The James Webb Revolution

25 Years, $10 Billion, and a Mirror the Size of a Tennis Court

The James Webb Space Telescope is the product of an international collaboration involving NASA, the European Space Agency, and the Canadian Space Agency — and represents the largest, most complex space observatory ever launched. Its primary mirror spans 6.5 metres, assembled from 18 hexagonal gold-coated beryllium segments, and operates at a temperature of minus 233 degrees Celsius — colder than most of deep space — to detect the faint infrared heat signatures of the earliest galaxies. The telescope orbits at the second Lagrange point, 1.5 million kilometres from Earth, a gravitationally stable location where it remains permanently shaded from the sun, Earth, and moon by a five-layer sunshield the size of a tennis court.

Its capabilities are staggering in comparison to its predecessors. Where the Hubble Space Telescope's primary mirror was 2.4 metres, JWST's is nearly three times larger — and because it operates in the infrared rather than visible light, it can peer through the dust clouds that obscure star-forming regions and detect the light from the very first generations of stars and galaxies, whose visible wavelengths have been stretched by the universe's expansion into the infrared over 13 billion years of cosmic time. The telescope began science operations in mid-2022, and within months had already produced findings that sent cosmologists back to their equations.

Galaxies Too Massive, Too Early — and the Models That Cannot Explain Them

The Lambda-CDM model — the Standard Model of cosmology — makes specific, well-tested predictions about how structure in the universe forms. In the aftermath of the Big Bang, dark matter halos should clump under gravity over hundreds of millions of years, then slowly accumulate ordinary matter to form the first stars, stellar clusters, and eventually galaxies. The first galaxies should have been small, dim, irregular objects. Massive, bright, structured galaxies like our Milky Way should require billions of years to assemble. This is the model that governed cosmology for decades.

Within months of beginning science operations, JWST had discovered objects that violated it. Astronomers identified multiple massive, luminous galaxies existing just 500 to 700 million years after the Big Bang — objects comparable in stellar mass to the Milky Way today, assembled in a small fraction of the time the standard model predicts would be required. A 2023 Nature paper described these as "impossible galaxies" — their existence implied either that early star formation was dramatically more efficient than models assumed, that dark matter behaves differently on small scales, or that some entirely new physical process was at work in the early universe. As of 2025, the debate has not been resolved, and the impossible galaxies continue to accumulate in the JWST catalogues.

The "Little Red Dots" — Ancient Supermassive Black Holes That Have No Business Existing

Alongside the too-massive galaxies, JWST has catalogued a population of mysterious compact objects astronomers have nicknamed "Little Red Dots" — sources that appear disproportionately luminous and red in the near-infrared, typically found at redshifts corresponding to the universe's first 1.5 billion years. Spectroscopic follow-up has revealed that a large fraction of these objects contain actively accreting supermassive black holes — black holes already weighing millions or billions of solar masses when the universe was less than a tenth of its current age.

This poses an acute theoretical problem. Black holes grow by accreting material, and there is a physical limit — the Eddington limit — on how fast that process can proceed. A black hole cannot simply grow arbitrarily fast without blowing away the very material feeding it. Working backwards from the masses observed by JWST, many of these black holes would have needed to grow faster than the Eddington limit permits, starting from massive "seeds" that current models of stellar evolution struggle to produce. In 2025, a landmark paper identified the earliest known black hole candidate — an object showing signs of active accretion just 570 million years after the Big Bang. The universe, it appears, assembled its most extreme objects faster than any model predicted.

The Hubble Tension — How Fast Is the Universe Actually Expanding?

The Hubble constant — the rate at which the universe is expanding — is one of cosmology's most fundamental measurements. It can be determined in two independent ways: by observing the cosmic microwave background (the afterglow of the Big Bang) and working forward through the standard cosmological model, or by directly measuring the recession speed of nearby galaxies using calibrated distance ladders built from Cepheid variable stars and Type Ia supernovae. Both methods are well-tested and widely accepted. For decades, astronomers assumed that when precision improved sufficiently, the two measurements would converge.

They have not. The early-universe method yields a Hubble constant of approximately 67 kilometres per second per megaparsec. The local measurement method consistently returns approximately 73 kilometres per second per megaparsec. The discrepancy is roughly 9% — and its statistical significance has grown to 5-sigma, the threshold at which physicists declare a discovery. At the January 2025 meeting of the American Astronomical Society, Nobel laureate Adam Scolnic declared the Hubble tension a crisis: not merely an anomaly to be watched, but a fundamental challenge to the Standard Model of cosmology.

