Abstract
We show that the usual formulation of the cosmological constant problem
breaks down when the effect of the huge fluctuations in quantum vacuum
stress-energy tensor is considered. Even if one has successfully fine-tuned the
bare cosmological constant in the Einstein equations to the required accuracy
of $10^-122$, the fluctuations would still cause the universe to explode. The
fluctuations would also produce a large positive contribution to the averaged
macroscopic spatial curvature of the Universe. In order to cancel this
contribution, the bare cosmological constant has to take large negative values,
and if it is large enough, the spacetime structure would be similar to the
cyclic model of the universe in the sense that at small scales every point in
space is a "micro-cyclic universe" which is following an eternal series of
oscillations between expansions and contractions. Moreover, due to the weak
parametric resonance effect caused by the fluctuations of the quantum vacuum
stress-energy, the size of each "micro-universe" increases a tiny bit at a
slowly accelerating rate during each micro-cycle of oscillation. Accumulation
of this effect over the cosmological scale gives an accelerating universe. More
importantly, the extreme fine-tuning of the cosmological constant is not
needed. This resolves the cosmological constant problem and suggests that it is
the quantum vacuum fluctuations serve as the dark energy which is accelerating
the expansion of our Universe.
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