Gravitational Wave Observations

Black Holes Merger

Note4Students

From UPSC perspective, the following things are important :

Prelims level : Black Holes

Mains level : Black holes merger

Billions of years ago, a collision between two black holes sent gravitational waves rippling through the universe. In 2019, signals from these waves were detected at the gravitational wave observatory LIGO (United States) and the detector Virgo (Italy).

Try this PYQ:

Q.Recently, scientists observed the merger of giant ‘blackholes’ billions of light-years away from the Earth. What is the significance of this observation?

(a) ‘Higgs boson particles’ were detected.

(b) ‘Gravitational waves’ were detected.

(c) Possibility of inter-galactic space travel through ‘wormhole’ was confirmed.

(d) It enabled the scientists to understand ‘singularity’.

Why in news?

  • The cause of curiosity is the mass of one of the parent black holes, which defies traditional knowledge of how black holes are formed.

What exactly was detected?

  • It was a signal from a gravitational wave, a relatively new field of discovery.
  • Gravitational waves are invisible ripples that form when a star explodes in a supernova; when two big stars orbit each other; and when two black holes merge.
  • Travelling at the speed of light, gravitational waves squeeze and stretch anything in their path.

Detecting gravitational waves

  • Gravitational waves were proposed by Albert Einstein in his General Theory of Relativity over a century ago.
  • It was only in 2015, however, that the first gravitational wave was actually detected — by LIGO. Since then, there have been a number of subsequent detections of gravitational waves.
  • The signal detected at LIGO and Virgo, as described by the LIGO Collaboration, resembled “about four short wiggles” and lasted less than one-tenth of a second.

Where did it come from?

  • Subsequent analysis suggested that GW190521 had most likely been generated by a merger of two black holes. The signal likely represented the instance that the two merged.
  • It was calculated to have come from roughly 17 billion light-years away, and from a time when the universe was about half its age.

Some questions to verify

  • The findings led to further questions.
  • One of the two merging black holes falls in an “intermediate-mass” range — a misfit that cannot be explained by traditional knowledge of how black holes form.

Why is it unusual?

  • All the black holes observed so far belong to either of two categories.
  • One category ranges between a few solar masses (one solar mass is the mass of our Sun) and tens of solar masses. These are thought to form when massive stars die.
  • The other category is of supermassive black holes. This range from hundreds of thousands, to billions of times that of our sun.
  • According to traditional knowledge, stars that could give birth to black holes between 65 and 120 solar masses do not do so — stars in this range blow themselves apart when they die, without collapsing into a black hole.

Observing for the first time

  • In the merger leading to the GW190521 signal, the larger black hole was of 85 solar masses —well within this unexpected range, known as the pair-instability mass gap.
  • It is the first “intermediate-mass” black hole ever observed. (In fact, the smaller black hole to is borderline, at 66 solar masses.)
  • The two merged to create a new black hole of about 142 solar masses. Energy equivalent to eight solar masses was released in the form of gravitational waves, leading to the strongest ever wave detected by scientists so far.

Possible reasons for its formation

  • The researchers suggest that the 85-solar-mass black hole was not the product of a collapsing star, but was itself the result of a previous merger.
  • Formed by a collision between two black holes, it is likely that the new black hole then merged with the 66-solar-mass black hole — leading to gravitational waves and the signal received by LIGO and Virgo.
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