Much of the universe behaves as if there is more matter there than we can see. Dark matter explains this by claiming that matter exists that we can not see, and there is evidence of this idea. In contrast, evidence for recognizing dark matter has behaved differently: everything we have done to seek it has gone empty-handed.
The last piece of Void was released this week, and it seems like this is a problem. Stop one of the possible remaining explanations for dark matter: black holes that formed shortly after the Big Bang and have since structured the universe. While previous studies seemingly excluded larger versions of these original black holes, the new study closes the window on something more massive than a large asteroid. And this was achieved with just a single night telescope time.
From the Beginning
Black holes would seem to be fantastic candidates for dark matter because they are black and therefore hard to spot. But there are a number of reasons why they do not work very well. While light can not escape from a black hole, it is often produced in amazing amounts by the material outside of a black hole. So it's not clear if a large number of black holes could somehow go undetected.
Then there is the timing. The only black holes that we know exist are formed by the death of stars. Yes, the cosmic microwave background and the structure of the universe itself indicate that dark matter was there from the beginning.
The latter question was dealt with by so-called "black holes". It was thought that these formed early in the history of the universe, when the density of matter was much higher and continues to this day. Modeling their formation indicated that under the right conditions, enough could form to account for dark matter.
The evidence, however, was not good for the idea. Some of this evidence comes from LIGO's detection of gravitational waves produced by collisions with black holes. If the original black holes are heavy enough, the rate of defections would be much higher than they are. Other limits have been set by studies of so-called "microlenses". We have identified a number of cases in which massive objects twist the tissue of the space to form a lens and magnify galaxies in the distance behind the lens-forming object. Smaller objects can do this on a smaller scale, temporarily lightening individual stars.
If common black holes were common, they would create many of these smaller microlens events and we should be able to recognize them. However, there were very few events in the search for them, as well as the Kepler Planet Hunting Telescope, which stared at a large field of stars for several years. Taken together, these indicated that all the original black holes must be extremely weird and weigh less than our sun.
But these small, original black holes could not be ruled out for theoretical reasons. As a result, a group of Japanese researchers attempted to exclude them for observation.
What & # 39; s Suprime?
Finding microlens events is relatively easy: just stare at a row of stars and wait for one of them to fade when a dense object moves across your line of sight. The challenge is to make sure you're looking at enough stars to make recognition likely. Previous polls used one of the dwarf galaxies that orbit the Milky Way to deliver the stars. The duration of the microlens event then depends on the mass of the object that processes the lens (smaller means a shorter event), and the speed with which it crosses our line of sight, which in turn is related to the distance between us and the object ,
The Japanese researchers decided to get bigger with some new telescope hardware. The hardware is a so-called Hyper Suprime Cam, an 870 megapixel monster attached to an 8-meter telescope. When properly configured, it can capture the entire Andromeda galaxy in a single frame, approximately every 90 seconds. This is so fast that even a black hole can be detected several times during the microlens. To ensure that they could capture as many events as possible, the researchers were given a full night with the telescope and an observation time of seven hours.
If they count, they would cover much smaller masses, we've already looked at each other. At the bottom, this would result in black holes of about 1
So many pixels
The Hyper Suprime cam has many pixels Andromeda has even more stars. To detect individual lens events, the team used software that compared consecutive images and highlighted pixels with highlighted changes. After an observation night, more than 15,500 events had to be sorted. These included things like variable stars, star lights and solar eclipses in binary systems. By focusing on short-term changes, the number could be reduced to around 12,000 while looking for symmetrical "bumps" where the light intensity increases and decreases, giving the researchers a total of 66 possible microlens events.
You simply manually checked each event. Most are just image artifacts caused by bright stars moving near pixel boundaries. At least one was an asteroid. By the time all of them were examined, there was only one candidate that could possibly be a microlens event. It was not a great game, but there were no other obvious explanations.
These observations could have caused microlenses through ancient black holes in the Milky Way or in Andromeda. So if there were a significant population of these black holes, the telescope would have had to absorb more. The conclusion, then, is that if original 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 original black holes – the authors suggest that they can tighten the probability even further with another 10 days observation time. However, the study reduces the already low chances that they are the cause of the effects of dark matter.
Nature Astronomy 2019. DOI: 10.1038 / s41550-019-0723-1 (About DOIs).