Posted on Mar 19, 2018
When giant stars die, they don’t just fade away. Instead they collapse in on themselves, leaving behind a compressed stellar remnant, usually a city-size, superdense ball of neutrons appropriately called a neutron star. In extreme cases, however, most theorists believe an expiring giant star will form a black hole—a pointlike “singularity” with effectively infinite density and a gravitational field so powerful that not even light, the fastest thing in the universe, can escape once falling in.
Now a new study is reinvigorating an alternate idea, reports Charles Q. Choi in Scientific American, that objects with names such as “black stars,” or “gravastars,” might exist midway between neutron stars and black holes. If real, these exotic stellar corpses should appear nearly identical to black holes save in one key way—they could not irretrievably swallow light.
There are good reasons to seek such alternatives, because black holes raise a host of theoretical problems. For instance, their singularities are supposedly hidden by invisible boundaries known as event horizons. Throw something into a black hole, and once it passes the event horizon it should be gone—forever—with no hope whatsoever of return. But such profound annihilation clashes with other long-cherished laws of physics that suggest the destruction of information is impossible, including information encoded within anything falling into black holes.
Conceived and developed across the past two decades, in part to sidestep such conundrums, models of black stars and gravastars postulate these objects would lack singularities and event horizons. But questions have lingered as to whether such objects could actually form—and remain stable after they did. New research from theoretical physicist Raúl Carballo-Rubio at the International School for Advanced Studies in Italy provides a novel mechanism that might allow black stars and gravastars to exist.
Carballo-Rubio investigated a strange phenomenon known as quantum vacuum polarization. Quantum physics, the best description yet of how all known subatomic particles behave, suggests reality is fuzzy, limiting how precisely one can know the properties of the most basic units of matter—for instance, one can never absolutely know a particle's position and momentum at the same time. One strange consequence of this uncertainty is that a vacuum is never completely empty but instead foams with so-called “virtual particles” that continuously fluctuate into and out of existence.
In the presence of gigantic amounts of energy of the sort produced by the collapse of a giant starprevious research found these virtual particles can polarize, or arrange themselves depending on their properties, much as magnets are divided into north and south poles. Carballo-Rubio calculated the polarization of these particles can produce a surprising effect inside the powerful gravitational fields of dying giant stars—a field that repels instead of attracts.
The image of Supernova 1987A at top of the page combines data from NASA's orbiting Chandra X-ray Observatory and the 8-meter Gemini South infrared telescope in Chile, which is funded primarily by the National Science Foundation.
The X-ray light detected by Chandra is colored blue. The infrared light detected by Gemini South is shown as green and red, marking regions of slightly higher and lower-energy infrared, respectively. The core remains of the star that exploded in 1987, a possible black star, is not visible here. The ring is produced by hot gas (largely the X-ray light) and cold dust (largely the infrared light) from the exploded star interacting with the interstellar region.
"Supernova 1987A is changing right before our eyes," said Dr. Eli Dwek, a cosmic dust expert at NASA Goddard Space Flight Center in Greenbelt, Md. For several years Dwek has been following this supernova, named 1987A for the year it was discovered in the Large Magellanic Cloud, a neighboring dwarf galaxy. "What we are seeing now is a milestone in the evolution of a supernova."
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