4 ms·

I've always wanted to ask this really dumb question, so I'll just ask it here (laugh or answer, your choice): Would there be gravitational waves so huge that w

I've always wanted to ask this really dumb question, so I'll just ask it here (laugh or answer, your choice): Would there be gravitational waves so huge that we'd have trouble detecting them? As in, if there was a huge, massive gravitational wave and we were in the process of "riding" it how would we ever know or detect such a thing? Would this possibly (not plausibly) be an answer to some of the weirdness and contradictions we have observed in astronomy over the past few decades?

krastanov 3y ago
You can see "sensitivity" plots for the various methods we have for observing gravitational waves. The horizontal axis is usually in terms of the frequency of the wave and the vertical axis is how sensitive we are to them. This article is about "nanohertz" waves, which means their wavelength is roughly 10⁹sec × speed of light, i.e. 30ish light years. So, it seems we can detect really "big" gravitational waves. If you look through the aforementioned plots you will see that sensitivity drops off significantly for "more astronomical" wavelengths, so there are definitely scales at which we can not detect, as you suggested. There might be some interesting ways to introduce very long gravitational waves as solution of current discrepancies in our understanding of cosmology, but they are also probably introducing more discrepancies than they solve. I would guess people have considered these ideas, because "cosmological scale general relativity" is pretty heavily researched and mainstream.
raattgift 3y ago
> so huge that we'd have trouble detecting them 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 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?
- whoknowsidont 3y ago
  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.

krastanov 3y ago

You can see "sensitivity" plots for the various methods we have for observing gravitational waves. The horizontal axis is usually in terms of the frequency of the wave and the vertical axis is how sensitive we are to them. This article is about "nanohertz" waves, which means their wavelength is roughly 10⁹sec × speed of light, i.e. 30ish light years. So, it seems we can detect really "big" gravitational waves. If you look through the aforementioned plots you will see that sensitivity drops off significantly for "more astronomical" wavelengths, so there are definitely scales at which we can not detect, as you suggested. There might be some interesting ways to introduce very long gravitational waves as solution of current discrepancies in our understanding of cosmology, but they are also probably introducing more discrepancies than they solve. I would guess people have considered these ideas, because "cosmological scale general relativity" is pretty heavily researched and mainstream.

raattgift 3y ago

> so huge that we'd have trouble detecting them 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 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?

whoknowsidont 3y ago
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.