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LIGO Detects Gravitational Waves for Third Time

caltech.edu

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Re: LIGO Detects Gravitational Waves for Third Time

#11

Post by privong »

mturmon wrote: post #8
yodon wrote: post #5

Earlier quoted context omitted.

Events big enough to be detected are quite rare, but the phenomenon you are asking about is more a financial reality than a "convenient coincidence" as you put it. There is a pretty steep curve connecting sensitivity and cost, so when the team that built LIGO was designing it, they used the best available models of colliding black hole event rates to estimate the sensitivity required to deliver a conclusive result in…

Hmm...are the rates of black holes per volume well-constrained at all? I was under the impression that it's a possibility that dark matter consists prominently of primordial BHs? The truth or falsity of this would seem to have a big effect on rates.

> are the rates of black holes per volume well-constrained at all?

There are estimates. But two of the three LIGO detections are of black holes that are more massive than we had expected to exist (~ a few tens of solar masses). Previously we had convincing examples of black holes with ~ 1e6 solar masses. But since we didn't have any convincing observational detections of BHs with ~20-40 solar masses, it's safe to say that the volume density is poorly constrained for that mass range. At the high end we have a reasonable estimate of the volume density, because we think all galaxies with spheroid components have a black hole and that the black hole's mass is linked to the spheroid.

> I was under the impression that it's a possibility that dark matter consists prominently of primordial BHs?

It depends on what you mean by "primordial". Micro-lensing experiments (when a star is brightly made brighter by the gravitational focusing of light from an object passing between us and the star), mostly looking towards the LMC/SMC [e.g., 0] have tried to address this. My recollection is that there aren't enough stellar mass black holes around to account for all of dark matter. Assuming Hawking radiation exists, low-mass primordial black holes should have evaporated by now, leaving only the more massive ones. There's a range in between the two, but I'm not sure if you can fit enough of them in a galaxy to account for dark matter while still being consistent with the sensitivity of the microlensing surveys.

[0] https://en.wikipedia.org/wiki/Optical_Gravitational_Lensing_...

> The truth or falsity of this would seem to have a big effect on rates.

Possibly. Though in order to emit GWs, pairs of black holes have to become bound to each other. If black holes make up the dark matter halos, they probably have large velocities relative to each other, which would limit their ability to form bound pairs (though it is possible with 3-body interactions). I am not aware of estimates of the BH pair-formation rate in halos _if_ DM haloes are in fact made of black holes. But the event rate probably can't be extraordinarily high, otherwise we might expect to see dark matter halos becoming less massive as the Universe ages. Though there are many confounding factors that might hide such a signal.

Re: LIGO Detects Gravitational Waves for Third Time

#12

Post by cletus »

The numbers here are just staggering:

- Black hole merger occurred 3 billion light years away

- Two solar masses were converted to energy

- Briefly 10^34 megatons of energy were released every second

This is hard to intuitively wrap your head around because we think of space as constant. Something like this can distort space itself. Amazing stuff.

Re: LIGO Detects Gravitational Waves for Third Time

#13

Post by lnx01 »

shortstuffsushi wrote: post #9

Somewhat naive questions, as I know very little about astronomy. Do black holes "move?" How is it that they could merge if they're stationary, unless they're pulling each other in I guess? If black holes are indeed pulling in everything, does that mean the whole universe would eventually be one giant black hole?

Black holes have mass, just like any other object in the universe - like a star, or a planet, or the sun.

Consequently, they follow orbits just as any other mass would. In some cases, they're the local most massive object and any other masses move more in response. Other times they are near other black holes, and they orbit one another until they collide and merge.

What makes black holes different is their density. The mass a black hole has is confined in a point of zero height, width and length called a singularity. The consequences are as we know, not fully understood by current models of physics.

edit: spelling

They do indeed move. It's conceivable that a black hole of mass X could be observed orbiting a red super-giant star of mass 100X. I don't think this happens much though, but the universe is big so who knows.

Re: LIGO Detects Gravitational Waves for Third Time

#14

Post by privong »

yodon wrote: post #5
wolfram74 wrote: post #2

Anyone familiar with this branch of astronomy want to explain why one detection in a volume on the order of 27 billion cubic light years is reasonable? Are they still processing data and will find more events? Is the sensitivity highly anisotropic so the detection volume is significantly smaller? Or are events like this just really conveniently rare that we get about 1 every data gathering interval?

Events big enough to be detected are quite rare, but the phenomenon you are asking about is more a financial reality than a "convenient coincidence" as you put it. There is a pretty steep curve connecting sensitivity and cost, so when the team that built LIGO was designing it, they used the best available models of colliding black hole event rates to estimate the sensitivity required to deliver a conclusive result in…

> If you're getting 10 events/second with a device like this, you probably overpaid for sensitivity

Aside from issues processing and disentangling the overlapping events in a situation with that high of an event rate, more events would not be bad, so I'm not sure I'd call it "overpaying". Imagine the kind of population demographics that could be built up if we were detecting that many events.

Re: LIGO Detects Gravitational Waves for Third Time

#15

Post by mturmon »

privong wrote: post #11
mturmon wrote: post #8

Earlier quoted context omitted.

Hmm...are the rates of black holes per volume well-constrained at all? I was under the impression that it's a possibility that dark matter consists prominently of primordial BHs? The truth or falsity of this would seem to have a big effect on rates.

> are the rates of black holes per volume well-constrained at all? There are estimates. But two of the three LIGO detections are of black holes that are more massive than we had expected to exist (~ a few tens of solar masses). Previously we had convincing examples of black holes with ~ 1e6 solar masses. But since we didn't have any convincing observational detections of BHs with ~20-40 solar masses, it's safe to say…

Thanks a lot for this reply.

