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There is no single distance for all fast radio bursts (FRBs). Astronomers determine an individual FRB’s distance most reliably by localizing the radio pulse, matching it to a host galaxy, and measuring that galaxy’s cosmological redshift. The redshift is then interpreted with a specified cosmological model. If no host is securely identified, the burst’s dispersion measure (DM) can provide a rough estimate, but it is not a precise distance meter because electrons in the Milky Way, its halo, intergalactic space, and the host environment all contribute to the signal.
NASA’s 2026 report on Webb observations of FRB 20240304B gives a current example: its host galaxy has a redshift of z = 2.148, and NASA says the burst occurred about 3 billion years after the Big Bang. The report described it as the most distant known FRB at that time. That redshift should not be casually converted into one number of light-years without stating whether the result is a lookback, comoving, or luminosity distance and which cosmological parameters were used.
What determines how far away an FRB is?
Fast radio bursts are brief, extremely bright flashes of radio energy that originate outside the Milky Way. Their duration can be milliseconds to a few seconds, but the pulse carries several clues about its journey. The most dependable individual distance measurement follows a chain of evidence:
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- Associate the position with a host galaxy. The candidate galaxy must fall within the localization region and be consistent with follow-up observations. A redshift from an unrelated galaxy does not establish the FRB’s distance.
- Measure the host’s spectrum. Spectral lines shifted toward longer wavelengths provide the galaxy’s cosmological redshift.
- Apply a cosmological model. The measured redshift is converted into a distance definition—such as comoving, luminosity, or angular-diameter distance—or into a lookback time.
The first two steps are essential. Older single-dish detections could have positional uncertainties of a few arcminutes, which may contain many galaxies. Robust host association generally requires localization on scales of a few arcseconds or better, as discussed in the Astronomy & Geophysics review of FRB hosts.
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Why a host association is the key link
Redshift belongs to the galaxy whose spectrum was measured, not automatically to the radio pulse. If the burst position is imprecise, astronomers can mistake a foreground or background galaxy for the host. Interferometric follow-up solved this problem for the repeating source FRB 121102, which was localized and associated with a star-forming dwarf galaxy at z = 0.193. That example demonstrates the method; it is not the current record for distance.
What does redshift tell us?
Cosmological redshift describes how much the expansion of the universe has stretched a source’s light while it traveled to Earth. It is dimensionless: z = 0 would mean no cosmological wavelength stretching, while larger values generally indicate that we are seeing farther back in cosmic history.
For an FRB, astronomers normally measure the redshift from the host galaxy’s emission or absorption lines. The value is not itself a distance in miles, kilometers, or light-years. To obtain a distance, researchers must specify a cosmological model, including parameters such as the expansion rate and matter and dark-energy densities. Different distance conventions answer different questions:
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- Lookback time: how long ago the emitted radiation left the source, expressed as a time rather than a present-day separation.
- Comoving distance: a coordinate that factors out the expansion of space and is useful for comparing locations on large-scale maps.
- Light-travel distance: a simplified travel-time description that should not be treated as the source’s present physical separation.
- Luminosity distance: the distance inferred from how bright an object appears after accounting for cosmic expansion; it is used in many flux and energy calculations.
- Angular-diameter distance: the distance relevant to an object’s apparent angular size.
At modest redshift these quantities can be numerically similar, but at high redshift they diverge substantially. Therefore, writing that an FRB at a particular redshift is “X billion light-years away” without naming the convention can give a misleading impression of precision.
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The current high-redshift example: FRB 20240304B
NASA Science’s 2026 report on James Webb Space Telescope observations states: “They confirmed it has a cosmological redshift of 2.148 and that the FRB occurred just 3 billion years after the big bang.” The host is described as a small, actively star-forming dwarf galaxy. The quoted three-billion-year figure is a cosmic-time description: the burst’s radio emission was produced when the universe was about three billion years old. It is not, by itself, a statement that the source is three billion light-years from Earth today.
To publish a distance in gigaparsecs for this burst, an author would need to state the adopted cosmological parameters and whether the number represented comoving, luminosity, or another distance. The NASA report supplies the redshift and cosmic-time context, so those are the quantities that can be reported directly without adding an unstated conversion.
Can dispersion measure estimate an FRB’s distance?
Yes, but only approximately. As a radio pulse travels through ionized gas, its lower-frequency components arrive later than its higher-frequency components. The amount of delay reveals the dispersion measure, or DM: the integrated column of free electrons along the line of sight, conventionally reported in pc cm−3.
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| Contribution | What it represents | Why it creates uncertainty |
|---|---|---|
| Milky Way disk | Ionized gas inside our Galaxy | Models of Galactic electron density are imperfect, especially in complex directions. |
| Milky Way halo | Diffuse ionized gas surrounding the Galaxy | The halo’s density and extent are not known with one exact value. |
| Intergalactic medium (IGM) | Electrons between galaxies | The cosmic baryons are distributed unevenly, producing line-of-sight scatter. |
| Host galaxy and local environment | Gas in the FRB’s galaxy, nearby nebulae, or a source’s immediate surroundings | This component can vary widely and is often the least constrained before the host is studied. |
A common first step is to model and subtract the Milky Way contribution. The remaining “excess” DM is extragalactic, but it still mixes the halo, IGM, and host terms. Because those terms have not been fully disentangled, a large excess DM can indicate a distant source without uniquely determining its distance.
