Searching for extraterrestrial life has always been one of the focuses of astronomy. Many models and theories have been created just to explain how alien civilizations would work and why we may not have found them yet.
An astronomer just put one of those theories into question. Duncan Forgan at the University of St. Andrews in the United Kingdom has come up with a model that establishes how hard it is for the "zoo hypothesis" to be plausible.
Back in 1973, John Ball, a radio astronomer at MIT, explained why we haven't come into contact with aliens yet. He postulated that alien civilizations could be unilaterally avoiding us. Keeping Earth as a part of some kind of intergalactic zoo. So, in a sense, we'd be left alone because of alien "Don't Touch The Animals" signs.
Forgan starts from the assumption that several alien civilizations must have talked with one another and decided on the zoo rule, like how the UN gets together and sets policies.
Key to that set-up would be intergalactic communication, limited by the speed of light. Therefore, civilizations must exist long enough for the messages to go from sender to receiver and vice versa. Once again, assuming that alien communication would be limited by the speed of light.
Also, life can only be sustained in a galaxy-wide habitable zone, the two-dimensional annular stretching between six kiloparsecs and 10 kiloparsecs from the galactic center.
Using these assumptions, the computer program then places civilizations randomly in the galactic map and calculates the possibility for any two civilizations to actually meet, communicate, and come to an agreement.
The computer determined that at least 500 civilizations must exist for the zoo hypothesis to be possible. Any less and there are too many groups of civilizations too far from each other for a consensus to be made.
Then the computer determined how old the civilizations must be for an effective agreement to be made. It saw that if civilizations last less than a million years, there also be many civilizations groups, same as above. But if they last over 1 million, a single galactic club can be achieved.
Therefore, the galaxy must have at least 500 civilizations that exist for more than a million years for the zoo hypothesis to work. Pretty stiff odds, if we're honest.