Many mysteries about our universe haunt astronomers. Among those, one of the most perplexing is what they call “dark matter.” By studying the gravitational interactions and motions of galaxies, astronomers learned that most of the matter in our universe, about 86%, is composed of stuff that has no connection to light. It doesn’t emit, reflect, or absorb any type of electromagnetic radiation, including light, hence the moniker dark matter.
Astronomers
know it’s there. The evidence for it is overwhelming. Galaxies rotate so fast
that they should fly apart unless there is a lot more mass than can be
accounted for in stars, planets, gas clouds, and all the other stuff we can
see. But they have no idea what it is.
Some
astronomers have suggested that so-called primordial black holes (PBH) may be
responsible for dark matter. Black holes are the extreme members of the cosmic
zoo. Their gravity is so strong that nothing, not even light, the fastest thing
in the universe, can escape. Normal black holes form when giant stars explode as
a supernova, and the remaining core becomes so compressed by gravity that it
collapses into a black hole. These are typically 5 to 10 times the mass of our
sun. Black holes also do not emit or reflect any light.
PBHs
form quite differently. When the universe began in the Big Bang, the density of
matter was unimaginably high. Particles would constantly collide, sometimes
sticking together. Usually, they would break apart in another collision a tiny
fraction of a second later. Some areas of this infant universe were so dense
that matter could clump together so tightly that they could form black holes
millions of times smaller than a single proton. They may contain no more mass
than a moon, a mountain, or even an elephant. Once formed, the extreme gravity
holds them together regardless of how many collisions they may experience.
PBHs
may well exist in such numbers that they could fill space. They might well be
responsible for most or even all the dark matter that we know exists out
there. Sarah Geller, a theoretical physicist at the University of California at
Santa Cruz, co-authored a study of PBHs. They wondered how often one might pass
through our solar system. "If there are lots of black holes out there,
some of them must surely pass through our backyard every now and then,"
Geller said.
The
researchers wondered "what might happen if a black hole punched through
Earth's crust, or passed through our atmosphere, or left a crater on the
moon," Geller said. "We even asked ourselves what would happen if one
of these tiny black holes hit a human." After crunching the numbers, they
realized that the chances of one hitting something as small as a person, or
even our planet, were extremely tiny.
"We started thinking about the very
precisely measured orbits of objects in the solar system," Geller said. "They
could produce wobbles in the orbits of objects in the solar system that are big
enough for us to measure." They estimate that one could pass through the
inner parts of the solar system once every decade or so. The researchers aren’t
claiming that PBHs exist, that they make up any of the dark matter, or that
they are in or have passed through the solar system. But if they do exist in
sufficient numbers that some or many have visited our planets, we can detect
their gravitational pull.
Just
to ease your mind, a separate study found that even if a PBH passed through
your body, they are so tiny and so fast, they would likely cause no noticeable
effect on your body.
Each month, I write an astronomy-related column piece
for the Oklahoman newspaper. After
it is published there, I post that same column to my blog page.
This is reprinted with
permission from the Oklahoman and www.Oklahoman.com.
