מיזוג חורים שחורים: הזיהוי הראשון של תדרי צלצול מרובים   

Merger of two חורים שחורים has three stages: inspiral, merger and ringdown phases. Characteristic גלי כבידה are emitted in each phase. The last ringdown phase is very brief and encode information about properties of final חור שחור. Reanalysis of data from binary חור שחור 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 חור שחור 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 חור שחור formed after the merger ‘rings’ for some time emitting faint גלי כבידה 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 חור שחור determine ringdown frequencies. Hence, this development paves the way for use of ringdown frequencies to study properties of final חור שחור.  

חורים שחורים are massive objects with extremely strong gravitational fields. When two מסלול חורים שחורים spiral around each other and eventually coalesce, the fabric of שטח-times around them are disturbed that creates ripples of גלי כבידה radiating outwards. Since September 2015 when gravitational-wave astronomy began with LIGO’s first detection of גלי כבידה generated by merger of two חורים שחורים 1.3 billion light years away, merging חורים שחורים are now routinely detected almost once every week.   

מיזוג של חורים שחורים has three phases. When the two חורים שחורים are widely separated, they slowly מסלול each other emitting weak gravitational waves. The binary gradually moves to smaller and smaller מסלולים as the energy of the system is lost in the form of גלי כבידה. זה שלב ההשראה של התלכדות. הבא הוא שלב המיזוג כאשר השניים חורים שחורים get close enough to coalesce to form a single חור שחור with distorted shape. Strongest gravitational waves (GWs) are emitted at this stage which are now routinely detected and recorded by gravitational-wave observatories.  

שלב המיזוג מלווה שלב קצר מאוד שנקרא שלב ה-ringdown wherein the resulting single distorted חור שחור quickly achieves more stable spherical or spheroidal form. גלי כבידה 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 חור שחור ‘rings’ for some time emitting much fainter גלי כבידה before achieving symmetrical stable form.  

The faint multiple ringdown frequencies of גלי כבידה released during the ringdown phase of merger of two חורים שחורים were undetected so far.  

A research team has recently been successful in detecting multiple gravitational-wave frequencies in the ringdown stage of binary חור שחור 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 חור שחור 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 חורים שחורים are completely characterised by mass and spin and no other “hair” is needed to describe its features) and found nothing beyond general relativity.  

זוהי אבן דרך מכיוון שהייתה רווחה הדעה שתצפית בתדרי צלצול מרובים לא תתאפשר לפני שגלאי גלי כבידה מהדור הבא יהיו זמינים בעתיד.  

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מקורות:   

1. קפאנו, תקליטור ואח'. 2023. ספקטרום Quasinormal Multimode מחור שחור מופרע. מכתבי סקירה פיזית. כרך יד. 131, גיליון 22. 1 בדצמבר 2023. DOI: https://doi.org/10.1103/PhysRevLett.131.221402  
2. Max-Planck-Institut fürGravitationsphysik(Albert-Einstein-Institut), 2023. חדשות - למי מצלצל החור השחור. זמין ב https://www.aei.mpg.de/749477/for-whom-the-black-hole-rings?c=26160 

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