Scientists speculate that the mysterious “Phoebe” event is not actually a microlensing phenomenon caused by a primary black hole passing between the star and us. It may be due to the variability of the star itself, and they have found evidence to support this.
Primordial black hole
Scientists may have been too quick to announce the discovery of a primordial black hole. At least, that is what a new study by researchers at the University of Warsaw suggests. It was posted on the ArXiv preprint server on June 17.
Primordial black holes are a theoretically possible type of the most mysterious objects in the universe, which could have formed without the collapse of massive stars followed by the merger of their supermassive remnants. Such a scenario limits the lower mass threshold of the object to a few solar masses.
According to some cosmological models, conditions could have arisen in the early Universe under which ultra-dense fluctuations in matter density would have collapsed into black holes before stars and galaxies formed. Such objects are called primordial black holes. Because of this, they can be extremely small. The only problem is that scientists have not yet observed any, and therefore are not sure whether primordial black holes exist at all.
Because if they really exist, that could solve quite a few problems at once. In particular, primordial black holes are one of the candidates for dark matter, and if they exist, there’s no need to look for any more exotic particles. The only problem is finding something at distances measured in light-years that is smaller than Earth and, at the same time, emits no electromagnetic waves and reflects nothing.
Gravitational lens that wasn’t there
One of the keys to finding primordial black holes is their gravity, which deflects starlight. This creates a gravitational lensing effect, which scientists are generally familiar with from the distorted images of distant galaxies. However, a smaller-scale phenomenon—microlensing—is also possible.
Microlensing usually manifests as the flickering of a particular star for no apparent reason. And it was precisely such a case that scientists working with the DECam dark matter camera reported in May. This telescope does not focus intently on any single object, but instead detects the slightest changes as it scans the entire sky.
It observed one of the stars in the Large Magellanic Cloud temporarily dim. The event was named “Phoebe” and announced as the passage of a black hole with a mass roughly equal to that of the Moon, located between us and the star. And since such small objects cannot form through stellar collapse, it is considered a primordial black hole.
And now this sensational claim may be refuted. In a new study, scientists pointed out that, in addition to DECam, the OGLE automated optical sky survey system has been operating for many years. And if such transits of primordial black holes do occur, OGLE should also be detecting them regularly.
Scientists reexamined the DECam data and discovered that this same star had dimmed twice more over the course of that year. Black holes don’t pass by stars that often. In other words, the star itself is variable.
The reasons for the star’s dimming are unknown, but scientists are aware of dozens of different phenomena that could be responsible for it. So, most likely, this time it really wasn’t a primary black hole. However, that doesn’t mean we shouldn’t continue searching for them.
According to phys.org
