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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteFast radio bursts (FRBs) are extragalactic flashes, but there is no single distance for all of them. Astronomers determine an individual burst’s distance most securely by localizing it to a few arcseconds, identifying its host galaxy, and measuring that galaxy’s cosmological redshift. If no host is securely identified, the burst’s dispersion measure (DM)—the amount of free-electron material along the line of sight—can provide a rough estimate, but not a precise mileage figure. The Milky Way, its halo, intergalactic space and the host system all contribute to the measured DM.
What is a fast radio burst?
An FRB is a very brief, intense pulse of radio emission. The bursts last only milliseconds, yet they can be detected across cosmological distances. Their signals are dispersed: lower radio frequencies arrive later than higher frequencies because the pulse has crossed free electrons. That delay is scientifically valuable, but it does not by itself reveal a unique distance.
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Most established FRBs are extragalactic radio transients. “Extragalactic” means the source lies beyond the Milky Way; it does not specify whether the source is millions or billions of light-years away. To answer the distance question for one burst, astronomers need either a reliable host-galaxy measurement or a carefully modeled estimate from the pulse’s dispersion.
The most reliable route: host-galaxy redshift
1. Localize the radio burst
Radio telescopes first determine where the burst came from on the sky. Older single-dish detections could have positional uncertainties of a few arcminutes. That area may contain many unrelated galaxies, so it is usually too large for a secure host assignment. Interferometric observations can reduce the position to a few arcseconds or better, allowing astronomers to compare the location with optical and infrared galaxy images.
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2. Associate the burst with a galaxy
A small error region is necessary but not sufficient. Researchers must decide which galaxy, if any, is physically associated with the FRB rather than merely lying in the same line of sight. Repeating sources can be especially useful because repeated bursts allow precise interferometric follow-up. The repeating FRB 121102, for example, was localized and associated with a star-forming dwarf galaxy.
3. Measure the galaxy’s spectrum
Once a host is identified, astronomers observe its light and measure spectral lines. The fractional shift of those lines gives the galaxy’s redshift, written as z. A secure host association makes this an observationally anchored redshift for the FRB itself.
4. Use a cosmological model to infer distance
Redshift is not a distance unit. It describes how much the universe’s expansion has stretched the host galaxy’s light. Astronomers convert z into a distance only after choosing cosmological parameters and specifying which distance is meant. Luminosity distance, comoving distance, proper distance and lookback time are different quantities and can have different numerical values for the same redshift. Therefore, a redshift should not be turned into a single number of light-years without stating the cosmological model and distance definition.
What does redshift tell us?
For a cosmological source, a positive redshift means that spectral features are observed at longer wavelengths than they had when emitted. The effect primarily reflects the expansion of space during the light’s journey. A larger redshift generally indicates that we are seeing a more distant, earlier phase of cosmic history, but the exact distance-redshift relationship is model-dependent.
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Redshift also gives a time perspective. If an FRB host is measured at a high z, the burst is observed as it existed billions of years ago. That is why host spectroscopy does more than answer “how far away?” It places the event in the timeline of galaxy formation and star formation.
For nearby galaxies, simplified relationships between redshift and distance can be useful. At the redshifts of many FRBs, however, cosmic expansion has changed over time, so a simple linear conversion can be misleading. A responsible distance statement names the redshift, the adopted cosmology and whether the quoted result is a lookback time or one of the several cosmological distance measures.
FRB 20240304B: the current high-redshift example
NASA Science reported in 2026 that James Webb Space Telescope observations identified the host galaxy of FRB 20240304B and measured a cosmological redshift of z = 2.148. NASA’s report 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.
NASA called FRB 20240304B the most distant known FRB in that report. The “known” qualification matters: it describes the state of observations at the time of that report, not a permanent record. The reported result gives a redshift and a cosmic-time context; it does not, by itself, state a particular number of gigaparsecs or light-years. Supplying such a number would require specifying the cosmological parameters and the distance convention used.
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Can dispersion measure estimate an FRB’s distance?
Yes, but only approximately. Dispersion measure is the integrated column of free electrons between the source and Earth. It is usually expressed in pc cm−3. The frequency-dependent arrival delay lets radio astronomers calculate the total electron column through which the pulse traveled.
To estimate an extragalactic contribution, astronomers model and subtract the Milky Way’s interstellar electrons. The remainder is not automatically the intergalactic distance signal. It can include several components:
- Milky Way interstellar gas: electrons in the Galactic disk and other Galactic structures along the sightline.
- Milky Way halo: hot, diffuse gas surrounding the Galaxy, whose electron content is not known with perfect precision.
- Intergalactic medium (IGM): ionized gas between galaxies.
- Host galaxy and local environment: gas in the host, its halo, a star-forming region, or material near the FRB source.
The source and IGM contributions have not been fully disentangled for every burst. Electron densities vary from one sightline to another, and the host environment can add a large, source-specific amount. A burst with a high DM is therefore not guaranteed to be farther away than every burst with a lower DM.
