Five Surprising Truths About Black Holes From LIGO
“1.) The largest merging black holes are the easiest to see, and they don’t appear to get larger than about 50 solar masses. One of the best things about looking for gravitational waves is that it’s easier to see them from farther away than it is for a light source. Stars appear dimmer in proportion to their distance squared: a star 10 times the distance is just one-hundredth as bright. But gravitational waves are dimmer in direct proportion to distance: merging black holes 10 times as far away produce 10% the signal.
As a result, we can see very massive objects to very great distances, and yet we don’t see black holes merging with 75, 100, 150, or 200+ solar masses. 20-to-50 solar masses are common, but we haven’t seen anything above that yet. Perhaps the black holes arising from ultra-massive stars truly are rare.”
Well, the LIGO and Virgo collaborations have put their head together to re-analyze the full suite of their scientific data, and guess what they found: a total of 11 merger events, including ten black hole-black hole mergers. Now that we’re in the double digits, we can actually start to say some incredibly meaningful things about what’s present in the Universe, what we’ve seen, and what that means for what comes next. Which black holes are the most common? How frequently do these mergers occur? What’s the highest-mass black hole binary that LIGO could detect? And how many black holes do we expect to find when run III starts up in 2019?
The answers are coming! Come see what we know so far, and learn five surprising truths about black holes, courtesy of our findings from LIGO!
Seeing One Example Of Merging Neutron Stars Raises Five Incredible Questions
“1.) What is the rate at which neutron star-neutron star mergers occur? Before this event was observed, we had two ways of estimating how frequently two neutron stars would merge: from measurements of binary neutron stars in our galaxy (such as from pulsars), and from our theoretical models of star formation, supernovae, and their remnants. That gave us a mean estimate of around 100 such mergers every year within a cubic gigaparsec of space.
Thanks to the observation of this event, we now have our first observational rate estimate, and it’s about ten times larger than we expected. We thought we would need LIGO to reach its design sensitivity (it’s only halfway there) before seeing anything, and then on top of that we thought that pinpointing the location in at least 3 detectors would be unlikely. Yet we not only got it early, we localized it on the first try. So now the question is, did we just get lucky by seeing this one event, or is the true event rate really so much higher? And if it is, then what is it about our theoretical models that are so wrong?”
Now that we’ve observed merging neutron stars for the first time, in many different wavelengths of light as well as in gravitational waves, we’ve got a whole new world of data to work with. We’ve independently confirmed that gravitational waves are real and that we can, in fact, pinpoint their locations on the sky. We’ve demonstrated that merging neutron stars create short gamma ray bursts, and shown that the origin of the majority of elements heavier than the first row of transition metals comes primarily from neutron star-neutron star mergers. But the new discovery raises a ton of questions, too. Seeing this event has presented theorists with a number of new challenges, ranging from the event rate being some ten times as great as expected to much more matter being ejected than we’d thought. And what was it that was left behind? Was it a neutron star? A black hole? Or an exotic object that’s in its own class?
There are some great advances that the future will hold for gravitational wave and neutron star astronomy, but it’s up to theorists to explain why these objects behave as they do. Here are five burning questions we now have.
LIGO-VIRGO Detects The First Three-Detector Gravitational Wave
“When you have a signal appearing in one detector, you can gain a rough estimate of its distance from you (with uncertainties), but with no information about its direction. A second detector not only gives another distance estimate, but the time difference between the two signals gives you some information about distance, allowing you to restrict yourself to an “arc” on the sky. But a third detector, with a third time difference, allows you to pinpoint a single point, albeit with significant uncertainties. This is where the word “triangulation” comes from, since you need three detectors to pinpoint a location-of-origin. That’s exactly what VIRGO was able to give.”
For over a century after the publication of General Relativity, it was uncertain whether gravitational waves were real or not. It wasn’t until their first direct detection less than two years ago, by the LIGO scientific collaboration, that their existence was spectacularly confirmed. With the VIRGO detector in Italy coming online this year to complement the twin LIGO detectors, however, so much more became possible. An actual position in space could be identified for the first time, enabling a possible correlation between the gravitational wave sky and the electromagnetic one. The three-dimensional polarization of a gravitational wave could be measured, and compared with the predictions of Einstein’s theory. And gravitational wave signals can be teased out earlier and measured to smaller amplitudes than ever before. Not only have we just seen our fourth gravitational wave event, we’ve seen it in all three detectors.
