- 4477
- BLACKHOLES - what
have we learned? - James Webb Telescope detects most distant
black hole merger to date. Astronomers
find evidence for an ongoing merger of two galaxies and their massive black
holes when the universe was only 740 million years old. This marks the most
distant detection of a black hole merger ever obtained and the first time that
this phenomenon has been detected so early in the universe.
-
----------------------------------- 4477 - BLACKHOLES - what have we learned?
- Astronomers have found supermassive black
holes with masses of millions to billions times that of the sun in most massive
galaxies in the local universe, including in our Milky Way galaxy. These black
holes have likely had a major impact on the evolution of the galaxies they
reside in. Scientists still don't fully understand how these objects grew to
become so massive.
-
- The finding of gargantuan black holes
already in place in the first billion years after the Big Bang indicates that
such growth must have happened very rapidly, and very early. These observations have provided evidence
for an ongoing merger of two galaxies and their massive black holes when the
universe was just 740 million years old. The system is known as “ZS7”.
-
- Massive black holes that are actively
accreting matter have distinctive spectrographic features that allow
astronomers to identify them. They found evidence for very dense gas with fast
motions in the vicinity of the black hole, as well as hot and highly ionized
gas illuminated by the energetic radiation typically produced by black holes in
their accretion episodes. Webb also
allowed astronomers to spatially separate the two black holes.
-
- They found that one of the two black holes
has a mass that is 50 million times the mass of the sun. The mass of the other black hole is likely
similar, although it is much harder to measure because this second black hole
is buried in dense gas.
-
- These findings suggest that merging is an
important route through which black holes can rapidly grow, even at cosmic
dawn. Active, massive black holes in
the distant universe show that massive black holes have been shaping the
evolution of galaxies from the very beginning.
-
- Once the two black holes merge, they will
also generate gravitational waves. Events like this will be detectable with the
next generation of gravitational wave observatories, the upcoming Laser
Interferometer Space Antenna (LISA) mission, which was recently approved by the
European Space Agency and will be the first space-based observatory dedicated
to studying gravitational waves.
-
- Physicists consider black holes one of the
most mysterious objects that exist. Ironically, they're also considered one of
the simplest. For years, physicists have been looking to prove that black holes
are more complex than they seem. And, LISA will help us with this hunt.
-
- Research from the 1970s suggests that you
can comprehensively describe a black hole using only three physical attributes—their
mass, charge and spin. All the other properties of these massive dying stars,
like their detailed composition, density and temperature profiles, disappear as
they transform into a black hole.
-
- The idea that black holes have only three
attributes is called the "no-hair" theorem, implying that they don't
have any "hairy" details that make them complicated. For decades, researchers in the astrophysics
community have exploited loopholes or work-arounds within the no-hair theorem's
assumptions to come up with potential hairy black hole scenarios. A hairy black
hole has a physical property that scientists can measure that's beyond its
mass, charge or spin. This property has to be a permanent part of its
structure.
-
- A black hole containing the maximum charge
it could hold—called an “extremal charged black hole”—would develop
"hair" at its horizon. A black hole's horizon is the boundary where
anything that crosses it, even light, can't escape.
-
- Since astrophysical objects such as stars
and planets are known to spin, scientists expect that black holes would spin as
well, based on how they form. Astronomical evidence has shown that black holes
do have spin, though researchers don't know what the typical spin value is for
an astrophysical black hole.
-
- A gravitational wave is a tiny disturbance
in space-time typically caused by violent astrophysical events in the universe.
The collisions of compact astrophysical objects such as black holes and neutron
stars emit strong gravitational waves.
-
- They can measure these hairy attributes
from gravitational wave data for fast-spinning black holes. Looking at the
gravitational wave data offers an opportunity for a signature of sorts that
could indicate whether the black hole has this type of hair.
-
- LISA will look for gravitational waves,
and the data from the mission could help with the hairy black hole questions. LISA consists of three spacecrafts
configured in a perfect equilateral triangle that will trail behind the Earth
around the sun. The spacecrafts will each be 1.6 million miles apart, and they
will exchange laser beams to measure the distance between each other down to
about a billionth of an inch.
-
- LISA will detect gravitational waves from
supermassive black holes that are millions or even billions of times more
massive than our sun. It will build a
map of the space-time around rotating black holes, which will help physicists
understand how gravity works in the close vicinity of black holes to an
unprecedented level of accuracy.
