Gravitational waves and black holeDate: 18 November 2019 Tags: Space
Scientists studying black hole mergers and gravitational waves have discovered possibilities of existence of stars and other celestial bodies that can mimic gravitational waves.
In September 2015, the LIGO detectors made history by directly detecting for the first time the merging of two black holes. As they spiralled in towards each other and merged, the binary black holes let off characteristic gravitational wave signals.
The properties of the merging black holes, namely the masses and spins could be arrived at by looking at the initial part of the signal waveform.
Similarly, by carefully looking at the tail end, also known as the ring down part of the signal, the mass and spin of the final merged state (black hole) can be inferred.
Theoretically there are possibilities such as the so-called gravastars and boson stars which are black hole mimickers. For instance, a gravastar is a strange object that would have a core of exotic matter resembling dark energy with an external shell of normal star-like matter.
The spinning of the compact object has a different effect on it whether it is a black hole or, for instance, a gravastar.
The gravastar is filled with dark energy; it exerts a negative pressure on the outside. So when it spins it behaves differently from normal stars and black holes. When a normal star spins about an axis, it tends to bulge about the equator and get compressed at the poles.
However, for a gravastar this effect is just reversed. It gets compressed near the equator and bulges out at the poles. Thus their shapes change differently when spinning.
Any compact object, in general, can undergo deformations due to its spinning motion and these deformations are expressed in terms of spin-induced multipole moments.
For black holes, due to the existence of event horizon, any property measured from outside will depend on only its mass and spin, unlike other compact objects.
One property that can distinguish between a black hole and exotic object is known as spin-induced quadrupole moment.
This parameter takes the value 1 for a black hole. For other compact objects, the value of this parameter is different from 1 and will vary depending on the internal structure.
A black hole is a region of space time exhibiting gravitational acceleration so strong that no particles or even electromagnetic radiation such as light can escape from it.
In astrophysics, an event horizon is a boundary beyond which events cannot affect an observer on the opposite side of it. An event horizon is most commonly associated with black holes, where gravitational forces are so strong that light cannot escape.