What is the Big Bang?

The Big Bang model describes an earlier, much hotter and denser universe that expanded and cooled. It is not an ordinary bomb exploding into preexisting empty space. It explains large-scale cosmic development without by itself settling every question about an absolute beginning of time or why a universe exists.

Evidence for a hot cosmic past

1. Cosmic expansion Light from distant galaxies exhibits redshifts related to large-scale expansion. At appropriate cosmological scales, distance and recession are related. Locally gravitationally bound systems require different treatment: our galaxy and atoms do not simply swell at the same rate as cosmic expansion. 2. Cosmic microwave background Radiation from when the universe cooled enough for light to travel freely is now observed in microwaves. This imprint dates to roughly 380,000 years after the beginning of the modeled expansion and reveals early conditions. Small temperature differences help investigate the origins of large-scale structures. 3. Light elements The abundances of light nuclei formed during the early hot phase provide important tests. Hydrogen and helium made up much of the initial chemical composition. Heavy elements were not all produced then: later stellar processes also matter.

Development, age and open questions

4. Structure formation Small differences in matter distribution can grow under gravity. Stars and galaxies formed later. Observing distant objects means viewing past epochs because their light takes time to arrive. 5. Estimating age Within the standard cosmological framework, the universe is approximately 13.8 billion years old. This estimate combines observations with physical models rather than directly measuring one star’s age. The observable universe need not encompass the entire universe. 6. Established and open issues The nature of dark matter and dark energy, very early physics and tensions between measurements remain under investigation. Cosmic inflation is a family of early-universe explanations, not a claim that every detail has been directly verified. Open questions do not erase independent evidence for a hot, expanding past.