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A review in The Astronomy and Astrophysics Review gives the low-redshift approximation DMIGM ≈ 1000 z pc cm−3, under assumptions about the cosmic baryon distribution, ionization state and cosmology. This relation applies to the IGM component, not blindly to the total observed DM. It is a rough scaling, not a universal one-to-one conversion.
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For a DM-based estimate, astronomers must choose models for the Galactic contribution, the halo, the IGM baryon fraction and the host. Different reasonable assumptions can produce different redshifts and luminosity distances. The result is useful for ranking bursts by likely distance, selecting targets for follow-up and studying populations, but it is not equivalent to measuring a host-galaxy spectrum.
Host redshift versus DM: what each method can establish
| Method | What is measured | What it can establish | Main requirements | Important limitation |
|---|---|---|---|---|
| Host-galaxy redshift | Spectral-line shift from an identified host | An observationally anchored redshift for the individual FRB, from which a cosmological distance can be calculated | Accurate radio localization, credible host association and optical/infrared spectroscopy | Some bursts are too faint, poorly localized or difficult to associate with a unique galaxy |
| Dispersion measure | Total free-electron column along the line of sight | A model-dependent estimate of extragalactic redshift or distance when no host redshift is available | A measured pulse delay and models for Galactic, halo, IGM and host electrons | Host and IGM contributions vary and are not fully separable, producing substantial uncertainty |
How astronomers determine an FRB distance in practice
- Detect the burst over multiple radio frequencies. The frequency-dependent delay establishes the dispersion measure and helps characterize the pulse.
- Refine the position. Interferometric follow-up narrows the location from a broad single-dish error region to arcsecond-scale coordinates or better.
- Search deep optical and infrared images. Candidate galaxies are cataloged within the radio localization region, with attention to morphology, star formation and the probability of a chance alignment.
- Test the host association. Repeated bursts, persistent radio emission, offsets within the galaxy and consistent follow-up observations can strengthen or weaken the identification.
- Obtain spectroscopy. Emission or absorption lines in the candidate host provide a measured redshift when the galaxy is bright enough for spectroscopy.
- Calculate the requested distance quantity. A cosmological calculator or analysis code uses the measured redshift and stated parameters to report, for example, luminosity distance, comoving distance or lookback time.
- Use DM as a consistency check. The observed DM is compared with modeled Milky Way, halo, IGM and host contributions. A mismatch can reveal an unusual host environment or an incomplete model; it does not automatically invalidate the spectroscopic redshift.
Why FRB 121102 is an important historical example
FRB 121102 demonstrates why localization changes the distance problem. Once interferometric observations tied the repeating bursts to a star-forming dwarf galaxy, spectroscopy gave the host a redshift of z = 0.193. That value is a host measurement, not a DM-only inference.
The example also illustrates why older review articles should not be used as current FRB census reports. Their historical sample sizes and forecasts describe the evidence available when they were written. The method remains instructive, while statements about how many localized or repeating FRBs exist must be checked against current observations.
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Localized FRBs turn transient events into probes of otherwise difficult-to-see matter. The pulse passes through ionized gas, so its DM records an integrated electron column even when much of that gas emits little visible light. Comparing measured DMs with host redshifts allows researchers to test how baryons are distributed among galaxies, halos and the intergalactic medium.
Macquart and collaborators used a sample of localized FRBs to estimate the cosmic baryon density, reporting Ωb = 0.051 with uncertainties of −0.025/+0.021 in units of h70−1 at 95% confidence. That result is a population-level application of localized FRBs. It is not a direct distance measurement for FRB 20240304B, nor does it remove the host and halo uncertainties for every individual burst.
Common mistakes when quoting an FRB distance
- Calling redshift a distance: “z = 2.148” is a redshift. It needs a cosmological model and a stated distance definition before it becomes a distance value.
- Using total DM as an IGM measurement: the total includes the Milky Way, halo and host contributions.
- Assuming a larger DM always means a farther source: a dense host environment can raise DM without placing the FRB at a greater cosmological distance.
- Skipping localization: a galaxy inside a several-arcminute error region is not automatically the host.
- Presenting the low-redshift approximation as exact:
DMIGM ≈ 1000 z pc cm−3depends on ionization, baryon-distribution and cosmological assumptions. - Mixing distance conventions: luminosity distance, comoving distance and lookback time answer different questions and should not be compared as if they were interchangeable.
- Treating a record as permanent: “most distant known” is time-sensitive and should be tied to the dated observation or report that made the claim.
So, how far away is a fast radio burst?
It depends on the burst and on the evidence available. A securely localized FRB with a host spectrum has a measured redshift that can be converted into a cosmological distance under an explicit model. A burst without a host spectrum has a DM-based estimate with potentially large uncertainty because several regions of ionized gas contribute to the signal.
The clearest current example is FRB 20240304B: NASA’s 2026 report gives its host a redshift of 2.148 and places the event about 3 billion years after the Big Bang. The scientifically correct answer is therefore not a single universal number of light-years, but a method: localize the burst, verify the host, measure redshift, then state which cosmological distance was calculated. Use DM as a valuable estimate and probe of intervening matter, not as a precision odometer.
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