Almost every astronomical study is affected by an unpleasant bias called the Malmquist bias effect. The only solution is to keep studying.
In the 1970s and 80s, astronomers first began to identify a structure known as the Great Attractor, which is defined by the collective motion of all galaxies in our immediate vicinity of the cosmos.
For over a decade, however, many astronomers were not really convinced that the Great Attractor exists. Their skepticism was well-founded, because there is an observational effect that is widely used in astronomy, the so-called Malmquist effect. This effect is named after the Swedish astronomer Gunnar Malmquist, who first explained this effect in 1922. It is a special version of a far more common statistical effect known as selection bias.
There is a limit to the brightness of most astronomical surveys. There is a certain lower limit that represents the dimmest object that a given telescope can see with a given exposure. However, objects in the universe can be dark for two different reasons: because they are really dark by nature, or simply because they are far away. A typical survey of astronomical objects such as galaxies therefore preferentially selects closer and/or brighter ones. In the case of galaxies, the further we look away from the Milky Way, the more likely we are to capture only the brightest galaxies at that distance and miss all of their dimmer siblings.
This distortion could potentially distort our understanding of the wider universe, especially if we try to use the velocities of galaxies to map their overall motion. In those first surveys in the 1970s and 80s, many astronomers argued that we would only see the motions of the brightest galaxies, creating the illusion of a general flow toward the Great Attractor, and that a more complete survey of the local universe would balance everything out.
The solution? More investigations, more thorough and more comprehensive, which finally revealed the reality of the Great Attractor.
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