Every person I met and who knows my major says, " Oh.. Ms. Astrophysics, you might know everything about space". I am like yeah..maybe. And my friends are "Why don't you tell us about the most exciting topic for you in space ". I think you have made the biggest mistake by asking this question because that is a never-ending conversation. But 'The most exciting topic according to me is Black Holes'. The basic question I heard from them is, "Isn't it like everything is far in space then how do you know there is a black hole?" and "Most important, doesn't a black hole suck everything"...Uhmm....hold on, that's a lot!. Let me give you a quick drive to Black Hole.

What is black hole?

Blackholes are extremely dense, with such strong gravitational attraction that not even light can escape their grasp. Most famously, black holes were predicted by Einstein's theory of general relativity, which showed that when a massive star dies, it leaves behind a small, dense remnant core. If the core's mass is more than about three times the mass of the Sun, the equations show, that the force of gravity overwhelms all other forces and produces a black hole.

How do we know there is a black hole?

In April 2019 the EHT team unveiled the first picture of the blackhole of the supermassive Black hole M87 in galaxy M87 in Virgo weighting in at more than 6 billion solar masses.
The global-spanning EHT(event horizon telescope) obtained the first picture of black holes by using eight different radio telescopes pointed at the same object simultaneously and combing the information. That's a technique known as very long baseline interferometry. The individual telescope, working collectively attained an image sharpness, or resolution, equivalent to that of a telescope of vastly grater size-roughly the size of the earth itself.

Spiraling Black Holes

This artist's animation shows the merger of two black holes and the gravitational waves that ripple outward during the event. The black holes—which represent those detected by LIGO on Dec. 26, 2015—were 14 and 8 times the mass of the sun, until they merged, forming a single black hole 21 times the sun's mass. One solar mass was converted to gravitational waves. In reality, the area near the black holes would appear highly warped, and the gravitational waves would be difficult to see directly.

LIGO?Gravitational wave?

Gravitational waves are 'ripples' in space-time caused by some of the most violent and energetic processes in the Universe. When two black holes orbit their shared center of mass, they emit gravitational radiation, causing them to spiral even closer together.
In 2015, the facility made the first direct detection of gravitational waves the laser Interferometer Gravitational wave Observatory (LIGO), which consists of two large laboratories in Louisiana and Washington state that work together to make a detection. LIGO is an L-shaped interferometer, Mirrors at the ends of the arms reflect light in order to create an interference pattern called 'fringes' and a device called a photodetector senses the interference pattern, convert the light into an electrical signal which can then be analyzed.

Each L-shaped interferometer spans 4 kilometers in length and uses laser light split into two beams that travel back and forth through each arm, bouncing between precisely configured mirrors. Each beam monitors the distance between these mirrors, which, according to Einstein’s theory, will change infinitesimally when a gravitational wave passes by the instrument.

Description of image:- Simplified diagram of an Advanced LIGO detector (not to scale). A gravitational wave propagating orthogonally to the detector plane and linearly polarized parallel to the 4-km optical cavities will have the effect of lengthening one 4-km arm and shortening the other during one half-cycle of the wave; these length changes are reversed during the other half-cycle. The output photodetector records these differential cavity length variations. While a detector’s directional response is maximal for this case, it is still significant for most other angles of incidence or polarizations

Prof.Holz

“Everything about black holes is absurd. It’s very appealing to say they can’t possibly exist, except that both our theories and our observations show that they must and in fact do exist,”

Observation from LIGO..

Below are the signals recorded by the LIGO Hanford and Livingston detectors. Times are shown relative to September 14, 2015. When LIGO detects gravitational waves, it typically produces a graph or chart representing the strain caused by the passing gravitational wave. The strain is a measure of how much spacetime is stretched and compressed by the wave. In the plotted graph, the detected signals closely align with the anticipated gravitational wave pattern resulting from the merging of black holes. The data not only affirms the presence of this specific wave, validating our gravitational wave model, but it also serves as corroboration for the existence of black holes. Leveraging our knowledge about black holes and the expected signal from a merger, we successfully predicted the waveform, confirming the accuracy of our predictions.

"Uhmmm...Bro.. we understood what a blackhole is and yes we have core evidence of a black hole which is quite convincing. Since we have two different sorts of methods to detect a black hole. The most convincing one that I found was EHT(Event Horizon Telescope) because I am more of an observational person. I believe what I see. .Thank you for listening to my talk about Blackhole and letting me know what evidence convinces you the most.
!!!Proud of myself!!