What Specifications Does a Space Telescope Need to See the Earliest Light in the Universe

Key Takeaways

• Detecting cosmic dawn requires infrared sensitivity reaching wavelengths beyond 10 microns
• Mirror diameters of 6 meters or larger collect enough photons from objects 13+ billion light-years away
• Temperatures near absolute zero keep detectors sensitive enough to capture faint primordial signals

The Universe Has an Edge in Time

The universe began roughly 13.8 billion years ago in an event known as the Big Bang. For the first 380,000 years after that moment, the cosmos was a dense, searing plasma in which photons couldn’t travel more than a short distance before colliding with free electrons and scattering away. There was light everywhere, but it was imprisoned.

When the universe cooled enough for electrons and protons to combine and form neutral hydrogen atoms – a process cosmologists call recombination – the fog lifted. Photons could finally travel freely across space, and the radiation that flooded the universe at that moment has been streaming outward ever since. Today it’s known as the cosmic microwave background, or CMB, and it represents the most distant light any telescope can ever detect. It isn’t the light of stars or galaxies. It’s the glow of a cooling plasma, the universe’s very first transparent moment.

But there’s a second kind of “earliest light” that astronomers are actively pursuing, one that came much later and tells a different story. After recombination, the universe entered a period called the Cosmic Dark Ages, a stretch of hundreds of millions of years during which matter slowly clumped under gravity. No stars existed. No galaxies. The universe was dark in the literal sense.

Then, somewhere between 100 million and 500 million years after the Big Bang, the first stars ignited. These were Population III stars – enormous, short-lived objects made almost entirely of hydrogen and helium, lacking the heavier elements that later generations of stars would carry. Their ultraviolet radiation began to ionize the neutral hydrogen that had filled the universe since recombination, gradually burning away the cosmic fog in a process called reionization. This period – from the first stellar ignition through the complete reionization of the intergalactic medium – is known as Cosmic Dawn and the Epoch of Reionization. It’s the holy grail of observational cosmology, and seeing it requires a telescope with specifications that push engineering to its absolute limits.

What the Target Actually Demands From an Instrument

The CMB and the first stars present completely different observational challenges. The CMB is observed across the entire sky at microwave and millimeter wavelengths, and dedicated missions like COBE, WMAP, and Planck have already mapped it in extraordinary detail. Those missions used microwave antennae and bolometer arrays, not optical mirrors. Future CMB missions like CMB-S4 will push further by mapping polarization patterns potentially left by gravitational waves from cosmic inflation, but they operate in a wavelength regime entirely separate from what’s needed to see the first stars and galaxies.

The targets most cosmologists mean when they talk about “earliest light” in the context of a new infrared space telescope are the first galaxies and the sources driving reionization. These formed between redshift 6 and redshift 20 in the standard cosmological framework – meaning the universe was between 7% and 0.2% of its current size when that light was emitted.

That redshift number is everything. Redshift measures how much the universe has expanded since the light was emitted. A galaxy at redshift 10 emitted its light when the universe was about 480 million years old. Every photon it released has been stretched by the expansion of space, multiplying its wavelength by a factor of 11. A photon originally emitted as ultraviolet light with a wavelength of 121.6 nanometers – the Lyman-alpha line of hydrogen, one of the most important spectral features in early-universe astronomy – would arrive at Earth stretched to about 1.34 micrometers, squarely in the near-infrared portion of the spectrum. A galaxy at redshift 20 would push that same Lyman-alpha line to about 2.55 micrometers. Galaxies at earlier epochs require sensitivity beyond 3 micrometers or further into the mid-infrared.

This is why no optical telescope, however powerful, can detect the first stars and galaxies. Their light has been stretched entirely out of the visible range. An infrared telescope isn’t an upgrade for this science – it’s the only viable path.

Why Earth’s Atmosphere Disqualifies Ground-Based Observatories

Earth’s atmosphere is opaque across most of the infrared spectrum. Water vapor, carbon dioxide, and other atmospheric molecules absorb infrared radiation at multiple wavelength bands, leaving only narrow transmission windows where ground-based observation is even possible. For wavelengths above 2.4 micrometers, the atmosphere becomes increasingly hostile. Above about 5 micrometers, it’s effectively a wall.