-
- With LIGO making new observations every day
and LISA to offer a glimpse into the space-time around black holes, now is one
of the most exciting times to be a black hole physicist.
Is there anything stranger in
the universe than black holes?
-
- Black holes might have hair. In the 1960s, physicist John Wheeler
suggested that black holes "have no hair," meaning that each
particular cosmic object could only be distinguished from its brethren by its
spin, angular momentum and mass. Any other differentiating information about a
black hole is considered "hair," and is thought to disappear behind a
black hole's impenetrable event horizon, a boundary around the black hole
beyond which nothing, including light, can escape.
-
- In 2016, when famed physicist Stephen
Hawking proposed that black holes actually sport a hairdo made of ghostly,
zero-energy particles, and that this contains information about material the
black hole has consumed. This hypothesis has not been proven, but could help
solve a longstanding paradox about what happens to gas and dust that has fallen
into a black hole's maw.
-
- Nothing is supposed to be able to escape a
black hole's powerful gravitational grip. But that only applies to material
that has gotten extremely close to the hole's edge. Many black holes are, in
fact, surrounded by streams of gas and dust, which circle around the hole, like
water going down a drain.
-
- Friction in this material generates heat,
which creates churning, storm-like structures in the gas and dust. Recent
observations suggest that this motion also produces arching rings that surround
inner columns of matter, which shoots straight into the air, strongly
resembling fountains.
-
- Quantum mechanics provides another way for
particles to escape a black hole. According to theory, pairs of subatomic
particles are constantly blinking in and out of existence around a black hole's
event horizon. Every so often, the configuration is aligned in just the right
way to cause one of the partners to fall into the black hole. The particle's
identical associate is then propelled away at extremely high speed, robbing the
black hole of a tiny bit of energy.
-
- This produces what's known as “Hawking
radiation”, after Stephen Hawking, who discovered the phenomenon. Because
energy equals mass, this process actually can cause a black hole to shrink and
eventually evaporate away over very long periods of time.
-
- One of the problems with Hawking radiation
is that it causes conundrums for physicists. The subatomic particles produced
by this radiation are entangled, meaning that what happens to one is
immediately felt by the other. So what does the partner that didn't fall into
the black hole feel as its associate gets crushed into an infinitely dense
point? Nobody knows.
-
- One theory holds that the black hole
severs the particles' entanglement, an outcome that — according to the laws of
quantum mechanics – would produce an insane amount of energy. That, in turn,
would mean that all black holes are surrounded by roiling walls of fire.
-
- It's hard to square black holes' crushing
mass with the laws of quantum mechanics, which hold that information about
particles can never be destroyed. But material that slips beyond a black hole's
edge should become forever lost to the universe. This conundrum is known as the
“black hole information paradox”.
-
- Recent research suggests that information
that gets scrambled within a black hole could be passed to the outgoing
particle partners in Hawking radiation; however, no definitive answer to this
paradox has been found, to date.
-
- Shortly after the Big Bang, the universe
should have produced a multitude of tiny black holes. Because these features
would be massive objects that give off no light, some physicists have
conjectured that these “primordial black holes” could account for dark matter,
that mysterious material that the vast majority of matter in the cosmos is made
of.
-
- But the idea is controversial, given that
data from the Laser Interferometer Gravitational-Wave Observatory (LIGO) has
ruled out a universe filled with many minuscule black holes. Perhaps
medium-size black holes might still be lurking out there, though observations
suggest they would only make up, at most, 1% to 10% of dark matter.
-
- Black holes run into the problem of
'infinity”. A black hole's mass is
crushed to an infinitely dense point that's infinitely small in size.
Physically, this doesn't make any sense, so researchers have searched for
alternative frameworks to get a handle on black holes.
-
- One proposal is known as “quantum loop
gravity”, suggests that the fabric of space-time is curved very strongly near
the center of the black hole. This would result in part of the hole extending
into the future, meaning that matter getting sucked into it would time travel
forward.
-
- One highly-controversial observation
suggests that our universe is a latecomer. Earlier universes might have existed
before ours, and would have contained black holes. Prominent Oxford University
mathematical physicist Roger Penrose has argued that cosmic background
radiation contains imprints of these black holes from before time.
-
- Physicists are set to release the
first-ever image of a black hole. This supermassive beast lurks at the heart of
our Milky Way galaxy; capturing a photo of it has been the aim of the Event
Horizon Telescope. This instrument is actually a global network of radio
telescopes all over the Earth, which have combined their powers to zoom in
closer to the galactic center than ever before. The telescope should be able to
spot the black hole's shadow across its material surrounding, and images are
expected in 2024.
