The Emergence of Gravity and Dark Matter
One of the most exciting ideas in modern physics is that gravity is not a fundamental force, like electromagnetic or nuclear forces. Instead, it's an emergent phenomenon, that merely looks like a traditional force.
This approach, known as entropic gravity, has been championed by Prof. Dr. Erik Verlinde at the University of Amsterdam who put forward the idea in 2010. He suggested that gravity is an entropic force caused by changes in the information associated with the positions of material bodies. In his view, gravity is the manifestation of a deeper process, in the same way water is wet, but individual water molecules are not, or just as pressure and temperature are the result of the behavior of molecules.
According to Verlinde, "Most people, certainly in physics, think we can describe gravity adequately using Einstein’s General Relativity. But it now seems that we can also derive it from a microscopic formulation where there is no gravity to begin with. This is called ‘emergence’."
In his most recent work, Verlinde is focusing on dark matter. "We think we understand gravity in most situations," he says, "but when we look at galaxies and, on much larger scales, at galaxy clusters, we see things happening that we don’t understand using our familiar equations, like Newton’s equation of gravity or even Einstein’s gravity. So we have to assume there’s this mysterious form of matter that we cannot see, which we call dark matter. Now dark energy is even weirder, in the sense that we don’t even know what it consists of. It’s something we can put in our equations to make things work, but there’s really a big puzzle to be solved in terms of why it’s there and what it’s made of."
Physicists and astronomers have shown a lot of interest in Verlinde's work, but that hasn't stopped them from doing plenty of great experiments designed to find out where the dark matter might exist in the universe. Satellites are looking for it, as well as detectors in salt mines trying to catch the particles involved. And the big LHC accelerator at CERN in Geneva puts a lot of energy into colliding protons in an effort to produce candidates for dark matte.
According to Verlinde these quests, however interesting they may be, don't stand a chance, because in his theory the extra gravity in the universe emerges naturally from the calculations, without any specific particle needed. Verlinde's theory describes the universe as a kind of hologram, in which all information about masses is written on the imaginary edge of the universe. When two masses are taken apart, the change in this information produces a resistance, which can be felt as a weight. In a nutshell, one gets a gravitational universe as a 'hologram' of a non-gravitational universe.
Verlinde says: "The universe is expanding and the farther a galaxy is from us, the faster it moves away. At a certain point it moves away from us faster than light. That is our horizon. We can't see what happens beyond that. That physical horizon has the same properties as the horizon of a black hole."
At short distances, his theory neatly gives us Newton's laws, but for greater distances deviations occur because the universe is still expanding. Verlinde shows how these deviations explain exactly the rotation of galaxies, without any need for invoking dark matter.
One of the aspects that we are now coming to understand about gravity is that in the final theory, gravitation and even spacetime itself will be closely related to the mysterious quantum mechanical property known as entanglement. If gravity can emerge from a set of physical laws without gravity, then something special about that non-gravitational physics has to make it happen. Physicists have now found that that special something is quantum entanglement: the special correlations among quantum mechanical particles that defies classical description.
Of course the theory of entropic gravity is still highly controversial. Some physicists find it a superficial description of something obvious, others think it's downright wrong. But as long as every answer in cosmology seems to evoke new questions, to view the issues from a diversity of angles remains the right approach.