Beyond absorption, the atmosphere itself glows in the infrared. Molecules in the upper atmosphere radiate infrared photons continuously, creating a bright, fluctuating background that overwhelms faint astronomical signals. A galaxy from the epoch of reionization might emit light equivalent to a few dozen photons per second hitting a telescope mirror. Against the blazing infrared sky background produced by Earth’s atmosphere, detecting that signal is essentially impossible from the ground.

Space removes both problems at once. Above the atmosphere, every infrared wavelength is accessible, and the sky background drops to near zero in most wavelength bands. The sensitivity gain of a space telescope over a ground-based facility is not a factor of two or ten – it’s orders of magnitude in the infrared regime where the earliest light arrives. Some ground-based facilities like the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Cosmology Telescope can observe at specific millimeter and submillimeter wavelengths from high-altitude Chilean desert sites, but they target the CMB and cold dust emission rather than the direct starlight of the first galaxies.

The James Webb Space Telescope (JWST), launched in December 2021, was designed from the start to observe between 0.6 and 28.3 micrometers, covering near-infrared and mid-infrared wavelengths where light from the epoch of reionization arrives. Its performance since entering science operations in mid-2022 has exceeded pre-launch predictions in nearly every measured category, confirming that the cold, infrared space observatory architecture is the right approach for this science.

Mirror Size: The Fundamental Constraint

The single most important physical parameter of any telescope is its primary mirror diameter. Larger mirrors collect more light, and more light means the ability to detect fainter objects. The relationship is direct: doubling the mirror diameter quadruples the light-collecting area. A telescope with a 10-meter mirror gathers roughly 17 times as much light as one with a 2.4-meter mirror.

For early-universe science, this matters enormously. The first galaxies were small by modern standards – some were comparable in size to present-day globular clusters – and they were billions of light-years away. Their apparent brightness on the sky is vanishingly faint. Detecting a galaxy at redshift 12 with enough signal-to-noise to study its stellar populations and chemical composition typically requires hours of exposure time even with the largest current space telescope.

JWST’s primary mirror is 6.5 meters in diameter, made from 18 hexagonal beryllium segments coated with a thin layer of gold. That gold coating was specifically chosen because gold reflects infrared light with higher efficiency than silver or aluminum, which are standard coatings for optical mirrors. Each segment is about 1.3 meters across. Getting 18 separate mirror segments to act as a single coherent optical surface in space – where temperature fluctuations and vibrations can throw alignment off by nanometers – required developing new approaches to active optics engineering that simply didn’t exist before JWST was designed.

The Hubble Space Telescope, with its 2.4-meter mirror, could barely detect galaxies at redshift 6 to 7 with deep field imaging programs like the Hubble Ultra Deep Field and Extreme Deep Field. Pushing to redshift 10 or 15 requires the combination of aperture and infrared sensitivity that only a telescope like JWST or its successors can provide.

For seeing Population III stars or the very first galaxies at redshift 15 and beyond, the scientific community has discussed telescopes with mirrors in the 12- to 15-meter range. The LUVOIR (Large UV/Optical/IR Surveyor) concept, studied extensively during the 2020 NASA Decadal Survey process, proposed mirrors up to 15 meters in diameter. Such a telescope would gather roughly 5 times the light of JWST and reach objects JWST can only barely detect. The engineering challenge of folding a 15-meter segmented mirror into a rocket fairing and deploying it reliably in deep space has no precedent in the history of spacecraft development.

Wavelength Range: How Far Into the Infrared Is Far Enough

A telescope designed to see the epoch of reionization needs to cover the wavelength range where the most important spectral features from those redshifts actually appear. The two most useful spectral markers are the Lyman-alpha line at 121.6 nanometers rest wavelength and the Lyman break, a sharp cutoff in the spectrum of star-forming galaxies below 91.2 nanometers caused by absorption of ultraviolet photons by neutral hydrogen.

At redshift 10, Lyman-alpha falls at 1.34 micrometers. At redshift 20, it shifts to 2.55 micrometers. The Lyman break at redshift 10 falls at 1.0 micrometer, and at redshift 20 it’s at 1.92 micrometers. A telescope targeting the epoch of reionization needs sensitivity from roughly 1 micrometer to at least 5 micrometers, and ideally further into the mid-infrared.