This discovery is, indeed, something big, but there’s even bigger science to come in the future! Come see what this first three-detector gravitational wave event has given us!
How Uncertain Are LIGO’s First Gravitational Wave Detections?
“What’s vital to understand is that no one is claiming LIGO is wrong, but rather that one team is claiming that perhaps LIGO has room for improvement in their analysis. And this is a very real danger that has plagued experimental physicists and astronomical observers for as long as those scientific fields have existed. The issue is not that LIGO’s results are in doubt, but rather that LIGO’s analysis may be imperfect.”
Three times now, the LIGO collaboration has produced very strong evidence that black hole pairs, from across the Universe, inspiraled and merged, producing gravitational waves. The twin LIGO detectors in Hanford, WA and Livingston, LA each detected these signals, and the signals were correlated between both detectors. For the first time ever (and the second, and the third), we had directly detected gravitational waves. But last month, a team of independent scientists from Denmark attempted to reproduce LIGO’s analysis, and noticed something that shouldn’t be there: noise correlations between the two detectors. Noise is supposed to be uncorrelated, and yet the noise correlations peaked at the moment of the inspiral-and-merger event. It doesn’t mean that gravitational waves aren’t real, but it does mean that LIGO, perhaps, has room for improvement.
This has been a very controversial topic over the past few weeks; come learn where we are in this saga of science playing out in real-time!
Was It All Just Noise? Independent Analysis Casts Doubt On LIGO’s Detections
“After an effort of more than 100 years and a collaboration involving over 1,000 scientists, we all celebrated. It was February 11, 2016, and LIGO had just announced their first direct detection of gravitational waves. Analysis of the data attributed the signal to a black hole merger that happened several billion light years away. But what if there wasn’t a signal at all, but rather patterns and correlations in the noise that fooled us into believing we were seeing something that wasn’t real? A group of Danish researchers just submitted a paper arguing that the celebration might have been premature.”
It revolutionized our view of the Universe when the LIGO annoucements – and we’re up to three, now – came out. They indicated the direct detection of gravitational waves from merging black holes, teaching us about a new population of stellar remnants, confirming the existence of gravitational waves, and showcasing yet another victory for Einstein’s General Relativity. But it all rests on one critical assumption: that what LIGO detected was a gravitational wave signal, not just noise in the detector. A critical test of this is whether the noise is truly random between detectors, as one would expect, or whether the noise is somehow correlated between the detectors, which would run contrary to expectations. An independent team from Denmark, outside of the LIGO collaboration, put this idea to the test, and what they found has cast significant doubts on the LIGO results.
There’s a new debate brewing surrounding gravitational waves, and while LIGO isn’t giving the new analysis much credence, the importance of getting it right, publicly, is too great to ignore. Sabine Hossenfelder explains.
There is sound in space, thanks to gravitational waves
“These waves are maddeningly weak, and their effects on the objects in spacetime are stupendously tiny. But if you know how to listen for them — just as the components of a radio know how to listen for those long-frequency light waves — you can detect these signals and hear them just as you’d hear any other sound. With an amplitude and a frequency, they’re no different from any other wave.”
You’ve likely heard that there’s no sound in space; that sound needs a medium to travel through, and in the vacuum of space, there is none. That’s true… up to a point. If you were only a few light years away from a star, stellar remnant, black hole, or even a supernova, you’d have no way to hear, feel, or otherwise directly measure the pressure waves from those objects. But they emit another kind of wave that can be interpreted as sounds, if you listen correctly: gravitational waves. These waves are so powerful, that in the very first event we ever detected, the black hole-black hole merger we saw outshone, in terms of energy, all of the stars in the observable Universe combined. There really is sound in space, as long as you know how to listen for it properly.
Come learn about it, and catch a live event, live-blogged by me, this evening!