APOD: Ninety Gravitational Wave and... (2021 Dec 07)

[APOD Robot]

Ninety Gravitational Wave Spectrograms and Counting

Explanation: Every time two massive black holes collide, a loud chirping sound is broadcast out into the universe in gravitational waves. Humanity has only had the technology to hear these unusual chirps for the past seven years, but since then we have heard about 90 -- during the first three observing runs. Featured above are the spectrograms -- plots of gravitational-wave frequency versus time -- of these 90 as detected by the giant detectors of LIGO (in the USA), VIRGO (in Europe), and KAGRA (in Japan). The more energy received on Earth from a collision, the brighter it appears on the graphic. Among many science firsts, these gravitational-radiation chirps are giving humanity an unprecedented inventory of black holes and neutron stars, and a new way to measure the expansion rate of our universe. A fourth gravitational wave observing run with increased sensitivity is currently planned to begin in 2022 December.

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[alter-ego]

There're a few events that I can't identify the chirp. I wish there were dashed lines indicating them, and I'm interested in their confidence levels too.

[aljo]

I find it amazing that they can measure the masses of the two merging objects and the resulting object from those spectrograms! How is it possible?

[orin stepanek]

OK! We will be able to know the exact moment of the beginning of
the Universe! How does that help us? I'm dumbstruck!

We saw this kitty before; he (or she) looks dumbstruck also!

[neufer]

I find it amazing that they can measure the masses of the two merging objects and the resulting object from those spectrograms!

How is it possible?

1) The no-hair theorem states that all black holes are basically simple objects.

2) More massive BHs generate lower frequency chirps due to the time it takes light to traverse their combined Schwarzschild radii.

3) The detailed results closely match supercomputer simulations.

LIGO results provide some experimental evidence consistent with the uniqueness of the no-hair theorem:

<<The no-hair theorem states that all black hole solutions of the Einstein–Maxwell equations of gravitation and electromagnetism in general relativity can be completely characterized by only three externally observable classical parameters: mass, electric charge, and angular momentum. All other information (for which "hair" is a metaphor) about the matter that formed a black hole or is falling into it "disappears" behind the black-hole event horizon and is therefore permanently inaccessible to external observers. Physicist John Archibald Wheeler expressed this idea with the phrase "black holes have no hair", which was the origin of the name.

In a later interview, Wheeler said that Jacob Bekenstein coined this phrase: "Richard Feynman, objected to the phrase that seemed to me to best symbolize the finding of one the graduate students: graduate student Jacob Bekenstein had shown that a black hole reveals by nothing outside it what went in, in the way of spinning electric particles. It might show electric charge; yes, mass; yes, but no other features - or as he put it, "A black hole has no hair" - and Richard Feynman thought that was an obscene phrase and he didn't want to use it. But that is a phrase now often used to state this feature of black holes, that they don't indicate any other properties other than a charge and angular momentum and mass."

The first version of the no-hair theorem for the simplified case of the uniqueness of the Schwarzschild metric was shown by Werner Israel in 1967. The result was quickly generalized to the cases of charged or spinning black holes. There is still no rigorous mathematical proof of a general no-hair theorem, and mathematicians refer to it as the no-hair conjecture. Even in the case of gravity alone (i.e., zero electric fields), the conjecture has only been partially resolved by results of Stephen Hawking, Brandon Carter, and David C. Robinson, under the additional hypothesis of non-degenerate event horizons and the technical, restrictive and difficult-to-justify assumption of real analyticity of the space-time continuum.>>

[neufer]

OK! We will be able to know the exact moment of the beginning of
the Universe! How does that help us? I'm dumbstruck!

Astrophysicists need to synchronize their watches

Click to play embedded YouTube video.

Estimated values of the Hubble constant, 2001–2020. Estimates in black represent calibrated distance ladder measurements which tend to cluster around 73 km/s/Mpc; red represents early universe CMB/BAO measurements with ΛCDM parameters which show good agreement on a figure near 67 km/s/Mpc, while blue are other techniques, whose uncertainties are not yet small enough to decide between the two.

---

<<Multiple methods have been used to determine the Hubble constant. "Late universe" measurements using calibrated distance ladder techniques have converged on a value of approximately 73 km/s/Mpc. Since 2000, "early universe" techniques based on measurements of the cosmic microwave background have become available, and these agree on a value near 67.7 km/s/Mpc. (This is accounting for the change in the expansion rate since the early universe, so is comparable to the first number.) As techniques have improved, the estimated measurement uncertainties have shrunk, but the range of measured values has not, to the point that the disagreement is now statistically significant. This discrepancy is called the Hubble tension.

