Quasars are among the brightest and most distant objects visible in the observable universe. They are active galaxies containing black holes crunching their way through dense gas, dust and stellar material, forming discs and jets of plasma that release radiation.
Over the distance the light then has to travel, it is red-shifted in a number of ways. The expansion of space and time means that the quasar is moving away from us at a great velocity, inducing a Doppler redshift. That same expansion acts on the photons, redenning them more, interactions with dust can remove energy, also redenning the spectrum as well as the effects of gravitational time dilation. In the case of the dust, the effect is due to energy being lost to interim particles, in the case of the expansion of the universe, it is space-time stretching that does the deed, but in the main contributor, the Doppler shift, it is a change in time that turns the light to the redder side.
The typical example of how this works is imagining someone throwing balls at you at regular intervals. The balls hit you with the same frequency as they are thrown, if the person is sufficiently accurate and malicious. Now imagine they are moving towards you but maintaining that frequency. Each time they throw, they do so from a closer distance meaning the ball has less distance to cover at the same speed, so it will reach you in less time than it took the last ball, the frequency of balls hitting is greater than that of balls being thrown. In terms of light, this manifests as a blue shift, and the opposite, when the source is receding from you, is the red shift. The time between observed events (such as ball throws) increases with recession velocity (there’s an additional factor due to relativistic time dilation as well).
This has been tested in supernova, which follow the same processes during their explosions. The time taken for the various parts of a supernova to occur is indeed correlated to distance as suggested by the relativistic Doppler shift. However, Quasars have chosen to be different.
Mike Hawkins of the Royal Observatory in Edinburgh, UK looked at events where Quasars brightened. He took observations over a period of twenty-eight years and nine hundred quasars spanning a distance of between six and ten billion light years from the Earth. He examined the periods for which brightening events happened in the closer ones and that for those further away and discovered they were pretty much the same.
Now unless the events in the further away quasars happened to be quicker than the near ones by exactly the same amount as would be needed to combat the Doppler induced time lag, then there were a few possible explanations. The Universe isn’t expanding – contradicting all the other observational evidence that it is. Quasars red shifts are not related to distance – another theory blown out of the water by observations (indeed these observations are another way of testing the present problem). Or maybe these brightness changes, whose origins were assumed rather than expected on a specific theoretical basis, are actually to do with interactions between the light and stuff in between it and the Earth and not the quasars themselves.
One idea suggested is that the quasars have been microlensed by black holes of various sizes. The trouble is there shouldn’t be enough around (scientists estimate from observations that no more than a fifth of the dark matter in the galaxy can be due to small black holes roaming about). Hawkins hopes that images of quasar microlensing can be found in order to compare with his own brightening events to rule this explanation in or out.