4 ms·

Can you ELI5 why the fact we have evidence of Hellings and Downs correlations is mind-blowing?

danparsonson 3y ago
I was referring more to the measurement of slight variations in pulsar frequencies across decades used to detect background gravitation waves - I'm afraid I have no context besides this article to ELY5 the science they're trying to prove.
nonameiguess 3y ago
I would agree with the other comment that the fact we can measure gravitational waves at all using pulsar timing with data collected over decades is what is mind-blowing, more than the actual waves being detected. However, depending on their nature, there is a possibility that this is still a pretty significant event. When we observed the cosmic microwave background, because this is light emanating from everywhere at the very first time photons decoupled from ionized matter and the mean free path exceeded the particle horizon (the universe "became transparent" as they say), this is the furthest back you ever see using light. A much earlier event can possibly be observed via the cosmic neutrino background. The universe would have been transparent to neutrinos much earlier, when the electroweak force first decayed into the weak and electromagnetic forces. This is a big difference, fractional seconds after the Big Bang compared to I think 379,000 years or so for light. However, the very earliest event that could ever be observed by any means at all would be the cosmic gravitational background expected to exist when gravity first decoupled from strong and electroweak forces. If that's what this is, we're not seeing the literal beginning, but it's the closest we can ever get by any sensor we currently have the ability to conceive of.
- dredmorbius 3y ago
  One lesson from the history of science is that new developments in detection and sensing capabilities often herald tremendous increases in knowledge. Lenses, and the telescope and microscope, both advanced astronomy and biology tremendously. Interferometry grows out of optics, spectroscopy, and the ability to detect interference at scales. Chemical tests, electromagnetic principles, radiation, and the ability to detect electromagnetic radiation outside the visible spectrum (infrared, radio, ultraviolet, x-ray, gamma ray) had similarly large influences. Another area of progress is increased precision in measuring both space and time, which this discovery addresses. I've also done some fairly casual eyeball-assessments of trends in Nobel-awarded research in physics and chemistry since roughly 1960, which is a convenient demarcation between the period of profound discovery through the earlier part of the 20th century, with fundamental laws and particles making frequent mention, to the past 60 or so years in which it's often been measurement or emission capabilities (think LIGO and blue LEDs) which have been the subject of awards. In one sense, these seem less significant than the earlier work, and that's been my own tendency in ascribing heft. But if this does in fact represent a gestation period in which new observational tools and techniques are being cultivated, there might well be more impact than appears initially. We're in what may well be the early stages of gravitational wave measurement and detection, and we're figuring out how to "build" galactic-- and intergalactic-scale measurement "devices" which are necessary to detect this pervasive but very low-energy effect. It is exciting, and I'm thinking over the past record of sensing technologies to think how these might inform us. My initial read is that gravitational waves are very low frequency, which is to say, low resolution, but because they have extraordinarily high upper bounds on magnitude (unlike, say, magnetic or electromagnetic events associated with stars, galaxies, and even black-hole and neutron-star interactions, though the latter play a strong role in gravitational wave phenomena), what I suspect we'll find is a general sensing capability which can "see through" cosmic structures that are otherwise opaque to us (gravity isn't bothered by dust and other matter), and also provides a mechanism for directly observing dark matter, which is defined by the fact that it does interact gravitationally but does not with the electromagnetic spectrum. That's a lay view, no specific expertise, but informed by a model of technologies and capabilities afforded, one element of which is information, itself comprised of sensing, processing, storage and recall, and transmission.
- raattgift 3y ago
  > If that's what this is No, this is about the nanohertz gravitational wave "background", which comes from massive binaries with orbital periods of on the order of months to years. Sources are expected to be be randomly arranged in spherical coordinates (up to an unfortunately low radial distance cutoff), like the (low-redshift) galaxies they likely live in the middle of. Primordially, most still-viable types of inflation produce gravitational waves on all length scales, so some very short wavelength primordial GWs are in principle accessible to us despite the large cosmological redshift (z >> 10^10; 10^10 is about the redshift at neutrino decoupling, a good guess is that z ~ 10^25 is the ballpark for the latest high-amplitude-peak (~ 10^-17 Hz) inflation-produced gravitational radiation[1]). If multimessenger and other techniques can help subtract the "foreground" of binary black holes and the like, then perhaps we will see them as redshifted-to-light-year-wavelengths not traceable to a hard SMBH binary or foreground mimicers. The CMB, particularly in its B-mode polarization, is already a useful probe of primordial gravitational radiation as was anticipated at least as early as BOOMERanG's 2003 flight. A better measurement of (dust and gas, and also some gravitational lensing) foregrounds is important there too. [1] Grischchuk 2005 §3, most easily obtained at https://arxiv.org/abs/gr-qc/0504018v4 https://arxiv.org/abs/gr-qc/0504018v4 (see Comments section below abstract for its provenance). PS: Bécsy, Cornish & Kelley 2022 (open access at The Astrophysical Journal, but easiest to grab the arxiv version which matches the published paper) https://arxiv.org/abs/2207.01607 https://arxiv.org/abs/2207.01607 is a very good overview of the nanohertz GW background(s) and how they're accessible to pulsar timing arrays. A couple of the recent coordinated-release papers by the various intl PTA members are also good overviews https:///www.ipta4gw.org/ https:///www.ipta4gw.org/

