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!

Black Holes!


As a kid, I used to look up to the sky and try to count the number of stars. But I wasn’t as intrigued by what happened inside of them till I heard the story of black holes. For the first time, I was super intrigued by these seemingly super mysterious points in our universe. What are they? How are they made? What happens inside of them?

While the question of what happens inside of them might be the subject of another upcoming post, if scientists manage to crack that code, I did manage to learn about how they are theoretically predicted as well as experimentally formed. Lo and behold, stars had a lot to do with it!

In Einstein’s General Relativity, we can write down the Einstein’s equations and calculate the solutions to these equations. A particular class of solutions which are called as Vacuum Solutions, exist, among which the “Schwarzschild Solution” is a special one. This is a metric which describes the space-time around a spherically symmetrical mass in the universe and it looks like this:

If we calculate the poles of this equation, we find that the equation predicts an event horizon as well as a singularity! If a star many times the mass of the sun goes through the whole chain of nuclear fusion events and ends up quite heavy, it would at some point collapse under its own weight and would form a blackhole if it is trapped inside it’s own event horizon! At this point, the mass and gravitational force of this star becomes so high that not even light can escape the confines of the event horizon! And thus, we do not have a direct probe for the inside of this hole due to which it looks black to us!

And thus, it is a black hole!! And to top this post off, here is a picture of a blackhole swallowing a star!

Taken from here.