As of 2020, the cause of the discrepancy is not understood. In April 2019, astronomers reported further substantial discrepancies across different measurement methods in Hubble constant values, possibly suggesting the existence of a new realm of physics not currently well understood. By November 2019, this tension had grown so far that some physicists like Joseph Silk had come to refer to it as a "possible crisis for cosmology", as the observed properties of the universe appear to be mutually inconsistent. In February 2020, the Megamaser Cosmology Project published independent results that confirmed the distance ladder results and differed from the early-universe results at a statistical significance level of 95%. In July 2020, measurements of the cosmic background radiation by the Atacama Cosmology Telescope predict that the Universe should be expanding more slowly than is currently observed.>>

[Chris Peterson]

OK! We will be able to know the exact moment of the beginning of
the Universe! How does that help us?

The age of the Universe is a key input into our cosmological models. As a rule, for any evolving process knowing when it began is critical to a full understanding.

[Eclectic Man]

Question: Does the collision of two black holes (or neutron stars) radiate gravitational waves equally in all directions?

Just wondering if there was a difference between the energy radiated in the plane of their mutual orbit and the perpendicular, and if so how that affects the special image received here on earth.

[neufer]

Click to play embedded YouTube video.

Question: Does the collision of two black holes (or neutron stars) radiate gravitational waves equally in all directions? Just wondering if there was a difference between the energy radiated in the plane of their mutual orbit and the perpendicular, and if so how that affects the special image received here on earth.

Since the polar direction allows for the radiation of both polarizations the ("circularly polarized") polar radiation is twice as strong.

LIGO could be badly oriented such that it misses the "linearly polarized" radiation in the orbital plane of a BH merger

LIGO is always properly oriented to detect the "circularly polarized" polar radiation of a BH merger.

[Chris Peterson]

Question: Does the collision of two black holes (or neutron stars) radiate gravitational waves equally in all directions? Just wondering if there was a difference between the energy radiated in the plane of their mutual orbit and the perpendicular, and if so how that affects the special image received here on earth.

:arrow: Since the polar direction allows for the radiation of both polarizations the ("circularly polarized") polar radiation is twice as strong.

LIGO could be badly oriented such that it misses the "linearly polarized" radiation in the orbital plane of a BH merger :!:

LIGO is always properly oriented to detect the "circularly polarized" polar radiation of a BH merger.

Well, yes, but this doesn't quite answer the question. The polarization of the radiated waves isn't the same as the energy distribution (the radiation pattern). IIRC, a point mass oscillating between two points generates gravitational waves only on its motion axis. So I'd expect the radiation pattern of two colliding masses to radiate more energy on their orbital plane. During the actual merger, you get a sort of blob that's oscillating in all directions, which I'd expect to produce a complex, but nonisometric pattern.

[neufer]

OK! We will be able to know the exact moment of the beginning of
the Universe! How does that help us?

The age of the Universe is a key input into our cosmological models. As a rule, for any evolving process knowing when it began is critical to a full understanding.

Understanding the exact age of the Universe is probably less important than
understanding why science is giving us inconsistent results for the age of the Universe.

Ideally, this leads to new physics.

At a minimum, it indicates a flaw in one or more of our current procedures.

[Chris Peterson]

OK! We will be able to know the exact moment of the beginning of
the Universe! How does that help us?

The age of the Universe is a key input into our cosmological models. As a rule, for any evolving process knowing when it began is critical to a full understanding.

Understanding the exact age of the Universe is probably less important than
understanding why science is giving us inconsistent results for the age of the Universe.

Ideally, this leads to new physics.

At a minimum, it indicates a flaw in one or more of our current procedures.

Knowing the age of the Universe is critical to fine tuning our models, which is how we come to such new understanding.

[Fred the Cat]

We've progessed from bunts to home runs!

[neufer]

The age of the Universe is a key input into our cosmological models.
As a rule, for any evolving process knowing when it began is critical to a full understanding.

Understanding the exact age of the Universe is probably less important than
understanding why science is giving us inconsistent results for the age of the Universe.

Ideally, this leads to new physics.

At a minimum, it indicates a flaw in one or more of our current procedures.

Knowing the age of the Universe is critical to fine tuning our models, which is how we come to such new understanding.

“Then you should say what you mean,” the March Hare went on.

“I do,” Alice hastily replied; “at least—at least I mean what I say—that’s the same thing, you know.”

