How Fast Radio Bursts Solved the Missing Baryon Problem

Astronomers have finally located the missing baryonic matter (ordinary matter made of protons, neutrons, and electrons) that cosmological models predicted but could not account for in the nearby universe; this matter exists in the warm-hot intergalactic medium (WHIM), a diffuse plasma within the cosmic web's filaments that connects galaxy clusters, with temperatures around 1 million Kelvin and electron densities of about 10 particles per cubic meter, representing approximately 36% baryonic overdensity compared to the universal average, confirming that the missing matter was never truly missing but simply hiding in plain sight within these vast cosmic structures.

Filaments contain the missing baryons of the universe. For years, cosmologists faced a puzzle: the number of baryons inferred from the cosmic microwave background was higher than what telescopes observed in stars and galaxies, with roughly half missing. The solution lies in the Warm-Hot Intergalactic Medium (WHIM), diffuse gas at temperatures of 10^5 to 10^7 Kelvin inhabiting filaments and sheets. This gas is too hot to see in optical light and too diffuse to glow strongly in X-rays, evading detection until ultraviolet and X-ray spectroscopy revealed its fingerprints in absorption lines against background quasars, especially oxygen ions. Today, astronomers believe most missing baryons reside in the WHIM.

The 'missing baryons' problem - the discrepancy between predicted and observed ordinary matter - was solved through X-ray and UV observations. A significant portion of baryonic matter exists as extremely hot (100,000-10 million K) and rarefied gas filling cosmic web filaments and galactic halos. This Warm-Hot Intergalactic Medium (WHIM) is highly ionized, emitting primarily in soft X-ray and far UV wavelengths. Detection relies on absorption lines of highly ionized atoms (oxygen VII, VIII) in quasar spectra, confirming that voids between galaxies contain giant plasma reservoirs.

When astronomers go out and count the baryons they can actually find today in stars, in galaxies, in the hot gas between galaxies in clusters, they come up short, significantly short. Current surveys account for less than 60% of the baryons that should exist. Between 40 and 50% of all the ordinary matter in the universe is unaccounted for in present-day inventories. The missing matter is not dark matter—dark matter is a separate problem entirely, involving an unknown form of matter that does not interact with light at all. The missing baryons are ordinary matter, the kind that should glow, absorb light, emit X-rays, and show up in telescopes if it is hot enough or dense enough. The leading explanation is that the missing baryons are hiding in a vast, extremely thin, extremely hot web of gas strung between galaxies across intergalactic space. Scientists call this the warm-hot intergalactic medium. The gas in this web is thought to have temperatures between 100,000 and 10 million degrees, which makes it emit mainly X-rays and ultraviolet radiation. The problem is that this gas is so thin, spread so thinly across such enormous volumes of empty space that even at those temperatures, it barely glows.

The Warm-Hot Intergalactic Medium (WHIM) is a diffuse plasma of hot gas that fills the vast voids between galaxies. It was theorized to contain the remaining missing baryonic matter that couldn't be accounted for by observations of galaxies and galaxy clusters. The WHIM consists of gas heated to temperatures exceeding 1 million degrees Celsius, making it difficult to detect directly. Astronomers predicted that this hot gas would account for approximately half of the missing baryonic matter in the universe.

Fast radio bursts (FRBs) serve as powerful cosmic probes because their dispersion measure (DM), rotation measure (RM), and scattering properties accumulate information about all ionized material along their line of sight, enabling astronomers to map the cosmic web, study the circumgalactic medium around galaxies, and constrain cosmological parameters like the Hubble constant and baryon feedback efficiency. The observed DM from FRBs shows a tight correlation with redshift, matching theoretical predictions for the average cosmic dispersion measure, thereby confirming the existence of the missing baryons in the diffuse intergalactic and circumgalactic media. Additionally, RM measurements of FRB hosts reveal that typical star-forming galaxies have magnetic field strengths lower than the local Milky Way value, providing new insights into galactic magnetic field structure.

In 2020, a radio pulse from a known magnetar was detected at multiple wavelengths (X-rays and radio), strongly suggesting magnetars are FRB sources. The most distant FRB has redshift 2.1, showing the universe as it was 10.7 billion years ago. FRBs now serve as probes of the intergalactic medium—like cosmic dipsticks measuring electron distribution. By combining dispersion measures with redshifts, astronomers can map matter distribution and potentially measure the Hubble constant independently.