Re: LIGO Detects Gravitational Waves for Third Time

#16

Post by symmetricsaurus »

shortstuffsushi wrote: post #9

Somewhat naive questions, as I know very little about astronomy. Do black holes "move?" How is it that they could merge if they're stationary, unless they're pulling each other in I guess? If black holes are indeed pulling in everything, does that mean the whole universe would eventually be one giant black hole?

Black holes move in the same way as any other object of the same mass, for example a star.

When they merge they are typically orbiting each other already in a similar way to binary stars. When they orbit the system will lose energy to gravitational radiation causing the black holes to spiral inwards. (This happens for all orbits, including the Earth's). Eventually they merge sending out a tremendous amount of gravitational radiation.

The gravitional pull of a black hole is just as strong as another object of the same mass. The difference is that the mass is concentrated to a point in the center. This is what makes them impossible to escape from.

It was believed the the whole universe could maybe collapse back into one point (like a black hole). Now it is believed that the universe will keep expanding forever. Actually the universe seems to be expanding ever quicker due to dark energy. But no one knows what dark energy is.

Re: LIGO Detects Gravitational Waves for Third Time

#17

Post by yodon »

privong wrote: post #14
yodon wrote: post #5

Earlier quoted context omitted.

Events big enough to be detected are quite rare, but the phenomenon you are asking about is more a financial reality than a "convenient coincidence" as you put it. There is a pretty steep curve connecting sensitivity and cost, so when the team that built LIGO was designing it, they used the best available models of colliding black hole event rates to estimate the sensitivity required to deliver a conclusive result in…

> If you're getting 10 events/second with a device like this, you probably overpaid for sensitivity Aside from issues processing and disentangling the overlapping events in a situation with that high of an event rate, more events would not be bad, so I'm not sure I'd call it "overpaying". Imagine the kind of population demographics that could be built up if we were detecting that many events.

It's not that physicists wouldn't love to capture all those events, it's that the cost of building instruments like LIGO is nearly prohibitively high and the cost is a strong function of the sensitivity of the instrument. If you aim too high in your sensitivity aspirations, the cost hits a point where the experiment simply can't be funded.

Re: LIGO Detects Gravitational Waves for Third Time

#18

Post by teraflop »

shortstuffsushi wrote: post #9

Somewhat naive questions, as I know very little about astronomy. Do black holes "move?" How is it that they could merge if they're stationary, unless they're pulling each other in I guess? If black holes are indeed pulling in everything, does that mean the whole universe would eventually be one giant black hole?

> Do black holes "move?"

Yes, like any other massive objects, black holes can have velocity and momentum. Two black holes, or a black hole and another object like a star, can orbit each other in a way that almost follows Newton's laws.

> How is it that they could merge if they're stationary, unless they're pulling each other in I guess?

This gets at what makes LIGO's findings interesting. Two black holes merge if they fall into each other's event horizons. But Newtonian gravity predicts that, in isolation, this would never happen; two orbiting black holes would just maintain their elliptical orbit forever. (I'm hand-waving here, because Newtonian gravity can't properly model black holes at all.)

The theory of general relativity predicts that the gravitational curvature of the space around the black holes contains energy, similar to the energy in the electric field around a charged particle. And intense changes in curvature can create waves in the curvature of space, which carry away kinetic energy from the black holes and cause them to spiral into each other. Under normal conditions these waves are so unimaginably tiny that they're unmeasurable, but during a black hole merger, they become intense enough to be (barely) detected from billions of light-years away. This is what LIGO detected, confirming a long-known theoretical prediction of GR.

> If black holes are indeed pulling in everything, does that mean the whole universe would eventually be one giant black hole?

Not necessarily. Everything in the universe attracts everything else gravitationally, but that doesn't mean any two objects will inevitably collide. If they have enough energy to move apart faster than their common escape velocity, they are not gravitationally bound and will continue separating forever.

Re: LIGO Detects Gravitational Waves for Third Time

#19

Post by [deleted] »

mturmon wrote: post #8
yodon wrote: post #5

Earlier quoted context omitted.

Events big enough to be detected are quite rare, but the phenomenon you are asking about is more a financial reality than a "convenient coincidence" as you put it. There is a pretty steep curve connecting sensitivity and cost, so when the team that built LIGO was designing it, they used the best available models of colliding black hole event rates to estimate the sensitivity required to deliver a conclusive result in…

Hmm...are the rates of black holes per volume well-constrained at all? I was under the impression that it's a possibility that dark matter consists prominently of primordial BHs? The truth or falsity of this would seem to have a big effect on rates.

[deleted]

Re: LIGO Detects Gravitational Waves for Third Time

#20

Post by eanzenberg »

privong wrote: post #14
yodon wrote: post #5

Earlier quoted context omitted.

Events big enough to be detected are quite rare, but the phenomenon you are asking about is more a financial reality than a "convenient coincidence" as you put it. There is a pretty steep curve connecting sensitivity and cost, so when the team that built LIGO was designing it, they used the best available models of colliding black hole event rates to estimate the sensitivity required to deliver a conclusive result in…

> If you're getting 10 events/second with a device like this, you probably overpaid for sensitivity Aside from issues processing and disentangling the overlapping events in a situation with that high of an event rate, more events would not be bad, so I'm not sure I'd call it "overpaying". Imagine the kind of population demographics that could be built up if we were detecting that many events.

It's a balance between cost and sensitivity, and remember that 1 discovery would prove the experiment a "success". I think the way it was planned and executed was great. Also, I think the proof that gravitational waves is far more exciting than comparing gravity wave signatures among a sample of celestial collisions.
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