Second detection of gravitational waves from a binary coalescence

Breaking news are all around the globe after yesterday's press conference by the LIGO/Virgo collaboration, which announced a second very solid gravitational-wave event on Boxing Day, Dec 26 2015, dubbed GW151226.

The waveforms of the 3 events detected by LIGO during O1 (the first observation run). GW150914 and GW151226 are events which are very solidly detected (at more than 5 sigma), whereas LVT151012 has a (small) probability of being a statistical fluctuation. From this page. The inspiral phase of the new event GW151226 lasted much longer than the original GW150914 (about 80 cycles in total)

I warmly suggest you to check this beautiful multimedia page, made by Marc Favata and his group.

As the LIGO/Virgo collaboration put it, the era of gravitational-wave astrophysics is officially started!

One cannot get rich with fundamental physics, they said...

..unless you make a landmark discovery such as the first detection of gravitational waves. In such case, you might win a $3 million Breakthrough Prize in Fundamental Physics!

The three funding fathers of LIGO, Ronald P. Drever and Kip. S. Thorne and Rainer Weiss, are going to share $1 million, and the other other $2 million will be split among 1,012 scientists who authored the milestone article in Physical Review Letters and a list of key contributors to the theoretical and experimental understanding of gravitational waves (Luc Blanchet, Thibault Damour, Lawrence Kidder, Frans Pretorius,
Mark Scheel, Saul A. Teukolsky, Rochus E. Vogt) without which LIGO outstanding discovery would not have been possible.

As Richard Feynman brilliantly put it:

This also applies to the economical reward that might following great discoveries, and it is probably the reason why reckless and economically inconvenient science is pursed: because it is passion driven rather than money driven.

Kudos to the LIGO/Virgo Collaboration!

Can one hear the shape of a black hole? [*]

[Edit: see also the Synopsis in APS Magazine "Physics", the coverage and this interview (in Italian) by the Italian Institute for Astrophysics (INAF), and the stories in Physics World, NewScientist, Phys.org, Le Monde and Repubblica.]


An orchestra conductor can easily tell a gong from a bell just by their different sound. Can astronomers do the same and tell a black hole from another dark object just by detecting their different gravitational-wave signal? In our recent paper, Vitor Cardoso, Edgardo Franzin and I show that this might not be the case [preprint here].


Last February, the LIGO/Virgo Collaboration announced the first direct detection of gravitational waves by the two laser interferometers advanced LIGO. This historical discovery has been also welcomed as the first conclusive proof for the existence of black holes, the most extreme objects in the Universe. The detected signal --dubbed GW150914-- corresponds to the "pas de deux" of two massive objects, which inspiral around each other and eventually collide in a cosmic spacetime-quake. LIGO data firmly show that the two objects are extremely compact and way too massive to be neutron stars. While providing compelling evidence, this does not represent a bullet-proof confirmation of the existence of black holes by itself. After all, signatures of compact, dark and massive objects come routinely from electromagnetic observations with infrared and X-ray detectors.

What makes GW150914 really unique is that the gravitational-wave signal contains all the final stages of the cosmic evolution of the binary system: the two objects lose an enormous amount of energy through the emission of gravitational waves, approach each other and eventually merge to form a single compact object of about 62 solar masses. After the merger (which lasted only a few milliseconds!) the final object was highly distorted and underwent an adjustment phase known as the "ringdown", in which the object vibrates pretty much like a drum. Just like the notes of the drum depend on its properties (the shape, the size, the material), the "ringdown modes" should carry information about the very nature of the final object produced after the merger.

A comparison between the ringdown signal of a particle falling into a black hole (black dashed line) and the same particle falling into a wormhole (red line). The wormhole geometry is illustrated in the top right corner. The two signals are identical at early times and the "universal" ringdown waveform is associated to the particle reaching point "A" (the light ring). The real quasinormal modes of the wormhole appear only at late times, when the particle reaches the throat (point "B").

Black holes are snatches in the spacetime fabric and their rim ---known as the event horizon--- vibrates in a very peculiar way that was predicted after decades of restless work by using Einstein's theory of general relativity. Scientists hope that, by detecting events like GW150914, one would be able to identify the modes of vibration of the final black hole (the so-called "quasinormal modes") from the ringdown signal. Detecting the quasinormal modes will be the definitive proof that black holes are produced in a binary merger, precisely as predicted by Einstein's theory.


In our recent work (selected as an Editor's Suggestion and featuring the cover of the current issue of Physical Review Letters), we show that this paradigm is incorrect. The vibrations of very compact objects without an event horizon are dramatically different from those of black holes (their frequency is lower and they last much longer time) and, nonetheless, the ringdown signal produced by these "black-hole mimickers" is identical to that of a black hole.

