The Universe is vast – and strangely, half-empty. For decades, cosmologists have wrestled with the “missing baryon problem”, a puzzling deficit of ordinary matter…
Chasing Cosmic Shadows
While stars, planets, and cold gas only account for less than 10% of what theory says should exist, the rest appeared to be hiding, diffuse, elusive and undetectable by traditional means. Now, with the help of some of the cosmos’ brightest and shortest flashes – fast radio bursts (FRBs) – scientists are turning that darkness into light.
Baryons and the Cosmic Web
Unlike the enigmatic realms of dark matter and dark energy, baryons are ordinary matter: protons, neutrons and their assemblies into atoms.
From your skin to the rings of Saturn, these constituents form everything that is tangible. Yet despite their relative familiarity, they have proven remarkably evasive when trying to tally their cosmic abundance.
Theoretical models predicted that most of these baryons should float between galaxies in the Inter-Galactic Medium (IGM) or collect in galactic haloes.
Galactic Haloes
Galactic haloes are spherical regions of hot gas enveloping galaxies.
Those vast and virtually invisible components of galaxies extend well beyond their visible boundaries. While the spiral arms and bright cores of galaxies capture most of our attention, haloes are enormous spheroidal regions that envelop these structures like cosmic cocoons.
They are composed of dark matter, hot gas, and a sparse sprinkling of old stars and globular clusters, making them crucial to understanding the formation and evolution of galaxies.
Most of a galaxy’s mass is actually contained within its dark matter halo – an elusive substance that does not emit light but exerts its gravitational influence. These haloes provide the scaffolding that shapes galaxies and guides their motion. Embedded within this dark framework is hot gas that can reach millions of degrees, often glowing faintly in X-rays.
This matter reservoir may act as a galactic recycling system, supplying material for future star formation while also trapping remnants from supernova explosions.
Haloes also preserve a fossil record of galactic history.
Ancient stars and globular clusters, some of which formed during the galaxy’s infancy, float in this tenuous region, tracing paths that hint at past mergers and interactions. When galaxies collide or cannibalize their smaller neighbours, their haloes absorb these fragments, leaving ghostly trails of disrupted matter.
Typically, galactic haloes extend far beyond the visible parts of a galaxy, spanning up to 300,000 light-years or more in diameter. About 92 kiloparsecs.
But these vast gaseous reservoirs are so thinly spread that their light emission is negligible, making them as hard to detect as observing fog in moonlight.
A Cosmic Lighthouse: Fast Radio Bursts
Enter fast radio bursts or FRBs for short. These are brief millisecond-long pulses of intense radio energy from distant corners of the Universe.
Since their first detection in 2007, over a thousand FRBs have been recorded. Their origins remain mysterious, but their scientific utility is undeniable: they serve as cosmic beacons, piercing through intergalactic fog and revealing what lies hidden.
Fast Radio Bursts (FRBs)
Fast Radio Bursts (FRBs) are ultra-short, ultra-powerful pulses of radio waves originating from deep space. Each burst typically lasts only a few milliseconds, yet in that blink of time, it can release as much energy as the Sun emits over several days.
Their discovery in 2007 was serendipitous. And ever since, FRBs have puzzled and intrigued astrophysicists worldwide due to their intensity, brevity, and mysterious origins.
While some FRBs repeat, others do not.
Their progenitors are still debated, ranging from magnetars and collapsing neutron stars to more exotic suggestions like axion miniclusters.
One of the most promising explanations involves magnetars, a type of neutron star with an extraordinarily strong magnetic field. When stress builds in a magnetar’s magnetosphere, it may snap violently, emitting a tremendous radio pulse. Other hypotheses include cataclysmic events like mergers between neutron stars, the collapse of pulsars, or even more speculative phenomena. Some FRBs repeat over time, suggesting ongoing processes, while others are one-off events, perhaps tied to explosive cosmic transitions.
Beyond their mysterious origins, FRBs are incredibly useful scientific tools as cosmic probes due to their brightness, brevity and broadband nature.
As their signals travel across billions of light-years, they pass through clouds of ionized gas and intergalactic plasma. This interaction subtly alters their properties, allowing scientists to use them to trace the distribution of matter across the Universe, especially the elusive baryons that do not appear in stars or galaxies.
In this way, FRBs act like cosmic sonar, illuminating the dark spaces between galaxies.
Research began pinpointing fast radio burst sources with increasing accuracy. One 2020 breakthrough traced an FRB to a magnetar in the Milky Way, confirming the viability of at least one origin theory.
Meanwhile, discoveries of bursts emanating from massive elliptical galaxies – largely devoid of star formation – have challenged assumptions and expanded the search for alternative mechanisms. Each new detection deepens the mystery, while also widening the window into understanding the invisible scaffolding of the cosmos.
Each FRB’s signal gets dispersed as it travels toward Earth – its longer wavelengths (like red light) slowed more than shorter ones (like blue light). This dispersion depends directly on the amount and density of matter the signal passes through.
