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In GR, a decelerating universe raises a series of questions: the horizon problem (why are opposite sides of the sky similar when they are casually unconnected?), the smoothness problem (Why are the fluctuations ~ 10-5 just right to produce a universe with large scale structure and galaxies etc. yet not too great so all matter clumps together in a few hyper-massive BHs?), the density problem (Why is \Omega_{total} ~ 1?), which can only be answered by special pleading - i.e. by setting specific initial conditions that can perhaps only be explained by Anthropic reasoning.SpaceTiger said:It's not clear to me why. If LCDM is not claiming to explain the origin of fluctuations, why would a problem concerning the initial distribution of those fluctuations bring it into question?
Another answer is of course that the universe may not have been decelerating over most of its history. [Apart from the Inflation era: 10-35 sec to 10-33 sec, according to the mainstream model the universe has been decelerating from the Planck era t = 10-43 sec to t > 10+17 sec, when DE acceleration kicked in. The present age t ~ 4 x10+17 sec.]
The monopole problem is different in that it arises from the GUT, which predicts magnetic monopoles should be plentiful and detectable. A lack of their detection therefore requires an explanation, such as Inflation, which would have diluted their density to undetectable levels.
Another explanation is of course that the GUT is wrong and they never existed in the first place.
Inflation resolves these problems by injecting massive expansion at that early yet post-Planck era stage, which more than counteracts the effects of the subsequent deceleration. Without it the standard model has some explaining to do.
One resolution would be to have an unorthodox equation of state for DE in order to have an extended era of acceleration, i.e. a kind of 'smeared out' inflation, or indeed a strictly linear expansion

Garth
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