What gravitational-wave events reveal about the Universe | Hacker News Reader

What gravitational-wave events reveal about the Universe

nature.com

70 pointsheadalgorithm39 comments

I recently finished "The Cosmic Revolutionary's Handbook"[0], which can best be described as "Here's all the data, please, tell me how your crazy theory fits ALL of it, because it's really hard to do that". I really illuminates the difficulty in building any theory that fits all the data, and the incredible beauty that our current best theory of the universe truly is.

Gravitational wave events like these are just more data, making the challenge that much more difficult. And yet, the model still fits pretty well, barring a few pieces. But we don't throw the baby out with the bathwater, because it's still a really good model.

And the best part about that is that we can use the model we have to make predictions, and most of the time they're pretty good. Predictions about things we can never see or experience or know. We can see into the past and future.

[0]https://www.amazon.com/Cosmic-Revolutionarys-Handbook-Beat-B...

I'm skeptical. These days it's increasingly possible to create models with enough free parameters and complicated math that you can sift through noise with the right filters and get what you were looking for. This is even more troublesome for unimodal phenomena. There has only been a singular multimodal observation that correlates with a GW observation; and that's troubling especially since the number of unimodal observations is increasing, and so the statistics of how often we would expect random signals to correlate (with the three prongs of the detector, and then with a multimodal observation) is getting murkier and harder for third parties to review.

The truly disciplined experiment is to do a double-blinded study: over a timespan record events in secret. Then augment the GW data with 3x fictional events (say, fix the times and magnitudes but randomize azimuth/altitude). Challenge EM observers to correlate with these events, break the seal, and see if multimodal observations pass a simple chi-squared test.

Thanks for the read recommendation, it's now my next book on the list.

> It shows that, in some binary systems, the two black holes have misaligned axes of rotation, which would imply that they formed separately. But many other binaries appear to have roughly aligned axes of rotation, which is what astrophysicists expect when the two black holes began their lives as a binary star system.

This is mindboggling -- you can tell two black holes from thousand of light years away had misaligned axes of rotation by measuring the effect they have on lasers bouncing some mirrors.

s/thousands/billions/

That doesn't make it less mindboggling...

Billions of light years?

I thought the energy would be too faint, but given they found tens of mergers, it makes sense its actually billions, I don't suppose there are that many black hole mergers in our galaxy in this timeframe.

It really is billions of light years.

Can you imagine the amount of energy released that we can detect these waves billions of light years away? It's mind blowing.

In some of these mergers, two black holes combine but their total mass after merging is short by 1 to 5 solar masses. ALL of that mass was converted to energy to create the gravitational waves we picked up. Picture that- the entire sun converted to energy in the blink of an eye, times five.

I read some article that the distortions at 10.000km from the merger would be ~1mm, so you could feel them, but not be harmed by them.

In contrast, if all that energy was released as EM waves, you'd be cooked from far, far away.

Such peaceful are these gravitational waves.

I'm not so sure that a 1mm distortion would be safe. It's happening at the level of space itself, so it would change the way parts of molecules will relate to each other. It passes fast, but potentially you could fall apart at the molecular level.

I am not a physicist so I could be completely wrong about that. And I'm not sure if it matters since it's probably accompanied by photons and probably neutrinos in sufficient quantity to wipe you out at that distance. (Not from the merger itself, but from any matter in its halo.)

I seem to recall 3 billion light-years as the estimated distance of one of the early mergers detected. Can't cite a source, though...

I wonder how they focus the system on a particular area of the sky. Do they have an array of these systems?

There are three of them, two in the US and one in Italy. They can’t measure directionally individually, they just measure the gravitational waves at that location, but you can measure the different times a signal arrives at each and extrapolate from that.

I think also the strength of signal detected has to do with the orientation of the detector with respect to the direction the gravity waves are travelling. So you can use the relative strength of multiple detectors pointing in different directions to infer the direction to the source.

They actually can't! They're looking in all directions at once by having 2, and now three detectors on different parts of the planet.

So when one thinks it's seen a GW, it has to correlate the observance with the others, and then they can say "Okay, so given the timing, which way did it come from?" and get a very rough direction. If it's a potential neutron star merger, they can then have everyone swing their telescopes in that direction to hopefully see it happen.

Slightly off topic question, but I am curious about the total energy content off gravitational wave energy in the universe. Are there any estimates? Or a way to calculate it? Is this figure taken into account when calculating the total energy of the universe?

That's an interesting question. I've never thought about it or seen that taken into account (its not something we fit to when constraining cosmology)

I just looked around and LIGO has constrained Omega_GW < 1e-7 (they find no evidence, but that is the limit of their sensitivity). This is at least an order of magnitude smaller than Omega_Radiation and so will have a negligible effect on cosmology/total energy content.

https://arxiv.org/pdf/1612.02029.pdf

We have estimates of the energy or mass formed into black holes, and at the end of the day this is just black hole energy that happens to have been later converted into a different form.

Ignoring dark energy, we expect gravitational energy to be identical to non-gravitational energy; ie, “half”.

On passing around objects, would the gravity wave create an interference pattern that we would be able to detect given enough detectors?

Also I feel that sentence structure is horrible. If anyone can offer alternatives that are easier to read I would appreciate it.

Does gravity have a different speed through different objects? Like nothing I know about blocks or slows gravity.

Closest thing I could think of is a gravitational lense that allowed the waves to arrive at different times at your location.

> usually created by the merging of two black holes — have dramatically increased their sensitivity since the first identification was made in 2015. The growing data set is helping astronomy

That graph looks like a hockey stick. Is it really accelerating or are we just getting better at detecting it (this makes more sense)?

