Friday, April 2, 2010

Eternally Collapsing Object

Last week, I wrote about some fascinating applications of micro black holes and in my first sentence, I defined a black hole as an object that is so dense and compact that within a certain distance from it, its gravitational pull becomes so strong that it does not let even light to escape. This boundary is known as the black hole’s event horizon and anything that happens to enter it will never escape. By this definition, a black hole does not have a true physical surface and its event horizon is basically a region of space within which nothing, including light, can escape.

In this article, I investigate a kind of black hole that is very different from what was defined in the above paragraph and I will be using the phase “progenitor star” to define an object that is currently in the process of collapsing under its own immense gravity towards forming a black hole. As the progenitor star collapses, it never reaches a true black hole state, but instead becomes a General Relativistic Radiation Pressure Supported Star (GRRPSS). What a mouthful! Although this scenario is purely theoretical, its exciting properties are surely worth considering.

The name “General Relativistic Radiation Pressure Supported Star” somewhat speaks for itself. So, how does radiation pressure works? Take for example our Sun - a ferocious hot ball of hydrogen and helium with 333 thousand times the mass of the Earth, sitting in the middle of our Solar System. The enormous amount of radiation produced via nuclear fusion within the Sun’s core produces an incredible amount of radiation pressure which tries to blow the Sun apart, while gravity tries to crush the Sun inwards. It is this perfect balance between radiation pressure and gravity which gives our Sun the size that we constantly observe it to be.

As the progenitor star collapses, the gravitational field in the vicinity of the star becomes increasingly stronger as the star becomes ever more dense and compact. As the physical diameter of the progenitor star collapses and approaches the diameter of the event horizon for a black hole of its mass, radiation emitted from the surface of the progenitor star will become increasingly red-shifted. This means that the radiation emitted from the progenitor star’s surface gets stretch into ever longer wavelengths.

When the progenitor star collapses below one and a half times the diameter of the event horizon for a black hole of its mass, light emitted at or near the tangent to the star’s surface will not be able to escape into space and will eventually fall back to the surface. As the progenitor star collapses until its physical size approaches the diameter of the event horizon for a black hole of its mass, only light that is being emitted vertically upwards from the star’s surface will escape into space instead of falling back somewhere else on the star’s surface. Therefore, as the progenitor star collapses, only light that is emitted at an ever decreasing angle from the local vertical will escape into space and the self-gravitational trapping of radiation by the progenitor star becomes more and more effective.

The radiation pressure created from the self-gravitational trapping of radiation by the progenitor star prevents it from collapsing to a true black hole state. Instead, the progenitor star will continue collapsing as an incredibly hot ball of quark gluon plasma which asymptotically tends towards a true black hole state but never reaches it. In fact, such an object will appear totally dark since almost no radiation will be able to escape the super strong gravity of the star and it will appear very much like a true black hole.

It might also be true that as the collapsing General Relativistic Radiation Pressure Supported Star tends to become a true black hole, its lifetime in this phase becomes infinite. Such as object can be called an Eternally Collapsing Object (ECO) and this class of objects represents an alternative to conventional black holes.

1. Abhas Mitra and Norman K. Glendenning (2010), “Likely Formation of General Relativistic Radiation Pressure Supported Stars or Eternally Collapsing Objects”, arXiv:1003.3518v2
2. Abhas Mitra (2006), “Radiation Pressure Supported Stars in Einstein Gravity - Eternally Collapsing Objects”, arXiv:gr-qc/0603055v3
3. Abhas Mitra (2006), “Sources of Stellar Energy, Einstein- Eddington Timescale of Gravitational Contraction and Eternally Collapsing Objects”, arXiv:astro-ph/0608178v3