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The notion that the Cosmological Constant is nonzero is based on the idea that the vacuum can contain energy! In the vacuum, we know that fluctuations occur; in vacuum we find virtual pairs of particles continuously popping into and outof existence. The virtual pairs of particles cannot be measured directly, but their interactions (effects) can be measured and they can affect the evolution of the Universe.
This bizzarre notion that virtual pairs can just appear in vacuum has in fact been experimentally verified. The Casimir Effect has been observed. This occurs when two uncharged metal plates attract each other through the electromagnetic force, if they are placed close enough. This is odd. Also, there is something known as the Lamb Shift. The structure of a hydrogen atom (note atoms are held together by the electromagnetic attraction between the positively charged protons in their nuclei and the negatively charged electrons surrounding the nuclei) is found to be slightly altered by the appearance and disappearance of virtual pairs of particles. The seemingly odd notion of spontaneous creation and destruction of particle pairs in vacuum has been experimentally verified.
Strangely enough, it doesn't. In a quantum mechanical world, we cannot measure both the position and momentum of a particle to absolute precision simultaneously. At a given time, there is always an uncertainty as to where a particle resides and how much momentum it carries. The relationship between the uncertainty in the location and momentum of an object is given by the Heisenberg Uncertainty Principle,
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The production of virtual pairs of particles occurs spontaneously everywhere in the vacuum. What are the implications of this? Look at the panel to the left, as the piston pulls out of the cylinder (moves to the right), the number of virtual pairs grows as the vacuum grows. That is, as the volume grows, the energy (and therefore mass) of the vacuum get larger! This is strange and oppostite to what happens with a normal gas. When a normal gas expands, it cools, becomes less dense, and its pressure goes down; the pressure performs work (and loses energy) as it pushes the piston outward. For a vacuum, as the piston moves outward, the energy increases because the density of pairs remains the same. This is odd and technically means that the pressure associated with the vacuum is negative ===> its effective mass is negative and so the vacuum acts like anti-gravity. Such vacuum energy may be the controlling force in the current evolution of our Universe! |
To see how this works, let's consider a simple estimate for the vacuum energy. The appearance and disappearance of virtual pairs of particles must not be observable and so, this sleight of hand must take place over a Compton wavelength, the size of the region over which we cannot precisely pinpoint the location of a particle. So, Virtual Pairs of particles that form in vacuum, arise in boxes which are roughly the size of their Compton wavelengths. This allows us to estimate the density of Virtual Pairs of a given particle. If we then sum up the contribution to the vacuum from all of the known kinds of particles, an estimate for the Dark Energy can be made. Annoyingly, the estimate is
predicted to observed = 10120
Ooops! This is not good even by astrophysical standards. In order to bring the observed Cosmological Constant and its theoretical estimate into line requires that fluctuations in the vacuum be arranged so that there there is some fierce cancellation going on. This turns out to be the grand-daddy of the fine-tuning problems. Without fine-tuning, even if we consider only protons, we would predict rather fierce effects for even our everyday perceptions,
The effect on the Universe even on our level would be noticeable in that we could redshifts of objects as they move off into the distance! There clearly must be fine-tuning going on. This is the
