For years, scientists have been puzzled by a major mystery in astrophysics known as the missing baryon problem. While dark matter and dark energy make up a large part of the universe, ordinary matter—which includes everything we can see, like stars, planets, and even ourselves—has been hard to fully account for. This article explores what baryonic matter is, its historical study, and recent breakthroughs that have helped us understand its distribution in the universe.
Baryonic matter is the ordinary matter made up of protons and neutrons, forming the atoms that make up stars, planets, and living organisms. Scientists expect that baryonic matter constitutes about 5% of the universe. However, observations have shown that only about 2.5% of this matter is detectable, raising the question: where is the rest?
The origins of baryonic matter date back to the Big Bang. In the early universe, protons and neutrons were abundant in a hot, radiation-filled environment. As the universe expanded and cooled, these particles began to fuse, forming stable nuclei like helium-4. This fusion process was influenced by the density of matter at the time. By about 20 minutes after the Big Bang, the elemental abundances were set, resulting in a composition of roughly 75% hydrogen and 25% helium by mass.
In the late 1990s, scientists began a quest to account for all baryonic matter. They conducted a thorough survey, cataloging visible matter such as stars, galaxies, and gas clouds. Surprisingly, this survey showed that only about 20% of the expected baryonic matter was accounted for. The remaining ordinary matter was not dark matter but baryonic matter that remained undetected, often hidden in darkness.
To find these hidden baryons, astronomers used quasars—extremely bright objects powered by supermassive black holes. The light from quasars can illuminate neutral hydrogen gas along our line of sight, creating absorption lines in their spectra known as the Lyman-alpha forest. By analyzing these absorption lines, scientists could estimate the amount of neutral hydrogen and, consequently, the baryonic matter present in the universe.
Despite these efforts, a significant portion of baryonic matter remained unaccounted for. Computer simulations suggested that this missing matter exists as ionized particles spread thinly between galaxies, in a state called the warm-hot intergalactic medium (WHIM). This medium is characterized by low densities—about one to ten particles per cubic meter—and high temperatures ranging from 100,000 to 10 million Kelvin.
Detecting the WHIM has been challenging because its ionized state means it does not absorb or emit light like neutral hydrogen. Traditional observational techniques have struggled to identify this elusive matter.
In 2007, astronomers discovered fast radio bursts (FRBs), brief but intense pulses of radio waves from distant galaxies. These bursts can be incredibly powerful and last only milliseconds. Researchers realized that by analyzing the dispersion of these radio waves, they could infer the presence of ionized baryons along the path of the signal.
A recent study published in Nature used this method to measure the dispersion of several FRBs in relation to the redshift of their host galaxies. The results confirmed the existence of the missing baryons, estimating that roughly 50% of them reside in the WHIM. This finding aligns with earlier computer simulations and provides a clearer picture of the universe’s baryonic matter distribution.
The journey to uncover the missing baryons has been a testament to the power of scientific inquiry. While only a fraction of baryonic matter is found in stars and galaxies, the majority exists in less visible forms, such as the WHIM. This discovery not only validates decades of theoretical work but also highlights the ongoing quest for knowledge in understanding the universe’s composition. As scientists continue to explore these mysteries, they remain open to the unexpected, which often leads to new insights into the fundamental nature of our cosmos.
Calculate the percentage of baryonic matter that is detectable in the universe. Given that baryonic matter constitutes about 5% of the universe and only 2.5% is detectable, determine the percentage of baryonic matter that remains undetected. Discuss your findings with your peers and consider the implications for our understanding of the universe.
Simulate the process of element formation in the early universe using a computer model. Focus on the fusion of protons and neutrons to form helium-4 and other light elements. Analyze how changes in the density of matter affect the resulting elemental abundances. Present your simulation results and discuss how they align with the historical data on elemental composition.
Examine the spectra of quasars to identify absorption lines in the Lyman-alpha forest. Use these lines to estimate the amount of neutral hydrogen along the line of sight. Discuss how this analysis helps in understanding the distribution of baryonic matter in the universe. Consider the challenges faced in detecting baryonic matter using this method.
Explore the difficulties in detecting the warm-hot intergalactic medium (WHIM). Conduct a literature review on the methods used to identify ionized baryons in the WHIM. Present your findings and propose potential new techniques or technologies that could improve the detection of this elusive matter.
Analyze data from fast radio bursts (FRBs) to infer the presence of ionized baryons. Use the dispersion measure of FRBs to estimate the amount of baryonic matter along the signal path. Discuss how this method has contributed to solving the missing baryon problem and what future research could further refine these estimates.
Baryon – A baryon is a subatomic particle made up of three quarks, held together by the strong force, and is a type of hadron. Protons and neutrons are the most common baryons. – In the early universe, baryons combined to form the first atomic nuclei during the process known as nucleosynthesis.
Matter – Matter is anything that has mass and occupies space, consisting of particles such as atoms, molecules, and ions. – Dark matter is a form of matter that does not emit or interact with electromagnetic radiation, making it invisible and detectable only through its gravitational effects.
Universe – The universe is the totality of space, time, matter, and energy, including all galaxies, stars, and planets. – The observable universe is estimated to be about $93$ billion light-years in diameter.
Hydrogen – Hydrogen is the simplest and most abundant element in the universe, consisting of one proton and one electron. – Hydrogen fusion in the core of stars is the primary process that powers them, converting hydrogen into helium and releasing energy.
Galaxies – Galaxies are massive systems consisting of stars, stellar remnants, interstellar gas, dust, and dark matter, bound together by gravity. – The Milky Way is a barred spiral galaxy that contains our solar system.
Quasars – Quasars are extremely luminous active galactic nuclei, powered by supermassive black holes at their centers. – Quasars are among the most distant and energetic objects in the universe, often outshining entire galaxies.
Intergalactic – Intergalactic refers to the space or matter that exists between galaxies. – The intergalactic medium is composed mainly of ionized hydrogen and helium, with traces of heavier elements.
Medium – In physics, a medium is a substance or material that carries a wave or through which a wave travels. – The interstellar medium is the matter that exists in the space between the star systems in a galaxy.
Radio – Radio refers to the part of the electromagnetic spectrum with wavelengths longer than infrared light, used in astronomy to study celestial objects. – Radio telescopes have been instrumental in discovering pulsars and mapping the structure of our galaxy.
Bursts – In astronomy, bursts refer to sudden, intense emissions of energy, often associated with phenomena like gamma-ray bursts or fast radio bursts. – Gamma-ray bursts are the most energetic explosions observed in the universe, believed to be associated with the collapse of massive stars or the merger of neutron stars.
