"Did 'The Leftover of the Universe' Become Us? A Scenario of Matter Creation Depicted by Heavy Neutrinos"

Why Did Matter Remain in the Universe? The Mystery of Existence Held by the "Overlooked Particle"

The stars in the night sky, the smartphone in front of you, and our bodies are all made of matter. While this is an obvious fact, when considering the beginning of the universe, the very fact that "matter exists" becomes a significant mystery.

For every particle of matter, there is a corresponding antiparticle. When they meet, they annihilate each other, transforming into other particles or light. If matter and antimatter were created symmetrically in the early universe and maintained balance thereafter, it would be difficult to explain a universe with the large amount of matter we see today.

So, what disrupted this balance?

A study introduced by Live Science on October 26, 2026, seeks clues in particles that have been treated as secondary in some theoretical calculations. The candidate is the "right-handed neutrino," which is very heavy and interacts minimally with other particles.

However, it is important to note that this particle has not been discovered. The current study calculates what could have happened in the early universe if it existed.

The Neutrino That Slips Through Our Familiar World and Its Unknown Companion

Neutrinos are elementary particles that interact very weakly with matter. Often called "ghost particles," normal neutrinos have been observed.

One of the mysteries is their mass.

There are three types of neutrinos: electron, muon, and tau. They undergo "neutrino oscillation," changing types as they travel. This phenomenon indicates that neutrinos have mass. However, the minimal form of the Standard Model, which explains the fundamental behavior of elementary particles, cannot account for the mass of neutrinos.

One promising idea to fill this gap is the "seesaw mechanism."

It posits that the observed light neutrinos have a very heavy unknown partner. Simply put, the larger the mass of the partner, the smaller the mass of the observed neutrino can be. This relationship is named after a seesaw in a playground.

The right-handed neutrino discussed in this study corresponds to this heavy partner. "Right-handed" is a technical term describing a particle's property, not that it flies clockwise through space.

It is crucial not to confuse the observed neutrinos with the hypothesized heavy right-handed neutrinos. The existence of the former is confirmed, but the latter remains a theoretical candidate.

The Third Particle Omitted Because "Two Can Explain It"

The research was conducted by Rishav Roshan from the University of Southampton, UK, and Sudipta Show from the Indian Institute of Technology Kanpur. According to Live Science, the study was published in the journal Physical Review Letters on September 8, 2026.

In the commonly considered seesaw model, three heavy right-handed neutrinos are introduced.

However, the properties of observed neutrinos can be explained by focusing on two of them. For this reason, the remaining one has sometimes been omitted from calculations due to its extremely weak interactions.

This study re-examined that omission.

Just because its role in creating neutrino mass is small doesn't mean its impact on the universe is also small.

The difficulty in interacting with other particles also means it can last a long time without decaying. If a sufficient amount existed in the early universe, these long-lived particles might have created a period where they constituted a significant portion of the universe's energy.

100 Trillion Times the Mass of a Proton—Until the Heavy Particle Becomes the Main Character of the Universe

In a representative calculation example introduced by Live Science, the mass of the heaviest right-handed neutrino is set to about 100 trillion times that of a proton. This is not a value measured by observation but a condition used in the study.

Why could this particle become the main character of the universe?

The key lies in the difference in how energy decreases with the expansion of the universe.

Radiation, like light, decreases in density as space expands. Additionally, the wavelength is stretched, reducing the energy of each photon.

On the other hand, for sufficiently slowed heavy particles, the energy center is mass. Even though the particle density decreases with the expansion of space, the mass energy does not decrease in the same way as light.

As a result, the energy density of radiation decreases faster, and the proportion occupied by heavy particles increases relatively.

The study suggests that through this mechanism, a temporary "matter-dominated era" could have occurred in the early universe until the heavy neutrinos decayed.

The matter dominance here does not mean stars or rocks already existed. It means that the universe's energy was more carried by slowly moving heavy particles than by light.

If the Expansion of the Universe Changes, So Does the Remaining Matter

The intriguing aspect of this study is that it doesn't end with "there might have been many heavy particles."

In the early universe, there was a competition between the speed of particle interactions and the speed of the universe's expansion. For a reaction to work sufficiently and approach equilibrium, it must proceed faster than the environment changes due to expansion.

If heavy neutrinos carry much of the energy, the expansion rate compared at the same radiation temperature can be larger than when only radiation dominates. This means reactions that would normally work sufficiently might not reach equilibrium until a lower temperature.

This is a different story from the "accelerated expansion" due to dark energy discussed in the current universe.

The focus here was on reactions that distinguish between types of leptons. Leptons are a group of particles that include electrons and neutrinos.

The decay of heavy neutrinos creates an imbalance in the number of leptons and antileptons, which through other reactions, is converted into an imbalance on the matter side, such as protons and neutrons. This concept is called "leptogenesis."