FRBs serve as powerful cosmological tools even without knowing their exact origin. They can measure the baryon content of the universe, revealing that the 'missing baryons' exist in the Warm-Hot Intergalactic Medium (WHIM)—diffuse gas at about a million degrees filling vast volumes. FRBs can measure intergalactic magnetic fields through Faraday rotation, providing direct measurements along specific lines of sight. They can probe galaxy clusters and cosmic web filaments, measuring gas content and providing independent mass estimates. FRBs can measure the Hubble constant independently of Cepheids and supernovae, potentially resolving the 'Hubble tension.' They can test Lorentz invariance by measuring photon arrival time differences across cosmological distances. CHIME discovered FRBs with periods of 200 milliseconds to 2 seconds, interpreted as neutron star rotation periods, providing evidence for the magnetar model.

Fast Radio Bursts (FRBs) can be used to measure the Hubble parameter (expansion rate of the universe) by analyzing their dispersion measure, which quantifies how much radio pulse frequencies are delayed as they travel through ionized plasma. The dispersion measure increases with distance because electrons in the intergalactic medium cause frequency-dependent delays proportional to one over frequency squared. By measuring the dispersion measure of FRBs and identifying their host galaxies to determine redshifts, researchers can establish a distance-redshift relationship that constrains the Hubble constant. This method provides an independent cosmological probe that complements traditional methods like supernovae and CMB observations, potentially helping resolve the current Hubble tension.

The Hubble tension—the five-sigma discrepancy between Planck's CMB measurement (67 km/s/Mpc) and local distance ladder measurements (73 km/s/Mpc)—is the most significant outstanding problem in cosmology. FRBs provide an independent measurement method using the dispersion measure-redshift relationship to constrain baryon content and expansion history. Different Hubble constant values predict different dispersion measure-redshift relationships. The 2022 FRB-based measurement using ~12 localized FRBs gave H0 = 73 ± 12 km/s/Mpc, consistent with local measurements rather than Planck. The planned Square Kilometer Array will detect thousands of FRBs annually with arcsecond precision, enabling Hubble constant measurements with ~0.5 km/s/Mpc precision sufficient to definitively resolve the tension. FRB 20200120E, localized to a globular cluster in M81 (11.7 million light-years away), challenges the simple young magnetar model. Globular clusters are ancient stellar populations (10-13 billion years old) with no ongoing star formation, where young magnetars from massive star collapse should not exist. This requires alternative formation channels: compact object mergers (neutron star-neutron star, neutron star-white dwarf, or white dwarf-white dwarf mergers enhanced by high stellar density) or accretion-induced collapse of white dwarfs reaching the Chandrasekhar limit. FRBs are coherent radio emission with brightness temperatures of order 10^35 Kelvin, impossible for thermal sources. The emission must be coherent, produced by in-phase motions of charged particles whose radiation adds constructively. Leading models include synchrotron maser emission (relativistic electrons in magnetic fields producing coherent emission through maser-like amplification) and curvature radiation (charge bunches moving along curved magnetic field lines producing coherent emission through interference). FAST's sub-millisecond structure and spectral drift patterns constrain these models, with synchrotron maser emission from shock fronts in magnetar winds currently best constrained. The Deep Synoptic Array (DSA) at Owens Valley provides arcsecond precision localizations for every FRB in real time, enabling immediate multi-wavelength follow-up. The 2023 campaign targeting a repeating FRB in an edge-on spiral galaxy produced the most comprehensive multi-wavelength dataset ever assembled for a repeating FRB source. The cosmic web—the largest structure in the observable universe, consisting of filaments of galaxies and matter connected at nodes containing galaxy clusters, separated by voids—is the direct consequence of primordial density fluctuations from inflation. Mapping it requires probes of matter distribution on tens to hundreds of megaparsec scales. Current observations provide partial maps of luminous and dark matter, but the warm-hot intergalactic medium (WHIM)—diffuse plasma filling cosmic web filaments constituting ~40% of ordinary matter—is extraordinarily difficult to observe directly. FRBs probe this WHIM directly, with each line of sight producing dispersion measure contributions from the plasma in filaments. By comparing FRB dispersion measures at specific sky positions with galaxy distributions from surveys like DESI and Euclid, astronomers can reconstruct the three-dimensional distribution of the warm-hot intergalactic medium with completeness unmatched by other probes. The 2021 Connor and Ravi paper demonstrated statistical detection of excess FRB dispersion measure toward galaxy filaments—the first direct detection of the WHIM in cosmic filaments through FRB observations. With thousands of localized FRBs from the SKA, this program will produce the most comprehensive three-dimensional map of the universe's ordinary matter distribution ever assembled. Multi-messenger coordinated observation of events in multiple physical channels has produced major astrophysics results. GW170817 (2017) demonstrated this power, confirming neutron star mergers produce short gamma-ray bursts and heavy elements (gold, platinum, uranium) via r-process nucleosynthesis, and providing the first standard siren Hubble constant measurement. FRBs are the next major multi-messenger target. Real-time localization capabilities of DSA, CHIME, and successors combined with automated target-of-opportunity systems enable searches for multi-wavelength counterparts. If FRBs are magnetars, magnetar-producing events (supernovae, compact object mergers) should be observable in gravitational waves (LIGO/Virgo), neutrinos (IceCube/Hyper-Kamiokande), and gamma rays (Fermi).

