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Astrophysics: 3 recent discoveries that illuminate our vision of the universe

Gravitational waves: a new era for astronomy

Isabelle Dumé, Science journalist
On November 3rd, 2021 |
4 min reading time
Paul Ramond modifiée
Paul Ramond
Post-doctoral Fellow in Astrophysics at Université Paris Dauphine-PSL
Key takeaways
  • Predicted by Albert Einstein in 1916, gravitational waves can only be produced by accelerating very massive objects (such as black holes) at close to the speed of light.
  • Gravitational waves from a “binary system”, or pair of black holes, were first observed in 2015, thanks to the LIGO observatory in the USA.
  • Since this first observation, about 50 more coalescence events have been detected, advancing many fields of research and the emergence of “gravitational astronomy”.
  • LIGO is a collaborative project involving more than 1,000 researchers and engineers in over 20 countries.
  • The next most promising future project is the European Laser Interferometer Space Antenna (LISA) observatory which should be operational in 2034.

The first ever direct obser­va­tion of grav­i­ta­tion­al waves in 2015 by the LIGO Sci­en­tif­ic Col­lab­o­ra­tion in the US is undoubt­ed­ly one of the biggest sci­en­tif­ic dis­cov­er­ies of the last decade, or even this cen­tu­ry. Six years lat­er, what can we say about these waves, and why is it so impor­tant to study them?

Grav­i­ta­tion­al waves (GWs), first pre­dict­ed to exist by Albert Ein­stein in 1916, allow for a brand-new way of look­ing at the uni­verse. Before their detec­tion, astronomers could only observe the sky using vis­i­ble light, and oth­er types of elec­tro­mag­net­ic radi­a­tion (includ­ing infrared, ultra­vi­o­let and gam­ma rays). 

While light is the prop­a­ga­tion of elec­tro­mag­net­ic fields vibrat­ing in space and time, GWs are com­plete­ly dif­fer­ent: they are rip­ples in the very fab­ric of space-time itself. They can thus be emit­ted by non-lumi­nous objects.

Studying black hole mergers

Cre­at­ing such rip­ples in the (rather rigid) fab­ric of space-time is not easy though. Indeed, GWs can only be pro­duced by accel­er­at­ing very small and extreme­ly mas­sive objects close to the speed of light. The best can­di­dates are there­fore black holes (which are the most com­pact objects in the uni­verse), and cer­tain very dense stars known as neu­tron stars (which are between 1.4 and 2.4 solar mass­es with a diam­e­ter of less than 20 km). To com­pare, the Sun has a diam­e­ter of 1.39 mil­lion kilometres.

In gen­er­al, an iso­lat­ed black hole does not pro­duce GWs. It needs a com­pan­ion to which it remains bound for a long time (much like Earth is bound to the Moon) to form what is called a bina­ry sys­tem. As they are extreme­ly dense, the black holes deform space-time in their vicin­i­ty as they orbit each oth­er, gen­er­at­ing GW rip­ples that prop­a­gate across the uni­verse at the speed of light.

As it emits these GWs, the bina­ry los­es some of the ener­gy that binds the black holes, which end up spi­ralling ever clos­er to each oth­er. This infer­nal waltz pro­duces more and more intense GWs (that can trav­el bil­lions of light-years across the uni­verse) until the black holes even­tu­al­ly merge. From time to time, one of these bina­ries pro­duces GWs with an ampli­tude that is just large enough to be detect­ed when it reach­es Earth, even though the sig­nal is extreme­ly weak.

The LIGO detection

The first sig­nal from such a bina­ry black-hole coa­les­cence, which occurred approx­i­mate­ly 1.3 bil­lion light-years from Earth, was detect­ed in Sep­tem­ber 2015 by an instru­ment called LIGO (for Laser Inter­fer­om­e­ter Grav­i­ta­tion­al-Wave Obser­va­to­ry)12. The coa­les­cence includes the “inspi­ral” (when the black holes become clos­er), the “merg­er” (when they touch) and the “ring­down” (when the new­ly formed, big­ger black hole relax­es into a steady state). 

a sin­gle black hole of 62 solar mass­es. The 3 solar mass dif­fer­ence was entire­ly con­vert­ed into grav­i­ta­tion­al ener­gy car­ried by the GWs.

