Cosmology from quantum fluid (no "big bang")

[Philip Thrift]

https://arxiv.org/abs/1404.3093 :

Cosmology from quantum potential

Ahmed Farag Ali, Saurya Das

It was shown recently that replacing classical geodesics with quantal (Bohmian) trajectories gives rise to a quantum corrected Raychaudhuri equation (QRE). In this article we derive the second order Friedmann equations from the QRE, and show that this also contains a couple of quantum correction terms, the first of which can be interpreted as cosmological constant (and gives a correct estimate of its observed value), while the second as a radiation term in the early universe, which gets rid of the big-bang singularity and predicts an infinite age of our universe.

https://www.physics-astronomy.org/2019/08/no-big-bang-quantum-equation-predicts.html

No Big Bang? Quantum Equation Predicts Universe Has No Beginning

...

In their paper, Ali and Das applied these Bohmian trajectories to an equation developed in the 1950s by physicist Amal Kumar Raychaudhuri at Presidency University in Kolkata, India. Raychaudhuri was also Das's teacher when he was an undergraduate student of that institution in the '90s.

Using the quantum-corrected Raychaudhuri equation, Ali and Das derived quantum-corrected Friedmann equations, which describe the expansion and evolution of universe (including the Big Bang) within the context of general relativity. Although it's not a true theory of quantum gravity, the model does contain elements from both quantum theory and general relativity. Ali and Das also expect their results to hold even if and when a full theory of quantum gravity is formulated.

In addition to not predicting a Big Bang singularity, the new model does not predict a "big crunch" singularity, either. In general relativity, one possible fate of the universe is that it starts to shrink until it collapses in on itself in a big crunch and becomes an infinitely dense point once again.

Ali and Das explain in their paper that their model avoids singularities because of a key difference between classical geodesics and Bohmian trajectories. Classical geodesics eventually cross each other, and the points at which they converge are singularities. In contrast, Bohmian trajectories never cross each other, so singularities do not appear in the equations.

In cosmological terms, the scientists explain that the quantum corrections can be thought of as a cosmological constant term (without the need for dark energy) and a radiation term. These terms keep the universe at a finite size, and therefore give it an infinite age. The terms also make predictions that agree closely with current observations of the cosmological constant and density of the universe.

In physical terms, the model describes the universe as being filled with a quantum fluid. The scientists propose that this fluid might be composed of gravitons—hypothetical massless particles that mediate the force of gravity. If they exist, gravitons are thought to play a key role in a theory of quantum gravity.

In a related paper, Das and another collaborator, Rajat Bhaduri of McMaster University, Canada, have lent further credence to this model. They show that gravitons can form a Bose-Einstein condensate (named after Einstein and another Indian physicist, Satyendranath Bose) at temperatures that were present in the universe at all epochs.

Motivated by the model's potential to resolve the Big Bang singularity and account for dark matter and dark energy, the physicists plan to analyze their model more rigorously in the future. Their future work includes redoing their study while taking into account small inhomogeneous and anisotropic perturbations, but they do not expect small perturbations to significantly affect the results.

 "It is satisfying to note that such straightforward corrections can potentially resolve so many issues at once," Das said.

(The cosmos is made of fluid? So Thales was right.)

@philipthrift

[Lawrence Crowell]

One obvious possible problem is that this employs Bohm QM. If you take the Klein-Gordon equation and do the Bohm calculation you find that the putative particle moves faster than light.This is one reason it is so commonly said Bohm QM is not compatible with special relativity. Then to try to work with general relativity is going to be a far bigger tangle to work through.

LC

[Philip Thrift]

The "quantum potential"  should be approached instead via

Feynman Paths and Weak Values

Robert Flack, Basil J. Hiley

https://pdfs.semanticscholar.org/5e42/85d4e8ca27499a0e6a3b24e5d4b5111bb40d.pdf

(Published in Entropy 2018)

There has been a recent revival of interest in the notion of a ‘trajectory’ of a quantum particle. In this paper, we detail the relationship between Dirac’s ideas, Feynman paths and the Bohm approach. The key to the relationship is the weak value of the momentum which Feynman calls a transition probability amplitude. With this identification we are able to conclude that a Bohm ‘trajectory’ is the average of an ensemble of actual individual stochastic Feynman paths. This implies that they can be interpreted as the mean momentum flow of a set of individual quantum processes and not the path of an individual particle. (This enables us to give a clearer account of the experimental two-slit results of Kocsis et al.)

The approach outlined in this paper shows that the basic assumption made in Bohmian mechanics, namely, that each particle follows one of the ensemble of ‘trajectories’ (calculated by Philippidis et al.) cannot be maintained. Rather the trajectories should be interpreted as a statistical average of the momentum flow of a basic underlying stochastic process.

@philipthrift

[Brent Meeker]

On 8/24/2019 6:11 AM, Philip Thrift wrote:

The "quantum potential"  should be approached instead via

But Feynman paths can cross.

Brent

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[Philip Thrift]

But

a Bohm ‘trajectory’ is the average of an ensemble of actual individual stochastic Feynman paths

@philipthrift