What JWST Found — and Why It Made Things Worse

Before JWST launched, many cosmologists hoped it might resolve the Hubble tension by revealing systematic errors in the Hubble Space Telescope's Cepheid measurements — errors caused by the crowding and dust confusion that can affect optical observations. JWST's infrared capabilities allow cleaner Cepheid measurements, removing the most plausible instrumental explanation for the discrepancy. In 2023 and 2024, JWST revisited the Cepheid distance ladders used to anchor the local Hubble constant measurement. The result was not resolution but confirmation: JWST's more precise measurements agreed with Hubble's, eliminating measurement error as an explanation and leaving the tension fully intact.

The implication is profound. If neither measurement is wrong, then something in the Standard Model of cosmology — Lambda-CDM, which has successfully explained virtually every large-scale observation for decades — must be incomplete. The leading hypotheses range from early dark energy (a phase of accelerated expansion in the early universe not captured by current models), to self-interacting dark matter, to entirely new physics beyond the current cosmological framework. As of 2026, no consensus has emerged, and the Hubble tension remains the most significant unresolved problem in observational cosmology.

How JWST Analyses Exoplanet Atmospheres — Transmission Spectroscopy

When an exoplanet transits its host star — passing between the star and Earth — a thin sliver of starlight passes through the planet's atmosphere. Different molecules absorb different wavelengths of light, and JWST's extraordinary infrared sensitivity allows it to read the chemical fingerprint left in that light. This technique, called transmission spectroscopy, had been attempted with Hubble and Spitzer, but with limited sensitivity and wavelength coverage. JWST extended the technique into a new regime of precision: for the first time, detailed molecular inventories of exoplanet atmospheres became achievable, not merely theoretical exercises.

Early results were rich. By 2023, JWST had detected carbon dioxide, water vapour, sulphur dioxide, and methane in the atmospheres of multiple exoplanets — the first clear detections of several of these molecules beyond our solar system. Sulphur dioxide was particularly interesting: its presence in the atmosphere of WASP-39b was attributed to photochemistry driven by the host star's ultraviolet radiation, confirming that exoplanet atmospheric chemistry can be complex and dynamic rather than static. Each detection validated the technique and calibrated expectations for the more challenging target class: smaller, cooler, potentially habitable worlds.

K2-18b — The Strongest Hint of Life Beyond Earth Yet Recorded

In April 2025, a team led by Nikku Madhusudhan at the University of Cambridge announced findings from JWST observations of K2-18b — a "hycean world" candidate, a class of exoplanet theorised to host deep liquid water oceans beneath a hydrogen-rich atmosphere, orbiting its M-dwarf star in the habitable zone 120 light-years from Earth. The JWST data showed spectral features consistent with the presence of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS) — molecules that on Earth are produced almost exclusively by biological processes, primarily marine phytoplankton.

The detection reached 3-sigma statistical significance — substantial but below the 5-sigma threshold conventionally required for a confirmed scientific discovery. The Cambridge team was appropriately cautious, noting that 16 to 24 additional hours of JWST observation time would be needed to approach confirmation-level significance. The scientific community was immediately divided. Independent analyses, including one from Oxford, found the signal consistent with a flat line at lower confidence thresholds, and several biosignature researchers argued the announcement exceeded what the data could support. As of early 2026, additional JWST observations of K2-18b are scheduled, and the result remains one of the most consequential open questions in science: is what JWST saw biology — or noise?

The Roman Space Telescope and the Wide-Field Survey

NASA's Nancy Grace Roman Space Telescope, due for launch in 2027, will complement JWST rather than replace it. Where JWST delivers extraordinary depth on small patches of sky, Roman is a wide-field survey instrument — capable of imaging areas 100 times larger than JWST's field of view in a single exposure. Its primary missions include a dark energy survey using Type Ia supernovae and gravitational weak lensing — potentially the most powerful independent test of the Hubble tension yet attempted — and a microlensing survey designed to detect free-floating planets and cold exoplanets that transit methods cannot find. Roman will not answer whether K2-18b is alive. But it may tell us how common ocean worlds are, and whether the conditions for life as we understand it exist across the galaxy at the scale the statistics demand.

The Extremely Large Telescope and Direct Imaging

The European Southern Observatory's Extremely Large Telescope (ELT), currently under construction on Cerro Armazones in Chile's Atacama Desert, will be the world's largest optical telescope when it begins operations around 2028 — with a 39-metre segmented primary mirror. The ELT's first-light instrument METIS will directly image the atmospheres of rocky planets around nearby stars, including TRAPPIST-1 system members, in the thermal infrared. Direct imaging — actually seeing the planet rather than watching it transit — sidesteps JWST's atmospheric sensitivity limitations and will allow characterisation of planetary atmospheres without requiring perfect orbital alignment with Earth. If K2-18b's biosignature hint strengthens with additional JWST data, the ELT will be the instrument most likely to either confirm or decisively refute biological activity beyond our solar system.