<p>The gravitational wave events that are being detected regularly now last a few seconds - they are the last few seconds of a process that took millions of years, as the two objects first orbited their center of mass from large distances, and slowly, slowly got closer together by constantly radiating very very low frequency, low amplitude gravitational waves. Finally, when they get really close and are rotating really fast, the frequency is high enough and the strength strong enough just before merging that normal GW detectors pick up the final moment.</p>
<p>So there are many <em>many</em> more systems in the early stages of GW radiation, radiating very tiny GW.</p>
<p>Think of it like this, there are about 7,900,000,000 living people on Earth. While only a handful are taking their dying breath at any moment, billions are breathing.</p>
<p>The GW events that are being detected are the occasional &quot;dying breaths&quot; of rotating pairs of objects, but the NANOGrav background comes from the billions still alive and rotating.</p>
<p>These make up the very faint GW background that NANOGrav and the other pulsar timing arrays say they have found strong evidence for. They chose the nanoHz part of the spectrum as the one they decided they could separate best from instrumental noise. The signal they have is supposed to be the sum of many, many different oscillations at the same time, all at slightly different frequencies.</p>
<p>Basically they have evidence for a signal at a confidence level that indicates it is likely gravitational waves -- not just noise.</p>
<p>So no <strong>one can not pick out one of these signals and say &quot;there it is!&quot;.</strong> The observations were made via long term (12-15 year) monitoring of very tiny shifts in signals from 60 to 70 pulsars from widely distributed directions.</p>
<p>Those tiny shifts in arrival times vary a bit more than expected based on the equipment they use, so they say they've detected GW background by using very complicated statistical analysis.</p>
<p>I posted a question a while ago <a href="https://astronomy.stackexchange.com/q/54149/7982">Main assumptions for ~4 sigma confidence superimposed gravitational waves from lots and lots of distant supermassive black hole pairs were detected?</a> but a moderator told me</p>
<blockquote>
<p>I'd recommend narrowing this down; there are a large number of very different assumptions and statistical methods used. An answer addressing all of them might be comparable in length to the paper itself. I'd also recommend making it clear that it hasn't been confirmed that, even if the signal is indeed a background, the background is actually from supermassive black holes; other models work fine (Check out <a href="https://arxiv.org/abs/2306.16219" rel="nofollow noreferrer">the paper on explanations with new physics</a> for more information.).</p>
</blockquote>
<p>and I realized how complicated the whole thing was so I (at least temporarily) deleted my own question and decided to wait until I could really dig deeper.</p>
<p><strong>Of course this is similar to how the <a href="https://en.wikipedia.org/wiki/Cosmic_microwave_background" rel="nofollow noreferrer">Cosmic Microwave Background</a> was first detected as well!</strong> It was simply microwave noise at a level higher than could be explained by the noise level of their equipment.</p>
<p>That elevated noise that others might have dismissed was the key to understanding the Big Bang. So while we can doubt that their complicated statistical analysis is conclusive, we should still take the result very seriously.</p>