JWST’s NIRCam instrument covers 0.6 to 5 micrometers with its two modules, while MIRI (the Mid-Infrared Instrument) covers 5 to 28.3 micrometers. This architecture was deliberately chosen to catch early-universe galaxies at the expected wavelengths. The NIRSpec spectrograph covers 0.6 to 5.3 micrometers and can observe up to 100 galaxies simultaneously through a programmable array of micro-shutters – a capability that dramatically accelerates spectroscopic surveys of the early universe compared to any previous instrument.

For even earlier objects, or for the faint gas emission surrounding the first galaxies, wavelengths beyond 10 micrometers become scientifically relevant. The Origins Space Telescope concept, proposed during the 2020 Decadal Survey study process, targeted wavelengths from 2.8 to 588 micrometers specifically to detect the cold gas and dust emission associated with the first star-forming regions. It was not selected as the top priority, but the science case for far-infrared observations of the early universe remains an active area of planning.

Angular Resolution: Seeing Structure at Cosmic Distances

Even with the right wavelength coverage and a large mirror, a telescope can only be useful if it can resolve the features it’s trying to study. Angular resolution – the ability to distinguish two closely spaced objects – is governed by the relationship between mirror size and wavelength. Larger mirrors produce sharper images, but longer wavelengths work against resolution. The minimum angular separation a circular aperture can resolve is approximately 1.22 times the wavelength divided by the mirror diameter.

For JWST observing at 2 micrometers with its 6.5-meter mirror, the diffraction limit works out to about 0.065 arcseconds. That’s roughly 3 times sharper than Hubble in the near-infrared. A galaxy at redshift 10 occupies a physical scale of perhaps a few hundred parsecs across, which at that distance subtends less than 0.1 arcseconds on the sky. JWST can just barely resolve the spatial structure of such a galaxy, and early science results have shown measurable sizes for these objects – something that was well beyond Hubble’s reach at those wavelengths.

Mid-infrared wavelengths present a harder problem. At 10 micrometers with the same 6.5-meter mirror, the diffraction limit degrades to about 0.33 arcseconds, still adequate for detection but not for detailed structural mapping of individual galaxies. Far-infrared telescopes face a severe resolution penalty that has no easy solution. An observatory observing at 100 micrometers needs a mirror approximately 1,500 meters in diameter to match JWST’s resolution at 2 micrometers. This physical reality is why far-infrared space missions tend to function as survey instruments rather than telescopes capable of imaging morphological detail within individual high-redshift objects.

Operating Temperature: The Physics of Thermal Noise

Every warm object emits infrared radiation. A detector trying to measure faint infrared light from a galaxy 13 billion light-years away will be overwhelmed if the telescope’s own mirrors and structure are radiating at room temperature. This thermal noise problem, and solving it, demands engineering comparable in difficulty to building the mirror itself.

JWST’s solution involves two main components. The sunshield – a five-layer, tennis-court-sized structure made from Kapton film coated with aluminum and silicon – blocks thermal radiation from the Sun, Earth, and Moon, allowing the telescope’s cold side to passively cool to around 40 to 50 Kelvin (roughly -223 to -233 degrees Celsius). That’s cold enough for JWST’s near-infrared instruments to operate without any active refrigeration.

MIRI, the mid-infrared instrument, needs to go colder still – to about 6.4 Kelvin. At mid-infrared wavelengths, even a telescope sitting at 40 Kelvin radiates enough thermal photons to swamp the faint signals from early-universe objects. MIRI uses a dedicated cryocooler, a mechanical refrigeration system that cycles helium gas to extract heat and maintain the detector assembly near absolute zero. This cryocooler consumes significant electrical power and adds considerable complexity to the overall system, but there’s no alternative for genuine mid-infrared sensitivity.

Future telescopes targeting even longer wavelengths would need even colder operating temperatures throughout their entire optical structure – mirrors included. The Origins Space Telescope concept called for mirrors cooled to about 4.5 Kelvin, maintained by an active cryocooler system over the telescope’s full operational lifespan. No space telescope has yet operated with its primary mirror at such extreme temperatures over a multi-year mission. This remains one of the most significant unresolved engineering challenges in far-infrared astronomy.