“Not the same thing a bit!” said the Hatter. “You might just as well say that ‘I see what I eat’ is the same thing as ‘I eat what I see’!”

“You might just as well say,” added the March Hare, “that ‘I like what I get’ is the same thing as ‘I get what I like’!”

“You might just as well say,” added the Dormouse, who seemed to be talking in his sleep, “that ‘I breathe when I sleep’ is the same thing as ‘I sleep when I breathe’!”

“It is the same thing with you,” said the Hatter, and here the conversation dropped, and the party sat silent for a minute, while Alice thought over all she could remember about ravens and writing-desks, which wasn’t much.

The Hatter was the first to break the silence. “What day of the month is it?” he said, turning to Alice: he had taken his watch out of his pocket, and was looking at it uneasily, shaking it every now and then, and holding it to his ear.

Alice considered a little, and then said “The fourth.”

“Two days wrong!” sighed the Hatter.
“I told you butter wouldn’t suit the works!” he added looking angrily at the March Hare.

“It was the best butter,” the March Hare meekly replied.

“Yes, but some crumbs must have got in as well,” the Hatter grumbled: “you shouldn’t have put it in with the bread-knife.”

The March Hare took the watch and looked at it gloomily: then he dipped it into his cup of tea, and looked at it again: but he could think of nothing better to say than his first remark, “It was the best butter, you know.”

[heehaw]

OK, Art! Here's another: https://henry.pha.jhu.edu/alice.pdf

[VictorBorun]

1) The no-hair theorem states that all black holes are basically simple objects.

<<The no-hair theorem states that all black hole solutions of the Einstein–Maxwell equations of gravitation and electromagnetism in general relativity can be completely characterized by only three externally observable classical parameters: mass, electric charge, and angular momentum. All other information (for which "hair" is a metaphor) about the matter that formed a black hole or is falling into it "disappears" behind the black-hole event horizon and is therefore permanently inaccessible to external observers. Physicist John Archibald Wheeler expressed this idea with the phrase "black holes have no hair", which was the origin of the name.
There is still no rigorous mathematical proof of a general no-hair theorem, and mathematicians refer to it as the no-hair conjecture.>>

No one I ask tells me whether a fast moving BH can extract some mass from another BH it passes and thus allow some things escape from under the second event horizon (after its radius has been deminished).

I wish the answer would be yes, we can get something — but then the answer could be the other way round, that a fast moving BH would never manage to pass the other BH close enough to take some mass; that once the two event horizons touch the two BHs just have to merge whatever large energy and spin would have to be shedded by gravitational waves.

By the way, to quickly put a probe under an event horizon without wasting much time on Earth is easy: you just send your probe to a BH soon to merge with a large mass like another BH; the radius of the event horizon will grow and your probe gets under in no time.

[Astronymus]

Interesting how many lock quite alike.

[Chris Peterson]

No one I ask tells me whether a fast moving BH can extract some mass from another BH it passes and thus allow some things escape from under the second event horizon (after its radius has been deminished).

How could it? The entire mass of a black hole is in a dimensionless point (or maybe a subatomic-sized volume) at the center. There is nothing going on between there and the event horizon. Two black holes should be able to pass each other with their event horizons overlapping and continue on unchanged. The event horizon isn't part of the structure of a black hole.

[johnnydeep]

Late to this interesting discussion, but I was always perplexed by why, if gravitational waves are just "ripples in space-time", they should be limited to propagating at the speed of light. That is, no THING is physically moving through space, so the c speed limit should be irrelevant. I guess it's because a full description of relativity says that no INFORMATION can propagate faster than c, and GWs are conveying information. Then there's the still hypothetical (I think?) existence of gravitons, and if they are indeed found to be the PARTICLE mediators of the gravitational force, I suppose they WOULD be limited by c.

Also, GWs apparently have a wavelength and frequency in inverse proportion to each other. But this APOD also mentions their energy, which is represented by the brightness of the spectra. So, how does the energy relate to the wavelength and frequency? We say light of higher frequency (and shorter wavelength) has higher energy, don't we? Yet neufer's first post above says that larger mass BHs generate lower frequency GW "chirps", but those would also be more energetic, wouldn't they?

[neufer]

Late to this interesting discussion, but I was always perplexed by why, if gravitational waves are just "ripples in space-time", they should be limited to propagating at the speed of light. That is, no THING is physically moving through space, so the c speed limit should be irrelevant. I guess it's because a full description of relativity says that no INFORMATION can propagate faster than c, and GWs are conveying information. Then there's the still hypothetical (I think?) existence of gravitons, and if they are indeed found to be the PARTICLE mediators of the gravitational force, I suppose they WOULD be limited by c.

