When is it estimated that the Big Bang occurred?

The estimate that the Big Bang occurred around 14 billion years ago is based on several lines of evidence from modern cosmology. This timeframe is derived from observations of the cosmic microwave background radiation, which is considered a remnant of the early universe and supports the Big Bang theory. Measurements of the rate of expansion of the universe, known as the Hubble constant, also contribute to this estimate. Additionally, the ages of the oldest known stars and the distribution of galaxies in the universe further align with this timeline.

The other options do not accurately reflect current scientific consensus. For instance, suggesting an occurrence of 10 billion or 20 billion years ago exceeds or doesn't meet the evidence gathered from astronomical observations. Similarly, positing that the Big Bang happened 4.5 billion years ago is notably incorrect, as this time frame aligns more closely with the formation of our Solar System rather than the beginning of the universe itself.