The trajectory of FRB science is toward the precision era—transitioning from discovery to using FRBs as precision cosmological tools. The Square Kilometer Array (SKA), a distributed network of thousands of antennas across South Africa and Australia functioning as a single aperture with ~1 km² collecting area, will detect FRBs at rates of several thousand per year with arcsecond precision localizations. Over a 10-year survey, the SKA will accumulate tens of thousands of localized FRBs, enabling dispersion measure-redshift relationship measurements with uncertainties comparable to Planck CMB and supernovae distance ladder measurements. Beyond Hubble constant measurements, SKA FRB science will accumulate magnetic field maps from rotation measures across thousands of lines of sight through the intergalactic medium, providing the first comprehensive three-dimensional map of cosmic web magnetic field structure. This will constrain the origin of intergalactic magnetic fields—whether generated during reionization, galaxy formation, or primordial epochs. Primordial magnetic fields from inflation or early universe phase transitions would have statistical properties in rotation measure distributions distinguishable from reionization-generated fields. Multi-messenger coordinated observation of events in multiple physical channels has produced major astrophysics results. GW170817 (2017) demonstrated this power, confirming neutron star mergers produce short gamma-ray bursts and heavy elements (gold, platinum, uranium) via r-process nucleosynthesis, and providing the first standard siren Hubble constant measurement. FRBs are the next major multi-messenger target. Real-time localization capabilities of DSA, CHIME, and successors combined with automated target-of-opportunity systems enable searches for multi-wavelength counterparts. If FRBs are magnetars, magnetar-producing events (supernovae, compact object mergers) should be observable in gravitational waves (LIGO/Virgo), neutrinos (IceCube/Hyper-Kamiokande), and gamma rays (Fermi).

A new generation of radio telescopes is revolutionizing FRB research. The Square Kilometer Array (SKA) will combine thousands of antennas across continents to form a virtual dish with one square kilometer collecting area—detecting flashes 10 times fainter than current instruments while pinpointing locations to within fractions of an arcsecond. MeerKAT in South Africa can track known repeaters with split-second timing or sweep the sky for new transients. FAST in China, with its 500-meter aperture, is the world's largest single dish and can detect flashes as faint as a few millijansky milliseconds. FRBs serve as precision probes testing fundamental physics. By comparing photon arrival times at different energies, astronomers search for cracks in physics. If photons of different colors arrived at different moments not due to plasma but because the speed of light itself varied, it would overturn Lorentz invariance—a foundation of Einstein's relativity.

The Deep Synoptic Array (DSA) represents the first radio telescope purpose-built for FRB detection and localization. The DSA-110 prototype consists of ten 4.5-meter antennas operating at L-band (1.4 GHz) with a 1.3-kilometer baseline, achieving approximately 5 arcsecond localization accuracy. This interferometric array demonstrates technology for the full DSA-110 system, which will have 110 antennas on a T-shaped configuration plus 15 outriggers. The DSA-2000 concept proposes scaling to 2000 dishes for comprehensive all-sky surveys with cadence down to a few microjanskys RMS sensitivity. This represents a paradigm shift toward dedicated FRB instrumentation, enabling systematic studies of FRB populations, host galaxy characterization, and probing of cosmic baryons. The technology enables arcsecond-level localization without requiring repetition, fundamentally changing FRB science capabilities.

The Australian Square Kilometre Array Pathfinder (ASKAP) is a radio telescope array consisting of 36 antennas that detected FRB 180924 on September 24, 2018. A researcher developed a new technology that 'freezes' the detection, allowing the telescope to send precise coordinates (right ascension and declination) to other major telescopes worldwide. This technology enables astronomers to study the location of FRBs that occur in fractions of a second.

The Square Kilometre Array and similar projects will dramatically expand our ability to detect and study cosmic radio sources. Scientists expect that with the SKA, we will discover thousands of new FRBs, gain detailed information about the structure of molecular clouds, and potentially detect signals from extraterrestrial civilizations if they exist. The increased sensitivity will allow us to study phenomena that are currently invisible, opening new frontiers in our understanding of the universe.