Scientific Overview
The discovery of cosmic expansion stands as one of the most significant breakthroughs in human intellectual history. In 1929, American astronomer Edwin Hubble, by observing the spectra of distant galaxies, discovered an astonishing pattern: virtually all galaxies showed redshifted spectra, with the amount of redshift proportional to the galaxy's distance. This meant galaxies were receding from us, with more distant galaxies receding faster. This relationship is known as Hubble's Law: v = H₀ × d, where v is the recession velocity, d is distance, and H₀ is the Hubble constant.
Hubble's Law's implications extend far beyond galactic motion itself. It revealed not that galaxies are flying apart through fixed space, but that space itself is expanding — just as points on a balloon's surface move apart as the balloon inflates. This discovery fundamentally changed humanity's understanding of the cosmos: the universe is not static and eternal but dynamic with an evolutionary history.
The Big Bang Theory
If the universe is expanding, then rewinding time, the universe must have been smaller, hotter, and denser in the past. Tracing back approximately 13.8 billion years, all matter and energy were concentrated in a state of infinitely small volume with infinite density and temperature — the Big Bang singularity. The Big Bang was not an explosion at some point in space but the birth and expansion of space itself — time, space, and matter all came into being simultaneously at that moment.
Within the first few minutes after the Big Bang, the universe underwent a series of critical physical processes. About one second after the Big Bang, quarks combined to form protons and neutrons. About three minutes later, with temperatures dropping to approximately one billion degrees, protons and neutrons began forming light elements through nuclear fusion — primarily hydrogen, helium, and trace amounts of lithium. This process, called Big Bang Nucleosynthesis, precisely predicts the abundance ratios of hydrogen and helium in the universe (approximately 75% hydrogen and 25% helium). This prediction perfectly matches observations, making it one of the most powerful pieces of evidence for the Big Bang theory.
Cosmic Microwave Background Radiation
About 380,000 years after the Big Bang, the universe cooled to approximately 3000K, and electrons combined with atomic nuclei to form neutral atoms — a process called recombination. Before this, the universe was filled with free electrons that constantly scattered photons, making the universe opaque. After recombination, photons could travel freely, forming the Cosmic Microwave Background radiation (CMB) we observe today.
In 1964, Arno Penzias and Robert Wilson at Bell Labs accidentally discovered this radiation — an isotropic microwave background with a temperature of approximately 2.725K. The CMB is the most direct evidence for the Big Bang theory: it is a "photograph" of the infant universe, recording its state at 380,000 years old. The extremely tiny temperature fluctuations in the CMB (on the order of one part in 100,000) reflect small density variations in the early universe, which eventually evolved under gravity into today's galaxies, galaxy clusters, and large-scale cosmic structure.
The Fate of the Universe
The universe's ultimate fate depends on its total energy density. According to the Friedmann equations, if the universe's density exceeds the critical density, gravity will eventually overcome expansion, and the universe will stop expanding and begin contracting, ultimately collapsing into a "Big Crunch" — in some sense a symmetric end to the Big Bang. If density is less than or equal to critical density, the universe will expand forever.
A shocking 1998 discovery changed this picture: by observing Type Ia supernovae, two independent teams of astronomers found that the universe's expansion is accelerating. This implies some mysterious "dark energy" is driving accelerated expansion. Dark energy accounts for approximately 68% of the universe's total energy, but its nature remains unknown.
If dark energy-dominated accelerated expansion continues, the universe's fate will be "heat death": as expansion continues, all stars eventually exhaust their fuel and go dark, all material structures dissolve under cosmic expansion's dilution, and the universe ultimately reaches maximum entropy — a uniform, near-absolute-zero, eternally dark void. This is the ultimate manifestation of the second law of thermodynamics on a cosmic scale.
Application in the Three-Body Trilogy
The origin and ultimate fate of the universe form the grand backdrop of the third Three-Body novel, Death's End. Liu Cixin weaves profound cosmological contemplation into the narrative of civilizational rise and fall.
