 Quote by sharik007
General relativity fails to explain the ultimate end of the universe.
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And there's the case where the universe is at critical density and is globally flat; this geometry too leads to heat death. But general relativity doesn't
fail to explain the ultimate fate of the universe, any more than modern meteorology fails to explain the weather a month from now. If we knew precisely the current state of the universe (its density), we would be able to use general relativity to extrapolate its fate. It's not a failing of the theory, but a matter of our incomplete knowledge of the global curvature of the universe.
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Today's mordern theory is the M-theory and i think the only reason to know the end of this universe is known first how universe was created. According to concept of M-theory. The universe came to existance with the collusion of two multidimensional branes (branes are the multidimensional planes).
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That's not so much a prediction of M-theory as it is of braneworld models that are based on M-theory, like ekpyrosis. But, if I wish, I can base a braneworld model on M-theory that has no such collision.
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There energy of collusion created the matter and energy of this universe. Understanding this very sensitivily, when the two branes collide with each other it is obvious that after the collusion the branes move apart from each other and allowed this universe to expand freely without any disturbance of their density. Presently, the universe is expanding due to the spreading of the two branes from each other and minimizing the effect of their density on this universe. The decreasing effect of the density of two branes increases the expansion energy of this universe. Physicist called this dark energy. The reason of increasing dark energy is due to spreading of branes from eachother.
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The mere separation of branes along an extra dimension does not by itself impart an accelerated expansion on either of the branes. You still need to introduce some sort of nonzero vacuum energy that exists along the extra dimension (for example, a scalar field with nonzero vacuum could be chosen to parameterize this distance, like the so-called radion field in ekpyrosis). Then this vacuum energy leads to a nonzero effective vacuum energy on the branes, driving the accelerated expansion. But you still need this added ingredient of positive vacuum energy, just like in good old GR.
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Now when the two branes can come back again for collusion as it was at the time of big bang when they will come towards each other the effect of their density on this universe as a bubble will slow down its expansion and will make it to contract. The space of this universe will contract with all matter and energy and thus finally when the branes will again collide the universe contracts to zero volume and thats singularity. Thus this makes the universe cyclic.
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Yeah, but at the expense of introducing an incredibly
ad hoc, fine-tuned, and grotesque scalar potential to control this behavior of the branes. It is by no means natural, and is perhaps one of the main criticisms against the cyclic models.
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I hope this makes us to understand the ultimate demise of this universe through the study of this new modal from M-theory.
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Yes, it is one possibility. But M-theory is still speculation, as are the cosmologies built on it. Especially those that introduce fine-tuned degrees of freedom that are not themselves constrained or well-understood within M-theory.