Missing baryon problem

Space is filled with stuff.

Scientists know how much stuff was there at the start. They look at the early universe to find out. This stuff makes up a small part of space.

But when we look at space now, something is wrong. We see stars and planets. We can also see gas. Still, we cannot find all the stuff from the start.

Most of it is hiding. It is in thin, hot clouds between galaxies. These clouds form a web.

Scientists use special light to find these clouds. This helps them see the missing stuff. They are working hard to find it all.

Scientists study the universe to find all its matter. Matter is the "stuff" that makes up everything. Most of this stuff is made of baryons. Baryons are the tiny parts that build atoms.

When we look at the early universe, we see a lot of baryons. They made up about 4.8% of everything. But when we look at space today, we find a problem. We can only find about half of those baryons. This is called the missing baryon problem.

Where did they go? Scientists think they are hiding in the cosmic web. This is a web-like shape of matter between galaxies.

This web is called the Warm-hot intergalactic medium, or WHIM. The WHIM is made of very thin, hot gas. It is hard to see because it is so spread out. Scientists use special tools to find it. They look for light from the cosmic microwave background. This light scatters off the gas in the WHIM. They also look for oxygen in X-rays. These methods help scientists find the missing pieces of our universe.

Scientists have a big mystery to solve about the universe. This is called the missing baryon problem. Baryons are the tiny parts that build atoms. In the very early universe, baryons made up about 4.8% of everything.

However, when we look at the universe today, we cannot find them all. We can only find about half of the amount we expected. This is different from the dark matter problem. Dark matter is a different kind of stuff that makes up 26.8% of the universe. Baryons are much rarer, making up only about 5% of the total energy.

How do we know how many baryons there should be? We use two main ways to look at the early universe. One way is called Big Bang nucleosynthesis. This is the theory of how the first light elements were made. Higher amounts of baryons would create more helium and lithium. Another way is to study the cosmic microwave background, or CMB. This is the light left over from the start of the universe. Baryons interact with this light and leave a visible mark.

These two methods both show that baryons should make up about 5% of the universe.

Finding baryons in the modern universe is a very hard job. We can easily see bright things like stars and galaxies. But baryons can also hide in dark forms. They can be in black holes or planets. They can even be in very thin gas between stars.

Scientists use a method called gravitational microlensing to find dark objects. This happens when a dark object moves in front of a distant star. The object's mass distorts the light from that star. We can also use the Lyman-alpha forest to find gas clouds. These clouds are visible when they are backlit by stars.

Many scientists believe the missing baryons are in the cosmic web. This is a web-like shape of matter between galaxies. The gas in this web is called the Warm-hot intergalactic medium, or WHIM.

The WHIM has three different heat levels. The warm state has temperatures between 10 to the power of 5 and 10 to the power of 5.7 Kelvin. The hot state is even warmer. The very hot state can reach 10 to the power of 7 Kelvin. This gas is very thin and hard to see. It is mostly made of ionized hydrogen.

In 2017, two groups of scientists shared a big discovery. They found evidence that the missing baryons might be in the WHIM. They used the Sunyaev–Zel'dovich effect to study the gas. This happens when CMB light scatters off the gas in the web. This scattering creates very dim patches in the CMB light. These patches are hard to see without comparing them to galaxy maps.

In 2019, a group led by Orsolya E. Kovács found more clues. They used X-rays to see oxygen in the cosmic filaments. These new results help show that the WHIM might hold the missing pieces.

Cosmologists face a significant mystery known as the missing baryon problem. Baryons are the fundamental particles that constitute ordinary matter, such as atoms. By studying the early universe, scientists determined that baryons should account for approximately 4.8% of the total energy density. However, when astronomers conduct a census of the modern, observable universe, they find less than half of that expected amount. This discrepancy is a major unsolved problem in physics. It is important to distinguish this from the dark matter problem. Dark matter is non-baryonic and makes up about 26.8% of the universe, while baryons make up only about 5%.

To understand why baryons are missing, we must first know how many should exist. Scientists use two independent methods to measure the abundance of baryons in the early universe. The first is Big Bang nucleosynthesis, which is the theory of how the first light elements formed. This theory predicts that higher baryon counts produce higher ratios of helium and lithium relative to hydrogen. The second method involves analyzing the cosmic microwave background (CMB). The CMB is the leftover light from the early universe. Baryonic matter interacts with CMB photons and leaves a visible imprint on the radiation. Detailed analysis of the CMB power spectrum, specifically the second peak, provides a very precise baryon fraction of about 5%.

Finding baryons in the recent universe is difficult because they are not always luminous. While stars and galaxies are easy to see, baryonic matter can also exist in non-luminous forms. It can be found in black holes, planets, or highly diffuse interstellar gas. Astronomers use several techniques to locate these hidden particles. One method is the Lyman-alpha forest, which uses clouds of diffuse gas that are backlit by stars. Another is gravitational microlensing, where a dark object distorts the light of a distant source as it passes in front. Scientists also use the Sunyaev–Zel'dovich effect. This occurs when CMB photons interact with free electrons, leaving a detectable imprint regardless of the gas temperature.

Recent theories suggest the missing matter resides in the Warm-hot intergalactic medium, or WHIM. The Lambda-CDM model predicts that matter between galaxies forms web-like structures called filaments. The WHIM is a low-density medium with only 1 to 10 particles per cubic meter. This medium exists in three distinct thermal states. The warm state ranges from 10^5 to 10^5.7 Kelvin and contains neutral hydrogen. The hot state ranges from 10^5.7 to 10^6.3 Kelvin. Finally, the very hot state reaches 10^6.3 to 10^7 Kelvin. This very hot phase is usually found near the outskirts of galaxy clusters.

Detecting the WHIM is a technical challenge because it is mostly composed of ionized hydrogen. Astronomers often look for highly ionized oxygen, such as OVI or OVII, to find these baryons. In 2017, two research groups claimed to find evidence of missing baryons in intergalactic matter. They used the thermal Sunyaev–Zel'dovich effect to measure the density of cosmic filaments. They found that these strands contained about 30% of the required baryonic density. While this was a major step, it only described nearby galaxies. A 2021 article suggested that 50% of all baryons might exist outside dark matter haloes to explain the full deficit.

Specific measurements help us map where the known baryons are located. Currently, about 7% of baryons exist within stars and galaxies. The Lyman-alpha forest contains roughly 28% of the total baryons. The circum-galactic medium (CGM) is a large sphere surrounding galaxies with a radius between 70 and 200 kiloparsecs. The CGM accounts for about 5% of the universe's baryons. Additionally, the intracluster medium (ICM) makes up about 4% of the total baryon content. The ICM is a low-density environment with about 10^-3 particles per cubic centimeter.

New observations continue to support the existence of the WHIM. In 2019, a group led by Orsolya E. Kovács detected OVII absorption using the X-ray spectrum of 17 stacked quasars. This detection corresponded to the WHIM in filaments with high overdensity. In 2020, astrophysicists reported the first direct measurement of baryonic matter via X-ray emissions from cosmic web filaments. These findings are consistent with the idea that the WHIM holds the missing baryons. Scientists are now working to detect these particles with even higher levels of significance. This research helps complete our understanding of how matter is distributed across the cosmic web.