Gravitational waves can be arbitrarily long because patterns in the cosmic microwave background suggest the early universe had gravitational radiation with wavelengths of arbitrary size, including quite long waves. The metric expansion of space has stretched those "primordial gravitational waves" (PGW), so even short-wavelength-in-early-universe PGW can have periods of many years at about the present size of the universe.
Additionally, most models of cosmic inflation stretch PGW, with some of the stretched wavelengths being made longer than the distance to the cosmic horizon (Initially, before inflation, PGWs can have practically unlimited length or can be almost arbitrarily short, with the shortest winding up not so short once inflation has ended), so at modern times the super-horizon-length primordial waves would have periods of many billions of years.
Any masses in a ~binary orbit (stars to galaxies, down to grains of dust and up to collections of galaxies like in the local group, <https://en.wikipedia.org/wiki/Local_Group>, the "dumbbell" shape is relevant) generates gravitational waves with a frequency comparable to the orbital period. In our sky there are galaxies that appear to be in mutual orbits with orbital periods of many millions of years.
The pulsar timing arrays in the cartoons linked at the top of the page look for gravitational waves with periods of months to years. It takes several orbital periods to be statistically comfortable that a gravitational wave with that period (nanohertz frequency) has been detected. Those are probably generated by supermassive black holes in hard mutual orbits. If we soften such an orbit, increasing the semimajor axis (or radius for a circular orbit), the orbital period grows, so we have to watch the array of pulsars longer (and be more wary of so-called red noise and other long-term signal contamination).
> some of the weirdness and contradictions we have observed in astronomy
Unlikely. Do you have any particular weirdness and/or contradiction in mind?
Thank you for this incredibly informative and detailed answer! It's going to take a few passes (and following some links) to put everything together.
>Do you have any particular weirdness and/or contradiction in mind?
Not in particular or specifically. I was just imagining that if there were such a thing possible, it surely would create some weird anomalies in our measurements.
The idea that something could warp our spacetime so dramatically that we would perceive it as being the normal state of things, even though the "warp" is ultimately temporary, is for some reason incredibly interesting to me.
Gravitational waves can be arbitrarily long because patterns in the cosmic microwave background suggest the early universe had gravitational radiation with wavelengths of arbitrary size, including quite long waves. The metric expansion of space has stretched those "primordial gravitational waves" (PGW), so even short-wavelength-in-early-universe PGW can have periods of many years at about the present size of the universe.
Additionally, most models of cosmic inflation stretch PGW, with some of the stretched wavelengths being made longer than the distance to the cosmic horizon (Initially, before inflation, PGWs can have practically unlimited length or can be almost arbitrarily short, with the shortest winding up not so short once inflation has ended), so at modern times the super-horizon-length primordial waves would have periods of many billions of years.
Any masses in a ~binary orbit (stars to galaxies, down to grains of dust and up to collections of galaxies like in the local group, <https://en.wikipedia.org/wiki/Local_Group>, the "dumbbell" shape is relevant) generates gravitational waves with a frequency comparable to the orbital period. In our sky there are galaxies that appear to be in mutual orbits with orbital periods of many millions of years.
The pulsar timing arrays in the cartoons linked at the top of the page look for gravitational waves with periods of months to years. It takes several orbital periods to be statistically comfortable that a gravitational wave with that period (nanohertz frequency) has been detected. Those are probably generated by supermassive black holes in hard mutual orbits. If we soften such an orbit, increasing the semimajor axis (or radius for a circular orbit), the orbital period grows, so we have to watch the array of pulsars longer (and be more wary of so-called red noise and other long-term signal contamination).
> some of the weirdness and contradictions we have observed in astronomy
Unlikely. Do you have any particular weirdness and/or contradiction in mind?