Tag: primordial black holes

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.

Primordial Black Holes Might be Cosmic Gold Diggers

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Neutron stars might have black hole parasites in their cores (NASA’s Goddard Space Flight Center)

When the universe’s first black holes appeared is one of the biggest mysteries in astrophysics. Were they born immediately after the Big Bang 13.8 billion years ago? Or did they pop into existence after the first population of massive stars exploded as supernovas millions of years later?

The origin of primordial black holes isn’t a trivial matter. In our modern universe, the majority of galaxies have supermassive black holes in their cores and we’re having a hard time explaining how they came to be the monsters they are today. For them to grow so big, there must have been a lot of primordial black holes formed early in the universe’s history clumping together to form progressively more massive black holes.

Now, in a new study published in Physical Review Letters, Alexander Kusenko and Eric Cotner, who both work at the University of California, Los Angeles (UCLA), have arrived at an elegant theory as to how the early universe birthed black holes.

Primordial beginnings

Immediately after the Big Bang, the researchers suggest that a uniform energy field pervaded our baby universe. In all the superheated chaos, long before stars started to form, this energy field condensed as “Q balls” and clumped together. These clumps of quasi-matter collapsed under gravity and the first black holes came to be.

These primordial black holes have been singled out as possible dark matter candidates (classed as massive astrophysical compact halo objects, or “MACHOs”) and they may have coalesced to quickly seed the supermassive black holes. In short: if these things exist, they could explain a few universal mysteries.

But in a second Physical Review Letters study, Kusenko teamed up with Volodymyr Takhistov (also from UCLA) and George Fuller, at UC San Diego, to investigate how these primordial black holes may have triggered the formation of heavy elements such as gold, platinum and uranium — through a process known as r-process (a.k.a. rapid neutron capture process) nucleosynthesis.

It is thought that energetic events in the universe are responsible for the creation for approximately half of elements heavier than iron. Elements lighter than iron (except for hydrogen, helium and lithium) were formed by nuclear fusion inside the cores of stars. But the heavier elements formed via r-process nucleosynthesis are thought to have been sourced via supernova explosions and neutron star collisions. Basically, the neutron-rich debris left behind by these energetic events seeded regions where neutrons could readily fuse, creating heavy elements.

These mechanisms for heavy element production are far from being proven, however.

“Scientists know that these heavy elements exist, but they’re not sure where these elements are being formed,” Kusenko said in a statement. “This has been really embarrassing.”

A cosmic goldmine

So what have primordial black holes got to do with nucleosynthesis?

If we assume the universe is still populated with these ancient black holes, they may collide with spinning neutron stars. When this happens, the researchers suggest that the black holes will drop into the cores of the neutron stars.

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Alexander Kusenko/UCLA

Like a parasite eating its host from the inside, material from the neutron star will be consumed by the black hole in its core, causing the neutron star to shrink. As it loses mass, the neutron star will spin faster, causing neutron-rich debris to fling off into space, facilitating (you guessed it) r-process nucleosynthesis, creating the heavy elements we know and love — like gold. The whole process is expected to take about 10,000 years before the neutron star is no more.

So, where are they?

There’s little evidence that primordial black holes exist, so the researchers suggest further astronomical work to study the light of distant stars that may flicker by the passage of invisible foreground black holes. The black holes’ gravitational fields will warp spacetime, causing the starlight to dim and brighten.

It’s certainly a neat theory to think that ancient black holes are diving inside neutron stars to spin them up and create gold in the process, but now astronomers need to prove that primordial black holes are out there, possibly contributing to the dark matter budget of our universe.