-
- Black hole singularities continue to defy
physics. New research presents a bold
solution to this puzzle: Black holes may actually be a theoretical type of star
called a 'gravastar,' filled with universe-expanding dark energy.
-
- “Gravastars” are hypothetical astronomical
objects that were introduced in 2001 as alternatives to black holes. They can be interpreted as stars made of
vacuum energy or dark energy: the same type of energy that propels the
accelerated expansion of the universe.
-
- A black hole is predicted to be a point of
infinitely high density, called a “singularity”, where all the mass of the
black hole is concentrated, but fundamental physics teaches us that infinities
do not exist, and their appearance in any theory signals its inaccuracy or
incompleteness.
-
- The main advantage of gravastars is that
they do not have singularities. “Dark
energy” is the strange phenomenon thought to be responsible for the
accelerating expansion of the universe, but could it also be holding black holes
together, as gravatar theory suggests?
-
- Like ordinary black holes, gravastars
should arise at the final stage of the evolution of massive stars, when the
energy released during thermonuclear combustion of the matter inside them is no
longer enough to overcome the force of gravity, and the star collapses into a
much denser object.
-
- But in contrast to black holes, gravastars
are not expected to have any singularities and are thought to be thin spheres
of matter whose stability is maintained by the dark energy contained within
them. Using Einstein's theory, the huge masses of hot matter that surround
supermassive black holes would appear if these black holes were actually
gravastars. They also scrutinized the properties of "hot spots",
gigantic gas bubbles orbiting black holes at near-light speeds.
-
- Their findings revealed striking
similarities between the matter emissions of gravastars and black holes,
suggesting that gravastars don't contradict scientists' experimental
observations of the universe. They discovered that a gravastar itself should
appear almost like a singular black hole, creating a visible shadow.
-
- This shadow is not caused by the trapping
of light in the event horizon, but by a slightly different phenomenon called
the 'gravitational redshift,' causing light to lose energy when it moves
through a region with a strong gravitational field. When the light emitted from regions close to
these alternative objects reaches our telescopes, most of its energy would have
been lost to the gravitational field, causing the appearance of this shadow.
-
- The Event Horizon Telescope traceing the
lines of powerful magnetic fields spiraling out from the edge of the
supermassive black hole at the center of our Milky Way galaxy suggests that
strong magnetism may be common to all supermassive black holes.
-
- Sagittarius A* wasn’t the first black hole
whose shadow was imaged by the EHT. Back in 2019, astronomers showed off a
similar picture of the supermassive black hole at the center of the galaxy M87,
which is more than a thousand times bigger and farther away than the Milky
Way’s black hole.
-
- In 2021, the EHT team charted the magnetic
field lines around M87’s black hole by taking a close look at the black hole in
polarized light, which reflects the patterns of particles whirling around
magnetic field lines. Researchers used the same technique to determine the
magnetic signature of Sagittarius A*, or Sgr A* .
-
- Getting the image wasn’t easy, largely due
to the fact that Sgr A* was harder to pin down than M87. The EHT team had to
combine multiple views to produce a composite image. What we’re seeing now is that there are
strong, twisted and organized magnetic fields near the black hole at the center
of the Milky Way galaxy.
-
- Along with Sgr A* having a strikingly
similar polarization structure to that seen in the much larger and more
powerful M87* black hole, we’ve learned that strong and ordered magnetic fields
are critical to how black holes interact with the gas and matter around them.
-
- The structure of the magnetic fields around
Sgr A* suggests that the black hole is launching a jet of material into the
surrounding environment. Previous research has shown that to be the case for
M87’s black hole.
-
- The fact that the magnetic field structure
of M87* is so similar to that of Sgr A* is significant because it suggests that
the physical processes that govern how a black hole feeds and launches a jet
might be universal among supermassive black holes, despite differences in mass,
size and surrounding environment.
-
- In the seven years since the EHT began
gathering observations, the collaboration has been adding to its array of radio
telescopes, which is resulting in the production of higher-quality imagery. The
researchers aim to produce high-fidelity movies of Sgr A* that may reveal a
hidden jet. They’ll also look for evidence of similar polarization features
around other supermassive black holes.
-
-
May 22, 2024 BLACKHOLES
- what have we learned? 4477
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------ “Jim Detrick” -----------
--------------------- --- Wednesday, May 22, 2024
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