Cosmic neutrino background

Long ago, the world was very hot. Tiny bits called neutrinos were there too. These bits used to bump into things. Then the world grew and cooled down. The bits moved too far apart to touch. They started to fly through space alone. They have been traveling for a long time. They are very hard to catch. They move through us right now. We know they are out there. We can see how they change the light in space.

Long ago, the universe was very hot and crowded. Tiny particles called neutrinos lived there. These particles used to bump into each other all the time. But the universe was growing and cooling down. Soon, the neutrinos were too far apart to touch. They stopped bumping and started flying alone. Scientists call this state free streaming. This happened when the universe was only one second old.

These neutrinos have traveled through space for 13 billion years. They are very hard to find. They rarely touch normal matter. Even the strongest tools have trouble seeing them. We call this group the cosmic neutrino background.

We cannot see them directly yet. However, we have strong clues they are real. They left tiny marks on the cosmic microwave background. This is a glow of light left over from the Big Bang. We also see how they helped make light elements. These elements are the building blocks of stars. Scientists hope to catch them one day. One plan uses a special material called tritium. This might help us finally see these old particles.

The cosmic neutrino background is a special group of particles left over from the Big Bang. These particles are called neutrinos. Scientists sometimes call them relic neutrinos. They are part of the very first things that happened in our universe. While light from the early universe is still around, these neutrinos are much older. Light started traveling freely when the universe was 379,000 years old. However, these neutrinos separated from other matter when the universe was only one second old. Scientists believe they are everywhere in space right now. They are a huge part of our cosmic history.

To understand how they formed, we must look at the very early universe. It was once a very hot and crowded plasma. In this plasma, many different particles bumped into each other constantly. Around one second after the Big Bang, the universe began to expand and cool. This expansion pushed the particles further apart. Eventually, the neutrinos were too far apart to find each other and react. They stopped colliding and started flying alone through space. This state is called free streaming. Because the early universe was mostly the same everywhere, this happened everywhere at once.

We cannot measure these neutrinos directly with our current tools. They have very low energy, which makes them hard to catch. Even high-energy neutrinos are difficult to detect. These relic neutrinos have energies that are billions of times smaller. Still, we have strong indirect evidence that they exist. We can estimate their temperature by looking at the cosmic microwave background. This is a glow of light left over from the early universe. By using math about how energy stays balanced, scientists can guess the neutrino temperature. Right now, their temperature is estimated to be very low.

Scientists use several ways to prove these particles are real. One way is by looking at how the universe grew. The neutrinos helped decide how many light elements were made in the beginning. This process is called Big Bang nucleosynthesis. We also see their effects in the cosmic microwave background. These particles left tiny, subtle marks on that ancient light. In 2015, researchers reported they found these shifts. These findings matched the predictions made by the Standard Model of physics. The data from the WMAP and Planck spacecraft also helped confirm this.

One day, we might catch a neutrino directly on Earth. This is a very hard job because they rarely touch normal matter. One idea is to use a material called tritium. Scientists propose an experiment called PTOLEMY in Italy. It would use tritium to try and trigger a reaction. This reaction would create electrons that we could actually measure. A smaller version of this tool might be ready by 2027. If it works, we will finally see these travelers from 13 billion years ago. It would be a huge discovery for science.

The cosmic neutrino background, often called the CNB or relic neutrinos, is a theoretical field of radiation. This radiation consists of neutrinos that have existed since the very beginning of our universe. While the cosmic microwave background (CMB) provides a snapshot of the universe at 379,000 years old, the CNB is much older. These neutrinos decoupled, or separated from other matter, when the universe was only one second old. Because neutrinos rarely interact with normal matter, they have been traveling through space for over 13 billion years. Today, they are estimated to have a very low temperature of roughly 1.95 K.

To understand the origin of the CNB, we must look at the early universe. At the start, the universe was a very hot, dense plasma. In this state, particles constantly collided and reacted with one another. This created a state of equilibrium, where particles were balanced through constant interaction. Around one second after the Big Bang, this equilibrium was disrupted. As the universe expanded and cooled, neutrinos, electrons, and positrons were spread too far apart to continue reacting. This process is known as decoupling. Once they decoupled, the neutrinos entered a state called free streaming, where they simply moved through space without further collisions.

Scientists cannot measure the temperature of these neutrinos directly, but they can calculate it using physics. The neutrinos and photons (light particles) were once in thermal equilibrium. However, before the photons decoupled from matter, electrons and positrons annihilated. This means they combined to produce more photons, which raised the temperature of the light. By using the principle of conservation of entropy, scientists can estimate this heating. Entropy is a way to measure the energy distribution in a system. Because the photons were heated by this annihilation event, they are now hotter than the neutrinos. Based on the current CMB temperature of 2.725 K, the neutrino temperature is calculated to be about 1.95 K.

There is strong indirect evidence that the CNB exists, even if we have not seen it directly. One major piece of evidence comes from Big Bang nucleosynthesis (BBN). This is the process that created light elements in the early universe. The expansion rate of the universe during BBN was influenced by the energy density of these neutrinos. By measuring the amounts of helium and deuterium in the universe, scientists can infer the presence of neutrinos. Current measurements of these elements align well with the predictions made by the Standard Model of particle physics.

Another way we see the CNB is through its effect on the cosmic microwave background. The presence of neutrinos affects how matter clusters and how the CMB looks. Specifically, neutrinos contribute to the radiation energy density of the universe. This density affects the timing of matter-radiation equality. Neutrinos also create anisotropic stress, which can dampen certain oscillations in the CMB spectra. Data from the WMAP and Planck spacecraft have provided tight bounds on these effects. These observations help confirm that there are likely three types of neutrino species, as the Standard Model predicts.

Directly detecting these relic neutrinos is an immense challenge for modern science. Most neutrinos we detect come from high-energy sources like the Sun or nuclear reactors. The CNB neutrinos are non-relativistic, meaning they move much slower, and they have extremely low energy. Their energy is roughly $10^{10}$ times smaller than the neutrinos we can currently catch. This creates a "lose-lose" situation for detectors. The lower a neutrino's kinetic energy, the less likely it is to interact with matter. Even if an interaction occurs, the signal is incredibly faint and difficult to distinguish from background noise.

One proposed solution for direct detection is the PTOLEMY experiment located in Italy. This experiment aims to capture relic neutrinos using a target of tritium, which is a form of hydrogen. When a neutrino is captured by tritium, it triggers an induced form of beta decay. This reaction produces an electron that can be measured. The challenge is that natural beta decay also produces electrons, creating a massive amount of background noise. To succeed, the detector must have excellent energy resolution to separate the signal from the noise. A demonstrator for this technology is expected to be ready by 2027.