One question for me is how do they know which collision is BH+BH, BH+star, Or star+star? That sounds incredibly interesting detail

> That graph looks like a hockey stick. Is it really accelerating or are we just getting better at detecting it (this makes more sense)?

I suspect it is a piecewise combination of two linear functions. There was probably a ~fixed sensitivity across Runs 1-2, with some upgrades before Run 3[0], which resulted in an improved sensitivity for that run. So in effect the slope of detections/time is steeper with the improved sensitivity.

[0] https://indico.cern.ch/event/577856/contributions/3422625/

For the first question: I think a big part is that other detectors came online, providing much more confidence that LIGO measurements were real gravitational waves.

For the second question: they can infer the masses of the objects in question from the gravitational waves. Generally if the mass is < 2.5 solar masses it has to be a neutron star, and if it’s > 5 it must be a black hole. Keep in mind this isn’t rigorous! The theoretical maximum for neutron stars is a little less than 3 solar masses, and the estimated minimum for a stellar black hole is much more empirical - I think the smallest known is about 3.5. There’s a lot of interesting physics between 2.5 and 5 solar masses - perhaps this is how we’ll discover “quark stars.”

I believe our detectors are not sensitive enough to detect the acceleration of normal stars in any physically plausible scenario - I think they would have to be accelerating way too fast to generate waves that are indistinguishable from noise.

Could a gravity whip of a large star around the a galactic central black hole make a big enough ripple?

The material in stars is too spread out over a large volume, and stars move too slowly for these systems to detect them. Black hole collisions events last on the order of about 10 milliseconds and emit up to about 40% of their combined mass in gravitational energy. Nothing a star can do is fast or energetic enough to register at those scales.

I am not sure and am only a layperson on this (math grad school, physics undergrad) - conceptually you could think of a blue giant orbiting at 0.99c, but

a) the orbit would be so large that the actual acceleration is fairly small, or

b) the centripetal forces of a small, relativistic orbit would rip the star into pieces. In particular this would dramatically reduce the effective density of the accelerating mass.

Having enough gravitational force to retain coherent structure while accelerating quickly enough to generate gravitational waves seems like a tough circle to square.

Sensitivity is getting much better, vibrational isolation, processing, this run I believe even includes elements that reduce the noise produced from quantum uncertainties [0]. The uncertainty principle means the product of two measurements (positionmomentum, timeenergy, conjugate variables in general) can't be more precise than a certain threshold, so if you only care about one value of that pair, you can dump all the uncertainty into the unmeasured quantity and get past what might otherwise be considered a barrier.

[0]https://astronomy.com/news/2019/12/new-technology-improves-g...

surely the latter, and that is what the sentence implies: "observatories [...] have dramatically increased their sensitivity"

Looks like things started to get _much_ better after Virgo joined. I assume that is because of the increased "baseline" (angular resolution) and because two different instruments at different locations makes it easier to detect & reject spurious signals.

> and because two different instruments at different locations

Virgo is the 3rd actually. LIGO consists of 2 separate facilities. But yes, having more detectors decreases the degrees of freedom when calculating the origin of the signal.

The number of detected events depend on the volume of space surveyed. So, for some increase in detector sensitivity x you get x^3 more events, roughly speaking.

We’re just getting better at detecting them.

> One question for me is how do they know which collision is BH+BH, BH+star, Or star+star

IANAA, but I imagine that they can detect the source of the waves and check electromagnetic observations from the same area. BH+BH = no electromagnetic radiation, star+star = electromagnetic radiation from 2 stars.

Of course, it's not easy to detect such fine measurements and correlate between different instruments.

No, the distinction between BH and NS can be made by just looking at the shape of the gravitational wave (stress as a function of time). A BH and a NS of the same mass will have different densities, so they produce very different ripples as they merge.

Of course, this can be corroborated with electromagnetic observations. But, to my knowledge, not all NS-NS mergers are witnessed in telescopes for various technical limitations.

I don't think that is right, though I'm not a GW expert so please tell me if I'm wrong.

I think they know the difference just by looking at the mass. i.e. we think it is hard to form black holes smaller than ~3.3ish solar masses and we don't think neutron stars can be more massive than 2.2 solar masses.

This is why we get articles like [1] where there is an issue when we think we've found something between those numbers.

And yes, the foolproof way of checking whether a NS was involved is to follow up with telescopes. But the constraints on position from GW aren't always good and so you can't always find it.

[1] https://www.sciencemag.org/news/2020/06/gravitational-waves-...

Ok, I'm actually only 90% wrong! It turns out that there is in fact some information about the BH-NS distinction in the gravitational signal itself, but it is very weak and hasn't been detected yet. It is expected to be detected in the future though.

https://twitter.com/di_goldene_pave/status/13223229779841433...

I didn't know about that, that's really cool! Thanks for letting me know.

Yikes, on reflection I think you're right. Sorry about that. Thanks for the correction!

Questions 1. How these detectors read distance of the event? In order to assert that the rate peaked 8*10^9 years ago, we'd need to know distance. Surely it's not based on amplitude 2. Galaxies collide. Is there reason to think that many galaxies contain a central supermassive (10^9 solar mass) black hole, and that such holes have collided as well? Perhaps too rare to be seen in five years?

1. The frequency evolution of the measures waveform depends on the mass of the objects. Once you know the mass you can use the amplitude to infer the distance.

2. Galaxies do contain supermassive black holes at their centers. However the frequency response of ground based detectors preclude observation. When space based detectors are operating we will see those collisions.

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