The low-redshift DM–redshift approximation
The Astronomy and Astrophysics Review gives the approximate relation DMIGM ≈ 1000 z pc cm−3 at low redshift, under assumptions about the cosmic baryon distribution, ionization state, and cosmology. Rearranging it can provide an order-of-magnitude redshift estimate when no host spectrum is available.
This relation applies to the intergalactic component, not automatically to the total observed DM. It also becomes less reliable when the host contributes a substantial or poorly known column, when the line of sight passes through unusual structures, or when the redshift is outside the regime for which the approximation was intended. DM-based luminosity-distance calculations should therefore be labeled estimates, not measurements equivalent to a host-galaxy spectrum.
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Host redshift versus DM: which method is better?
| Criterion | Host-galaxy redshift | DM-based estimate |
|---|---|---|
| What is measured | Spectral-line wavelength shift from an identified host | Integrated free-electron column inferred from radio-frequency delays |
| Best use | Distance or cosmic-time inference for an individual, securely localized FRB | Approximate distance ranking or statistical inference when a host is unavailable |
| Main requirement | Arcsecond-scale localization, credible host association, and optical or infrared spectroscopy | A measured DM plus models for Galactic, halo, IGM, and host contributions |
| Principal uncertainty | Incorrect or ambiguous host identification and the chosen cosmological model | Unknown partition of the total DM and variation in electron density along the sightline |
| Typical result | An observationally anchored redshift that can be converted using stated cosmology | A broad, model-dependent redshift or distance range |
Neither method is “just a calculation.” The host-redshift route requires substantial observing time and accurate localization, while the DM route requires astrophysical modeling. When both are available, the measured redshift calibrates how much of the DM came from the IGM and host, improving models for future bursts.
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How astronomers determine an FRB’s distance step by step
1. Detect and characterize the radio pulse
Radio facilities record the burst’s arrival time across many frequency channels. The frequency-dependent delay yields the total DM, while the burst’s width, polarization, and possible repetition guide follow-up observations. At this stage, the DM can suggest that the event is extragalactic, but it does not identify a unique distance.
2. Obtain interferometric localization
Arrays of radio antennas compare the signal’s arrival at separated dishes. This greatly improves angular precision over a single-dish detection. A position accurate to a few arcseconds can reduce the list of plausible galaxies; sub-arcsecond positions are even more valuable for crowded fields or faint hosts.
3. Search for a persistent or transient counterpart
Optical, infrared, and radio imaging are used to locate galaxies in the error region. For a repeating FRB, multiple bursts can strengthen the position and permit deeper searches. A candidate host must be evaluated against the localization uncertainty rather than selected solely because it is the nearest visible galaxy.
4. Measure the candidate host’s spectrum
Spectroscopy identifies familiar atomic features and compares their observed wavelengths with laboratory values. The fractional shift gives the galaxy’s redshift. Imaging alone generally cannot provide the same secure measurement, especially for a faint dwarf galaxy.
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5. Convert redshift only after defining the output
Researchers then choose a cosmological model and report the relevant quantity: lookback time, comoving distance, luminosity distance, or another convention. A careful result states the model, parameter values, and uncertainty, rather than presenting an unqualified “distance.”
6. Use DM as a consistency check and a science measurement
The observed DM is compared with the expected Milky Way, halo, IGM, and host contributions. A mismatch can reveal an unusually dense source environment, an under-modeled Galactic sightline, or assumptions that need revision. DM is therefore valuable even when the host redshift is already known.
Why FRB distances matter beyond a single burst
Localized FRBs turn otherwise fleeting radio flashes into probes of matter between galaxies. Once a host redshift is known, the total DM can be compared with the distance expected from cosmology. Across a sample, this tests how baryons are distributed through the IGM, including diffuse material that is difficult to detect by starlight alone.
Macquart and collaborators used localized FRBs to estimate the cosmic baryon density, reporting Ωb = 0.051 with −0.025/+0.021 uncertainties in units of h70−1 at 95% confidence. That result illustrates a population-level application: the value comes from combining many localized bursts and their dispersion measures, not from treating one FRB’s DM as an exact distance gauge.
Common mistakes when reporting an FRB distance
- Calling redshift a distance. Redshift is a wavelength-stretching measurement. A distance requires a cosmological model and a named distance convention.
- Using total DM as though it were DM from the IGM alone. Galactic, halo, and host electrons must be modeled separately.
- Assigning a host without adequate localization. A galaxy somewhere in a large error region may be unrelated to the burst.
- Converting a high redshift into light-years without qualifications. At values such as z = 2.148, lookback, comoving, and luminosity distances are not interchangeable.
- Treating a historical example as a current record. FRB 121102 at z = 0.193 explains host association, whereas NASA’s 2026 report identifies FRB 20240304B as the most distant known FRB at that time.
- Assuming every FRB has a measured host redshift. Many events remain too poorly localized or too faint for a secure host identification, so their distances remain model-dependent.
So, how far away is a fast radio burst?
An FRB can be anywhere from relatively nearby on cosmological scales to an epoch when the universe was only a few billion years old. The scientifically defensible answer for an individual event is the distance or cosmic time derived from a securely associated host’s redshift, with the cosmological model and distance definition stated. When that evidence is unavailable, DM can indicate that the burst is extragalactic and provide a rough estimate, but its uncertainty reflects the unknown electron content of several different regions along the path.
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