The theory of dark fluid is a new and alternative theory to account for a pair of mysterious puzzles in astrophysics which have previously been "solved" through the supposition of a pair of other unseen forces, dark matter and dark energy. As it has been articulated in the past several years, the theory of dark fluid argues that space-time behaves like a fluid, flowing and coagulating around objects in space to produce the effects previously attributed to dark matter.
- The "Dark" Problems -
During the twentieth century, it seemed that human understanding of the universe grew exponentially, so that, several decades ago, it would have been accepted confidently that we understood that we lived in a universe which had exploded outwards from an initial singularity in the Big Bang, was expanding at a gradually decreasing rate because of the force of gravity tugging everything inwards again, and would eventually collapse again into a central singularity. (This collapse, then, could prompt a new Big Bang.)
Two key problems remained, however. The first was that, beginning with the study of rotation rates of other galaxies, it became apparent that the mass of the physical matter present - in the form of stars and gas - could not begin to explain the rate of rotation. It followed that there must be additional mass present which somehow could not be seen. The response was to posit the existence of a new class of matter which could not be seen through electromagnetic radiation, but nevertheless existed - called "dark" matter because of our inability to see it. Dark matter is estimated to have a mass equal to several times that of regular physical matter, and its existence has been supported, though not confirmed, through gravitational lensing experiments.
The second problem was that, since the 1990s, supernova analyses have confirmed that the universe's expansion is accelerating, not decelerating (as normal theories of gravity say it must). Further studies suggest that, even with dark matter added to regular matter, we can only account for about one-quarter of what mathematics suggests is the total amount of mass and energy in the universe. The solution to this new problem was to posit the existence of another mysterious "dark" force: dark energy.
- Dark Fluid -
Dark matter and dark energy both provide theoretical solutions to the empirical problems that have been identified - but they are still only theories. In recent years an alternative explanation has emerged: the theory of dark fluid. Unlike dark matter or dark energy, the dark fluid theory does not suggest that there are new and unseen types of matter or energy. Instead, building on the Einsteinian concept of the fabric of spacetime, it suggests that space works like a fluid, flowing around and between objects. Like any other fluid (like water), space can therefore pool or coagulate in unusual densities around objects which obstruct its flow. Elsewhere, like the intergalactic medium where there is virtually no matter to obstruct its flow, space expands and thins out.
The consequences, according to proponents of the theory, suggest that the effects we now attribute to dark matter and dark energy are simply illusions of this deeper behaviour of space. Matter-rich areas seem to have invisible dark matter present, because in fact they are surrounded by massive eddies in the fabric of spacetime. In the negligible-mass intergalactic medium, space flows unobstructed, thins, and stretches out, producing the effects attributed to dark energy. Other applications, like modified Newtonian dynamics, can also allegedly be seen to be special cases following the deeper logic of dark fluid theory.
In a sense, dark fluid is thus a limited form of the much-sought-after "theory of everything": a grand theory which will explain and subsume a range of phenomena previously attributed to a set of multiple distinct forces. Grand theories are the holy grail of modern science - but, like the grail, many turn out to be merely mythical. The coming years will show whether dark fluid gains broader acceptance in the academy, and accumulates empirical support, or whether it is set aside and relegated to the dustbin of failed theory.