Also, GWs apparently have a wavelength and frequency in inverse proportion to each other. But this APOD also mentions their energy, which is represented by the brightness of the spectra. So, how does the energy relate to the wavelength and frequency? We say light of higher frequency (and shorter wavelength) has higher energy, don't we? Yet neufer's first post above says that larger mass BHs generate lower frequency GW "chirps", but those would also be more energetic, wouldn't they?

Mass-less (spin 2) gravitons have the nominal quantum relationship: E = hν = hc/λ

• where h = 6.626×10⁻³⁴ Joule⋅seconds/

Total gravitational radiation power corresponds to:
the total number of emitted gravitons per second times their minuscule quantum energies.

The Hulse–Taylor (pulsar plus neutron star) binary are in a pair of 27,900 second period elliptical orbits.
By transforming its kinetic energy into (mostly ~13,950 second [i.e., 27.9 AU] long) gravitational waves
the Hulse–Taylor binary emits ~ (7.35 × 10²⁴ Joules/second)(13,950 seconds) / (6.626×10⁻³⁴ Joule⋅seconds)

~ 1.55 × 10⁶² gravitons/second

Orbital decay of PSR B1913+16. The data points indicate the observed change in the epoch of periastron with date while the parabola illustrates the theoretically expected change in epoch according to general relativity.

---

<<In the Hulse–Taylor binary the pulsar and its neutron star companion both follow elliptical orbits around their common center of mass. The period of the orbital motion is 7.75 hours, and the two neutron stars are believed to be nearly equal in mass, about 1.4 solar masses. The orbit has decayed since the binary system was initially discovered, in precise agreement with the loss of energy due to gravitational waves described by Albert Einstein's general theory of relativity. The ratio of observed to predicted rate of orbital decay is calculated to be 0.997 ± 0.002. The total power of the gravitational waves emitted by this system presently is calculated to be 7.35 × 10²⁴ watts. For comparison, this is 1.9% of the power radiated in light by the Sun. The Solar System radiates only about 5,000 watts in gravitational waves, due to the much larger distances and orbit times, particularly between the Sun and Jupiter and the relatively small mass of the planets. With this comparatively large energy loss due to gravitational radiation, the rate of decrease of orbital period is 76.5 microseconds per year, the rate of decrease of semimajor axis is 3.5 meters per year, and the calculated lifetime to final inspiral is 300 million years.>>

Note that even passing interstellar objects (such as our own Oumuamua) lose a tiny amount
of kinetic energy due to gravitational Bremsstrahlung of very low frequency/energy gravitons.

[VictorBorun]

No one I ask tells me whether a fast moving BH can extract some mass from another BH it passes and thus allow some things escape from under the second event horizon (after its radius has been deminished).

How could it? The entire mass of a black hole is in a dimensionless point (or maybe a subatomic-sized volume) at the center. There is nothing going on between there and the event horizon. Two black holes should be able to pass each other with their event horizons overlapping and continue on unchanged. The event horizon isn't part of the structure of a black hole.

But suppose the donor BH is yet not all in the center of its event horzon; suppose there are some ring singularity or more complex cluster of singular points, filaments and branes, and perhaps some matter is still swarming around that central region deep under the event horizon

[alter-ego]

No one I ask tells me whether a fast moving BH can extract some mass from another BH it passes and thus allow some things escape from under the second event horizon (after its radius has been deminished).

How could it? The entire mass of a black hole is in a dimensionless point (or maybe a subatomic-sized volume) at the center. There is nothing going on between there and the event horizon. Two black holes should be able to pass each other with their event horizons overlapping and continue on unchanged. The event horizon isn't part of the structure of a black hole.

But suppose the donor BH is yet not all in the center of its event horzon; suppose there are some ring singularity or more complex cluster of singular points, filaments and branes, and perhaps some matter is still swarming around that central region deep under the event horizon

Online discussions definitively say the regardless of relative velocity between two black holes, if their even horizons overlap at all, their fate is sealed in a merger.
Black holes: Is merger inevitable when horizons touch?
If two black hole event horizons overlap (touch) can they ever separate again?
Maybe a way to think about this is once a bridge is formed between two separate event horizons, there is only one, contiguous event horizon, and since the relative BH velocity cannot reach the speed of light, neither BH can escape the common event horizon, however small the bridge is. I don't know, but superficially, breaking a closed-surface event horizon seems contrary to BH fundamentals.

