Astrophysicists need to choose between dark matter or modified gravity to explain the Universe. But a strange new duality may mean they can have both
The debate over the wave or particle-like nature of light consumed physicists for 300 years after Isaac Newton championed particles and Christian Huygens backed the idea of waves. The resolution, that light can be thought of as both a wave and a particle, would have astounded these giants of physics, as indeed, it does us.
What shouldn't surprise us, though, is that other seemingly intractable arguments might be similarly resolved.
But exactly this may be in store for the dark matter conundrum which has puzzled astrophysicists for almost 80s years, according to Chiu Man Ho at Vanderbilt University in Nashville and a couple of buddies,
The problem is that galaxies rotate so fast that the matter they contain ought to fly off into space. Similarly, clusters of galaxies do not seem to contain enough mass to bind them together and so ought to fly apart. Since this manifestly doesn't happen, some force must be holding these masses in place.
Astrophysicists have put forward two explanations. The first is that these galaxies are filled with unseen mass and this so-called dark matter provides the extra gravitational tug. The second is that gravity is stronger at these intergalactic scales and so does the job by itself, an idea called modified Newtonian dynamics or MOND.
There is no love lost between the dark matter proponents and their MONDian counterparts: both say the other is wrong and scour the Universe in search of evidence to damn their opponents. Neither side has convincingly crushed the other's argument so far but all concerned seem to agree that when one triumphs, the other will be ground underfoot.
Perhaps there's another possibility, however: that they're both right.
What makes this possible is a new approach to gravity in which it is an emergent phenomenon related to entropy. We looked at this a few months ago here
The basic idea is that parts of the Universe have different levels of entropy and this creates a force that redistributes matter in a way that maximises entropy. This force is what we call gravity.
So far, this approach has assumed a simple Universe. But cosmologists know that our Universe is not only expanding but accelerating away from us. What Chui and co have done is derive gravity as an emergent force using the same entropic approach but this time in a Universe that is accelerating.
The result is a form of gravity in which parameters for acceleration and mass share a strange kind of duality: either the acceleration term can be thought of as modified as in MOND; or the mass term can be though of as modified, as in the dark matter theory.
In effect, Chui and co are saying that dark matter and MOND are two sides of the same coin.
Interestingly, the effect of each type of modification seems to be scale dependent. In this theory, the MONDian interpretation works at the galactic scale while the dark matter interpretation works best at the scale of galactic clusters.
That's actually how the observational evidence pans out too. MOND seems to better explain the real behaviour of galaxies while the dark matter approach better explains the structure of galaxy clusters.
Could it be that both are manifestations of the same thing? Only the brave or foolish would rule it out. And stranger things have happened in physics, as Newton and Huygens would surely attest to.