black hole merger

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A black hole merger is the final stage of the evolution of a binary system of two compact objects. The loss of orbital energy through gravitational wave radiation leads to the shrinking of the orbit, and in the final fractions of a second the objects collide, forming a single black hole, while up to several percent of their total mass-energy is emitted as gravitational radiation.

History

The existence of black holes was predicted by Einstein long ago, but the first merger was detected only in 2015 by the LIGO detectors — it's like hearing the impact of two giant invisible hammers from a billion light-years away.

How it works

Similar to merging whirlpools in a bathtub: small vortices approach and merge. Only in space these 'whirlpools' of pure gravity move at nearly the speed of light under the influence of Newton's law of gravity, and then Einstein's spacetime distortions.

💡 The first recorded merger, GW150914, converted about three solar masses into gravitational wave energy in a fraction of a second — more than the entire universe emitted in light during that same time.

Black hole merger is a process in which a close pair of stellar-mass (or supermassive) black holes loses energy and angular momentum due to the emission of gravitational waves. The inspiral (slow approach) is described by the post-Newtonian approximation of General Relativity, followed by the dynamical merger with the formation of a common quasi-stationary region with an event horizon, and the final ringdown phase when the perturbed black hole settles to a Kerr state. The observed signal covers all three stages and allows extraction of masses, spins, and distance to the system.

History

The theoretical possibility of mergers had been considered since the 1960s, but only in 2015 did the LIGO collaboration (including Rainer Weiss, Kip Thorne, and others) directly detect gravitational waves from the merger of a pair of black holes with masses ~36 and 29 solar masses. This discovery confirmed the predictions of General Relativity and launched gravitational-wave astronomy.

How it works

The orbital dynamics are governed by the radiation of gravitational waves, which carry away energy and angular momentum. The rate of approach increases as the distance decreases, and the frequency of the emitted waves rises (chirp). At the moment the event horizons touch, the merger occurs, and the resulting black hole emits a characteristic damped signal — quasinormal modes. The entire process is modeled numerically based on Einstein's equations.

\mathcal{M} = \frac{(m_1 m_2)^{3/5}}{(m_1+m_2)^{1/5}}

m1 and m2 are the masses of the spherical black holes in units of solar mass; \(\mathcal{M}\) is the chirp mass, a combination measured from the rate of increase of the signal frequency.

Open problems & fun facts

nature of the final black hole and its quasinormal modes; formation of supermassive black holes through mergers; origin of component spins and their role in the dynamics; agreement of numerical models with analytical formulas for extreme parameters

💡 Before the merger, the orbital speed of black holes can reach 0.6 times the speed of light, and the gravitational wave frequency — hundreds of hertz, becoming audible in the sound range after conversion.

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black hole merger

History

How it works

History

How it works

History

How it works

Open problems & fun facts

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