宇宙の起源に関する疑問は、現代物理学の最も深遠なテーマの一つです。誰も実際に見たことのない「宇宙の起源」をどうやって解明するのか、という疑問に答えるためには、私たちが持つ知識と技術の限界を押し広げなければなりません。
観測と理論の融合
宇宙の始まりを理解するためには、観測データと理論物理学の融合が必要です。特に、遠方の天体からの光や放射を分析することで、宇宙の歴史を遡ることができます。Ia型超新星の観測は、宇宙が加速膨張していることを示し、これはダークエネルギーの存在を示唆しました。この発見により、1998年に2つの研究グループがノーベル物理学賞を受賞しました。
ダークエネルギーの謎
現在、宇宙全体の約70%を占めるとされるダークエネルギーの正体は、未だ解明されていません。宇宙が加速膨張している原因であるとされますが、そのエネルギースケール（約0.002eV）を説明する理論は存在しません。ダークエネルギーが時間とともに変化する可能性や、未知のスカラー場（仮にφと呼ぶ）によるものなど、様々な仮説が立てられていますが、いずれも確証は得られていません。
宇宙の未来と人間原理
現在の観測データから、宇宙がこのまま加速膨張を続ければ、銀河は遠ざかり、恒星は燃え尽き、最終的にはブラックホールも蒸発すると予測されています。このシナリオにおいて、ダークエネルギーがどのように振る舞うかが重要な鍵を握っています。
一方で、ダークエネルギーの謎を解くための一つのアプローチとして「人間原理」があります。これは、宇宙の物理法則や定数が、人間が存在し得る条件を満たすように設定されているという考え方です。強い人間原理は、宇宙の性質が人間の存在によって決まっているかもしれないという、哲学的な概念です。
新しい物理学の必要性
ダークエネルギー問題の科学的な解決には、現代物理学の枠組みを超えた新しい理論の発見が必要です。既存の理論では説明できないエネルギースケールや現象を解明するためには、未発見の物理法則があるかもしれません。これは、科学の進歩において非常に重要な挑戦です。
結論
宇宙の起源とダークエネルギーの謎を解くことは、現代物理学の最前線の課題です。観測技術の向上と新しい理論の発見が求められています。人間原理は一つの仮説に過ぎず、最終的な解決には科学的探求が不可欠です。これからの研究により、宇宙の謎が解明されることを期待しています。
The Origin of the Universe
How can we unravel the origin of the universe, which no one has ever seen?
13.8 billion years ago, the universe was born from a tiny clump of energy smaller than a point. From there, matter, Earth, and life emerged, leading to our existence. However, if we calmly think about it, one might wonder how we can unravel the "origin of the universe," which no one has ever seen.
In this series, we will guide you through the great mysteries of the universe, based on "The Origin of the Universe and Matter," thoroughly explained from basics to the cutting edge by leading researchers.
This article is an excerpt and re-edited from "Understanding the 'Invisible World': The Origin of the Universe and Matter" edited by the High Energy Accelerator Research Organization's Institute of Particle and Nuclear Studies (Blue Backs).
Dark Energy, Which Accounts for 70% of the Universe
As stated in the previous article, observations suggest that dark energy or the cosmological constant currently makes up about 70% of the universe. This abundance of dark energy, similar to inflation, is causing the accelerated expansion of the universe we see today.
Observing light from Type Ia supernovae explosions reveals that the rate of acceleration has been increasing the closer we get to the present age of the universe, compared to when the universe was between one-third to one-half its current size. Type Ia supernovae occur when a white dwarf, one possible final stage of a star, accumulates gas and explodes upon exceeding a critical mass. The discovery of the universe's accelerating expansion, published simultaneously in 1998, earned Dr. Saul Perlmutter and his team in the U.S., and Dr. Brian Schmidt and Dr. Adam Riess and their team in Australia and the U.S., the Nobel Prize in Physics in 2011.
The Accelerating Expansion of the Universe Revisited
If we assume dark energy is truly a constant, it means that it was an incredibly small amount in the early universe. It is believed that some physical process planted this tiny seed at the beginning of the universe. It is also expected that in the near future, dark energy will comprise 100% of the universe's energy and become completely dominant. However, the origin of this smallness cannot be explained by modern physics. It remains unresolved, indicating the need for the discovery of new physical theories. In the last section of this chapter, we will discuss the anthropic principle as a non-scientific solution, which posits that the existence of humans might determine the nature of the universe.
The energy scale of dark energy is about 0.002 eV, which is the lowest energy state vacuum. Current physics cannot explain this energy scale. The standard model of particle physics does not know of any phase transition occurring at this 0.002 eV scale.
What is Dark Energy?
In particle physics terms, the cosmological constant is thought to be an unknown scalar field settled at the bottom of its potential energy. The exact nature of this scalar field, hypothesized as "φ," remains completely unknown. Its potential energy density scale is about 0.002 eV.
If we introduce a wider concept where dark energy is not a perfect constant but changes gradually, the cosmological constant becomes part of the broader concept of dark energy. This interpretation is not ruled out by observations.
The Future of the Universe
If we assume dark energy's energy density remains nearly constant, the future of the universe can be predicted as follows:
- Approximately 4 billion years later, our galaxy will merge with the Andromeda Galaxy to form a giant galaxy called "Milkomeda."
- About 5 billion years from now, the sun will die, potentially engulfing the Earth.
- Around 1.4 trillion years in the future, Milkomeda will become a lone galaxy due to accelerated expansion.
- About 1 trillion years later, all stars will have burned out.
- Roughly 10^34 years from now, protons will decay into positrons and other particles according to grand unified theory predictions, meaning no more ordinary matter will exist.
- Around 10^83 years from now, supermassive black holes at the centers of galaxies will evaporate, leaving the universe devoid of celestial bodies.
Some researchers propose an even more exotic model where dark energy increases over time, causing a "Big Rip" where all celestial bodies are torn apart.
The Remaining Big Problem
The dark energy problem, particularly its energy scale of approximately 0.002 eV, is an issue that current physics cannot explain. This extremely small value requires precise tuning, and if it were about a thousand times larger, the universe would have expanded too quickly for galaxies or Earth to form.
One philosophical approach to this problem is the anthropic principle, applying René Descartes' "I think, therefore I am" to cosmology, suggesting that the universe's laws are as they are because humans exist to question them.
Adopting the multiverse theory, our universe might be just one among a vast number of universes, each with different physical laws. If so, only in those universes where conditions allow for the emergence of humans would such questions arise.
Conclusion
Currently, there seems to be no other way to resolve the dark energy problem scientifically, except by applying the anthropic principle. However, the introduction of new physical laws would mark a true victory for science, potentially solving this profound issue.