Detector Technology: Counting Photons From Across Cosmic Time

The detectors that record the light collected by the mirror are where the physics of early-universe astronomy meets the limits of materials science. Near-infrared detectors for space telescopes are typically Mercury Cadmium Telluride (HgCdTe) arrays. JWST uses detectors fabricated by Teledyne Technologies with pixel counts up to 4096 x 4096 per array and sensitivity extending to 5 micrometers. These detectors must operate at temperatures below 40 Kelvin to suppress dark current – the thermal electrons that mimic real photon detections and add noise to the signal.

For mid and far-infrared wavelengths, the detector physics changes substantially. JWST’s MIRI instrument uses arsenic-doped silicon (Si:As) detectors that can respond to photons out to about 28 micrometers. Beyond that wavelength range, different semiconductor materials are needed, and beyond 100 micrometers, the technology shifts to bolometers – devices that measure the tiny temperature rise caused by absorbed photons rather than directly converting photons to electrical charges. Bolometers are used extensively by CMB and submillimeter telescopes and represent a completely different detection philosophy from the photovoltaic devices used in near-infrared instruments.

The sensitivity metric that ultimately matters for faint source detection is a figure of merit for astronomical instruments called the Noise Equivalent Flux Density (NEFD). JWST’s NIRCam can detect sources as faint as a few nanoJansky in a few hours of exposure – a nanoJansky being one billionth of a Jansky, itself a unit equal to 10^-26 watts per square meter per hertz. The faintest galaxies seen in the JWST deep fields sit at luminosities of about 2 to 10 nanoJansky, right at the edge of what the instrument can reliably record.

Getting to even fainter and earlier sources requires reducing the noise floor further. That means larger mirrors (more photons per unit time), colder detectors (less thermal noise), and longer exposure times. Detector arrays have maximum sizes constrained by fabrication technology. Temperature has a hard physical floor at absolute zero. Exposure times compete with other science programs for telescope time. The path to detecting individual Population III stars, if they can be detected at all without extreme lensing, almost certainly runs through gravitational magnification rather than direct imaging.

The Role of Gravitational Lensing in Early-Universe Observation

Gravitational lensing is both a fortunate accident and a planned observing strategy. When a massive foreground object – a galaxy or galaxy cluster – lies between a telescope and a very distant target, the mass of the foreground object bends the paths of the background object’s light rays, amplifying its apparent brightness and sometimes producing multiple distorted images. Total magnification can reach factors of 10, 50, or even higher in exceptional cases.

JWST has used galaxy clusters like SMACS 0723 and Abell 2744 as natural gravitational lenses to peer behind them at galaxies at redshifts above 10. In the first year of science operations, multiple candidate galaxies at redshift 12 to 16 were identified in lensed fields, with spectroscopic confirmation obtained for several. These detections represented a direct measure of how far the existing JWST specification can reach under optimal conditions.

For a future telescope trying to detect individual Population III stars – which may be possible in strongly lensed arcs where magnification exceeds 100 – the combination of a large primary mirror of at least 6 to 10 meters, coverage from 1 to at least 5 micrometers, and precise pointing stability near the critical angular scales set by lensed arc widths forms the baseline specification. Pointing stability requirements are not trivial. JWST holds its pointing to within about 0.006 arcseconds over extended observing periods – a level of stability that required entirely new reaction wheel isolation systems and a precise guide star architecture developed specifically for this mission.

Spectroscopic Capability: Reading the Chemical History of the Cosmos

Detecting a very distant galaxy is one thing. Understanding what it’s made of and what it’s doing requires spectroscopy – dispersing the light into its component wavelengths and reading the absorption and emission lines that reveal temperature, composition, velocity, and ionization state. For early-universe science, spectroscopy is how redshift gets confirmed. Photometric redshift estimates derived from broad-band colors can be significantly wrong, and only a spectrum showing recognized spectral features at shifted positions settles the question.

