What is Dark Fluid?
There is so much darkness in the universe, no more so than man’s knowledge about what is out there and what it is made of. We used to think we knew the Standard Model, but every so often a new theory arises and filters through the scientific consciousness and darkens the cosmic waters again. Enter: Dark Fluid.
First, we had Dark Matter to explain the fact that there aren’t enough visible stars or gas in the structure of galaxies to explain their high rate of rotation. This non-visible Dark Matter is so-called, because it is unseen and it does not interact with the light-bearing electromagnetic force. Second, came Dark Energy to explain that the universe was not only expanding, but accelerating in its expansion. This Dark Energy was accounted for by the fact that Type Ia supernovae in distant galaxies were fainter, and thus further away than expected. Lastly, there is Dark Flow, the controversial observation that certain galaxy clusters, indifferent to the prevailing cosmic microwave background radiation, are moving toward one specific universal location. Now we have another dark and mysterious conundrum regarding the universe.
Dark Fluid seeks to explain Dark Matter and Dark Energy as emergent properties of Dark Fluid. Theorised in 2008 by Dr HongSheng Zhao, a British astrophysicist at the University of St Andrew’s School of Physics and Astronomy, he proposed that Dark Matter and Dark Energy are not separate physical phenomena with separate origins, but are “specific sub-effects of new extended laws of gravity at very large scales.”
Dark Fluid sees the Einsteinian spacetime fabric as a fluid that flows, condenses, or expands just like any other fluid. Around matter, Dark Fluid slows down and thickens, drawing more space around it, intensifying the gravity around it, much as Dark Matter is supposed to do. Also, where there is less matter, as in interstellar space, Dark Fluid expands, stretching away from itself becoming like a repulsive force, which is the same effect as Dark Energy. Dark Fluid could also be the model behind Inflation (after the Big Bang), Quintessence (a negative pressure scalar field), MOND (Modified Newtonian Dynamics) and more.
But what is Dark Fluid made of? Is it the same stuff theorised to comprise Dark Matter? Are neutrinos, neutralinos, or gravitinos the missing matter, clumping and expanding to cause the emergent properties seen as dark matter and dark energy? Is there a prevailing neutralino field or force that is the key to understanding gravity on large scales? Dr Zhao believes efforts to find large dark-matter particles with the Large Hadron Collider at CERN will fail as Dark Fluid would be a low-energy particle, too low and undetectable for the Large Hadron Collider. However, Dark Fluid may point to the absence of dark-matter particles. Dark Fluid may not be particles of energy, but just a modification of the law of gravity.
So if Dark Fluid is a modification of gravity then are gravitons its primary component? If neither gravitons nor neutrinos, or neutralinos are discovered or are not responsible for any of the Dark materials, then could a new particle, a hypothesised graviphoton perhaps (which can be repulsive in force), be responsible? How can a force be attractive and repulsive at the same time? Would Dark Fluid matter change, depending if it manifested as Dark Matter or Dark Energy? Can Dark Fluid be hot, cold or warm like Dark Matter? Just as water can freeze, become liquid, and turn into gas; the same material under different conditions, can Dark Fluid have differing effects and manifestations under different cosmic conditions? If space is basically a super-fluid stretching and contracting according to the matter around it, Dark Fluid could cause varying effects which to the outside observer would seem to be several different manifestations, when in fact it is just the same phenomenon reacting in different ways.
In this writer’s speculative opinion, Dark Fluid’s properties are similar, yet different to water. Assuming Dark Fluid concentrates around other matter; its relative particle temperature should fractionally increase due to pressure, friction, drag, density, etc, thus it should expand like liquid water or gas. Hence, around large cosmic structures like galaxies there should be less gravitational effects/less density (Dark Matter) but more energy/more velocity (Dark Energy). Galaxies should have flown apart. But the opposite is true. The gravitational force, though weak, must be binding the Dark Fluid as a cohesive unit. This manifests as Dark Matter.
Liquids contract in the cold. Hence, interstellar space should have large masses of cold, clumped Dark Fluid and be less energetic. But the opposite is true. Dark Fluid expands in the ‘colder’ regions of interstellar space as when water turns to ice. Thus Dark Fluid’s density is lower than surrounding space, thus it floats away at an accelerated rate. Gravity, as a weak force, cannot bind the particles together. This manifests as Dark Energy.
So is Dark Fluid the answer? Whatever the properties of Dark Fluid and its relationship to Dark Matter and Dark Energy, there is much to ponder. Many different cosmological models turned out to be part of the same String Theory. Dark Matter and Dark Energy may be strings on the same Dark Fluid bow. Maybe Dark Flow is Dark Fluid heading down the universal drain. At the moment, Dark Fluid is an elegant theory, which answers many questions, is testable through existing models, and makes predictions. How long that lasts is a Dark Mystery.
Gilliland, B. 2010. A shot in the dark: Has Jupiter led science astray? Metro, June 18, 2010, p 22-23.