Cosmic Time – From the Big Bang to the Eternal Future

Impey, Chris

Cosmology presents intriguing issues for the understanding and tracking of time. The big bang theory stipulates that the universe came into being 13.8 billion years ago, starting from a situation of unimaginable temperature and density. The age of the universe is highly constrained by the model, as long as there are good measurements of the current expansion rate (the Hubble constant), and amounts of baryonic matter, dark matter, and dark energy. A crucial cross-check on the model age comes from stellar chronometers and the ages of the oldest stars in the Milky Way. Landmarks in evolution reach back to 380,000 years (recombination), 10,000 years (matter domination), and a few minutes (light element creation) after the big bang. Whereas time in Newtonian cosmology is absolute, linear, and eternal, time in the modern cosmology is governed by Einstein’s general relativity, a geometric gravity theory which embodies a profound connection between space and time. In general relativity, objects travel on paths called world lines in 4-dimensional spacetime. Relativity defines proper time as time measured by an observer with a clock following a world line. A clock in motion relative to the observer or in a different gravity situation will not measure proper time. The concept of time rests on the cosmological principle – the assumption the universe is homogeneous and isotropic on large scales. If that is true, there are well-defined relationships between time, scale factor, and temperature going all the way back to the first fraction of a second after the big bang. Time in the far future of the universe can be measured in terms of physical processes – the spinning down of pulsars and the evaporation of black holes. There are still profound physical and philosophical issues raised by the definition of clocks and observers in cosmology.

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