JWST’s NIRSpec is the most powerful near-infrared spectrograph ever flown in space. Its micro-shutter assembly can open individual shutters in a 342 x 342 grid to admit light from up to 100 pre-selected targets simultaneously, enabling multiplexed spectroscopy across a field of view of roughly 3.4 x 3.6 arcminutes. A single NIRSpec observation covering several hundred galaxies in one pointing would have required years of individual observations with any previous instrument.

For future telescopes, multiplexing this spectroscopic capability scales with the number of independently controllable apertures. Proposed next-generation instruments explore technologies including digital micromirror devices (DMDs), which are essentially repurposed display chip arrays, and fiber-optic positioning systems. The Euclid space telescope, launched by the European Space Agency in July 2023, uses a slitless grism spectrograph to survey millions of galaxies from redshift 0.9 to 1.8, though its near-infrared coverage stops at 2 micrometers and it’s not optimized for the earliest cosmic epochs.

A telescope targeting redshifts above 15 would need a spectrograph covering at least 1.5 to 6 micrometers with resolving power R = 1000 to 2000 (where R equals wavelength divided by the minimum resolvable wavelength difference) to separate and identify common spectral features. Higher resolving power helps study gas kinematics and detect faint oxygen and carbon emission lines that trace metallicity – the abundance of elements heavier than helium – in the first galaxies. Even a faint trace of oxygen or carbon in an otherwise pristine galaxy is diagnostic of whether that object formed before or after the first cycle of stellar nucleosynthesis enriched the surrounding gas.

Stability, Stray Light, and the Engineering Reality

Sensitivity and wavelength coverage don’t count for much if the telescope is overwhelmed by stray light or if pointing jitter smears the images. Stray light enters through imperfect baffling – when sunlight or earthshine reflects off internal surfaces into the detector focal plane. JWST’s sunshield does double duty here: it keeps the telescope cold and shields the optical assembly from solar radiation. But stray light from the Milky Way’s own infrared emission – including zodiacal light and galactic cirrus – is unavoidable at some level regardless of which direction the telescope points.

JWST’s Continuous Viewing Zone – the region of sky where it can observe year-round without the Sun or Earth crossing its field of view – covers about 5% of the sky at any given time. This constrains where deep early-universe fields can be placed. JWST’s deep field programs are concentrated at high ecliptic latitudes where zodiacal light is minimal. A more capable successor telescope would ideally have a larger continuous viewing zone, which argues for orbits further from the Earth-Moon system or for a telescope with a deployable sunshield that can reorient faster.

The L2 Lagrange point – a gravitational equilibrium location about 1.5 million kilometers from Earth in the direction away from the Sun – has become the standard destination for large infrared space telescopes. JWST, Euclid, and the planned Nancy Grace Roman Space Telescope all operate at or near L2. The location keeps the Sun, Earth, and Moon in roughly the same direction (behind the sunshield) and provides a thermally stable environment. The major limitation is serviceability: L2 is far beyond the reach of any crewed spacecraft currently in operation, meaning any hardware failure that occurs there is permanent.

Hubble’s longevity – it has operated since April 1990 and received five servicing missions through May 2009 – was enabled by its low Earth orbit at approximately 547 kilometers altitude. The trade-off between infrared performance and serviceability has been resolved in favor of performance for every telescope in the current generation. This is the right engineering decision for the science, but it carries a real risk that no one should minimize.

How Operational Telescopes Compare on the Key Specifications

The table below captures the key specifications of telescopes relevant to early-universe observation, from Hubble to proposed next-generation concepts.

How JWST Has Already Reshaped the Picture

JWST began full science operations in July 2022 and has already delivered results that forced revisions to theoretical models of early galaxy formation. Multiple confirmed galaxies at redshift above 10 have been identified, with the current spectroscopic record-holder at redshift 14.32 – a galaxy designated JADES-GS-z14-0 confirmed in 2024, corresponding to a lookback time of about 13.5 billion years and a universe age of just 290 million years at the time the light was emitted.

This galaxy contained more stars, more oxygen, and more spatial structure than theoretical models had predicted for such an early epoch. Its mass was estimated at several hundred million solar masses, and it was actively forming new stars at a rate of about 20 solar masses per year. Neither its size nor its metallicity were supposed to exist at that point in cosmic history according to pre-JWST simulations. Models are being refined rather than discarded, but the implication is that the first galaxies built up their stellar populations faster than simulations had suggested, and it raises direct questions about what happened in the universe’s first 200 million years before this galaxy’s light was emitted.

