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Fast radio bursts (FRBs) let astronomers weigh otherwise hard-to-see cosmic gas by measuring how much ionized material a radio pulse crosses. The measurement is a dispersion measure (DM): an integrated count of free electrons along one line of sight. It is not a picture of the gas or a three-dimensional map. By combining many bursts with known locations, host galaxies and redshifts—and by estimating what the Milky Way, hosts and intervening structures contribute—researchers can infer how ordinary matter is distributed between galaxies.
What an FRB measures
An FRB is a brief burst of radio emission arriving with a range of frequencies. As the signal travels through ionized matter, its lower-frequency components are delayed more than its higher-frequency components. Astronomers measure this frequency-dependent arrival-time difference and summarize it as a dispersion measure.
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In physical terms, DM tracks the free-electron column along the path from the source to Earth. The longer or more electron-rich the traversed material, the greater its contribution to the column. The 2020 study led by James P. Macquart described the method as determining “the electron column density along each line of sight and account[ing] for every ionized baryon.” That is the technique’s power: it is sensitive to ionized ordinary matter even when the gas is too diffuse or faint to observe directly. The practical inference still requires separating the source environment and foreground contributions from the cosmic signal (Macquart et al., Nature, 2020).
A DM is an integrated measurement, much like a total counted along a route. It does not say exactly where each electron was encountered. Two very different paths through the universe can produce similar totals, and one burst cannot reveal the full structure of the material along its route. FRBs become a way to study cosmic gas when astronomers compare many such columns and use independent information about the bursts and intervening galaxies.
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Why ionized gas matters to the missing-baryon problem
Baryons are ordinary matter: the matter that makes stars, planets, people and gas. A cosmic inventory based only on bright stars and readily observed gas can miss a substantial amount because ordinary matter is not all packed into luminous objects. Some is spread thinly through intergalactic space or resides in gas around galaxies and larger structures. Ionized diffuse gas can be difficult to detect with ordinary imaging, so it can be undercounted when astronomers tally matter from light alone.
“Missing baryons” refers to this accounting problem: matter expected in the universe’s ordinary-matter budget that is not fully accounted for in the visible components being counted. It does not mean that the matter has vanished. FRB dispersion offers an independent way to detect its free electrons. Because a burst’s DM samples the entire route to Earth, a population of well-characterized bursts can test whether the matter budget includes the diffuse ionized component.
The method is complementary to observing gas directly. A telescope image can show where detectable emission lies; a DM supplies a column total even when the gas itself is not visually distinct. The two kinds of evidence answer different questions, and an FRB column alone cannot assign every electron to a particular cloud, galaxy or intergalactic region.
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How astronomers turn a burst into a cosmic gas measurement
The observed DM is not automatically the amount of intergalactic gas. It combines electrons encountered in several environments: our own Milky Way, material near the FRB, the FRB’s host galaxy, and structures between the host and the Milky Way. The host and its local environment can contribute, as can intervening galaxy halos, groups and other foreground structures. A high DM therefore signals a large total electron column, not a unique location or cause.
- Measure the burst’s dispersion. Observers compare arrival times across radio frequencies and determine the burst’s DM. The result is a sightline-integrated quantity, not a spatially resolved observation.
- Localize the source and identify its host where possible. A position accurate enough to associate a burst with a galaxy helps establish the source’s context and, when the host redshift is available, how far the burst traveled. The landmark 2020 census used localized FRBs for its cosmic estimate.
- Estimate foreground and source contributions. Researchers model the Milky Way contribution and account for the host, local environment and any intervening halos or groups. These components must be considered rather than assigning the entire extragalactic DM to the intergalactic medium.
- Compare multiple sightlines. A sample lets researchers look for the relationship between the bursts’ DMs and their locations or redshifts, and test whether the aggregate columns fit a proposed cosmic gas distribution. Individual paths differ, so the inference is statistical.
- Interpret the result for a defined component or inventory. A cosmic baryon census, an analysis of foreground galaxy halos and a study of gas in groups do not measure the same thing. Their conclusions depend on which component is targeted and how the other contributions are modeled.
That sequence explains why localization and sample construction matter as much as the initial timing measurement. Without a useful source position or host information, it is harder to put a burst’s integrated column into a cosmic context. Without a model for foreground and host contributions, a measurement can be attributed to the wrong component.
What the 2020 FRB census established
Macquart and colleagues used localized FRBs to estimate the universe’s baryon density. They reported Ωb = 0.051−0.025+0.021 h70−1 at 95% confidence. In that sample and analysis, the estimate was consistent with values inferred from the cosmic microwave background and Big Bang nucleosynthesis (Nature, 2020).
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This was important because it demonstrated that localized bursts could be used for a cosmic baryon census, not merely for describing radio transients. The result supported the conclusion that FRB dispersion can account for ionized baryons that are difficult to see directly. It was an estimate with a broad stated uncertainty from a particular sample and model, not an exact constant or a claim that every baryon had been individually located.
