The history of the Universe, from cosmic inflation through the Big Bang to the present day, is well understood. However, whether the Universe is infinite or not remains an unresolved question.
Approximately 13.8 billion years ago, our Universe began with the hot Big Bang, which created a primordial mix of particles and antiparticles that eventually formed the stars, planets, and galaxies we see today. This Big Bang followed an earlier phase known as cosmic inflation, but only the very last fraction of a second of this inflation is visible in our observable Universe. While our observations are limited to what we can see within this observable part, we remain uncertain about the nature of the regions beyond it—whether they are finite or infinite. Although our observational tools and techniques are advanced, they have limits. We cannot see beyond the cosmic microwave background, which represents the first formation of stable, neutral atoms about 380,000 years after the Big Bang. Before this, there is evidence of a cosmic neutrino background emitted just a second after the Big Bang, and a hypothesized gravitational wave background from the inflationary phase preceding the Big Bang, which might also be detectable.
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