TY - GEN
T1 - Cosmic time
T2 - Symposium on The Science of Time, 2016
AU - Impey, Chris
N1 - Publisher Copyright:
© 2017, Springer International Publishing AG.
PY - 2017
Y1 - 2017
N2 - Cosmology presents intriguing issues for the understanding and tracking of time. The big bang theory says that the universe began 13.8 billion years ago, in a situation of almost unimaginable temperature and density. The age of the universe is highly constrained by the world model, as long as there are reliable 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 cosmic evolution reach back to about 380,000, years (recombination), 40,000, years (matter domination), and a few minutes (light element creation) after the big bang. Time in Newtonian cosmology is absolute, linear, and eternal, whereas time in the modern cosmology is governed by Einstein’s general relativity, a geometric theory which embodies a profound connection between space and time. In relativity, objects travel on paths called world lines in four-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 time concept rests on the cosmological principle—the assumption that 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.
AB - Cosmology presents intriguing issues for the understanding and tracking of time. The big bang theory says that the universe began 13.8 billion years ago, in a situation of almost unimaginable temperature and density. The age of the universe is highly constrained by the world model, as long as there are reliable 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 cosmic evolution reach back to about 380,000, years (recombination), 40,000, years (matter domination), and a few minutes (light element creation) after the big bang. Time in Newtonian cosmology is absolute, linear, and eternal, whereas time in the modern cosmology is governed by Einstein’s general relativity, a geometric theory which embodies a profound connection between space and time. In relativity, objects travel on paths called world lines in four-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 time concept rests on the cosmological principle—the assumption that 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.
KW - Big bang
KW - Cosmic time
KW - Cosmology
KW - Expansion
KW - Universe
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U2 - 10.1007/978-3-319-59909-0_1
DO - 10.1007/978-3-319-59909-0_1
M3 - Conference contribution
AN - SCOPUS:85031304972
SN - 9783319599083
T3 - Astrophysics and Space Science Proceedings
SP - 1
EP - 13
BT - The Science of Time 2016 - Time in Astronomy and Society, Past, Present and Future
A2 - Gabor, Pavel
A2 - Arias, Elisa Felicitas
A2 - Combrinck, Ludwig
A2 - Hohenkerk, Catherine
A2 - Seidelmann, P. Kenneth
PB - Springer Netherlands
Y2 - 5 June 2016 through 9 June 2016
ER -