 |
This artist’s impression video shows the path of the star S2
as it passes very close to the supermassive "black hole" at the
center of the Milky Way. (Image credit: ESO/M. Kornmesser) |
The black hole in our galaxy could be something quite
different.
In the summer of 2014, astronomers watched with bated breath
as G2, a cloud of gas, swung perilously close to a supermassive black hole in
the Milky Way's center. There were no sparks, and there was no feeding frenzy.
Instead, G2 sped through the event uninjured, escaping what astronomers had
said would be a near-death experience.
But, because black holes are enormous bullies, the fact that
the gravity well overlooked the gassy bystander was startling. It appeared to
be impossible. Astronomers now believe that the supermassive black hole at the
heart of our galaxy is actually a fluffy ball of dark matter rather than a
black hole. According to new study, this unusual concept can account for the
"impossible" encounter as well as all galactic center observations —
and then some.
Icarus and the black hole
Astronomers have long suspected that a supermassive black
hole exists in the center of the Milky Way, known as Sagittarius A*. Of course,
they won't be able to view the black hole because it doesn't emit any light of
its own. Instead, they deduce its existence by observing the movements of the
S-stars, a cluster of stars. The S-stars orbit a hidden, unseen center object,
and astronomers can determine the mass and size of that core object by tracing
their orbits over time.
A black hole, with a mass estimated to be greater than 4
million times that of the sun, is the most plausible candidate for that
concealed central object. But the S-stars aren't the only visitors to our
galactic metropolis. There are also clumps of gas, one of which, called G2,
garnered significant attention. Soon after the clump was discovered decades
ago, astronomers understood that G2's orbit would put it uncomfortably close to
the black hole — close enough for the black hole's tremendous gravity to tear
the gas cloud apart.
However, the gas appeared to survive the closest approach of
G2 to the black hole in 2014, when it came within 260 AU of the giant.
Make it fluffier
G2's longevity is most likely due to the fact that it is
more than just a regular gas cloud. What is its secret superpower? A star or
two could be hidden inside the cloud, and the star's gravity would keep the
entire structure intact as it passed near the black hole.
However, there is another, more extreme explanation: the
supermassive black hole may not be a black hole at all. Maybe it's a clump of
dark matter that's hazy.
Dark matter is the name given by astronomers to an invisible
substance that accounts for more than 80% of the universe's mass. It doesn't
appear to interact with light in any way - it doesn't glow, absorb, reflect, or
refract light — thus we can't see it. Its gravity, on the other hand, makes its
presence felt. Multiple, independent lines of evidence have all shown that dark
matter makes up the majority of the universe's mass.
According to one idea, dark matter is comprised up of an
exotic, hitherto undiscovered particle known as "darkinos." According
to the hypothesis, the darkino is a fermion, a sort of particle. Fermions
include electrons, protons, quarks, and neutrinos, all of which share the
property of being unable to share the same state. In other words, a particular
volume can only hold so many fermions (this is in contrast to the bosons, which
you can shove as many as you want into a given volume).
If dark matter is made up of darkinos, and darkinos are
fermions, these dark matter particles would only concentrate to a certain
extent near the galaxy's core. This would imply that instead of a supermassive
black hole with a well-defined event horizon, a large ball of densely packed
darkinos exists. The boundary of this darkino ball would be a little hazy,
because not everyone can join the party at the very center, just as partygoers
standing in line outside the local disco.
Keep it consistent
The gravitational pressures at the galaxy's center would be
softer since the huge darkino ball would be fuzzy, letting gas clouds like G2
to persist in their orbits.
However, there's more to our galaxy's core — and to our
views of the galactic core — than G2. There's also the matter of the S-stars.
Any radical theory attempting to substitute a supermassive black hole with
something else must produce predictions that are consistent with the
observations.
And a recent study backs up this claim. The team of
astrophysicists, led by Eduar Antonio Becerra-Vergara of the International
Center for Relativistic Astrophysics in Italy, discovered that by replacing the
supermassive black hole with a ball of darkinos with the right mass and
velocity, they were able to replicate all of the S-stars' observed motion. In
some circumstances, their model may be able to fit observed orbits better than
standard black hole calculations.
However, the outcome is insignificant. The black hole model
is extremely simple: to predict how S-stars should behave, you only need to
plug in two numbers: the black hole mass and spin. The darkino model, on the other
hand, has a lot more parameters, which allows for more fine-tuning, and the
researchers discovered the best possible combination of darkino properties.
Future observations will be the true test. If dark matter is
made up of darkinos, a model that accurately depicts what's going on in the
galactic center should also be able to duplicate all of the various dark matter
measurements found across the cosmos. This would include figuring out why
galaxies rotate faster than they should for their mass.
The new study is published in the journal Monthly Notices of
the Royal Astronomical Society Letters in the May issue.
Originally Published By Live Science
0 Comments