Somewhere in the millimeter-wave static that washes over southern Arizona, there is a faint signal from galaxies that lit up when the universe was less than a billion years old. A Caltech-led team believes their new instrument can find it. Called TIME, for Tomographic Ionized-carbon Mapping Experiment, the spectrometer completed its first on-sky observations during a 2021 to 2022 commissioning run at the Arizona Radio Observatory’s 12-meter telescope on Kitt Peak. The results, published on the arXiv and now informing the next phase of observations as of June 2026, show that TIME can do exactly what its designers need: pull a specific spectral line out of a crowded sky and separate overlapping signals that would otherwise blur together.
That ability is the foundation for something far more ambitious. TIME was built to detect the collective glow of ionized carbon from the earliest star-forming galaxies, objects so distant and so numerous that no existing telescope can pick them out one by one.
Catching light from the first billion years
Most galaxy surveys work by pointing a telescope at a patch of sky and cataloging individual sources. TIME takes a fundamentally different approach called intensity mapping. Rather than resolving single galaxies, it measures the total light from every galaxy in a given volume of space, including the countless faint ones below the detection threshold of even the most powerful observatories.
The target is a specific spectral fingerprint: the 158-micron emission line of singly ionized carbon, written as [CII]. This line is one of the brightest signals that star-forming gas produces, and it acts as a tracer of how vigorously galaxies were converting raw material into stars. Because the universe is expanding, light from distant galaxies gets stretched to longer wavelengths. A [CII] line emitted during the epoch of reionization, roughly 600 million to one billion years after the Big Bang, arrives at Earth shifted into the millimeter-wave band that TIME is tuned to observe.
By scanning across frequencies, TIME can effectively look at different slices of cosmic history. According to the Caltech project page, the instrument is designed to map fluctuations in the redshifted [CII] line across redshifts of approximately 5 to 9, corresponding to a period when ultraviolet radiation from young galaxies was stripping electrons from hydrogen atoms throughout the intergalactic medium. That process, known as reionization, transformed the universe from opaque to transparent, and understanding how much star formation drove it remains one of the central open questions in cosmology.
As a bonus, carbon monoxide (CO) rotational lines from galaxies at lower redshifts, roughly 0.5 to 2, also fall within TIME’s frequency band. That lets the instrument probe molecular gas during the era when galaxies were assembling the bulk of their stellar mass, giving astronomers two epochs of cosmic history for the price of one survey.
What the commissioning run proved
Before chasing a signal no instrument has ever detected, the team needed to show that TIME’s hardware works under real observing conditions. The commissioning campaign did that in a demanding environment: the Sagittarius A molecular cloud complex at the center of the Milky Way, a region packed with overlapping emission from dust, hot gas, and molecules.
After calibrating on Jupiter, TIME turned to the Galactic Center and produced frequency-resolved maps that recovered the 12CO(2-1) spectral line. More importantly, the instrument separated thermal dust emission from free-free radiation, the glow produced when electrons scatter off ions, using differences in how each component’s brightness changes with frequency. That spectral-index analysis is precisely the kind of signal separation TIME will need for its cosmological targets, where foregrounds from the Milky Way and from lower-redshift galaxies will sit in front of the high-redshift [CII] background.
Because the Sgr A region has been mapped extensively by other telescopes at many wavelengths, TIME’s results can be cross-checked against independent data. The agreement gives the commissioning paper real weight: it is a proof of concept grounded in observation, not a theoretical forecast.
The hard part is still ahead
Recovering a bright molecular line from the Galactic Center is a different challenge from extracting a faint statistical signal from the edge of the observable universe. Several hurdles stand between the commissioning success and the science TIME was built to deliver.
The most fundamental is sheer sensitivity. The [CII] intensity-mapping signal is expected to be extraordinarily faint, requiring long integrations on blank-sky fields far from the bright Galactic plane. TIME’s array of 1,920 transition-edge-sensor detectors spread across 60 spectral channels was engineered for that task, but on-sky calibration stability beyond the Jupiter observations has not been publicly characterized. Gain drifts, atmospheric fluctuations, and instrumental systematics that barely matter when mapping a bright source like Sgr A become critical over the hundreds of hours of integration that a deep extragalactic survey demands.