[neufer]

Online discussions definitively say the regardless of relative velocity between two black holes, if their even horizons overlap at all, their fate is sealed in a merger.
Black holes: Is merger inevitable when horizons touch?
If two black hole event horizons overlap (touch) can they ever separate again?
Maybe a way to think about this is once a bridge is formed between two separate event horizons, there is only one, contiguous event horizon, and since the relative BH velocity cannot reach the speed of light, neither BH can escape the common event horizon, however small the bridge is. I don't know, but superficially, breaking a closed-surface event horizon seems contrary to BH fundamentals.

Us old-timers were taught that event horizons were an artifacts of a poor coordinate system...yet it is still the only coordinate system available to scientific observers. I have no idea whether the event horizon is a hot fire wall or a frozen hologram (or both... or neither) and prefer to avoid the whole subject altogether in order to prevent severe brain freezes. Hawking said that it is a radiating black body and that's good enough for me.

• 1) A free falling coordinate system will generate a BH singularity in time T₁.
  2) In my coordinate system a BH will eventually evaporate in time T₂.

I have no idea, whatever, how to synchronize these two coordinate systems to determine if T₂>T₁ or T₁>T₂

[Guest APOD Watcher]

While reading today's APOD explanation, I encountered the sentence,

"Humanity has only had the technology to hear these unusual chirps for
the past seven years, but since then we have heard about 90 -- during
the first three observing runs."

What caught my attention was the phrase, "during the first three
observing runs".

Are not these facilities in near continuous use?

If not, how long are these observing runs?

Ninety waves in seven years is not all that big a number (given the scale
of the universe and the number of black holes to be found in it), but if the
runs are only six months out of each year, the number is much more
impressive.

Are you able to enlighten me at all?

Thanks for any help you can offer.

[neufer]

Are not these facilities in near continuous use?

If not, how long are these observing runs?

• The last observing run lasted almost a year.
  The next observing run won't start for another a year.

<<Initial LIGO operations between 2002 and 2010 did not detect any gravitational waves. After 2010, LIGO went offline for several years for a major upgrade, installing the new Advanced LIGO detectors in the LIGO Observatory infrastructures. By the time the LIGO Laboratory started the first observing run 'O1' with the Advanced LIGO detectors in September 2015, the LIGO Scientific Collaboration included more than 900 scientists worldwide. The first observing run operated at a sensitivity roughly 3 times greater than Initial LIGO, and a much greater sensitivity for larger systems with their peak radiation at lower audio frequencies. On 11 February 2016, the LIGO and Virgo collaborations announced the first observation of gravitational waves. The signal was named GW150914. The waveform showed up on 14 September 2015, within just two days of when the Advanced LIGO detectors started collecting data after their upgrade. On 15 June 2016, LIGO announced the detection of a second gravitational wave event, recorded on 26 December 2015, at 3:38 UTC. The signal was named GW151226.

The second observing run (O2) ran from 30 November 2016 to 25 August 2017, with Livingston achieving 15–25% sensitivity improvement over O1, and with Hanford's sensitivity similar to O1. In this period, LIGO saw several further gravitational wave events: GW170104 in January; GW170608 in June; and five others between July and August 2017. Several of these were also detected by the Virgo Collaboration. Unlike the black hole mergers which are only detectable gravitationally, GW170817 came from the collision of two neutron stars and was also detected electromagnetically by gamma ray satellites and optical telescopes.

The third run (O3) began on 1 April 2019 and was planned to last until 30 April 2020. Future observing runs will be interleaved with commissioning efforts to further improve the sensitivity. It is aimed to achieve design sensitivity in 2021. On 6 January 2020, LIGO announced the detection of what appeared to be gravitational ripples from a collision of two neutron stars, recorded on 25 April 2019, by the LIGO Livingston detector. Unlike GW170817, this event did not result in any light being detected. Furthermore, this is the first published event for a single-observatory detection, given that the LIGO Hanford detector was temporarily offline at the time and the event was too faint to be visible in Virgo's data. The Kamioka Gravitational Wave Detector (KAGRA) in Japan became operational on 25 February 2020, likely improving the detection and localization of future gravitational wave signals. However, KAGRA does not report their signals in real-time on GraceDB as LIGO and Virgo do, so the results of their observation run will likely not be published until the end of O3. The LIGO-Virgo collaboration ended the O3 run early on March 27, 2020 due to health concerns from the COVID-19 pandemic.

On 15 November 2021, LIGO announced that "we plan to start the O4 Observing run in mid-December 2022".>>