Merger of two černé díry has three stages: inspiral, merger and ringdown phases. Characteristic gravitační vlny are emitted in each phase. The last ringdown phase is very brief and encode information about properties of final černá díra. Reanalysis of data from binary černá díra merger event GW190521 has provided, for the first time, evidence of signature aftershocks of the merger in the form of two separate faint ringdown frequencies produced by the resulting single černá díra as it settled down to a stable symmetrical form. This is the first detection of multiple gravitational-wave frequencies in the ringdown stage. Just like a bell ‘rings’ for some time after being stuck, the resulting single distorted černá díra formed after the merger ‘rings’ for some time emitting faint gravitační vlny before achieving symmetrical stable form. And, just the way shape of the bell determines the specific frequencies with which the bell rings, similarly, as per no-hair theorem, mass and spin of the černá díra determine ringdown frequencies. Hence, this development paves the way for use of ringdown frequencies to study properties of final černá díra.
Černé díry are massive objects with extremely strong gravitational fields. When two obíhající černé díry spiral around each other and eventually coalesce, the fabric of prostor-times around them are disturbed that creates ripples of gravitační vlny radiating outwards. Since September 2015 when gravitational-wave astronomy began with LIGO’s first detection of gravitační vlny generated by merger of two černé díry 1.3 billion light years away, merging černé díry are now routinely detected almost once every week.
Sloučení společnosti černé díry has three phases. When the two černé díry are widely separated, they slowly oběžná dráha each other emitting weak gravitační vlny. The binary gradually moves to smaller and smaller oběžné dráze as the energy of the system is lost in the form of gravitační vlny. To je inspirační fáze koalescence. Další je fáze fúze když ti dva černé díry get close enough to coalesce to form a single černá díra with distorted shape. Strongest gravitational waves (GWs) are emitted at this stage which are now routinely detected and recorded by gravitational-wave observatories.
Po fázi fúze následuje velmi krátká etapa tzv fáze vyzvánění wherein the resulting single distorted černá díra quickly achieves more stable spherical or spheroidal form. Gravitační vlny emitted in the ringdown phase are damped and much fainter than the GWs released in the merger phase. Just as a bell ‘rings’ for some time after being stuck, the resulting single černá díra ‘rings’ for some time emitting much fainter gravitační vlny before achieving symmetrical stable form.
The faint multiple ringdown frequencies of gravitační vlny released during the ringdown phase of merger of two černé díry were undetected so far.
A research team has recently been successful in detecting multiple gravitational-wave frequencies in the ringdown stage of binary černá díra merger event GW190521. They searched for individual fading tones in the ringdown frequencies without considering any relation with frequencies and damping times and were successful in identifying two modes implying the resulting deformed černá díra emitted at least two frequencies after the merger. This was predicted by Einstein’s general relativity hence the result confirms the theory. Further, researchers compared the frequencies and damping times of the two ringdown modes found in the merger event to test the “no-hair theorem” (that černé díry are completely characterised by mass and spin and no other “hair” is needed to describe its features) and found nothing beyond general relativity.
Toto je milník, protože se všeobecně mělo za to, že pozorování četných vyzváněcích frekvencí nebude možné, dokud nebudou v budoucnu dostupné detektory gravitačních vln nové generace.
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Zdroje:
- Capano, CD et al. 2023. Multimódové kvazinonormální spektrum z narušené černé díry. Fyzické kontrolní dopisy. sv. 131, Vydání 22. 1. prosince 2023. DOI: https://doi.org/10.1103/PhysRevLett.131.221402
- Max-Planck-Institut fürGravitationsphysik (Albert-Einstein-Institut), 2023. Novinky – Pro koho zvoní černá díra. Dostupné v https://www.aei.mpg.de/749477/for-whom-the-black-hole-rings?c=26160
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