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The main experiments study different evidence from the early universe: CMB projects map temperature and polarization patterns in ancient light; DESI measures galaxies and their redshifts to trace cosmic expansion; and the James Webb Space Telescope (JWST) observes distant galaxies in infrared light. They are complementary, not competing versions of one experiment: each measures a different observable and answers a different set of questions.
At a glance: what each experiment measures
| Experiment or project | What it observes | Main early-universe question |
|---|---|---|
| BICEP/Keck, Simons Observatory, CMB-S4 | Millimeter-wave cosmic microwave background (CMB) temperature and polarization | Could primordial gravitational waves have left a detectable polarization pattern? What can the CMB reveal about matter, relic particles and cosmic evolution? |
| DESI | Galaxy positions, redshifts and clustering | How has the universe expanded, and how has its structure grown? |
| JWST | Near- and mid-infrared light from distant galaxies and other objects | How did the first galaxies form and evolve? |
How CMB experiments look for evidence from the earliest light
The CMB is leftover radiation from the Big Bang. Ground-based millimeter-wave telescopes map its temperature and polarization across the sky. Polarization experiments focus in part on a subtle pattern called B-mode polarization. Primordial gravitational waves, if present, could have produced such a pattern during the universe’s early expansion. Finding that signal would support models of inflation, but the cited project sources describe a search, not an established detection.
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BICEP/Keck: a focused polarization search
BICEP/Keck experiments observe from the South Pole and concentrate on CMB polarization. NIST describes the BICEP Array as four upgraded telescopes using five observing wavelengths, following earlier BICEP and Keck instruments. The challenge is distinguishing a possible primordial signal from other sources of polarization, rather than treating every B-mode pattern as evidence for gravitational waves. NIST’s BICEP Array overview.
Simons Observatory: mapping the millimeter-wave sky
The Simons Observatory is located at Cerro Toco in Chile’s Atacama region. Its program uses millimeter-wave observations to study the CMB and other signals, with scientific questions spanning the beginning of the universe, neutrino masses, dark matter, cosmic acceleration, and galaxy and cluster evolution. The project also describes high-resolution maps of matter and gas.
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A 2019 technical paper gives a planned design with six frequency bands centered at 27, 39, 93, 145, 225 and 280 GHz, and an initial configuration of three small-aperture telescopes and one large-aperture telescope. Those are design and forecast details in the paper, not a report of achieved present-day performance. Simons Observatory science goals and forecasts (2019).
CMB-S4: a next-generation collaboration
CMB-S4 describes its primary mission as searching for primordial gravitational waves through their possible B-mode imprint in CMB polarization. Its other science themes include time-variable millimeter-wave astronomy, mapping matter through gravitational lensing and scattering, and investigating light relics and the dark universe.
Foregrounds are central to this work: emission from the Milky Way can resemble or obscure a primordial signal, and gravitational lensing can also generate B-modes. CMB-S4 identifies both as sources that complicate the search. Its project description presents a next-generation mission and its goals, not a completed detection. CMB-S4 science program.
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How DESI uses galaxies to trace expansion and structure
The Dark Energy Spectroscopic Instrument (DESI) is a galaxy survey. It measures galaxy positions and redshifts, then uses patterns in their distribution to study cosmic expansion, clustering and dark energy. Redshift indicates how much light from a galaxy has been stretched as the universe expanded; paired with distance information, it helps build a history of expansion.
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BAO: an early sound wave preserved in galaxy clustering
Baryon acoustic oscillations (BAO) began as sound waves moving through the hot plasma of the early universe. Once atoms formed and radiation stopped pushing the plasma, a faint preferred distance scale remained. Astronomers can detect that scale statistically in the later distribution of galaxies. DESI uses BAO to infer distance as a function of redshift; it does not photograph the Big Bang itself.
DESI also uses redshift-space distortions—the effects of galaxy motions on their observed distribution—to study how structure grows and to test gravity. The collaboration describes its cosmology mission as investigating how dark energy’s density evolves and affects matter clustering. DESI’s explanation of its cosmology mission.
What DESI says it aims to measure
DESI’s official science page says the project aims to measure positions and receding velocities for about 40 million galaxies and constrain expansion over the past 11 billion years. These are the project’s stated goals on the page accessed in 2026; the page does not display a publication year. DESI science overview.
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The James Webb Space Telescope collects near- and mid-infrared images and spectra. Because light from distant objects is stretched toward longer wavelengths as the universe expands, infrared observations help researchers study very distant galaxies and their properties. JWST’s approach is to examine the light from individual objects, rather than map the CMB’s broad patterns or statistically chart millions of galaxy positions as DESI does.
NASA lists searching for the first galaxies and studying galaxy evolution among Webb’s goals. The universe began 13.8 billion years ago, a contextual figure NASA gives in its overview; it is not a result measured by Webb alone. Webb data are archived at the Mikulski Archive for Space Telescopes and are publicly and freely accessible online after any applicable proprietary period. NASA’s Webb early-universe overview. NASA’s Webb mission page.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to compare the experiments fairly
- Observable: CMB projects measure temperature and polarization; DESI measures galaxy positions, redshifts and clustering; JWST analyzes infrared light from distant objects.
- Question: CMB polarization can test for a possible primordial gravitational-wave signal. DESI traces expansion history and structure growth using galaxy patterns. JWST investigates how early galaxies formed and evolved.
- Platform and wavelength: BICEP/Keck and Simons Observatory are ground-based millimeter-wave projects; DESI is a spectroscopic survey; JWST is a space telescope observing in near- and mid-infrared bands.
- Systematic uncertainties: CMB analyses must separate galactic foregrounds and lensing from the signal of interest. Galaxy-survey conclusions depend on sample selection and modeling of clustering and galaxy motions. Infrared galaxy studies depend on interpreting faint, redshifted light and spectra.
- Status and evidence: Separate stated goals and planned designs from achieved measurements. For example, the Simons Observatory frequency bands above come from a design-and-forecast paper, while CMB-S4 presents its work as a next-generation mission.
Sky coverage, angular resolution, frequency bands and redshift range describe different aspects of an experiment. A larger number or wider range in one category does not, by itself, make one project a substitute for another.
Why the approaches complement one another
The CMB records conditions in the early universe through patterns in background radiation. BAO preserves a statistical trace of early sound waves in the later galaxy distribution. JWST examines the galaxies that formed and evolved as the universe developed. Taken together, these methods connect early physical conditions, the growth of cosmic structure and the history of galaxies—while answering distinct questions with distinct data.
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