Whether JWST can reach further and confirm a galaxy at redshift 20 or beyond – corresponding to the universe’s first 180 million years – is an open question. Photometric candidates exist in the existing deep field data. Spectroscopic confirmation does not. Some of those candidates may turn out to be lower-redshift interlopers with unusual spectral energy distributions. The evidence suggests JWST is probably operating close to its practical sensitivity limit for sources at redshifts above 15 without gravitational lensing assistance. It may detect Population III star clusters if they happen to undergo a gravitationally lensed transient event during the telescope’s lifetime, but reliably surveying the universe at redshift 20 almost certainly requires a next-generation instrument.

What a Next-Generation Telescope Would Need

Pinning down a specification for a telescope that could observe the epoch of Population III star formation requires being specific about the science goal. Detecting photometric candidates at redshift 15 to 20 needs a telescope with sensitivity roughly 10 to 50 times better than JWST at 3 to 5 micrometers. Confirming those objects spectroscopically at useful signal-to-noise requires another factor of 5 to 10 on top of that.

These numbers translate to mirrors in the range of 15 to 20 meters, assuming comparable detector performance and operating temperature to JWST. A 15-meter mirror has 5.3 times the collecting area of JWST’s 6.5-meter mirror, meaning it collects 5.3 times as many photons per second from the same source. Combined with modest improvements in detector efficiency and noise floor, a 15-meter telescope could achieve in a few hours the sensitivity that JWST needs hundreds of hours to reach for the faintest sources.

The wavelength range would need to extend from about 0.5 to at least 5 micrometers as a baseline, with a strong science case for extending coverage to 10 micrometers or beyond for mid-infrared continuum emission and molecular hydrogen emission from cooling primordial gas clouds. Operating temperatures would mirror JWST’s architecture: passive cooling to 35 to 40 Kelvin for the near-infrared instrumentation, active mechanical cooling to below 7 Kelvin for the mid-infrared suite.

Angular resolution at 3 micrometers with a 15-meter mirror would reach about 0.05 arcseconds – enough to resolve the internal structure of galaxies at redshift 12 to 15 and study the spatial distribution of star-forming regions within them. Spectroscopic multiplexing capabilities of 1,000 or more simultaneous targets would be needed to survey the early universe efficiently, given the small number density of very high-redshift galaxies per square degree of sky.

Pointing stability requirements scale with the science goals. For gravitational lensing observations that might detect individual stars, stability of 0.001 to 0.005 arcseconds (1 to 5 milliarcseconds) over integration times of hours would be needed. That’s an order of magnitude more demanding than standard JWST operations for the most extreme science cases, and it pushes into territory where entirely new spacecraft attitude control systems would be required.

The 21-Centimeter Line and the Case for Radio Observation

While the discussion of space telescopes naturally centers on optical and infrared instruments, there’s a separate observational window on the earliest light that operates in the radio domain. Neutral hydrogen emits and absorbs radiation at a rest wavelength of 21.1 centimeters – the 21-centimeter line. During the Cosmic Dark Ages and into the epoch of reionization, the universe was filled with neutral hydrogen, and detecting this hydrogen in absorption or emission against the CMB background would provide a three-dimensional map of the universe’s neutral gas structure from the Dark Ages through reionization.

At the relevant redshifts (6 to 30), the 21-centimeter line is stretched to frequencies between 50 and 200 megahertz – frequencies overlapping commercial FM radio broadcasting on Earth. This creates a severe radio frequency interference problem for ground-based detectors. The Hydrogen Epoch of Reionization Array (HERA) in South Africa and the Low Frequency Array (LOFAR) in the Netherlands are both attempting to detect the epoch of reionization signal from the ground, shielded as much as possible from human-made radio interference.