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Why nearby galaxy halos can change a sightline
Some FRBs pass through gas surrounding foreground galaxies before reaching Earth. Such a halo adds electrons to the measured column, so a burst whose sightline intersects a nearby galaxy may have a different DM from a burst that does not. In 2022, Connor and Ravi analyzed 474 events from CHIME/FRB Catalog 1, selecting bursts likely to intersect nearby foreground galaxy halos. They found an excess DM greater than 90 pc cm−3 for the galaxy-intersecting sightlines relative to non-intersecting ones, at greater than 95% confidence (Connor and Ravi, Nature Astronomy, 2022).
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The excess is evidence that foreground environments matter, but it does not isolate a single galaxy’s halo as the only cause. The authors cautioned that “The excess is larger than expected for the diffuse gas surrounding isolated galaxies, but may be explained by additional contributions from gas surrounding galaxy groups, including from the Local Group.” A line of sight categorized as crossing a galaxy halo can also pass through a broader group environment. That complicates any attempt to assign the full excess to one isolated halo.
This result addresses a narrower question than the 2020 census. Rather than estimate the total cosmic baryon density from localized bursts, it tests whether galaxy-associated foregrounds produce additional dispersion. It shows why halo and group contributions belong in the accounting, while also illustrating how a DM alone cannot identify precisely which structure supplied each electron.
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Later work broadens the questions FRB samples can address. The available 2025 Nature Astronomy abstract reports a late-Universe baryon-abundance measurement of Ωb h70 = 0.051 ± 0.006 and describes partitioning baryons among the intergalactic medium, clusters and galaxies. That is a different reported parameterization and scope from the 2020 result; the abstract-level information does not establish all methodological details needed for a like-for-like comparison (Nature Astronomy, 2025).
A 2026 Nature Astronomy preview reports the use of 114 FRBs to study baryon-density fluctuations and halo gas fractions. This points toward a shift from estimating an overall abundance to probing how baryons are distributed and how gas relates to structure formation. The preview supports describing the study’s sample and broad focus, but does not provide enough detail here to state its errors, model assumptions or specific conclusions (Nature Astronomy, 2026).
| Study | Question and sample | Reported result | What it does not mean |
|---|---|---|---|
| Macquart et al., 2020 | Cosmic baryon census using localized FRBs | Ωb = 0.051−0.025+0.021 h70−1, 95% confidence | Not a direct 3D map or an exact universal value independent of sample and model. |
| Connor and Ravi, 2022 | Foreground galaxy halos; 474 CHIME/FRB Catalog 1 events likely to intersect nearby halos | Excess DM greater than 90 pc cm−3 at greater than 95% confidence | Not a clean measurement of isolated galaxy halos alone; group gas may contribute. |
| Nature Astronomy, 2025 | Late-Universe baryon abundance and partition among IGM, clusters and galaxies | Ωb h70 = 0.051 ± 0.006, as reported in the indexed abstract | Not directly interchangeable with the 2020 parameterization; detailed methods are not established by the abstract information available. |
| Nature Astronomy, 2026 | Baryon fluctuations and halo gas fractions, using 114 FRBs | The preview identifies the sample size and broad research focus. | A precise result, uncertainty or methodological comparison is not established by the preview information. |
What FRB dispersion can—and cannot—tell us
What it can reveal
- It can measure the integrated free-electron column through a path that includes faint or diffuse ionized matter.
- Across a localized, characterized sample, it can help estimate the cosmic baryon abundance and test models of where ordinary matter resides.
- Comparisons between sightlines can expose additional contributions associated with foreground galaxies and broader structures such as groups.
- With larger samples and suitable source information, the method can be extended from a total inventory toward questions about fluctuations and the division of gas among environments.
What it cannot reveal by itself
- It cannot photograph the intergalactic medium or show the exact position of every electron on a sightline.
- It cannot assign all extragalactic DM to the IGM, because the host, local environment, foreground halos and groups can also contribute.
- One burst cannot provide a complete baryon census. Population-level conclusions depend on the sample and on models for different components and sightline variation.
- A reported baryon-density estimate should not be treated as a direct count of every missing baryon or as a result that is automatically comparable with other studies using different definitions or assumptions.
How to understand the answer to “Where are the missing baryons?”
FRBs help answer that question by showing that the matter inventory cannot be limited to stars and conspicuous gas. Their dispersive delays reveal free electrons accumulated across otherwise hard-to-observe paths. When enough bursts have useful localization and redshift information, the measured columns can be compared with models of the Milky Way, hosts, halos, groups and the wider intergalactic medium to infer how much ionized matter is present and how it is distributed.
The most accurate mental model is a network of cosmic sightlines, not a camera. Every burst adds a line-integrated constraint; every new sample adds statistical leverage, while the task of separating overlapping gas components remains central. FRBs therefore turn the “missing” baryons into a measurable accounting problem: not by showing all the gas at once, but by measuring electrons along many paths and carefully determining which parts of the universe those electrons can belong to.
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