An interesting historical byline was the debate over the distance of quasars in the 1960s. What made quasars so unusual was that their spectrum resembled absolutely nothing that had been seen before. A typical quasar spectrum has a broad continuum with a number of emission lines on top of it. This should have made it fairly easy to identify the composition of quasars since the emission lines tell you what elements are nearby, but the observed emission lines matched up with no known elements. Maarten Schmidt realized that the most prominent emission lines could be those of hydrogen if the recessional velocity was about 15% of the speed of light.
This sparked a big debate in astronomy because whether you believed Schmidt or not, something crazy had to be going on. If Schmidt was right, these objects would have to extremely far away --- much further than anything that had ever been observed before. And if they were that distant and that bright, that would imply that they would have to be extraordinarily luminous. The timescale of the variability of the spectrum implied that whatever the source was, it had to be less than the size of Solar System. Yet if they were really that far away, something of that volume had to outshine an entire galaxy!
On the other hand, if Schmidt was wrong and the quasars were really local, you still had to explain just what exactly they were. One plausible explanation was that the emission lines were generated deep in a gravitational well (i.e., there was gravitational redshift rather than cosmological redshift). But no known objects could stably produce emission at such specific gravitational redshifts.
Ultimately the cosmological view of quasars prevailed because theorists could show that accretion of gas onto a supermassive black hole could produce the required luminosity in the required volume, whereas there was no good explanation for how you could produce a non-cosmological quasar.
The story of non-cosmological quasars turns out to be an excellent example of science progressing one funeral at a time. Even when most of the field had moved on by the late 1970s and 1980s, a small cadre of vocal astronomers continued to insist on the non-cosmological quasar model even into the 1990s until they started to die. Halton Arp was the most well known of these (he is better known for the Arp Atlas of Peculiar Galaxies) and even published a book in 1987 arguing in favor of the non-cosmological model.
This sparked a big debate in astronomy because whether you believed Schmidt or not, something crazy had to be going on. If Schmidt was right, these objects would have to extremely far away --- much further than anything that had ever been observed before. And if they were that distant and that bright, that would imply that they would have to be extraordinarily luminous. The timescale of the variability of the spectrum implied that whatever the source was, it had to be less than the size of Solar System. Yet if they were really that far away, something of that volume had to outshine an entire galaxy!
On the other hand, if Schmidt was wrong and the quasars were really local, you still had to explain just what exactly they were. One plausible explanation was that the emission lines were generated deep in a gravitational well (i.e., there was gravitational redshift rather than cosmological redshift). But no known objects could stably produce emission at such specific gravitational redshifts.
Ultimately the cosmological view of quasars prevailed because theorists could show that accretion of gas onto a supermassive black hole could produce the required luminosity in the required volume, whereas there was no good explanation for how you could produce a non-cosmological quasar.
The story of non-cosmological quasars turns out to be an excellent example of science progressing one funeral at a time. Even when most of the field had moved on by the late 1970s and 1980s, a small cadre of vocal astronomers continued to insist on the non-cosmological quasar model even into the 1990s until they started to die. Halton Arp was the most well known of these (he is better known for the Arp Atlas of Peculiar Galaxies) and even published a book in 1987 arguing in favor of the non-cosmological model.