Scientists looking to capture evidence of dark matter -- the invisible substance thought to constitute much of the universe -- may find a helpful tool in the recent work of researchers from Princeton University and New York University. The team unveiled in a report in the journal Physical Review Letters this month a ready-made method for detecting the collision of stars with an elusive¹（难懂的） type of black hole that is on the short list of objects believed to make up dark matter. Such a discovery could serve as observable proof of dark matter and provide a much deeper understanding of the universe's inner workings.

 

Postdoctoral researchers Shravan Hanasoge of Princeton's Department of Geosciences and Michael Kesden of NYU's Center for Cosmology and Particle Physics simulated the visible result of a primordial²（原始的） black hole passing through a star. Theoretical remnants of the Big Bang, primordial black holes possess the properties of dark matter and are one of various cosmic objects thought to be the source of the mysterious substance, but they have yet to be observed.

 

If primordial black holes are the source of dark matter, the sheer number of stars in the Milky³ Way galaxy⁴ -- roughly 100 billion -- makes an encounter inevitable⁵, the authors report. Unlike larger black holes, a primordial black hole would not "swallow" the star, but cause noticeable vibrations⁶ on the star's surface as it passes through.

 

Thus, as the number of telescopes and satellites probing distant stars in the Milky Way increases, so do the chances to observe a primordial black hole as it slides harmlessly through one of the galaxy's billions of stars, Hanasoge said. The computer model developed by Hanasoge and Kesden can be used with these current solar-observation techniques to offer a more precise method for detecting primordial black holes than existing tools.

 

"If astronomers⁷ were just looking at the sun, the chances of observing a primordial black hole are not likely, but people are now looking at thousands of stars," Hanasoge said.

 

"There's a larger question of what constitutes dark matter, and if a primordial black hole were found it would fit all the parameters⁸ -- they have mass and force so they directly influence other objects in the universe, and they don't interact with light. Identifying one would have profound implications for our understanding of the early universe and dark matter."

 

Although dark matter has not been observed directly, galaxies⁹ are thought to reside in extended dark-matter halos based on documented gravitational effects of these halos on galaxies' visible stars and gas. Like other proposed dark-matter candidates, primordial black holes are difficult to detect because they neither emit nor absorb light, stealthily traversing the universe with only subtle gravitational effects on nearby objects.

 

Because primordial black holes are heavier than other dark-matter candidates, however, their interaction with stars would be detectable¹⁰ by existing and future stellar observatories¹¹ , Kesden said. When crossing paths with a star, a primordial black hole's gravity would squeeze the star, and then, once the black hole passed through, cause the star's surface to ripple¹² as it snaps back into place.

 

"If you imagine poking¹³ a water balloon and watching the water ripple inside, that's similar to how a star's surface appears," Kesden said. "By looking at how a star's surface moves, you can figure out what's going on inside. If a black hole goes through, you can see the surface vibrate."

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