LIGO is a col­lab­o­ra­tive project with over 1000 sci­en­tists and engi­neers from more than 20 coun­tries, and three of its mem­bers were award­ed the 2017 Nobel Prize in Physics3. It took near­ly 50 years of intense research to build the GW detec­tors, and in June 2016 the researchers announced that they had observed a sec­ondary bina­ry black hole coa­les­cence4. The obser­va­tion was made on 26 Decem­ber 2015, and this time, the black holes were about 1.4 bil­lion light-years away. Rough­ly 50 such merg­er events have been detect­ed since this time. All these dis­cov­er­ies great­ly advanced many research fields and kicked off the era of grav­i­ta­tion­al-wave astronomy.

Tiny length changes

Instru­ments like LIGO and oth­er ground-based GW detec­tors, such as Vir­go in Italy and Kagra in Japan, rely on an advanced sens­ing method called laser inter­fer­om­e­try. This tech­nique has long been used to detect dif­fer­ent sorts of sig­nals, but it had nev­er been pushed to the lim­it need­ed to detect the very weak sig­nals that GWs produce. 

The LIGO facil­i­ty basi­cal­ly works by send­ing twin laser beams down two 4 km-long “arms” arranged in an L‑shape and kept under a near-per­fect vac­u­um. The beams are reflect­ed by mir­rors pre­cise­ly posi­tioned at the ends of each arm.  As a GW pass­es through the obser­va­to­ry, it caus­es extreme­ly tiny dis­tor­tions in the dis­tance trav­elled by each laser beam. The instru­ment is thus able to mea­sure the local con­trac­tion and expan­sion of space-time caused by the GW.

The extreme sen­si­tiv­i­ty of the instru­ment means that it is prey to all sorts of exter­nal vibra­tions (such as those from planes fly­ing by and waves on a dis­tant shore). LIGO engi­neers there­fore had to design sev­er­al inge­nious noise-reduc­tion sys­tems that not only great­ly enhance the pre­ci­sion of the detec­tors, but also allow them to dif­fer­en­ti­ate between ter­res­tri­al arte­facts and the pre­cious GW signals.

By mea­sur­ing how long it takes for the laser beams to trav­el along an arm, researchers can extract infor­ma­tion such as the fre­quen­cy and ampli­tude of the GW from the sig­nal. These quan­ti­ties are of major impor­tance since they con­tain key phys­i­cal infor­ma­tion about the source of the wave, such as its dis­tance from Earth and its posi­tion in the sky, as well its mass and whether it is a black hole or a neu­tron star.

Future detectors

The char­ac­ter­is­tics of inter­fer­om­e­ters like LIGO makes them sen­si­tive only to grav­i­ta­tion­al waves with­in a cer­tain fre­quen­cy band, from about 10 Hz to 10 kHz, which cor­re­sponds to black holes of about 10 to 100 solar masses. 

To extend this fre­quen­cy range, the most promis­ing future project is the Laser Inter­fer­om­e­ter Space Anten­na (LISA)5. This Euro­pean space-based obser­va­to­ry, due to come online in 2034, will tar­get fre­quen­cies in the low­er mil­li­hertz range to detect waves from the merg­er of much big­ger black holes. These “super­mas­sive” objects are found at the cen­tres of most galax­ies – includ­ing our Milky Way – and have mass­es that are mil­lions or even bil­lions of times that of the Sun.

LISA should also be able to observe “asym­met­ric” pairs, such as a neu­tron star orbit­ing a super­mas­sive black hole, and even the so-called “cos­mic grav­i­ta­tion­al-wave back­ground”, which is very impor­tant for cos­mol­o­gy since it con­tains infor­ma­tion about the pri­mor­dial GW cre­at­ed right after the Big-Bang6. Far more pre­cise than its ter­res­tri­al cousins, LISA will be a mil­lions-of-kilo­me­tre-long instru­ment con­sist­ing of three tiny robots posi­tioned in an equi­lat­er­al tri­an­gle pat­tern in solar orbit just behind Earth.


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