Why does the universe expand?
Quick Answer
Space itself is stretching, a process first recognized from galaxy redshifts and later explained by modern cosmology.
The Full Story
The universe expands because the fabric of space is increasing in scale over time. Astronomers first noticed in the 1920s and 1930s that distant galaxies show redshift, meaning their light is stretched toward longer wavelengths. This led to the conclusion that galaxies are moving apart as space expands, not simply flying through static space. The expansion began with the hot Big Bang about 13.8 billion years ago and continues today. Observations in the late 1990s showed that the expansion is accelerating, which is often attributed to dark energy.
Key Facts
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"Why is the universe getting bigger every second?"
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