Kip Thorne among the 100 most influential people...

....and the one with the best outfit on Time!

Kip Thorne, our bet for the next Nobel Laureate in Physics, more badass than Walter White.

Hey Grandma look: I finally study something that exists!

So much has been already written about today's announcement of the first direct discovery of the gravitational waves from two merging black holes by the advanced LIGO detector. Some examples:

[1], [2], [3] (this beautiful piece by my friend and colleague Emanuele Berti)

In Italian press: [1], [2], [3]

Exactly 100 years ago, Albert Einstein proved that his theory of gravity, General Relativity, predicted the existence of gravitational waves.

This is nothing but one of the most outstanding historic discoveries in science. However, since you can find much deeper posts on this topic, here are some random, not-so-serious, thoughts hastily written down while watching today's live streaming announcement (sorry for typos, this is written on the wings):

1) Look Grandma: I finally study something that exists!
[Last time someone told my grandma that maybe black holes didn't exist after all.. she literally cried!]

2) Wow, my field of research has finally become mainstream! [is this good of bad? Anyway, the largest lecture hall at the Physics Department at Sapienza was full 30 mins before the live streaming..that's pretty uncommon for something related to gravity...]

3) The first direct detection of gravitational waves is really great, but what is emitting these waves is even more interesting: these are two black holes orbiting each other, loosing energy through gravitational-wave emission and finally merging to form a final big black hole. All of this is beautifully predicted by Einstein's theory of General Relativity and required several decades of theoretical, experimental and computational work.

4) Now everyone claims they have predicted that the signal from a binary black hole merger would have been detected. Truth is, just 6 months ago nobody would have bet on this particular source.

5) Related to this, isn't it amazing how physics works? It takes just a single observation to completely change the paradigm that theorists have built over decades. Just 6 months ago very few people would have predict that LIGO -even in the case of a detection- would have been able to test General Relativity or just to make some science or astrophysics out of this discovery. Well, judging from the result of the paper published today (and the companion papers to come) this expectation was completely wrong.

6) Finally, my bets for the Nobel Prize (in random order)

Rainer Weiss, one of the founders of LIGO

Roy Kerr, who discovered the unique solution of General Relativity describing a spinning black hole

Kip Thorne, one of the cofounders of LIGO, and of the fathers of modern General Relativity
(plus one of the creator of the movie Interstellar)

Listening to Gravitational Waves [1]: a very simple analogy!

Universe is a Jungle and Gravitational Waves are sounds of the animals in it

Close your eyes and imagine you are in a helicopter flying over a very big beautiful jungle in the heart of Africa. Open your eyes now! What you see is like the following pictures: a lot of trees and plants which you see them as a big, green picture and call it “jungle”!

What you see from your helicopter above a huge jungle in Africa

The ground is so covered by trees and plants such that there is no way to see the animals who are living in this jungle from your helicopter flying ~500m above the ground. You also can’t hear their voices. Even though you do believe that lions, tigers, elephants, monkeys, and etc are living down there and make some sounds some times naturally, based of what Mr. Einstein have told you. You have never seen these animals before but according to Mr. Einstein, they should sound like followings: tigers [listen], lions [listen], elephants [listen], monkeys [listen].

There are some animals under the jungle trees that there is no way to see them from your helicopter, you just can hear their voices

However you can not see the lions in the jungle because they have hidden by many trees, but you are able to see if some birds are flying around your helicopter or even very far away, but above the trees.

You are able to see if some birds are flying around your helicopter or even far away, but above the jungle trees.

Unwanted sounds from your helicopter: noises!

Unfortunately, you can not hear any animal voice neither birds nor lions. Noise of your helicopter engine [listen] is the only sound that you can hear (however you probably can hear your friend beside you when he is shouting in your ear). With the assumption of a completely silent helicopter, not all the animals have strong voices which can reach you at the helicopter. Even if the animals make strong enough voices, in reality, what you hear is helicopter noise plus an extremely weak voice of an animal. Therefore it would be so difficult to recognize the animal voice in presence of such a high level of noise. In the best circumstances, you will need a super-duper high-tech artificial ears to filter out animal voice from helicopter noise. To do this, indeed, it will be required to know how does the animal voice that you are looking for look like. You can not use voice of monkey and look for voice of lion in the data! Fortunately, by experience, we do know what does the voice of a typical lion look like, however the African lion voice might be a little different. But it doesn’t matter that much, it perfectly works for our purpose.