Foreground removal will also grow more complex. In the Galactic Center test, TIME exploited clean spectral-index differences between dust and free-free emission. For the cosmological survey, the foreground cocktail includes Galactic dust, synchrotron radiation, and CO line emission from intermediate-redshift galaxies that can mimic the [CII] signal. Cosmologists have developed techniques such as multi-line masking and cross-correlation with galaxy catalogs from other surveys to isolate the target signal, but TIME has not yet demonstrated those methods on real data.
There is also the question of whether the signal itself matches predictions. Semi-analytic models calibrated on lower-redshift CO observations suggest a certain amplitude for the [CII] power spectrum at redshifts of 6 to 7, but those models carry large uncertainties because they extrapolate gas properties across billions of years. A stronger-than-expected signal would imply a larger molecular gas reservoir during reionization and faster star formation than current models assume. A weaker signal could point to more modest star formation or to a higher fraction of ultraviolet photons escaping young galaxies into the intergalactic medium, a parameter that directly affects reionization models.
Published technical documents also carry some ambiguity. One set of SPIE proceedings describes a spectral range of 183 to 326 GHz with a resolving power of roughly 100, per a CaltechAUTHORS record, while a later paper lists 186 to 324 GHz and a resolving power near 200. The differences likely reflect design evolution between early prototypes and the as-built instrument, but no single post-commissioning document has reconciled the figures. Until one does, forecasts that depend sensitively on bandwidth or spectral resolution should be treated as approximate.
Where TIME fits in a crowded field
TIME is not the only experiment chasing line-intensity mapping signals. The CONCERTO instrument at the APEX telescope in Chile targets similar [CII] redshifts using a different detector technology, and the CCAT collaboration is building the 6-meter Fred Young Submillimeter Telescope on Cerro Chajnantor to conduct wide-field intensity mapping surveys at overlapping wavelengths. At lower redshifts, the COMAP experiment in California’s Owens Valley is already producing early CO intensity-mapping results.
What distinguishes TIME is its combination of broad spectral coverage and the specific redshift window it targets. If multiple experiments detect the same [CII] power spectrum independently, the result becomes far more convincing than any single measurement. Cross-correlation between TIME’s maps and galaxy catalogs from the James Webb Space Telescope or the Vera C. Rubin Observatory could further strengthen detections by confirming that the signal traces known large-scale structure.
The commissioning results position TIME as a credible participant in that effort. The instrument has shown it can do spectroscopy in a real observing environment, separate overlapping emission components, and produce maps consistent with independent data. Those are necessary conditions for the extragalactic program, even if they are not sufficient.
What a detection would actually mean
If TIME eventually measures the [CII] power spectrum at redshifts of 6 to 9, the implications reach well beyond a single spectral line. The amplitude and shape of that signal encode information about how many galaxies existed during reionization, how much gas they contained, and how efficiently they formed stars. Combined with CO measurements at intermediate redshifts from the same instrument, the data would trace the rise and evolution of cosmic star formation across more than 10 billion years.
That kind of census has never been done with intensity mapping at these distances. Traditional surveys, no matter how deep, are biased toward the brightest galaxies. Intensity mapping captures everything, offering a more complete accounting of the universe’s total light output during its formative era. For cosmologists trying to understand how the first galaxies ended the cosmic dark ages, that completeness matters enormously.
For now, TIME has cleared its first major test. The commissioning maps from the Galactic Center confirm that the instrument’s core capability, frequency-resolved imaging that can untangle overlapping astrophysical signals, works as designed. The next step is pointing it at a patch of apparently empty sky and listening for the faint, redshifted whisper of carbon ions in galaxies that burned bright when the cosmos was still young.
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*This article was researched with the help of AI, with human editors creating the final content.
