Everything about ECO (Eternally Collapsing Object)

Most people might heard about black holes which is a hot topic in modern science. But, there is an another candidate against black hole which is called ECO (Eternally Collapsing Object). This concept was first proposed by an Indian astrophysicist Dr. Abhas Kumar Mitra in 1998. According to him, true black holes do not exist in this real universe. Whether we have heard and saw the picture of the black hole was nothing but an ECO. But till now, the scientific community has not accepted his theory due to some reasons. After a few years later, two scientists Darryl Leiter and Stanley Robertson proposed almost same idea about black holes. But they called it MECO (Magnetospheric Eternally Collapsing Object) as they have their own magnetic field. But what are these ECOs? Let us discuss a little about it.

ECO or MECO:

To understand about ECO, we have to first understand the concept of life cycle of stars. The life cycle of stars has been already explained in the article Life of Stars. Simply, when the amount of helium in a giant star (~100 times larger than our sun) becomes very less, then due to its strong gravity, it overcomes every forces of it and collapses to a certain point resulting a supernova explosion. Actually for giant stars, this supernova explosion is called Hypernova Explosion. After this massive explosion, the mass and density of the core becomes infinite and hence it warps the surrounding space-time fabric to an infinitely small point called Singularity. Thus its gravitational field becomes so strong that even light cannot escape through it. Simply, this is the concept of a black hole.

But, according to Dr.Abhas Mitra, true black hole does not exist. It can be called as Quasi Black Hole, that means, such types of objects are in the path of becoming black holes, but in the real time they have not formed yet. In his research paper Mass of Schwarzschild Black Holes Is Indeed Zero And Black Hole Candidates Are Quasi Black Holes, he mentioned that the process of forming ECO is also almost same as the formation of black holes. But in ECO, unlike black holes there does not form any singularity and event horizon in finite time. 

When supernova explosion occurs at the end of a massive star, due to its strong gravitational force, it starts compressing to a certain point. But, it never turns to singularity. Due to compression, the matter and density of the star becomes denser and hotter and hence it glows even more brightly. When its interior approaches Eddington Limit (Maximum brightness limit upto which a star can achieve due to its total balance between the force of radiation acting outward and the gravitational force acting inward), then the outward radiation becomes sufficient to counter the inward compression of the star. Thus it slows down the compression process and it will take infinite time to create a singularity at the center. That means, an observer will see it as continuously compressing object forever. 

Why Abhas Mitra proposed the idea of ECO:

In newtonian mechanics, time and space is absolute. So, if a star starts to collapse then at a finite time it will create singularity point resulting the formation of black hole. But from relativity point, when a star starts collapsing due to its own gravity, then the space-time fabric will also being warped by the star continuously. Hence the effect of time dilation will be there, that is why it is totally unknown in which time the star will create a singularity. Due to this, some famous physicists like Einstein, Nathan Rosen, Sir Arthur Eddington and Paul Dirac proposed that a star can never form a singularity at any finite point. 

At the beginning of 2014, well known physicist Stephen Hawking also published a research paper by describing the impossibility of forming black hole. He said that, in the real universe black holes does not exist. But Dr. Mitra predicted that 13 years before by introducing ECO. According to all these physicists, a star can only collapse to its Schwarzschild Radius, not beyond it. That is why Paul Dirac said like this,"The mathematicians can go beyond this Schwarzschild radius, and get inside, but I would maintain that this inside region is not physical space, because to send a signal inside and get it out again would take an infinite time, so I feel that the space inside the Schwarzschild radius must belong to a different universe and should not be taken into account in any physical theory. So from the physical point of view, the possibility of having a point singularity in the Einstein field is ruled out. Each particle must have a finite size no smaller than the Schwarzschild radius."

Why does ECO look black:

We all know why does a black hole look black. This is because, its gravity is so strong that even light cannot escape from it. But ECOs are extremely hot and relativistic radiation pressure supported stars which have radius greater than the Schwarzschild Radius. That means, ECOs emit their energy continuously according to the mass energy equivalent condition E=mc². When they emit energy, then they decrease in size and hence the gravitational force becomes stronger. Thus, this process occurs up to infinite time resulting the increase of gravitational force. Thus, the acceleration due to gravity and gravitational red shift at its surface becomes very large (even greater than pulsar or neutron stars) but not infinite. That is why, the light emitted from ECOs surfaces will never reach to any observer and it will look like a black ball.

Differences between ECO and black hole:

Though their formation process is almost same, they have some major differences. Such as-

1. ECOs are quasi black holes whereas black holes form at the end of a giant star by collapsing to a certain point.
2. ECOs are extremely hot and relativistic radiation pressure supported stars. Also, they have physical surfaces wheres, black holes do not have any physical surfaces.
3. ECOs do not have any event horizon whereas, black holes have an invisible boundary called event horizon.
4. Light can escape from ECO whereas, light cannot escape from black hole after passing event horizon.
5. Laws of physics work properly in case of ECOs whereas, laws of physics fail in case of black holes.

Why this concept has not been accepted yet:

In scientific community, there are some scientists which agreed with Dr. Mitra, but till now, it has not been accepted completely. The main reason behind this is that, it is not an alternative theory of black hole. According to the scientific community, the object which Dr. Mitra was talking about is nothing but a black hole. If the object keeps collapsing to an infinite time, that means the collapsing rate is so small that, for an observer the process will be almost stopped. Hence, it does not make any sense. Dr. Mitra's theory only says that, the star will being collapsed to an infinite time and hence does not create singularity in finite time.

Again, let's take a hypothetical situation. If an observer moves to the very near to its surface, then will the observer see the same thing as a distant observer? Answer is no. In that frame of reference, the observer will see that ECO forming a singularity at finite time which is nothing but a black hole. Thus, this theory does not work in this situation. 

But, every theory has some pros and cons. The laws of physics may not work in some particular situations. So, by considering that, it can be hoped that more research will be conducted in this topic in distant future.