In fact, voice, or basically sound wave, is not more than some simple mechanical waves in the air. When you speak, your vocal cords shake the air around and make some mechanical waves in air which can be heard by your friend’s ears. Waves, in all forms, need an environment to travel through. If you speak in vacuum [suppose your body doesn’t implode in vacuum!] nobody can hear you. Gravitational waves, in the other hand, are some kind of waves which are produced by moving super-massive sky-objects like black holes, and neutron stars. Instead of air in the case of sound waves, gravitational waves travel trough the fabric of spacetime and affect masses in the field. Gravitational waves are prediction of Einstein’s general theory of relativity. If strong gravitational waves go through your body, you will experience a situation like the following picture. Although, effects are super exaggerated in this picture. 

When strong gravitational waves go through your body. (different polarization)

When you speak your vocal cords shake the air around and make sound waves

Getting back to the analogy of Gravitational Waves

Above described circumstances in jungle is very much similar to what we study in the field of gravitational waves. Universe is the jungle, and Earth is your helicopter in this case. You can look at the sky and are able to see some celestial objects like planets, stars, comets and etc with naked eye or even with modern telescopes. They are the birds above the trees that we can see them but can’t hear them. But this is not everything which exists in the jungle. There are some animals hidden by trees that there is no way to see them even with modern telescopes [Gravitational Wave Sources]. The only way to detect them is listening to their voices [Gravitational Wave Signals]; however they are extremely weak compared to helicopter noise [Detector Noise]. You need to use your super-duper high-tech artificial ears [Gravitational Wave’s Detectors] to get some data, which is basically background noise plus some signal. People use Matched Filtering methods to filter out the signal from data. Post-Newtonian theory, for example, is your knowledge about the voice of a typical lion and if you want to know exactly the model of African lion voice you should use Numerical Relativity.

Universe as a jungle

Celestial objects as jungle animals including birds: stars, supernovas, galaxies, and lions [can’t see but can hear]: black holes and neutron star binaries

We actually use Post-Newtonian formalism to study the sound of a particular source: two angry lions fighting! [compact binary systems, e.g. two extremely massive black holes orbiting each other]. This is the most promising source of gravitational waves that ground current detectors on the earth can detect. One of our motivations to study this system is to find out if Mr. Einstein was right [2]. Gravitational wave astronomy will open a new window to the universe. In addition to the animals that we expect to detect their voices, some strange creatures, might be heard by this new generation of astronomy that we never expect. Somebody may has been “LOST” in spacetime from many years ago. Who knows?

We might hear voice of strange creatures (like super-massive BHs) in the jungle (universe) that we have never been aware of them before Gravitational Waves Astronomy

A good source: two angry lions fighting! [compact binaries]

Imagine in 3014, when kids will be learning Quantum Mechanics at kindergartens, they will learn the sounds of cosmic creatures at elementary schools. Latest modelings show that the voice of some sky animals are expected to be as followings.

Tigers: Equal mass binary gravitational waves: Two black holes, each of 50 solar masses [Listen] 

Lions: Extreme mass ratio binary gravitational waves: Initially circular orbit, into rapidly spinning BHs [Listen] 

Elephants: Extreme mass ratio binary gravitational waves: Initially circular orbit, into slowly spinning BHs [Listen] 

Monkeys: Extreme mass ratio binary : Initially highly eccentric orbit, into rapidly spinning BH [Listen]

Next generation of astronomers will “listen” to the sky instead of looking. Photo: My imaginary son which is an old fashion astronomer at right, and his son which is a modern astronomer at left, looking and listening to the sky from MIR-III space station. ©Photo by: my lovely great-granddaughter, who is not an astronomer but a poet.

References

[1] The title is borrowed from a Bernard Shutz's talk, "Gravitational Waves: Listening to the Music of the Spheres", public talk at Washington University in St. Louis, 2010.
[2] Title of a book by C.M. Will, “Was Einstein Right?”, 2nd Edition, Basic Books, New York, 1993.

LIGO Magazine

Here the annoucement by Gabriela Gonzalez (LSC spokesperson):

The US-based Laser Interferometer Gravitational-Wave Observatory (LIGO) is supported by an international group of more than 800 scientists from about 80 institutions, the LIGO Scientific Collaboration (LSC, http://www.ligo.org).

Following the examples of other large science projects we decided to create the LIGO Magazine, a regular publication to exchange information, news and stories from and with the LSC community. The inaugural issue is now available, with more than 30 pages of entertaining stories and fascinating photos from the installation of the Advanced LIGO detectors.

We believe that the magazine is also a good way to find out more about LIGO and the gravitational wave community, and to satisfy your general interest in science and technology. Therefore the magazine is available as a free download (pdf file) at: http://www.ligo.org/magazine/

We hope you find it inspiring and entertaining. Let us know what you think and do tell your friends and colleagues by forwarding this link to them.