Half of the universe's ordinary matter has long been missing, but recent astronomical advancements have shed light on this elusive component. Astronomers have harnessed the power of fast radio bursts (FRBs) to detect the universe's missing normal matter for the first time. Unlike dark matter, which constitutes about 85% of the universe's mass but remains invisible due to its non-interaction with light, this missing matter is composed of atoms (baryons) that do interact with light, yet have remained too faint to observe until now.
The absence of this ordinary matter has posed a significant challenge in cosmology, often overshadowed by the more enigmatic dark matter. However, the missing baryonic matter problem has persisted due to its distribution, which is incredibly sparse across halos surrounding galaxies and in the diffuse clouds drifting through intergalactic space. A recent breakthrough by a team of astronomers has successfully identified and quantified this missing matter by using FRBs to illuminate these tenuous structures.
FRBs are brief pulses of radio waves, lasting only milliseconds, yet in that fleeting moment, they can emit an amount of energy equivalent to what the sun produces over 30 years. The origins of these bursts remain largely mysterious, as their short duration and singular occurrences make it difficult to trace them back to their sources. Nevertheless, astronomers have recognized their potential in measuring the matter between galaxies for some time.
Out of thousands of FRBs identified, only a select few are suitable for this purpose. For an FRB to effectively gauge the matter between itself and Earth, it must have a known point of origin and a precise distance from our planet. So far, astronomers have achieved this localization for approximately 100 FRBs. In their study, team leader Liam Connor from the Center for Astrophysics, Harvard & Smithsonian (CfA), and his colleagues utilized 69 FRBs, sourced from distances ranging from 11.7 million to an astonishing 9.1 billion light-years away. Notably, the distant FRB 20230521B represents the farthest FRB source ever discovered.
Of the 69 FRBs analyzed, 39 were identified by a network of 110 radio telescopes at Caltech's Owen Valley Radio Observatory, known as the Deep Synoptic Array (DSA). This array was specifically designed to spot and localize FRBs to their respective galaxies. Following this localization process, instruments at Hawaii's W. M. Keck Observatory and the Palomar Observatory near San Diego were utilized to measure the distances between Earth and the galaxies hosting these FRBs.
Many of the remaining FRBs were discovered by the Australian Square Kilometre Array Pathfinder (ASKAP), a cutting-edge network of radio telescopes in Western Australia renowned for its proficiency in FRB detection and localization. As FRBs traverse various forms of matter, the light they emit is split into different wavelengths—akin to sunlight passing through a prism to create a rainbow. The degree of this separation allows astronomers to estimate the amount of matter present in the clouds or structures that the FRBs encounter.
According to the findings, approximately 76% of the universe's normal matter is located within the intergalactic medium—the expansive space between galaxies. An additional 15% is found in the vast, diffuse halos surrounding galaxies, while the remaining 9% is concentrated within galaxies themselves, primarily in the form of stars and cold galactic gas. This distribution aligns with predictions made by advanced simulations of the universe's evolution, marking a significant milestone as the first observational evidence of this phenomenon.
The implications of this research could lead to a deeper understanding of galaxy formation and growth. For Ravi, a co-author of the study, this discovery is just the beginning. The next phase includes the anticipated construction of Caltech's new radio telescope, DSA-2000, set to be built in the Nevada desert. This advanced radio array is expected to detect and localize as many as 10,000 FRBs annually, further enhancing our understanding of these powerful radio waves and their role in uncovering the universe's baryonic matter content.
