if every Mass-containing-body has its own gravity, then why does the universe expand and why does it not collapse upon itself? I WANT A GOOD AND PROPER EXPLANATION !!
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Answer:
The field equations are differential equations and cannot be helpful in determining the topology of the universe. But there are only two topologic options, a sphere or a hypersphere. A three-dimensional spherical structure embedded in an infinite flat environment leads to contradictions with observations and with plausible scenarios. A hypersphere topology corresponding to the surface of a four-dimensional sphere avoids those contradictions but needs a plausible explanation of the red shift.
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.
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