Primordial Black Holes Probably Don’t Pack a Dark Matter Punch

Waiting for the Andromeda galaxy’s stars to twinkle may have extinguished hope for tiny black holes being a significant dark matter candidate

Should a black hole drift in front of a star, it could trigger a microlensing event, so astronomers set out to estimate the number of primordial black holes in Andromeda [Kavli IPMU]

Using the Andromeda galaxy as a huge detector, astronomers have taken a stab at seeing the unseeable — possibly disproving a hypothesis first put forward by the late Stephen Hawking 45 years ago.

According to Hawking’s work, the universe should be filled with black holes that were formed at the beginning of time, when the universe was a chaotic soup of energy just after the Big Bang. Known as “primordial” black holes, these ancient objects are hypothesized to invisibly occupy modern galaxies, including our own, boosting their dark matter mass.

These black holes aren’t the supermassive monsters that lurk in the centers of most galaxies; they’re not even stellar-mass black holes, formed after massive stars go supernova. Primordial black holes are much smaller than that, having leaked most of their mass via Hawking radiation since their formation 13.8 billion years ago. They should, however, still have powerful gravitational effects on the space surrounding them and, in new research published last week in the journal Nature Astronomy, an international team of researchers have leveraged these hypothetical black holes’ space-time-warping powers to reveal their presence.

Or not, as it turns out.

Central to this study is the effect of microlensing. This astronomical method relies on an object passing between us and a distant star. It has been used to great effect when detecting distant exoplanets, or rogue brown dwarfs wandering through interstellar space. Should one of these objects drift directly in front of a star, its gravitational field can create a magnification effect that briefly brightens the star’s light. The gravitational field creates a natural “lens” out of space-time itself, a prediction that arises from Einstein’s general relativity.

The effect of gravitational microlensing on a star in the Andromeda galaxy should a primordial black hole drift in front [Kavli IPMU]

It stands to reason that even though primordial black holes don’t generate any light themselves, if you stare at at entire galaxy for long enough, you should see a lot of twinkling stars, or microlensing events caused by the hypothetical swarm of primordial black holes the galaxy should contain. Count the number of events, and you can take a statistical stab the total number of primordial black holes in a galaxy like Andromeda, thereby providing an estimate as to how much of the universe’s missing dark matter mass is made up from these objects.

Using the power of the Subaru telescope in Hawaii, the researchers put this to the test, capturing 190 consecutive images of Andromeda over seven hours during one night with the observatory’s Hyper Suprime-Cam digital camera. If Hawking’s theory held, the telescope should have recorded approximately 1,000 microlensing events caused by primordial black holes with a mass of less than our moon drifting in front of Andromeda’s stars. Alas, only one microlensing event was detected that night. From this observation campaign alone, the researchers estimate that primordial black holes make up no more than 0.1 percent of the total dark matter mass in our universe.

Although this elegant study doesn’t necessarily disprove the existence of primordial black holes — one single event is interesting, but not compelling — it does put a wrench in the idea that they dominate the mass holed up in dark matter. So, the quest to understand the nature of dark matter grinds on and, with the help of this study, astronomers have now narrowed down the search by removing primordial black holes from the dark matter equation.

One thought on “Primordial Black Holes Probably Don’t Pack a Dark Matter Punch”

  1. Very interesting write-up. I had never heard of primordial black holes and it’s truly amazing that we were able to detect even just one given that they are less massive than the moon and light years away.

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