If the point at which lepton types need to be distinguished changes, the calculation of the final remaining imbalance also changes.

In other words, the heaviest particle changes how the universe expands, and within that environment, the lighter right-handed neutrinos create a matter imbalance. Such a division of roles can be considered.

On the other hand, when the heavy particles finally decay, there is an effect of diluting the already created imbalance. The study shows that even including this effect, there are conditions that can explain the observed matter imbalance.

On Social Media: "Too Difficult" and "Headlines Are Too Big"

Articles covering the same paper were shared on other media platforms and on the overseas forum Reddit. The reactions observed there were not only about interest in the research content but also questions about how it was communicated.

One issue was confusion over technical terms.

There were posts quoting the paper's description and stating they couldn't understand what it meant. Even starting to read from the familiar and grand question of the universe's existence, terms like "Yukawa interaction," "equilibration," and "lepton flavor" can suddenly create a sense of distance.

Another cautious reaction was towards the headlines in the media.

There were comments pointing out that explaining the mechanism by which matter remained is different from explaining why anything exists in the first place. There were also posts noting that the idea of linking neutrino mass and matter-antimatter imbalance has existed for some time.

Meanwhile, there were comments likening human existence to the "remainder" of calculations or joking about the difficulty of neutrinos to react.

These reactions were confirmed on a specific forum dealing with the same research and do not represent the overall evaluation on social media. Nonetheless, it shows that while grand headlines can spark curiosity, they can also lead to losing sight of the actual scope of the research.

It's Still Too Early to Say "The Mystery of Existence Is Solved"

This study addresses the issue of how the imbalance between matter and antimatter was formed.

Why was the universe itself born? Why do physical laws exist? Why do life and consciousness arise? This study does not directly answer such questions.

Also, caution is needed with expressions like "everything disappears in annihilation with antimatter." In annihilation, energy doesn't vanish but transforms into other forms like light. The mystery is not that we narrowly escaped literal nothingness but how the large amount of matter we see today remained.

The novelty of this study lies in showing that by reintroducing particles often omitted in existing theories, the expansion history of the early universe and the timing of reactions change, altering the conditions for creating matter imbalance.

Of course, there are conditions for this conclusion. A sufficient amount of heavy particles must have been created in the early universe and survived for a long time. The historically possible scenario in calculations does not necessarily mean it was the actual history of the universe.

Clues Lie in the Traces Left in the Universe Rather Than the Particles Themselves

The hypothesized particles are extremely heavy, making it difficult to create them directly with current particle accelerators. Thus, investigating the traces left in the universe becomes a candidate method.

Live Science introduces gravitational waves from the early universe as one method of verification.

If there was a temporary matter-dominated era, there might be characteristic changes in the distribution of the strength of gravitational waves from the early universe across different frequencies. Hypothetical "cosmic strings" producing gravitational waves are among the candidates.

However, this is not a prediction that they will definitely be observed. The visible traces vary depending on the source of the gravitational waves, particle mass, lifespan, and other conditions. Even if features are found, they need to be compared with other explanations.

For Japanese readers, the point where the term neutrino broadens the perspective to the entire history of the universe is intriguing. The properties of tiny, invisible particles and the conditions for the existence of stars and galaxies are connected within the same question.

This study is not a declaration of solving the mysteries of the universe. However, it indicates a new direction for verification, suggesting that what was thought to be negligible in calculations might have played an indispensable role in the universe as a whole.

Sources

1. Live Science:
   Introducing interviews with researchers, setting representative masses, impacts on the early universe, and the potential for verification through gravitational waves.
   https://www.livescience.com/physics-mathematics/particle-physics/a-neglected-particle-from-the-dawn-of-time-could-explain-why-everything-in-the-universe-exists

2. Roshan and Show's Research Paper: Public Preprint
   "Delayed Charged Lepton Yukawa Equilibration and Thermal Leptogenesis." The primary source of research on how long-lived heavy right-handed neutrinos affect the expansion and reaction equilibrium of the early universe. The public version is from 2025 and has a different title from the 2026 published paper.
   https://arxiv.org/abs/2510.23712arxiv.org

3. DOI of the Paper Published in Physical Review Letters
   The original article cites "Delayed charged Lepton Yukawa equilibration in minimal seesaw," Volume 137, 111802.
   https://doi.org/10.1103/nfkx-q6z7

4. Reddit・r/science2: Reactions to Articles Covering the Same Research
   A thread sharing an article from The Independent. Observations of confusion over technical terms, questions about the scope of headlines, and humorous reactions. These are not comments directly attached to the Live Science article.
   https://www.reddit.com/r/science2/comments/1xch2w8/scientists_reveal_new_theory_of_why_we_exist/reddit.com