In the novel's cosmology, the universe was originally a ten-dimensional, idyllic existence. The Big Bang created a universe rich in dimensions with elegant physical laws. However, as civilizations arose and the Dark Forest theory took effect, the universe's dimensions were progressively reduced by dimensional strikes. Each use of dimensional weapons collapsed part of the universe from higher to lower dimensions — from ten to nine, to eight, down to the three-dimensional universe we see today. The universe's current state is not the result of natural evolution but the accumulated trauma of warfare among countless civilizations.
This premise creates a meaningful dialogue with Big Bang theory. In standard cosmology, the universe's evolution from the Big Bang's unified state to today's complex structures is a natural physical process. In Liu Cixin's cosmology, this process has been distorted by civilizational intervention — the universe's physical constants and dimensional structure are no longer "native" but have been altered by intelligent life.
The novel's concept of "Returners" (归零者) directly connects to the universe's ultimate fate. The Returners are one or more super-civilizations whose goal is to return the universe to its pre-Big Bang state — to make the expanding universe contract again, undergo the "Big Crunch," then be reborn in a new Big Bang. The Returners broadcast their message to the entire universe: they hope all civilizations that have created pocket universes will return the mass within those pocket universes to the main universe, because pocket universes have "stolen" mass from the main universe. If enough stolen mass is not returned, the main universe may never achieve sufficient density to trigger contraction and restart.
This involves a profound cosmological question: the relationship between the universe's mass-energy conservation and its ultimate fate. In real physics, if the universe's total mass-energy density exceeds the critical density, it could indeed stop expanding and begin contracting at some future point. The Returners' logic is: mass transferred by countless civilizations into pocket universes has reduced the main universe's total density below the critical value, causing the universe to expand forever and ultimately reach heat death — the ultimate death of all life. Only by returning this mass could the universe possibly contract and start anew.
The moral dilemma Cheng Xin and Guan Yifan face in their pocket universe is a microcosm of this cosmological predicament. Their pocket universe — an independent spacetime bubble created for them by an advanced civilization — contains mass extracted from the main universe. The Returners' call demands they return this mass, sacrificing their safe haven in exchange for a slim hope of universal restart. Ultimately, Cheng Xin and Guan Yifan choose to return most of the pocket universe's mass to the main universe, keeping only a small drift bottle containing ecosystem information, to float into the restarted new universe.
Through this narrative, Liu Cixin explores the ultimate existential question: when the universe itself faces death, what meaning do individual civilizations and individual lives hold? Heat death is an absolute, irreversible end that dissolves all meaning and value. The Returners' effort represents the final attempt to resist ultimate nihilism — sacrificing present existence for the possibility of future universal rebirth. It is simultaneously the grandest altruism and the most desperate gamble.
Real-World Scientific Extensions
Cosmic expansion and Big Bang theory are supported by extensive observational evidence. Precise CMB measurements (particularly from WMAP and Planck satellites) provide extremely accurate parameter constraints for the standard cosmological model. The nature of dark energy remains one of contemporary physics' greatest mysteries.
Regarding the universe's ultimate fate, current observational evidence favors eternal expansion and heat death. However, some theoretical physicists have explored "cyclic universe" models — the universe may undergo periodic expansion and contraction, with each "Big Bounce" producing a new universe. Penrose's Conformal Cyclic Cosmology (CCC) proposes that one universe's heat death state is mathematically equivalent to another Big Bang's initial state. While these theories remain observationally unconfirmed, they share intriguing resonances with the Returners' universal restart concept in the Three-Body trilogy.
The universe's multidimensional structure is also a core topic of string theory. String theory predicts the universe has 10 or 11 dimensions, with extra dimensions compactified (curled up to extremely small scales). While the Three-Body trilogy's premise of the universe reducing from ten to three dimensions is science fiction, it shares deep conceptual connections with string theory's discussions of dimensions and compactification.