A space-based 21-centimeter observatory – or better yet, one placed on the far side of the Moon, permanently shielded from Earth’s radio emissions by 3,400 kilometers of solid lunar rock – would represent an enormous scientific leap for this field. China’s Chang’e 4 mission carried a low-frequency radio antenna to the lunar far side in January 2019, demonstrating the basic concept. A full-scale radio array on the lunar far side would have access to the cleanest radio environment in the inner solar system and could potentially map the 21-centimeter signal from the Cosmic Dark Ages back to redshifts of 50 to 100 – corresponding to just 50 million years after the Big Bang.

This approach is complementary to infrared space telescopes rather than competitive with them. The 21-centimeter line traces hydrogen gas; infrared observations trace the stars. Together they could reconstruct the sequence of events from the first density fluctuations in the neutral hydrogen fog through the formation of the first stars and the burning away of the intergalactic neutral gas.

The Funding and Political Reality

Telescopes at the scale needed to observe Population III stars directly are not close to being approved, funded, or built. JWST cost approximately 10 billion US dollars over a 25-year development period, and that cost did not scale simply with mirror diameter – it reflected the extraordinary complexity of designing a cryogenic, deployable observatory that had to work perfectly on the first try, with no possibility of repair. A 15-meter segmented mirror infrared telescope at L2 would likely cost in the range of 20 to 50 billion dollars and require a development period of 20 to 30 years from program start, placing it in the late 2040s or 2050s at the earliest.

The NASA Decadal Survey for Astronomy and Astrophysics 2020, known as Astro2020, recommended the Habitable Worlds Observatory as the next flagship mission. That telescope focuses on exoplanet atmospheres in the optical and near-infrared range rather than early-universe cosmology. The next opportunity for early-universe science to emerge as a flagship priority is the Decadal Survey process that will produce its report around 2030 to 2032. Whether a very large infrared telescope becomes the community’s stated priority depends partly on what JWST finds in the next several years, particularly whether those high-redshift photometric candidates get confirmed or progressively disproven.

The European Space Agency has its own large-scale planning process and has historically partnered with NASA on major missions. JWST is a partnership between NASA, ESA, and the Canadian Space Agency. Any next-generation early-universe telescope would almost certainly require similar international cooperation to distribute the cost across multiple national space programs.

Summary

Seeing the universe’s earliest light is a problem of layered specifications, each one demanding engineering at the boundary of what’s physically possible. The mirror must be large enough – at least 6 meters, and ideally 12 to 15 meters for the most distant targets – to gather sufficient photons from objects billions of light-years away. The wavelength range must extend from near-infrared through mid-infrared so that light stretched by cosmic expansion still falls within the detector’s sensitive window. Operating temperatures must sit near absolute zero to prevent the telescope’s own thermal emission from masking the signals it’s meant to measure. The observatory must be in space, where the atmosphere’s infrared opacity and self-emission are simply absent. Detector arrays must be sensitive enough to register individual photons from cosmological distances, and spectrographs must be capable of multiplexed observations covering hundreds of targets simultaneously to make surveys of the early universe feasible within a finite mission lifetime.

JWST meets most of these specifications better than any telescope before it, and its early results have already reshaped understanding of when and how the first galaxies formed. But it probably can’t consistently confirm galaxies above redshift 16 without gravitational lensing, and the era of Population III stars remains just beyond its reach. The next step requires a mirror in the 12 to 15-meter class, deeper mid-infrared sensitivity, and the same cold, space-based platform that makes JWST work. Whether the funding and institutional will exist to build such a telescope is a different question from whether the physics allows it. The physics does allow it. Whether that telescope actually gets built in the 2040s or 2050s is uncertain in a way that isn’t diplomatic hedging – it reflects the real and documented difficulty of sustaining multi-decade, multi-billion-dollar scientific programs across changing administrations, competing budget priorities, and shifting political environments.

There is a compelling argument that a civilization capable of building JWST is also capable of building its successor, and that the scientific return – seeing the very moment the universe’s first stars switched on – is worth the cost. That argument deserves to be made plainly and persistently in the years ahead.

Appendix: Top 10 Questions Answered in This Article

What is the earliest light in the universe that a telescope could detect?

The earliest detectable light is the cosmic microwave background (CMB), released approximately 380,000 years after the Big Bang when the universe cooled enough for neutral hydrogen to form. For infrared space observatories, the earliest practical targets are the first stars and galaxies, which formed between 100 million and 500 million years after the Big Bang during an era called Cosmic Dawn.