CC BY-SA 4.0
2023-09-12T23:34:39.493Z
it means when they reach to our earth in very low strain or in this paper they detect  directly low strain ?
Hamidreza Abdollahi
CC BY-SA 4.0
2023-09-12T23:37:21.680Z
@HamidrezaAbdollahi no, "The observations were made via long term (12-15 year) monitoring of very tiny shifts in signals from 60 to 70 pulsars from widely distributed directions." NANOGrav measures the timing of signals from pulsars. If a very low frequency GW passes through the space between us and the pulsar, the strain will cause a tiny shift in the time it takes to reach us. So the big space between us and the pulsars is the "detector". It's not local. If the GW period is 1 year, the GW wavelength is one lightyear for example.
uhoh
CC BY-SA 4.0
2023-09-13T09:33:28.157Z
a) Please do not call this the "NANOgrav background". There are four international collaborations (NANOgrav, EPTA, PPTA, and CPTA) monitoring pulsars for passing gravitational waves each of which announced similar results (with varying degree of certainty) one the same day.
TimRias
CC BY-SA 4.0
2023-09-13T09:34:25.150Z
b) It is important to note that it is not clear what the source of the measured signal is.
TimRias
CC BY-SA 4.0
2023-09-13T09:37:12.947Z
c) If it is indeed the stochastic background of GWs, note that the sources would be supermassive black hole binaries, not the stellar mass binaries currently observed by ground based instruments.
TimRias
CC BY-SA 4.0
2023-09-13T15:33:48.750Z
@TimRias Thanks; the title of the paper linked in the OP's question is [The **NANOGrav** 15 yr Data Set: Evidence for a Gravitational-wave **Background**](https://iopscience.iop.org/article/10.3847/2041-8213/acdac6) so I just went with that. I'm pretty certain you have a lot to add here, so why not post an additional answer? See my [comment](https://astronomy.stackexchange.com/questions/54691/nano-gravitational-wave/54697?noredirect=1#comment123220_54691).
uhoh
CC BY-SA 4.0
2023-09-12T23:25:47.923Z
2023-09-13T00:02:13.477Z
2023-09-13T00:02:13.477Z
uhoh
<p>very recent paper for the first time in history admit and reveal low frequency with low strain of ripple in space-time in universe
here is source:
<a href="https://iopscience.iop.org/article/10.3847/2041-8213/acdac6" rel="nofollow noreferrer">https://iopscience.iop.org/article/10.3847/2041-8213/acdac6</a></p>
<p>can someone briefly write here about its methodology and result</p>

CC BY-SA 4.0
2023-09-12T20:14:03.870Z
The "nano" in the name refers to the frequency (in nanohertz) This is a low frequency,  (between one cycle a month, and one cycle a decade) and not a tiny wave.  They are emitted by some of the largest objects in the universe: binary supermassive black holes.
James K
CC BY-SA 4.0
2023-09-12T23:07:24.217Z
how can logically a supermassive object like black hole oscillate once per decade?!!
Hamidreza Abdollahi
CC BY-SA 4.0
2023-09-12T23:10:37.053Z
Two black holes, orbiting each other at a certain distance would do so
James K
CC BY-SA 4.0
2023-09-12T23:11:59.887Z
other mentions of NANOGrav here https://astronomy.stackexchange.com/search?q=nanograv
uhoh
CC BY-SA 4.0
2023-09-12T23:46:00.753Z
There was a lot of news coverage of the results -- of NANOGrav's papers and the papers from the other PTAs. It might help answerers a bit if you could say what in particular has confused you, since the pipelines are pretty complex.
HDE 226868
CC BY-SA 4.0
2023-09-13T01:56:57.477Z
@HamidrezaAbdollahi thanks for accepting my answer, but I wonder if it makes sense to wait a few days to see if more (and more authoritative) answers are posted? It's up to you of course, but your question has only made it half way around the Earth so far (12 hours) and many readers haven't even seen it yet.
uhoh
<gravitational-waves>
Nano Gravitational wave
Hamidreza Abdollahi
# Nano Gravitational wave

(Asked by: username_0 on 2023-09-12)

very recent paper for the first time in history admit and reveal low frequency with low strain of ripple in space-time in universe
here is source:
<https://iopscience.iop.org/article/10.3847/2041-8213/acdac6>

can someone briefly write here about its methodology and result

**Tags:** gravitational-waves

> **Comments:**
> **username_1**: The "nano" in the name refers to the frequency (in nanohertz) This is a low frequency,  (between one cycle a month, and one cycle a decade) and not a tiny wave.  They are emitted by some of the largest objects in the universe: binary supermassive black holes.  
> **username_0**: how can logically a supermassive object like black hole oscillate once per decade?!!  
> **username_1**: Two black holes, orbiting each other at a certain distance would do so  
> **username_2**: other mentions of NANOGrav here https://astronomy.stackexchange.com/search?q=nanograv  
> **username_3**: There was a lot of news coverage of the results -- of NANOGrav's papers and the papers from the other PTAs. It might help answerers a bit if you could say what in particular has confused you, since the pipelines are pretty complex.  
> **username_2**: @HamidrezaAbdollahi thanks for accepting my answer, but I wonder if it makes sense to wait a few days to see if more (and more authoritative) answers are posted? It's up to you of course, but your question has only made it half way around the Earth so far (12 hours) and many readers haven't even seen it yet.  

