What if the
center of our galaxy isn’t a supermassive black hole after all, but instead, a
massive amount of dark matter? That would flip our long-held understanding of
the Milky Way, but in a new study, scientists from Italy, Argentina, and
Colombia say the evidence stacks up.
The idea of
a supermassive black hole at the center of the Milky Way is well-established,
based partly on the orbit of specific stars like S0-2. Scientists study these
objects in orbit as a way to extrapolate what they’re actually orbiting
around—in this case, “a supermassive black hole 4 million times the mass of the
Sun,” ScienceAlert explains.
That
supermassive black hole is called Sagittarius A*. S0-2 and other established
stars are pulled every which way in their orbits around Sagittarius A*, into
extreme orbits that scientists measure in order to get some idea of what the
supermassive black hole is all about. Think about how soap suds behave as water
swirls down your drain and what that tells you about what the drain is doing.
Even if you couldn’t see the drain directly, the behaviors around it would give
you clues.
Scientists are still conducting that kind of research, but a new-to-us class of space objects is casting a shadow over the black hole theory. These objects “look like gas but behave like stars,” physicist Andrea Ghez told ScienceAlert in 2020. There are six “G” objects of this kind, with orbits ranging from 170 to 1,600 years long.
Recently,
their wonky orbits have led to a new, competing theory to describe the center
of the Milky Way.
In 2014,
scientists observed object G2 passes its closest point to Sagittarius A* and
become stretched out and distorted—a phenomenon known as drag. This, scientists
from the International Center for Relativistic Astrophysics in Italy say, is a
sign that Sagittarius A* might be something other than a supermassive black
hole.
Best-fit
orbits for the 17 best-resolved S-stars orbiting Sagittarius A*.
In 2020, the
same research team published findings that G2 and S2 might be experiencing the
same different kind of gravitational pull near Sagittarius A*. They say the way
these objects behave is more consistent with a specific kind of dark matter, In
their side-by-side analysis of black hole versus dark matter as the
explanation, dark matter was a better statistical fit.
Dark matter
is matter that we can’t see, but we can measure it through its effect on
gravity and objects around it. Dark matter makes up 30 percent of the mass of
our universe, yet it’s invisible and, so far, not directly observed. But its
presence, along with the related idea of dark energy, makes up a total of 99.5
percent of the mass of the universe around us and provides the missing piece to
a ton of questions in physics and cosmology.
Dark matter
can and does turn into black holes, but the scientists posit that Sagittarius
A* is instead a blob-like mass that will require a lot more material in order
to turn into a black hole. For now, it could just be a dense blob that still
attracts nearby objects as a black hole does. And if it is a kind of proto
future black hole, that could explain how supermassive black holes form in the
first place—something scientists have been puzzled by for decades.
In this newstudy, accepted for publication by MNRAS Letters, the Italian, Argentinian, and
Colombian scientists explain that not just the directly affected G2 and S2
space objects line up with the dark matter theory. They’ve expanded their study
to the closest, most well-understood stars orbiting Sagittarius A* and found
those stars also behave consistently with a dark matter model.
So, what do
these findings mean? Well, when it comes to the swirling mysteries at the heart
of the galaxy, all news is good news. These scientists have put out an
ambitious new theory, and subsequent work will either back them up or
respectfully disagree, citing different analysis and observation. That will
continue to lead toward a better understanding of supermassive black holes and
of Sagittarius A* in particular, and could help to solve the mystery.