A Brief History of Cosmic Expansion

The expansion history of the universe can be roughly divided into four key stages:

1. Inflationary Epoch (10⁻³⁶ s → 10⁻³² s)
In far less than an instant, the universe expanded by at least 10²⁶ times at a speed far exceeding that of light. This solved problems such as the uniformity of the cosmos and seeded the quantum fluctuations that would later form large-scale structures.

2. Decelerating Expansion Phase (~10⁻³² s → ~6 billion years ago)
After inflation ended, the universe was filled with matter and radiation. The pull of gravity caused the expansion to continuously slow down. During this period, atoms, stars, and galaxies were born.

3. Accelerating Expansion Phase (~6 billion years ago → present)
Dark energy became dominant, producing an "anti-gravity" effect that drove the expansion to accelerate again. Galaxies are moving away from each other at increasing speeds. This is the current phase we are in.

4. Future Possibilities (still debated)
The mainstream prediction is continued acceleration leading to "heat death." However, recent studies such as those by the Korean team suggest that dark energy may have already begun to decay, and the expansion might have entered a new decelerating phase. This remains to be verified.

In short: Inflation → Braking → Accelerating again. Whether the intensity of this "second acceleration" is now weakening is one of the frontier puzzles of modern cosmology.

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Let me know if you'd like a more concise or a more formal/academic version.