The blink of distant quasars may hint at one of the cosmos’s big mysteries
Prof Chris Lintott is an astrophysicist and co-presenter on The Sky at Night
Most cosmologists believe that dark matter, the mysterious substance that seems to account for six-sevenths of the matter in the Universe, exists in the form of myriad weakly interacting massive particles, or WIMPs.
The WIMPs won out over ideas of a cosmos filled with planets or black holes (collectively known, appropriately, as MACHOs, Massive Compact Halo Objects) because of a set of experiments that used modest telescopes to stare at stars in the Milky Way’s halo. The aim was to spot the effect of a passing MACHO on a background star; if the alignment was perfect, gravitational lensing would cause the background star to brighten and then fade in a predictable pattern.
The physics is sound – Einstein’s theory of relativity tells us that light should be bent by massive objects in just this way. But very few events were seen, so there simply aren’t enough free-floating planets or black holes surrounding our Galaxy to account for the dark matter we think must be there. If free-floating, planet-sized objects exist, we should detect them passing between us and distant objects too – and variations in the brightness of quasars, extremely distant objects, have been attributed to the presence of hosts of passing planets in the distant Universe.
Quasar clues
In this month’s paper, the authors aim to reconcile this explanation for quasar flickering with the apparent absence of such planets around the Milky Way. First, they set out to distinguish the effects of lensing by dense objects from changes due to the quasar itself, which might brighten and fade as material swirls onto the accretion disc around its central black hole.
The trick is to use a system like Q2237+0305, where, thanks to lensing from a nearby galaxy, we see four separate images of
