Hypothetical Cosmological Model

Our universe does not expand forever in one direction. Instead, it follows the laws of energy conservation and the balance between gravity and repulsion, undergoing an eternal cyclic motion of expansion and contraction. Throughout this process, with no external interference, the universe behaves as an isolated system. The total amount of dark matter and dark energy remains constant, strictly obeying the First Law of Thermodynamics. Their mutual transformation strictly follows the mass-energy equivalence. The expansion and contraction of the universe are simply the result of this mass-energy conversion and the alternating dominance of gravity and repulsion.

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I. The Beginning of a New Expansion: The Rebound

The universe initially exists in a dense, high-gravity, compact state, with dark matter absolutely dominant. At the end of the previous contraction, a vast amount of dark matter was compressed to its extreme and, following the mass-energy equivalence, transformed into dark energy, generating tremendous repulsion — like a spring compressed to its limit suddenly snapping open. This is the "Big Rebound." The residual repulsive force of this dark energy pushes the universe into a new expansion.

However, because the matter density is still extremely high and gravity is far stronger than repulsion, the expansion speed is continuously suppressed. The universe thus enters the early decelerating expansion phase.

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II. Early Decelerating Expansion: Gravity Dominates, Speed Slows

During this phase, the universe has extremely high matter density, and gravity is much stronger than dark energy repulsion. Although space expands outward, its speed is constantly held back by gravity, slowing down continuously. At the same time, matter gradually clumps together under gravity, forming galaxies, galaxy clusters, and other large-scale structures.

As the universe continues to expand, space is stretched ever larger, the overall matter density is progressively diluted, and gravity weakens accordingly.

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III. Accelerating Expansion Phase: Dark Energy Dominates, Galaxies Move Apart

When gravity falls below a certain critical threshold, dark matter can no longer maintain a stable bound state. It then undergoes a dissipative transformation, slowly turning into dark energy (following the mass-energy equivalence). The proportion of dark energy gradually rises, repulsion in the universe increases, and begins to overpower gravity.

As a result, the expansion speed changes from slow to fast, entering the accelerating expansion phase. This is the current state of our universe — galaxies are moving away from each other at an ever-increasing speed due to repulsion.

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IV. Expansion to the Extreme: Dark Energy Becomes Thin, Expansion Stops

As the universe continues to accelerate, space becomes more and more empty. Both matter density and dark energy density are diluted to extreme levels.

When the density of dark energy itself drops extremely low, the repulsion it produces becomes too weak to counter gravity. The expansion of the universe then gradually slows down until it stops completely. With expansion halted and gravity still dominant, the universe turns to contraction.

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V. Contraction Phase: Dark Energy Turns Back into Dark Matter, Contraction Accelerates

During contraction, space is continuously compressed, and matter density rises again. The once extremely thin, nearly dissipated dark energy becomes unstable under the high-pressure environment and, following the mass-energy equivalence, condenses back into dark matter.

As the proportion of dark matter steadily increases, gravity further strengthens, and the contraction speeds up. The universe is like a movie of expansion played in reverse: galaxies approach each other, space shrinks, and overall temperature and density rise continuously.

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VI. Returning to the Starting Point, Triggering the Next Rebound

The universe contracts all the way back to a dense, high-gravity, compact state — similar to the beginning of the previous cycle. At that point, almost all energy exists in the form of dark matter, and dark energy is compressed to its limit.

Under this extreme compression, dark matter again undergoes a violent transformation, turning back into dark energy and releasing a powerful repulsive force. This triggers a new "Big Rebound," and the universe begins expanding once more.

Thus the cycle repeats forever: expansion → deceleration → acceleration → thinning → stop → contraction → conversion → rebound → expansion again...

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In one sentence:
Every reincarnation of the universe is a grand dance of dark matter and dark energy ebbing and flowing, with gravity and repulsion alternating dominance. There is no absolute beginning and no eternal end — only an eternal cycle.