Question

why is the universe expanding at a rate such that it avoids its own collapsing?​

Answer 1

Answer:

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Explanation:

The expansion of the universe can be explained by General Relativity using the Friedmann-Robertson-Walker metric, which contains a scale factor a(t) that determines the size of cosmos. Upon solving Einstein's field equations for ordinary matter (eg. dust), the scale factor is found to increase as t to the two-thirds power. Thus, GR predicts the universe expands from a point, called the big bang, and its expansion slows down with time. This solution does not involve pressure, as dust is assumed to be pressureless.

However, in the 1990's it was discovered through Supernova Type Ia observations that the expansion of the universe is not slowing down, but is accelerating. Cosmic acceleration at first appeared to contradict GR. A number of modified gravity theories have been proposed to explain it, but none have gained general acceptance. The most favored theory today is the Lamda-Cold-Dark-Matter (LambdaCDM) model, whereby cosmic acceleration is described by adding a cosmological constant Lambda to Eintein's field equations. This constant is often interpreted as "Dark Energy", a phantom substance with pressure equal to minus the mass-energy density. The model has a number of outstanding problems, and is the subject of ongoing research.