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Investigation of the gravitational property of the quantum vacuum may explain the accelerating expansion of the universe.
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The source of dark energy is a long-standing mystery that ranks among the most enigmatic of outstanding questions in science. Since it was determined that the expansion of the universe appears to be accelerating, there has been an ongoing effort to determine the source of this expansive force.
Before it was even known that the universe appears to be expanding, Einstein understood that his gravitational solution of general relativity would require some counteracting force to keep the universe from collapsing in on itself (assuming a static-state universe). Einstein introduced the cosmological constant, a term with an undefined source but which counteracts large-scale gravitational attraction that would otherwise cause a steady-state universe to collapse. With Edwin Hubble's discovery of an apparently expanding universe, the cosmological constant was dropped. However, this idea has seen a recent revival in order to explain the accelerating expansion, yet the source of the force remains undefined and the constant itself must be fine-tuned to an extreme accuracy in order to give the correct values for what is observed.
Given the seeming contrived mechanism of the cosmological constant (which is one reason it was abandoned by Einstein in the first place), and the peculiarities of other potential explanations, such as scalar fields with a negative pressure, many have sought to explain the source of the accelerating expansion using principles that are already relatively known in physics but that may require a fresh new appraisal.
One such method has been undertaken by the physicist William Unruh, renowned for his work describing the thermal properties of the vacuum in non-inertial frames -- whereby an observer in an accelerating frame will detect a non-zero temperature of the ambient space as the accelerating frame causes the quantum vacuum to appear to emit black-body radiation.
Unruh and his doctoral students have shown that the extremely large energy density expectation value of the quantum vacuum, which is several orders of magnitude larger than the sum of the energy of every particle in the universe, is not as catastrophic as has been largely presumed. The extremely large energy density of the vacuum has led many physicist to assume that the calculation must somehow be fallacious, although no one has been able to determine why or how this is so.
Now, Unruh and his research team have shown that the spacetime fluctuations of the vacuum are almost completely balanced between expansive and contractive fluctuations of the gravitational field. There is however a slightly greater degree of the expansive fluctuations that result in a net force favoring expansion at the cosmological scale.
The results suggest that there is no need for the cosmological constant and that so called dark energy may indeed be sourced in the gravitational effects of the ubiquitous energy of space -- the quantum vacuum.
Unruh and his research team are not the first to take this particular approach to solving the problem of dark energy, as physicist Nassim Haramein and astrophysicist Dr. Amira Val Baker have described how the extremely large energy expectation value of the quantum vacuum is reconciled with the seeming exceedingly small energy density of the cosmological constant. As described in the latest study, the difference between these two values represents a 122 order of magnitude discrepancy in vacuum expectation values between that of dark energy and quantum field theory, the so-called 'vacuum catastrophe'.
In a presentation at the Royal Society in London, Haramein & Val Baker unveiled findings in a poster session in which Haramein's quantized solution to gravity, when applied to the mass-energy density of the universe, resolves the 122 order discrepancy of the standard cosmological model.
The study reveals that when the geometrical solution of the proton rest mass, Haramein's quantized solution to gravity, is scaled with the radius of the universe the exact value of the energy density of the cosmological constant is found. The presentation was well received, and the poster session can be found on the HIUP website (the paper is undergoing preparations for publication). Haramein and Val Baker's study is perhaps one of the first demonstrations of a physical mechanism of how the energy density expectation value from quantum field theory is not in disagreement with the dark energy value predicted for the universe, and thus one of the first fully consistent physical explanations for the expansion rate of the universe.
If free space has a net expansive force, it is interesting to speculate what spacetime geometry and torque associated with elementary particles may have on the vacuum fluctuations, whereby they result in a net quantum gravitational attractive force between matter. If it is large enough to drive apart galaxies at the cosmological scale, perhaps it is strong enough to bind subatomic particles and nuclei together at the atomic scale?