---

## 1 answers

### Answer by username_2 on 2023-09-12. Score: 2

The gravitational wave events that are being detected regularly now last a few seconds - they are the last few seconds of a process that took millions of years, as the two objects first orbited their center of mass from large distances, and slowly, slowly got closer together by constantly radiating very very low frequency, low amplitude gravitational waves. Finally, when they get really close and are rotating really fast, the frequency is high enough and the strength strong enough just before merging that normal GW detectors pick up the final moment.

So there are many *many* more systems in the early stages of GW radiation, radiating very tiny GW.

Think of it like this, there are about 7,900,000,000 living people on Earth. While only a handful are taking their dying breath at any moment, billions are breathing.

The GW events that are being detected are the occasional "dying breaths" of rotating pairs of objects, but the NANOGrav background comes from the billions still alive and rotating.

These make up the very faint GW background that NANOGrav and the other pulsar timing arrays say they have found strong evidence for. They chose the nanoHz part of the spectrum as the one they decided they could separate best from instrumental noise. The signal they have is supposed to be the sum of many, many different oscillations at the same time, all at slightly different frequencies.

Basically they have evidence for a signal at a confidence level that indicates it is likely gravitational waves -- not just noise.

So no **one can not pick out one of these signals and say "there it is!".** The observations were made via long term (12-15 year) monitoring of very tiny shifts in signals from 60 to 70 pulsars from widely distributed directions.

Those tiny shifts in arrival times vary a bit more than expected based on the equipment they use, so they say they've detected GW background by using very complicated statistical analysis.

I posted a question a while ago [Main assumptions for ~4 sigma confidence superimposed gravitational waves from lots and lots of distant supermassive black hole pairs were detected?](https://astronomy.stackexchange.com/q/54149/7982) but a moderator told me

> I'd recommend narrowing this down; there are a large number of very different assumptions and statistical methods used. An answer addressing all of them might be comparable in length to the paper itself. I'd also recommend making it clear that it hasn't been confirmed that, even if the signal is indeed a background, the background is actually from supermassive black holes; other models work fine (Check out [the paper on explanations with new physics](https://arxiv.org/abs/2306.16219) for more information.).

and I realized how complicated the whole thing was so I (at least temporarily) deleted my own question and decided to wait until I could really dig deeper.

**Of course this is similar to how the [Cosmic Microwave Background](https://en.wikipedia.org/wiki/Cosmic_microwave_background) was first detected as well!** It was simply microwave noise at a level higher than could be explained by the noise level of their equipment.

That elevated noise that others might have dismissed was the key to understanding the Big Bang. So while we can doubt that their complicated statistical analysis is conclusive, we should still take the result very seriously.

> **Comments:**
> **username_0**: it means when they reach to our earth in very low strain or in this paper they detect  directly low strain ?  
> **username_2**: @HamidrezaAbdollahi no, "The observations were made via long term (12-15 year) monitoring of very tiny shifts in signals from 60 to 70 pulsars from widely distributed directions." NANOGrav measures the timing of signals from pulsars. If a very low frequency GW passes through the space between us and the pulsar, the strain will cause a tiny shift in the time it takes to reach us. So the big space between us and the pulsars is the "detector". It's not local. If the GW period is 1 year, the GW wavelength is one lightyear for example.  
> **username_4**: a) Please do not call this the "NANOgrav background". There are four international collaborations (NANOgrav, EPTA, PPTA, and CPTA) monitoring pulsars for passing gravitational waves each of which announced similar results (with varying degree of certainty) one the same day.  
> **username_4**: b) It is important to note that it is not clear what the source of the measured signal is.  
> **username_4**: c) If it is indeed the stochastic background of GWs, note that the sources would be supermassive black hole binaries, not the stellar mass binaries currently observed by ground based instruments.  
> **username_2**: @username_4 Thanks; the title of the paper linked in the OP's question is [The **NANOGrav** 15 yr Data Set: Evidence for a Gravitational-wave **Background**](https://iopscience.iop.org/article/10.3847/2041-8213/acdac6) so I just went with that. I'm pretty certain you have a lot to add here, so why not post an additional answer? See my [comment](https://astronomy.stackexchange.com/questions/54691/nano-gravitational-wave/54697?noredirect=1#comment123220_54691).  

---

