Gravitational Wave Observations

Black Hole, Resolving the Mistry

Note4Students

From UPSC perspective, the following things are important :

Prelims level : space developments

Mains level : space developments,black hole, merging of Stars, Energy, Gravitation

Black Hole

Context

  • For the very first time, scientists noted that this observation of the Laser Interferometer Gravitational Wave (LIGO) observatories coincided with the measurements made by other telescopes that measured visual and electromagnetic signals.

What is Black hole?

  • A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.
  • Because no light can get out, people can’t see black holes. They are invisible. Space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently than other stars.

Black Hole

What is the background?

  • LIGO Observations: In 2017, astrophysicists observed an unusual feat among the stars. The Laser Interferometer Gravitational Wave(LIGO)observatories recorded a signal which indicated that two massive and dense stellar bodies had merged to form a third body, likely a black hole.
  • Generation gravitational waves: In the process they gave off vibrations that quite literally shook the universe and its very fabric of space time.
  • Neutron stars: Scientists, piecing together evidence from complementary measurements, surmised that the event they had observed was of two neutron stars merging and forming a black hole and, in the process, giving off light.

Black Hole

What are the observations through telescopes?

  • The matter moving faster than light: An unusual jet of matter was observed that gave an illusion of travelling faster than light. These were all exciting phenomena observed for the very first time by telescopes and observatories.
  • Confirmation by Hubble Space Telescope: Now, using data that had been recorded by the Global astro metric Interferometer for Astrophysics (GAIA) spacecraft and Hubble Space Telescope instruments, scientists have confirmed that the above picture is correct. They have made it more precise and descriptive.
  • Seven times the speed of light: In a paper published in Nature, they describe measuring the “apparent speed” of the jet to be about seven times the speed of light.
  • Lorenz factor: They have also measured more accurately a factor called the Lorenz factor which scales with the actual speed of the particles in the jet. Unlike earlier estimates which placed this factor at about 4, the present paper estimates this factor to be over 40. This is because they measure the speed of the relativistic jet to be close to 0.9997c, where “c” is the speed of light.
  • Clarity about the source as neutron star in block hole generation: This resolves the earlier fuzziness about what the source was and puts the source clearly as massive neutron stars merging to give a black hole and throwing off relativistic jets of particles in the process.

Black Hole

Merging of Neutron stars

  • Born out of Supernova explosion: Neutron stars are stellar corpses, left behind after a star has undergone a supernova explosion and reached the end of its lifetime. They are extremely dense, containing more mass than the sun in a sphere that is a few tens of kilometre wide.
  • Produces fast moving material: This has been seen in many active galactic nuclei  galaxy centres that harbour black holes and binary star systems within our galaxy, where one of the stars is a black hole. “Mostly, black holes are responsible for producing such fast moving material

Why present observations about black Hole are significant?

  • Estimating changing position of sky: The present measurements and observations made with GAIA data are extremely challenging. They amount to measuring the position of an object in sky coordinates. These authors measured a change in sky position one millionth the span of the full moon. Normally, if one were making these measurements from earth-based telescopes, it would require data from radio telescopes spaced apart by intercontinental distances.
  • VLBI technique: This technique is called Very Long Baseline Interferometry (VLBI) and was used in the earlier papers. “Here, the authors could beat VLBI in precision because they calibrated Hubble Space Telescope data with GAIA, which is a precision astrometry mission.
  • It’s an estimate not a measurement: the researchers used both their Hubble Space Telescope and GAIA optical position measurement along with the earlier VLBI position measurement to get a better estimate of the speed of the source and angle (viewing angle) with which it is travelling with respect to us on earth. This estimate requires plugging in equations of the special theory of relativity. “So, it is an estimate as opposed to a measurement.
  • Improvement in estimation: we have learnt that neutron star mergers can result in material moving with speeds as high as 0.9997c.Earlier results using Very Long Baseline Interferometry had pegged this value at about 0.938c. with the new results this lower limit has been improved. Even earlier, with VLBI, it was understood that it was a neutron star merger that produced such ultra-relativistic material. Before the VLBI results, there were several models that could replicate the observations.
  • Explanation using ultra realistic material: The observations could be explained both by ultra-relativistic material and non relativistic material, with some differences in assumptions. That study indicated that the observed gamma ray bursts were produced along with the ultra-relativistic material.

Conclusion

  • Current discovery strengthens the hypothesis that such neutron star mergers are responsible for a class of gamma ray bursts. Gamma ray bursts are flashes of extreme gamma ray photons that release a huge amount of energy nearly 1047 They come from different galaxies in the universe and are observed here quite frequently.

Mains Question

Q. What is the neutron star and how the merger of two stars produces a black hole? How LIGO and Hubble space telescope are useful in demystifying the black Hole phenomenon?

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