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What Is Emergent Gravity Otherwise Known as Dark Gravity?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

Dark matter and dark energy remain some of the biggest mysteries in physics. For decades, scientists have attempted and toiled largely in frustration as theory after theory has bitten the dust. This darkness just seems to be beyond current scientific tools. But what if we are looking at the picture wrong? Maybe our idea of missing things out there is just incompleteness in a current theory we don’t have enough knowledge on. Enter the alternate theories, and one of the most intriguing is dark gravity.

Dark Gravity Physics

Work by Erik Verlinde seems to show that dark energy and dark matter don’t really exist. He took a look at one of the clues for dark matter: gravity. By examining how this weak force operates on larger scales, one can see the theories don’t predict what we see and hence the need for a dark material to fill in the void. Galaxies are too light without it, star movement is all wrong, and the gravitational pulls we see would result from nothing if relativity was solely operating (O’Connell, Maartens).

But Verlinde has a solution to save gravity and eliminate the unnecessary fluff. He postulates that gravity is really a property that arises from the field of statistics – that is, particle interactions or the kinetic energy model for thermodynamics. By examining the entropy associated for a portion of de-Sitter space and how it’s affected when matter is present near it (like with gravity), Verlinde was able to draw parallels between this dark gravity and the dark energy’s accelerated expansion of the Universe. For a given region, we can talk about a holographic layer for a space that conveys the information of the space on its surface. When sufficient matter is present, entropic effects are minimized as entanglements settle out, our layer separating spaces breaks down and so we get Newtonian gravity. But when we have little matter over a large space, our entropic effects are not mitigated and we get dark energy behavior as the region expands. And when this emergent gravity effect interplays with large amounts of matter on a macroscale, we get dark matter behavior. The information isn’t just on the surface in that layer, it’s in the space itself. Verlinde initially developed a gravity model based on this concept in 2010 that accurately predicted Newtonian and Einstein’s gravity, but in 2017 he was able to extend this dark gravity model to large scales and demonstrate that this was sufficient to provide the forces scientists have seen. Dark energy is really just an emergent feature of space-time gravitational effects on a microscopic scale that grow to a macroscopic effect (Lee "Emergent," Kruger, Wolchover, Skibba, O’Connell, Delta, Mosher).

Alexander Peach (Durham University) extended this work to consider what happens with emergent/non-emergent regions of space that are separated by a holographic layer break down. That holographic boundary deals with information of the emergent space as conveyed to the non-emergent (in the form of gravity) with a reduction of a degree a usual consequence of this. If we have a massive particle in proximity to this layer then any changes to its position will correlate to how the layer’s entropy is. It’s essentially an emergent force happening to our separated region, and the work by Peach shows that for a critical radius, the holography collapses and violates our physical laws…unless it’s non-holographic beyond that point, but still separated. We therefore have found the boundary when we transition from holography to non-holographic emergent spaces. Couple this with the changes in entropy and thermodynamics as the region grows and we have a new, bulk-like explanation which accounts for the layer’s collapse. That is, it’s a dark matter explanation from an emergent dark gravity scenario that Verlinde’s work only brushed over and gives a new explanation for the dark matter properties that the emergent dark gravity is attributed to. It should be noted that the most basic formula of Verlinde’s which uses anti-deSitter space (not like our reality) was developed, so it remains to be seen how a more complicated model will hold up but this holographic work does reflect our reality better and is a step in the right direction. It really hits home how the information of gravity isn’t on our layers but in the space itself because that holographic layer collapses. This extension also gives a network approach to mapping out the effects predicted by the theory (Peach, Delta, Mosher).

Testing it Out

To see if dark gravity has any merit, we need some evidence for it. Observations by Margot Brouwer (Leiden Observatory) and team were done on gravitational lensing objects to find the mass of 33,613 galaxies, as recorded by GAMA and KiDS arrays. With these in mind, they ran all the necessary parameters into both dark matter and dark gravity models, and wouldn’t you know it: They both gave the same result (O’Connell, Mosher).

So, it’s a start. Let’s see where this takes us.

Works Cited

Delta Institute for Theoretical Physics. “New theory of gravity might explain dark matter.” Phys.org. Science X Network, 08 Nov. 2016. Web. 06 Mar. 2019.

Lee, Chris. "Diving Seep into the World of Emergent Gravity." arstechnica.com. Kalmbach Publishing Co., 22 May 2017. Web. 10 Nov. 2017.

Kruger, Tyler. "The Case Against Dark Matter. Astronomy.com. Kalmbach Publishing Co., 07 May 2018. Web. 10 Aug. 2018.

Maartens, Roy. “Dark Energy and Dark Gravity.” Doi:10.1088/1742-6596/68/1/012046.

Mosher, Dave. “Astronomers found evidence for a ‘dark’ gravitational force that might fix Einstein’s most famous theory.” Businessinsider.com. Insider, Inc., 14 Dec. 2016. Web. 06 Mar. 2019.

O’Connell, Cathal. “New theory of ‘dark gravity’ passes first test, but Einstein’s still on top.” Cosmosmagazine.com. Cosmos. Web. 05 Mar. 2019.

Peach, Alexander. “Emergent Dark Gravity from (Non)Holographic Screens.” arXiv:1806.1019v1.

Skibba, Ramin. "Researchers Check Space-Time to see if It's Made of Quantum Bits." quantamagazine.com. Quanta, 21 Jun. 2017. Web. 27 Sept. 2018.

Wolchover, Natalie. "The Case Against Dark Matter." quantamagazine.com. Quanta, 29 Nov. 2016. Web. 27 Sept. 2018.

© 2020 Leonard Kelley