Universe, Contracting or Expanding

In the 1920s the physicists Alexander A. Friedmann (1888–1925) and George Lemaître (1894–1966) succeeded in describing the global dynamics of the universe by simplifying the field equations of the general theory of relativity with the help of symmetry assumptions. The equations they found do not allow for a stable, static universe. This result led Albert Einstein to add a so-called cosmological constant to his equations in order to achieve an equilibrium state for the universe. In 1926, however, the American astronomer Edwin P. Hubble (1889–1953) observed all distant galaxies as moving away from the earth, the more distant stars receding the fastest. After this discovery, the expansion of the universe was considered a fact, and even Einstein finally removed the cosmological constant from his field equations. There are nevertheless local deviations from the global expansion, if the gravity between close-by objects manages to overcome the global Hubble drag. For example, the Milky Way and the Andromeda galaxy are approaching each other at the rate of some dozen miles per second to merge in 2 to 4 billion years.

It is generally thought that right after the big bang, the beginning of space and time, the universe ran through a very short period of extremely fast expansion called inflation. This very enormous expansion passed over to the moderate one still taking place today. One usually assumes a phase transition of a spatial homogenous scalar quantum field that exerts a negative pressure as a reason for inflation. The detailed origin and course of inflation is still a matter of speculation.

Based on numerous observations, especially by the NASA space probe WMAP, cosmologists are today in a position to characterize the state of the universe via just a few parameters. The most important of these are the mean cosmic density ω; the Hubble parameter H0, characterizing the current expansion rate; and finally the cosmological constant, which is, along with the mean density ω, crucial for the future of the cosmic expansion. Depending on the values of these numbers we live in a universe that either will bring its expansion to a halt and maybe even convert it to contraction in a far distant future-or that will expand forever. The connection between H0, ω, and ω on the one hand and the history and fate of the universe on the other is given in a simple way by the Friedmann -Lemaître equations. Following the parameters, it is widely assumed today that out universe will even accelerate its expansion in the future, instead of slowing it down.

[Page 1393]This knowledge makes any philosophical speculation needless concerning the so-called arrow of time. In his most famous book, A Brief History of Time, astrophysicist Stephen Hawking speculates about the direction of time and a decrease of entropy in a contracting universe. The possibility of a time inversion is, however, generally not accepted. Rather, the “cosmological arrow of time” is interpreted in a way that the particular size of an exclusively expanding universe allows us to distinguish both globally and uniquely between the past and the future.

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