot cross it. But it is getting closer than ever before.

There is something philosophically unsettling about this limit of the observable universe. Not because the universe ends there, but because it is a boundary that light itself cannot cross for us. We are, in a sense, trapped inside a cosmic bubble defined by the speed of light and the age of the universe. No telescope, no matter how powerful, will ever be able to see beyond it. It is one of the few absolute limits of human knowledge.

James Webb: A Window into the Big Bang

The James Webb Space Telescope is not just the most powerful telescope ever built. It is an optical time machine, an instrument that transforms the finite speed of light and the expansion of the universe into tools for cosmic archaeology. By capturing light stretched into the infrared spectrum by billions of years of travel through an expanding universe, it offers us snapshots of the universe at ages we thought were out of reach. Galaxies 13.5 billion years old, stars formed a few hundred million years after the Big Bang, and the atmospheres of planets hundreds of light-years away.

We are made of the past that James Webb observes

There is a personal dimension to what James Webb does. The atoms that make up your body were forged in stars that lived and died billions of years ago—in galaxies that resemble those James Webb is observing today in their earliest stages. Looking at the JWST images is, in a way, like looking into the cosmic workshop where your own atoms were forged. The past that the telescope is peering into is not a foreign past: it is our own. The next image the James Webb Space Telescope releases may be the oldest light ever captured by humankind. And it will tell us something essential about who we are and where we come from.

By Maxime Marquette, columnist