From UPSC perspective, the following things are important :
Prelims level : Gravastars
Mains level : Gravastars, Black Holes
- According to some scientists, the universe contains not just black holes but many exotic objects, such as gravastars and boson stars which are black hole mimickers.
Why study Gravastars?
- In September 2015, the LIGO detectors in the US made history by directly detecting for the first time the merging of two black holes.
- Since then, LIGO, joined by other detectors around the world, has gone on to detect eleven events of which one is the merger of two neutron stars and the remaining ten, of pairs of black holes (binary 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.
- The question emerges – whether other exotic objects exist that may act as black hole mimickers and give off similar signals.
What is Gravastar?
- 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.
- There are no observational evidences for their existence till date, but then, there were not too many ways in which one could look for them.
How it is distinguished from black hole?
- The spinning of the compact object has a different effect on it whether it is a black hole or, for instance, a gravastar.
- Since 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.