I was referring more to the measurement of slight variations in pulsar frequencies across decades used to detect background gravitation waves - I'm afraid I have no context besides this article to ELY5 the science they're trying to prove.

nonameiguess 3y ago

I would agree with the other comment that the fact we can measure gravitational waves at all using pulsar timing with data collected over decades is what is mind-blowing, more than the actual waves being detected. However, depending on their nature, there is a possibility that this is still a pretty significant event. When we observed the cosmic microwave background, because this is light emanating from everywhere at the very first time photons decoupled from ionized matter and the mean free path exceeded the particle horizon (the universe "became transparent" as they say), this is the furthest back you ever see using light. A much earlier event can possibly be observed via the cosmic neutrino background. The universe would have been transparent to neutrinos much earlier, when the electroweak force first decayed into the weak and electromagnetic forces. This is a big difference, fractional seconds after the Big Bang compared to I think 379,000 years or so for light. However, the very earliest event that could ever be observed by any means at all would be the cosmic gravitational background expected to exist when gravity first decoupled from strong and electroweak forces. If that's what this is, we're not seeing the literal beginning, but it's the closest we can ever get by any sensor we currently have the ability to conceive of.

dredmorbius 3y ago
One lesson from the history of science is that new developments in detection and sensing capabilities often herald tremendous increases in knowledge. Lenses, and the telescope and microscope, both advanced astronomy and biology tremendously. Interferometry grows out of optics, spectroscopy, and the ability to detect interference at scales. Chemical tests, electromagnetic principles, radiation, and the ability to detect electromagnetic radiation outside the visible spectrum (infrared, radio, ultraviolet, x-ray, gamma ray) had similarly large influences. Another area of progress is increased precision in measuring both space and time, which this discovery addresses. I've also done some fairly casual eyeball-assessments of trends in Nobel-awarded research in physics and chemistry since roughly 1960, which is a convenient demarcation between the period of profound discovery through the earlier part of the 20th century, with fundamental laws and particles making frequent mention, to the past 60 or so years in which it's often been measurement or emission capabilities (think LIGO and blue LEDs) which have been the subject of awards. In one sense, these seem less significant than the earlier work, and that's been my own tendency in ascribing heft. But if this does in fact represent a gestation period in which new observational tools and techniques are being cultivated, there might well be more impact than appears initially. We're in what may well be the early stages of gravitational wave measurement and detection, and we're figuring out how to "build" galactic-- and intergalactic-scale measurement "devices" which are necessary to detect this pervasive but very low-energy effect. It is exciting, and I'm thinking over the past record of sensing technologies to think how these might inform us. My initial read is that gravitational waves are very low frequency, which is to say, low resolution, but because they have extraordinarily high upper bounds on magnitude (unlike, say, magnetic or electromagnetic events associated with stars, galaxies, and even black-hole and neutron-star interactions, though the latter play a strong role in gravitational wave phenomena), what I suspect we'll find is a general sensing capability which can "see through" cosmic structures that are otherwise opaque to us (gravity isn't bothered by dust and other matter), and also provides a mechanism for directly observing dark matter, which is defined by the fact that it does interact gravitationally but does not with the electromagnetic spectrum. That's a lay view, no specific expertise, but informed by a model of technologies and capabilities afforded, one element of which is information, itself comprised of sensing, processing, storage and recall, and transmission.
raattgift 3y ago
> If that's what this is No, this is about the nanohertz gravitational wave "background", which comes from massive binaries with orbital periods of on the order of months to years. Sources are expected to be be randomly arranged in spherical coordinates (up to an unfortunately low radial distance cutoff), like the (low-redshift) galaxies they likely live in the middle of. Primordially, most still-viable types of inflation produce gravitational waves on all length scales, so some very short wavelength primordial GWs are in principle accessible to us despite the large cosmological redshift (z >> 10^10; 10^10 is about the redshift at neutrino decoupling, a good guess is that z ~ 10^25 is the ballpark for the latest high-amplitude-peak (~ 10^-17 Hz) inflation-produced gravitational radiation[1]). If multimessenger and other techniques can help subtract the "foreground" of binary black holes and the like, then perhaps we will see them as redshifted-to-light-year-wavelengths not traceable to a hard SMBH binary or foreground mimicers. The CMB, particularly in its B-mode polarization, is already a useful probe of primordial gravitational radiation as was anticipated at least as early as BOOMERanG's 2003 flight. A better measurement of (dust and gas, and also some gravitational lensing) foregrounds is important there too. [1] Grischchuk 2005 §3, most easily obtained at https://arxiv.org/abs/gr-qc/0504018v4 https://arxiv.org/abs/gr-qc/0504018v4 (see Comments section below abstract for its provenance). PS: Bécsy, Cornish & Kelley 2022 (open access at The Astrophysical Journal, but easiest to grab the arxiv version which matches the published paper) https://arxiv.org/abs/2207.01607 https://arxiv.org/abs/2207.01607 is a very good overview of the nanohertz GW background(s) and how they're accessible to pulsar timing arrays. A couple of the recent coordinated-release papers by the various intl PTA members are also good overviews https:///www.ipta4gw.org/ https:///www.ipta4gw.org/