A few weeks ago we discussed a new theory of gravity that has been hotly disputed and debated since it was formulated a few years ago. Now, a new set of astronomers has produced observations that are consistent with predictions made in that study, though their paper is yet to be peer-reviewed.
This has been seen as strong evidence for the new idea, called emergent gravity. The theory is proposed by Professor Erik Verlinde, a researcher from the Delta Institute for Theoretical Physics in Amsterdam, and its core idea is that gravity is not a real force but is just an effect due to the increasing entropy of the universe.
The evidence can, however, just as easily be explained by the current view of the universe, known as ΛCDM (Lambda-CDM) cosmology, which expects galaxies to be surrounded by dark matter, a substance that doesn’t interact with visible light and is invisible to our instruments.
So what’s the fuss all about? Dark matter being a theoretical particle requires several parameters that we are not sure of, while the new theory, emergent gravity (EG), comes from extensions of accepted physical formulas. So should we be excited? Not really.
The observations were possible thanks to gravitational lensing, which is the bending of light due to heavy objects. Gravitational lensing is predicted in general relativity and it has been used in the extensive testing of the standard cosmological model.
“Although there is no direct description of lensing and cosmology in EG yet, we can make a reasonable estimate of the expected lensing signal of low redshift galaxies by assuming a background ΛCDM cosmology,” the researchers said in the paper, which can be viewed on arXiv.org.
So emergent gravity wants to do away with general relativity and dark matter, but to be tested it requires dark matter and dark energy and general relativity. On top of that, while the ΛCDM predicts the real universe, emergent gravity is only applicable to isolated, spherical, and static systems.
Verlinde's paper, which is published in the Monthly Notices of the Royal Astronomical Society, claimed the theory passed its first astronomical test, but it had been tested before with less success. While emergent gravity indicates the same effects and results we have seen from general relativity and dark matter, in the quantum realms its predictions are significantly different. Experiments on the gravitational bound state of neutrons categorically disprove this theory.
As Carl Sagan said, "extraordinary claims require extraordinary evidence," and we are yet to see the required evidence for the idea of emergent gravity.