By measuring this effect precisely, astronomers can count the invisible baryons – not through sight, but through signal delay.
The Study: Painting the Fog
Led by Liam Connor of Harvard University and formerly Caltech, a team of astronomers meticulously analyzed 69 FRBs spanning distances from 11 million to 9.1 billion light-years.
These bursts were sourced from a number of observatories:
- Deep Synoptic Array (Caltech, California),
- W.M. (William Myron) Keck Observatory (Hawaii),
- Palomar Observatory (San Diego),
- Square Kilometre Array Pathfinder (Australia)
Their most distant find, FRB 20230521B, now holds the record for the farthest FRB ever detected.
Using these instruments, the researchers traced how each FRB signal slowed across wavelengths, effectively weighing the amount of matter encountered – a bit like counting fog droplets by how much they delay your flashlight.
As FRBs pass through clumps of plasma, gravitational lensing effects can amplify or distort their signals offering clues about small-scale turbulence, density fluctuations and temperature profiles of the IGM. Scattering tails and echo-like signals provide data on electron density and spatial distribution.
The Dispersion Measure
Fast radio bursts experience dispersion as they travel through ionized matter, with lower frequencies arriving later than higher ones.
The Dispertion Measure (DM) is the total number of free electrons per unit area along the line of sight between an observer and a radio source, typically a pulsar or FRB, which causes lower-frequency signals to arrive later than higher-frequency ones.
It is proportional to the integrated column density of free electrons along the line of sight.
By measuring how much each FRB signal was slowed down as it passed through space, Connor and his team tracked the gas along its journey.
FRBs act as cosmic flashlights. They shine through the fog of the intergalactic medium, and by precisely measuring how the light slows down, we can weigh that fog, even when it’s too faint to see.
Liam Connor, Assistant Professor of Astronomy, Harvard University, United States
This distribution lines up with predictions from advanced cosmological simulations, but it had never been directly confirmed until now.
The results were clear.
Approximately 76% of the Universe’s baryonic matter lies in the IGM. About 15% resides in galaxy haloes. And a small fraction is burrowed in stars or amid cold galactic gas.
This distribution lines up with predictions from advanced cosmological simulations, but it had never been directly confirmed until now.
It’s a triumph of Modern Astronomy. We’re beginning to see the Universe’s structure and composition in a whole new light, thanks to FRBs. These brief flashes allow us to trace the otherwise invisible matter that fills the vast spaces between galaxies.
Vikram Ravi, Assistant Professor of Astronomy, Caltech, California, United States
Where the Baryons are Hiding
The mapping revealed a new cosmic census:
This distribution confirms that the bulk of baryons drift in the vast spaces between galaxies – previously invisible, now sketched by FRB shadows. The result aligns with prior simulation-based estimates, finally grounding decades of theory in observational evidence.
Out on a WHIM
By all acounts, it looks like most of the Universe’s ordinary matter is chilling in the vast spaces that exist between galaxies.
The majority of baryonic matter is not locked up in stars or galaxies, it resides in the Warm-Hot Intergalactic Medium (WHIM).
The discovery points to a number of cosmological implications.
Galaxy Evolution and Beyond
Understanding baryon distribution helps refine models of galaxy formation and feedback processes:
- Gravitational pull gathers baryons into galaxies.
- Supermassive black holes and stellar explosions expel them, preventing overheating, acting as a kind of cosmic thermostat.
This feedback ensures galactic environments remain conducive to star formation and structure retention. The data implies such outflows are highly efficient, reinforcing the dynamic interplay between galaxies and the IGM.
A Tool for Mapping the Cosmic Web
Beyond solving the missing baryon problem, FRBs may enable detailed mapping of the cosmic web – the dark matter skeleton upon which galaxies are built. Though dark matter itself remains invisible, its shape influences baryonic flow.
FRBs, then, trace not just matter – but the structure that governs it.
Caltech is now planning a new radio telescope array in Nevada, poised to detect up to 10,000 FRBs per year, promising even sharper insight into the universe’s scaffolding.
Lighting up the Darkness
Fast radio bursts are more than astrophysical oddities.
They are cosmic lanterns, flashing through the abyss and revealing what once seemed unreachable. Thanks to this breakthrough, we now glimpse the architecture of the universe with new clarity – not by looking for what glows, but by measuring what delays the light.
The decades-old ‘missing baryon problem’ was never about whether the matter existed. It was always: Where is it? Now, thanks to FRBs, we know.
Liam Connor, Assistant Professor of Astronomy, Harvard University, United States
Fast Radio Bursts have become cosmic lighthouses, piercing the vast darkness of the Universe and tracing the faint fingerprints of its elusive baryonic matter. By measuring how these bursts are scattered and delayed across space, astronomers can map the hidden web of gas between galaxies – bringing visibility to what was once invisible.
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