Dark matter remains one of the main unsolved problems in modern physics. Despite the growing evidence for its existence coming from astronomical observations, all efforts to detect it in a lab on Earth have failed. One possible candidate for dark matter that can not be detected on Earth (and let’s hope it stays like that) are primordial black holes (or PBH). This type of black hole was created during the first moments of the universe and may have survived till today. PBH are invisible (they don’t emit light, or extremely low amounts if they are not very massive) and pretty much interact with the rest of the universe only through gravitational forces. This is basically the same behaviour as dark matter. Most types of PBH have been already ruled out but they can still exist in certain mass ranges (also, high spin PBH may not have been considered in detail in previous studies and may be harder to exclude). One of these possible mass range is about 30 solar masses (think LIGO) and the second one is around the mass of a brown dwarf or a planet. A new type of observation may be able to prove these masses and rule out the possibility that PBH could be a sizeable fraction of the dark matter. This observation relies on caustic crossing events like the Icarus and Iapyx events observed in the galaxy cluster MACS1149. The interpretation of these events is that a very distant and luminous background star (z=1.55) is moving in a region that lies very close to one of the caustics of the cluster (a caustic is a position which results in a large fraction of the light emitted from the star being focused to us at the focal point of the gravitational lens). As it moves, the light of the star gets amplified by the effect known as gravitational lensing. In its path to us, this light passes near stars (microlenses) in the galaxy cluster and the magnifcation changes depending on the distance to the microlenses. Caustics are normally assumed to be smooth curves. In the presence of microlenses, caustics are disrupted like in the figure accompanying this post that shows a caustic being blown up by many PBH, each with 30 times the mas of the sun (without the PBH the caustic would resemble a single straight line instead of the web shown in the figure). We have studied this new type of observations and shown that through continuous monitoring of caustic crossing events it is possible to constrain the fraction of dark matter in the form of microlenses. So far, preliminary results do not favour an scenario where even a modest fraction of the dark matter ( a few percent) can be made of massive PBH (~ 30 solar masses).
You can read the scientific papers in the links below.
Seen stars in motion
A wise man said once that ; “A picture is worth a thousand words“. The wiser man replied, “A movie is worth a thousand pictures“. The movies below show a few examples on how the flux of the background star would change as the star moves across the field of microcaustics in the cases where only stars (and remnants) in the cluster act as microlenses and in the case where 1% of the dark matter is in the form of PBH with 30 solar masses each. For the first four movies the star is made unrealistically large in order to better see the effect (R=70000 solar radii). The magnification does not show large fluctuations as a consequence of this extreme radius.
Video 1) Icarus event with ICL stars
Video 2) Iapyx event with ICL stars
Video 3) Icarus event with ICL stars and 1% dark matter as PBH
Video 4) Iapyx event with ICL stars and 1% dark matter as PBH
An even higher resolution of the effect can be found in the two videos below where the resolution is increased by a factor ~30 and a more realistic star with 1000 Rsun is considered star (this is a typical radius for a giant star) . The first movie considers the more likely scenario where the direction of motion of the star with respect to the cluster caustic is at an angle. The movie considers an angle of 30 degrees but the result would be very similar at any angle larger than few degrees. The second case considers the special case (unlikely) where the motion of the star is aligned almost perfectly with the direction of the cluster caustic. In this case the star approaches the caustics through the cusps of the caustics producing a different pattern in the magnification. The caustic map is shown in the right panel of the movie with the position of the background star shown as a cross. For these movies we only consider microlenses from the intracluster medium (i.e, no PBH) and the central microlens has a mass of one solar mass.
Video 5) Star travelling at an angle with the caustic.
Video 6) Star travelling parallel to the caustic.
Similar movies but with just one microlens can be found in the two links below.In tehse movies, three nearby background stars cross the same caustic from a single microlens having M=1 Msun. The movies show how the same microlens can produce very different magnification patterns depending on the trajectory of the background star.
You can read the scientific papers in the links below.
cosmicspectator 