WHITE HOLE: HISTORY, THEORY AND HOW IT IS FORMED - PHYSICAL - 2024

The white hole is a singularity of space - time, belonging to the exact solutions of the equations of general relativity. These singularities have what is called an event horizon. This means the presence of a barrier, which in a white hole nothing can penetrate from the outside. Theoretically, a white hole is a singularity that goes to the past.

At the moment no one has been able to observe any. But it is possible that we owe our existence to the most special of all: the Big Bang of 13.8 billion years ago can be considered as an event caused by a supermassive white hole.

A large mass like a planet can warp space-time. Source: Mysid

The theory of general relativity considers that space-time can be deformed by the effect of acceleration or by the presence of massive objects. It is the same theory that predicted the existence of black holes, of which white holes would be the counterpart. Therefore, the existence of these is considered equally possible.

Now, to form the space-time singularity some physical mechanism is required. In the case of black holes, the cause is known to be the gravitational collapse of a supermassive star.

But the physical mechanism that could form a white-hole-like singularity is not yet known. Although candidates have certainly emerged to explain their possible training, as will be seen shortly.

Differences between black holes and white holes

Many of the known black holes are the remnant of a supergiant star that has suffered an internal collapse.

When that happens, the gravitational forces increase to such an extent that nothing that gets close will be able to escape its influence, not even light.

That is why black holes are capable of swallowing everything that falls into them. On the contrary, nothing could enter a white hole, everything would be rejected or repelled from it.

Is the existence of such an object possible? After all, black holes remained for a long time as a mathematical solution to Einstein's field equations, until they were detected thanks to the gravitational and radiation effects they cause in their surroundings, and recently photographed.

In contrast, the white holes are still hidden from cosmologists, if they really exist.

History of its discovery

The theory of the existence of white holes started from the works of Karl Schwarzschild (1873-1916), a German physicist and the first to find an exact solution to Albert Einstein's relativistic field equations.

For this, he developed a model with spherical symmetry whose solutions have singularities, which are precisely black holes and their white counterparts.

Schwarzschild's work was not exactly popular, perhaps having been published during the First World War. It took a few years for two physicists to take it up independently in the 1960s.

In 1965 mathematicians Igor Novikov and Yuval Ne'eman analyzed the Schwarzschild solutions, but using a different coordinate system.

At that time the term white hole had not yet been coined. In fact, they were known as "lagging nuclei", and were considered unstable.

However, being the counterpart of black holes, the researchers tried to find a physical object whose nature was compatible with that predicted for white holes.

Quasars and white holes

The researchers believed they found it in quasars, the brightest objects in the Universe. These emit an intense flux of radiation detectable by radio telescopes, just as a white hole should.

However, the energy of quasars was eventually given a more feasible explanation, related to black holes at the center of galaxies. And so the white holes were again as abstract mathematical entities.

So even though they are known, white holes have received much less attention than black holes. This is due not only to the fact that they are believed to be unstable, which puts their actual existence in doubt, but also because there is no reasonable hypothesis about their possible origin.

In contrast, black holes arise from the gravitational collapse of stars, a physical phenomenon that has been well documented.

Possible finding of a white hole

Researchers are convinced that they have finally detected a white hole in a phenomenon called GRB 060614, which occurred in 2006. This phenomenon has been proposed as the first documented appearance of a white hole.

GRB 060614 was a gamma ray burst detected by Neil Gehrels' Swift Observatory on June 14, 2006, with peculiar properties. It challenged a previously held scientific consensus on the origins of gamma-ray bursts and black holes.

The Big Bang, which some believe was a supermassive white hole, may in turn have been the result of a supermassive black hole, at the heart of an unknown galaxy located in our parent universe.

One of the difficulties in observing a white hole is that all matter is expelled from it in a single pulse. So the white hole lacks the necessary continuity to be observed, while black holes do have enough persistence to be seen.

Theory

Einstein postulates that mass, time, and length are closely dependent on the velocity of the reference frame in which they are being measured.

In addition, time is considered as one more variable, with the same significance as the spatial variables. Thus, space-time is spoken of as an entity in which any event and all events take place.

Matter interacts with the fabric of space-time and modifies it. Einstein describes how this happens with a set of 10 tensor equations, known as the field equations.

Some important concepts in the theory of relativity

Tensors are mathematical entities that allow the temporal variable to be considered at the same level as the spatial variables. The well-known vectors such as force, velocity and acceleration are part of this expanded set of mathematical entities.

The mathematical aspect of Einstein's equations also involves concepts such as the metric, which is the distance in both space and time that separates two infinitesimally close events.

Two points in spacetime are part of a curve called a geodesic. These points are united a space-time distance. Such a representation of space time is observed in the following figure:

The shape of the cone is determined by the speed of light c, which is a constant in all reference frames. All events must take place within the cones. If there are events outside of them, there is no way of knowing, because information should travel faster than light to be perceived.

Einstein's field equations admit a solution with two singularities in an empty region (that is, without mass). One of those singularities is a black hole and the other is a white hole. For both there is an event horizon, which is a spherical boundary of finite radius that surrounds the singularity.

In the case of black holes, nothing, not even light, can get out of this region. And in white holes, the event horizon is a barrier that nothing can penetrate from the outside. The black hole solution in vacuum is in the light cone of the future, while the white hole solution is in the past region of the light cone.

The solutions of Einstein's equations that include a real black hole require the presence of matter, and in this case the solution that contains the white hole disappears. Therefore, it is concluded that as a mathematical solution, in the theory of singular solutions without matter, white holes do exist. But this is not the case when matter is included in Einstein's equations.

How is a white hole formed?

In 2014, theoretical physicist Carlo Rovelli and his team at the University of Aix-Marseille in France proposed that white holes can arise from the death of a black hole.

Already in the 1970s, the leading expert on black holes, Stephen Hawking, calculated that a black hole loses mass through the emission of Hawking radiation.

Calculations by Rovelli and his team indicate that such a radiation-loss contraction from a black hole could, in its final stage, produce a bounce that creates a white hole.

But Rovelli's calculations also indicate that in the case of a black hole with a mass equal to that of the Sun, it would take about a quadrillion times the current age of the Universe to form a white hole.

White holes and dark matter

One second after the Big Bang, fluctuations in density in a rapidly expanding Universe were able to produce primordial black holes (without the need for stellar collapse).

These primordial black holes are many, many smaller than those of stellar origin and can evaporate until they die to give way to a white hole in a time included in the life of the Universe.

Microscopic white holes can be very massive. For example, one the size of a grain of dust may have a greater mass than the Moon.

Rovelli's team even suggests that these microscopic white holes can explain dark matter, another of the most important cosmological mysteries.

Microscopic white holes would not emit radiation; and since they are smaller than one wavelength, they turn out to be invisible. This could be another reason that would explain why they have not yet been detected.

References

1. Battersby, S. 2010. Eternal black holes are the ultimate cosmic safe. Recovered from: newscientist.com.
2. Choi, C. 2018. White Holes may be the secret ingredient in mysterious dark matter. Recovered from: space.com.
3. Fraser, C. 2015. What are white holes ?. Recovered from: phys.org.
4. Masters, Karen. 2015. What is a white hole ?. Recovered from curious.astro.cornell.edu
5. Wikiwand. White hole. Recovered from: wikiwand.com