A night at the telescope excludes the idea of ​​black hole / dark matter

Much of the Universe behaves as if there was more material than we can see. Dark matter explains this by stating that there is a present matter that we can not see, and evidence has accumulated in favor of this idea. On the other hand, the proof of the identity of the dark matter behaved in the opposite way: all that we did to look for it appeared empty.

The last nothingness of emptiness was published this week and apparently puts an end to one of the possible explanations for dark matter: the black holes that formed shortly after the Big Bang and that structure since the universe. While previous studies seemed to exclude larger versions of these primordial black holes, the new study closes the window on anything more massive than a large asteroid. And all this was accomplished in one night only with the time spent at the telescope.

At the dawn of time

Black holes would seem to be fantastic candidates for dark matter, since they are black and therefore difficult to detect. But there are a number of reasons why they do not work particularly well. On the one hand, if the light does not escape a black hole, it is often produced in prodigious quantities by the material located just outside of a black hole. It is therefore not clear if a large number of black holes could remain undetectable.

Then there is the timing. The only black holes of which we know the existence were formed by the death of the stars. Yes, the cosmic microwave background and the very structure of the universe both indicate that there was dark matter from the beginning.

This last problem has been dealt with by what are called "primordial black holes". It was assumed that these formed early in the history of the Universe, when the density of matter was much higher, and persisted until today. Modeling their training suggested that, under the right conditions, enough could be formed to account for dark matter.

The evidence, however, did not support this idea. Part of this evidence comes from LIGO's detection of gravitational waves produced by black hole collisions. If primordial black holes are heavy enough, the rate of defections would be much higher than it is. Studies of what is called "microlens" have also set other limits. We have identified a number of cases where large objects twist the structure of space so as to form a lens, thereby magnifying the galaxies beyond the object that forms it. Smaller objects can do this on a smaller scale, causing temporary lightening of individual stars.

If primordial black holes were common, they would create a multitude of these smaller microlens events, and we should be able to spot them. However, they were very little sought after, like the Kepler planet hunting telescope, which for several years has observed a vast field of stars. Combined, they suggested that any primordial black hole must be extremely strange and weigh less than our Sun.

But these primordial black holes could not be excluded for theoretical reasons. This led a group of Japanese researchers to try to exclude them for reasons of observation.

What is it supreme?

The search for microlens events is relatively simple: just staring at a group of stars and waiting for one of them to clear and go dark when a dense object moves on your field of vision. The challenge is to make sure you look at enough stars to make detection likely. A number of previous studies have used one of the dwarf galaxies that gravitate around the Milky Way to provide the stars. The duration of the microlensing event will then depend on the mass of the object effecting the lens (smaller means a shorter event), as well as the speed at which it crosses our line of sight, which is itself bound at the distance that separates us. and the object.

Activated by new telescope equipment, Japanese researchers have decided to scale up. The hardware is called Hyper Hyper-Cime, an 870-megapixel monster connected to an 8-meter telescope. Configured correctly, it could capture the entire Andromeda galaxy in a single image every 90 seconds or so. It's fast enough that even a small black hole can be captured multiple times during the microlens. To make sure they can capture as many events as possible, the researchers had a full night with the telescope for themselves, with seven hours of total observation time.

According to them, this would allow them to cover masses much smaller than those we examined before. At the bottom of the list, this would give black holes of about 10^-14 solar masses, roughly in the vicinity of the mass of the asteroid Eros. Although black holes in this mass range can not be formed during star death, it is possible that they formed early in the Universe.

So many pixels

Although the Hyper Suprime-Cam has a lot of pixels, Andromeda still has more stars. To detect individual lens events, the team used software that compared consecutive images and highlighted pixels that had changes between them. After a night of sightings, more than 15,500 events had to be sorted. But that included things like variable stars, stellar flares, and eclipses in binary star systems. Focusing on short-term changes has reduced the number to around 12,000, while the search for symmetrical "bumps" in which the intensity of light increases and decreases has left researchers with 66 possible micro-lens events.

At this point, they simply examined each event manually. Most of them were simply images of artifacts caused by bright stars moving near the pixel boundaries; at least one was an asteroid. By the time all had been reviewed, there was only one candidate that could possibly be a microlens event. It was not a good match, but there was no other obvious explanation.

These observations should have captured the microlens caused by primordial black holes in the Milky Way or Andromeda. The telescope should have taken a lot more events. The conclusion is, therefore, that if primordial black holes formed early in the history of the Universe, they were not formed in sufficient numbers to account for dark matter. This may not be the end of the search for primordial black holes – the authors suggest that they may impose even stricter limits on probability with another 10 days of observation. But the study reduces the already slim chances that they are at the source of the effects of dark matter.

Nature Astronomy, 2019. DOI: 10.1038 / s41550-019-0723-1 (About DOIs).