December 18, 2019. A star of the great Magellanic cloud increases its brightness. It does so in a way that is intense enough not to go unnoticed by scientists analyzing the data from the Víctor M. Blanco telescope at the Inter-American Observatory of Cerro Tololo (Chile), but not so intense that it corresponds to an explosion. Rather, it is a gentle increase in brightness, followed by a symmetrical decrease in brightness. The entire process lasts 1 hour and baffles scientists, who baptize the object causing this phenomenon as Phoebe.
Since then, Phoebe’s origin has been a mystery. Now, the same scientists who made the discovery they have answers that point to what would be one of the oldest objects that have ever been detected.
Phoebe’s origin. There are three hypotheses for Phoebe’s origin. For one thing, it could be a free floating planet in the Milky Way. That is, a planet that was expelled from its solar system and now wanders through our galaxy. It could also be exactly the same, but in the Large Magellanic Cloud instead of the Milky Way.
Finally, it could be a primordial black hole. That is, a very small black hole that, instead of being formed by the collapse of a star, was caused by fluctuations in the density of matter in the cosmos during the first seconds of the Big Bang. The authors of the study that has just been published have calculated the probabilities of each hypothesis and the third one beats the rest by a factor of 100,000.
A gravitational microlens. While Phoebe’s origin has been a mystery all this time, it didn’t take long for scientists to understand the phenomenon that had caused the star’s brightness to fluctuate in 2019. It must have been gravitational microlensing.
This is a phenomenon which is formed when a very massive object is placed between our telescopes and another object. The mass of the central object is so great that its gravity is capable of bending space-time, forming a kind of lens that magnifies the image of what is behind it. On the other hand, if what is behind it is a very distant star, what is magnified is its brightness. That is why this increase in brightness occurred, because Phoebe was passing between the star and the telescopes of the Chilean observatory.
The key is in the duration. Previous studies with gravitational lensing show that the duration of the event can give us an idea of the mass of the body that causes the lens to form. The lighter the object, the faster it moves and the shorter the increase in brightness lasts. In this case, the phenomenon lasted an hour. It may seem like a lot to us, but in cosmic terms it is quite little. In fact, it is just above the detectable limit.
This tells us that the object that caused this increase in brightness must have been very light. According to calculations made by scientists at Swinburne University taking into account fluctuations in brightness, it would have approximately the mass equivalent to three moons.
A winning option. Black holes that form from stars usually have at least the mass of about 5 suns. 3 moons is much less. It is also too small an object to correspond to a planet wandering in the Milky Way or the large Magellanic cloud. This, together with the geometry of the event and the expected spatial distribution, has led the probability calculation to lean so clearly towards the primordial black hole.
Primordial black holes
Big news about something very small. Primordial black holes are theoretical phenomena. It is believed plausible that could have formed in the first seconds of the Big Bang, when fluctuations in the density of matter in the cosmos caused an accumulation of matter dense enough to collapse. Most of them would be very small. They would have most of the characteristics of a black hole, but radically smaller in size. They would form before there were stars or matter as we know it, but they could be related to one of the greatest mysteries of astrophysics: dark matter.
Only 5% of the cosmos is made up of “normal” atoms. The rest is unknown. One part is known as dark matter and another as dark energy. It is not known what they are, but one of the hypotheses about dark matter is that it could be composed in part of primordial black holes. Therefore, if it is shown that Phoebe is really a primordial black hole, we would perhaps be facing one of the first demonstrations of the composition of dark matter.
And now what? Logically, this is just the beginning. We will have to continue looking for more objects like Phoebe to be able to prove that these scientists are right. For this, You have to know well where to point the telescopes. To begin with, not any of them will do. They need to be sensitive enough to detect gentle changes in the brightness of stars. They also need to be able to focus on large fields of vision. And, if possible, focus on places with a large concentration of stars, since it is easier for the gravitational lensing phenomenon to occur there.
It is expected that some observatories, such as the Vera Rubin, will provide interesting data in this regard. Now we will have to analyze them and look for points in common with Phoebe. That December 18, 2019, a pandemic was brewing on Earth, but in space the clue could be jumping that would resolve one of the greatest mysteries in the history of astrophysics.
Image |Martin Bernardi |NASA
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