Why does a space telescope need infrared sensitivity to see the first stars and galaxies?

The expansion of the universe has stretched the light from the first stars to wavelengths far beyond the visible spectrum, placing it in the near and mid-infrared range. A galaxy at redshift 10 has its Lyman-alpha spectral line shifted to 1.34 micrometers; at redshift 20, that same line falls at 2.55 micrometers. No optical telescope can detect this light because it arrives as infrared radiation that optical detectors simply cannot register.

How big does a space telescope mirror need to be to study the first galaxies?

A primary mirror of at least 6 meters in diameter is needed to collect enough photons from galaxies at redshift 10 and above for meaningful scientific analysis. The James Webb Space Telescope’s 6.5-meter mirror can confirm galaxies spectroscopically to at least redshift 14. Detecting the very first Population III stars would likely require mirrors in the 12 to 15-meter range.

Why can’t ground-based telescopes observe the universe’s first galaxies?

Earth’s atmosphere absorbs most infrared wavelengths, particularly above 2.4 micrometers, blocking the redshifted light from early-universe galaxies before it can reach ground-based detectors. The atmosphere also emits its own infrared radiation, creating a bright background that overwhelms the faint signals from objects at cosmological distances. Only telescopes above the atmosphere have access to the full infrared spectrum without this interference.

What temperature must a space telescope operate at to detect the earliest light?

Near-infrared instruments must operate below about 40 Kelvin to suppress thermal detector noise. Mid-infrared instruments like JWST’s MIRI require temperatures around 6 to 7 Kelvin to prevent the telescope’s own emission from masking faint signals. Far-infrared observatories targeting wavelengths beyond 50 micrometers would require mirror temperatures below 4.5 Kelvin maintained by active cryocooler systems throughout the mission’s lifetime.

What is the Epoch of Reionization and why is it scientifically important?

The Epoch of Reionization was the period from roughly 150 million to 1 billion years after the Big Bang when radiation from the first stars and galaxies ionized the neutral hydrogen filling the universe. It marks the transition from a dark, opaque universe to the transparent cosmos that exists today. Studying reionization reveals how the first stars formed, how quickly they enriched the surrounding gas with heavy elements, and what role they played in shaping the large-scale structure of the modern universe.

What redshift corresponds to the most distant galaxies confirmed spectroscopically?

As of early 2025, the most distant spectroscopically confirmed galaxy is JADES-GS-z14-0 at redshift 14.32, corresponding to a time when the universe was approximately 290 million years old. This galaxy was confirmed by JWST’s NIRSpec spectrograph and contained unexpectedly high stellar mass and oxygen abundance for its epoch, prompting revisions to galaxy formation models.

What is the role of gravitational lensing in observing the early universe?

Gravitational lensing occurs when a massive foreground galaxy cluster bends the light from a background source, magnifying its apparent brightness by factors of 10 to 100 or more. JWST routinely uses massive galaxy clusters like Abell 2744 as natural lenses to amplify the light from distant background galaxies, enabling observations of objects that would otherwise be undetectable. Lensing is currently considered the most plausible path to detecting individual Population III stars with existing or near-future technology.

What orbit is best for a space telescope designed to see the early universe?

The Sun-Earth L2 Lagrange point, located 1.5 million kilometers from Earth on the side opposite the Sun, is the standard choice for large infrared space telescopes. A sunshield positioned at L2 can simultaneously block thermal radiation from the Sun, Earth, and Moon, enabling passive cooling to around 40 to 50 Kelvin. JWST, Euclid, and the planned Nancy Grace Roman Space Telescope all operate at or near L2 for this reason.

How much more powerful than JWST would a telescope need to be to see Population III stars?

Detecting Population III stars or confirming galaxies at redshift 15 to 20 spectroscopically would require sensitivity roughly 50 to 100 times greater than JWST at wavelengths of 3 to 5 micrometers. Achieving this translates to a primary mirror of 15 meters or larger, improved detector noise floors, and cryogenic operating conditions at least as stringent as JWST’s. Such a telescope is estimated to cost between 20 and 50 billion US dollars and would require 20 to 30 years of development from initial program approval to launch.