Tricksters in the Sky.


Following my last post on black holes, I want to write a small post focusing on what other kinds of objects could be in the sky. A more specific question is: What other objects does General Relativity (GR) predict?

Also, physicists have long known that GR as it stands currently, cannot account for all the matter in the universe. In fact, 85% of the universe must be what we call “Dark Matter“. Now, whether Dark Matter could be made up of many possible things. And it could very well be made up of “dark” bosonic particles.

Turns out there are many possible “compact” objects in the sky. Schwarzschild metric is the metric which describes the space-time around a spherical symmetric static object and turns out it can be applicable to different kinds of stars such as normal stars (Luminous stars made up of hydrogen and helium), which could turn into red and white dwarfs, neutron stars and some other hypothetical stars known as Boson Stars. Mind you, here we are assuming that the Standard Model of Particle Physics is the only valid model for these stars.

My personal favourites are called Boson Stars, especially because they can mimic black holes. So what are these stars and how are they different from the usual stars as we know them?

Boson Stars (BS) are stars which are made up of elementary particles called “Bosons”. They are solutions to Einsteins Equations which can be found using numerical GR. One of the simplest kind of Boson Stars would have no self interaction force between the bosons other than being held together by gravity. However, in order to stabilise them, you would probably require some kind of repulsive potential between the bosons in order to stabilise them against gravitational collapse due to gravity pulling everything in.

Such a simple BS, would not have any electromagnetic radiation or light being emitted which would make them very hard to detect. In fact, they can have masses close to masses of black holes and without having any light emission, might not be indistinguishable from them! And this is why they caught my interest. They are like silent dark spies in the night sky, steadily spinning around without any one knowing about their existence. How exciting is that!

In a recent paper, me and my collaborators modelled a very simple Boson Star with a repulsive potential which could be made up of “dark” Bosons. Since these do not have any electromagnetic coupling, when two BS which are in a binary system, rotating around each other, finally collapse, they will radiate energy in the form of gravitational waves, just like black holes. We calculated the amount of gravitational wave radiation would be emitted from mergers of binary Boson star systems in the universe such as this one and we discussed whether detectors such as LISA and EPTA would be able to detect them or not. Turns out in certain ranges LISA might just be lucky!

Here a few favourite things about Bosons Stars that people have said on the internet and some lists it has found a place on:

All I can say is, if you exist, you go girl!! I am rooting for your existence!