Dark matter is one of those terms that immediately piques the interest of anyone fortunate enough to hear it, but why? Perhaps it’s the fascination our species seems to have acquired for those mysterious, fantastical ideas, or even simply because it just sounds so cool.
But what exactly is dark matter? Simply put, nobody knows, which is where the name comes from. It’s a strange, unidentified form of matter that makes up roughly 27% of the mass and energy within the observable universe. It doesn’t interact in any way with or emit electromagnetic radiation, and as such is quite difficult to detect. The only reason we know it’s there is because of its gravitational effects on other things within the universe, and it is suspected that our current estimations of the mass of many celestial bodies may be too small because of only accounting for visible ‘luminous’ matter.
“It’s a strange, unidentified form of matter that makes up roughly 27% of the mass and energy within the observable universe”
It is hardly surprising then that a new model for gravity that does away with the existence of dark matter entirely caused ripples in the scientific community. Initially proposed in 2010 by Professor Erik Verlinde, string theory expert at the University of Amsterdam, this model posits gravity to be an ‘emergent phenomenon’, rather than a fundamental natural force.
Physicists have been struggling for decades to reconcile gravity with quantum mechanics, as the predictions of both highly regarded elements of modern physics do not combine properly when applied to extreme situations, be it during the Big Bang or when approaching black holes.
It’s certainly not the first time we’ve seen a non-standard answer to this question; Modified Newtonian dynamics (MOND) was introduced by Israeli physicist Mordehai Milgrom in 1983, to address the discrepancies between the measured size of galaxies and that predicted by conventional Newtonian mechanics. Instead of the gravitational force experienced by a star being proportional to its spin, Milgrom proposed that the force be proportional to the square of that value instead. While his calculations do technically hold true, scientists have yet to build a useful cosmological model in this way because it fails to account for the behaviour of large-scale galaxy clusters.
Verlinde’s hypothesis however considers gravity as a chaotic force that appears due to a difference in how much disorder there is within a system. His new theory begins entirely from first principles rather than fine-tuning existing formulae to fit new observations. Another key element is his version of the holographic principle, introduced by his tutor Hooft in 1999. Simply put, this concept means that every single piece of information in the universe can be fully described on an enormous hypothetical sphere around it. Verline has adjusted this idea with the caveat that some of this information must be contained within physical space itself.
Like any scientist, Verlinde has received challenges from fellow physicists. His theory relies on entropic gravity being correct, which is highly contested. As it makes no predictions and it is not falsifiable, which is one of the cornerstones of the modern scientific process. Furthermore, entropic processes should break quantum coherence (the idea that subatomic particles are able to ‘communicate’ with one another, much like a series of tuning forks all resonating together). Experimental observation of ultra-cold neutrons has shown that this coherence remains intact, potentially disproving the theory.
Verlinde remains resolute, however, and in this case heavyweight intellectuals backing either side of the argument has resulted in enormous breakthroughs in this area of science. This particular conflict has sparked interest within the community at large, and whether Verline is right or wrong, humanity has surely taken a good step forward in the process.