- 3843 - STRANGE BLACKHOLE - Buchdahl star densest object. - An elusive object in space has posed a riddle for scientists. It looks like a black hole. It acts like a black hole. It may even smell like a black hole. But it has one crucial difference: It has no event horizon, meaning that you can escape its gravitational clutches if you try hard enough.
----- 3843 - STRANGE BLACKHOLE - Buchdahl star densest object.
- It's called a Buchdahl star, and it is the
densest object that can exist in the universe without becoming a black hole
itself. Evidence for blackholes is everywhere we look, including the release of
gravitational waves when they collide and the dramatic shadows they carve out
of surrounding materials.
-
- Astronomers also understand how black holes
form: They are the remnants of the catastrophic gravitational collapse of
massive stars. When giant stars die, no force in nature is capable of
sustaining the stars' own weight, so these doomed behemoths just keep crushing
themselves to infinity.
-
- Astronomers know of “white dwarfs”, which
contain a sun's worth of mass in a volume equivalent to Earth, and we know of
“neutron stars”, which compress all that down even further into the volume of a
city. But we don't know if there's anything smaller still that avoids the fate
of becoming a black hole.
-
- In 1959, German-Australian physicist Hans
Adolf Buchdahl explored how a highly idealized "star", represented as
a perfectly spherical blob of material, might behave as it was compressed as
much as possible. As the blob got smaller and smaller, its density rose, making
its own gravitational pull even more intense. Using the tools of Einstein's
general theory of relativity, Buchdahl found an absolute lower limit to the
size of that blob.
-
- That special radius is equal to 9/4 times
the mass of the blob, multiplied byNewton's gravitational constant, all divided
by the speed of light squared.
-
- The Buchdahl limit is important because it defines
the densest possible object that can still avoid becoming a black hole. Below
that, the blob of material must always become a black hole, at least in the
theory of relativity.
-
- Finding exotic objects that come right to
the edge of that limit, “Buchdahl
stars”, has become a popular pastime of theorists. Buchdahl stars are "black hole
mimics" because their observable properties would be nearly identical,
studied what happens to the energy of a hypothetical star as it begins
collapsing into a Buchdahl star.
-
- As the star collapses, it picks up
gravitational potential energy, which is negative because gravity is
attractive. At the same time, the interior of the star gains kinetic energy as
all the particles are forced to jostle against each other in a smaller volume.
-
- By the time the star reaches the Buchdahl
limit the total kinetic energy was equal to half the potential energy.
-
- This relationship is known as the “virial
theorem”, and it applies to numerous situations in astronomy where the force of
gravity is in balance with other forces. This means that a Buchdahl star could
theoretically exist as a stable object with known, well-understood properties.
-
- This finding suggests that theoretical
Buchdahl stars may really be out there, and could lead to insights about the
inner workings of black holes. We can
interact with a Buchdahl star and study what it's made of, which may give us
clues as to what black hole interiors are like.
-
- To date,2023, there is no known arrangement
of matter that can create a Buchdahl star. Further research will be needed to
discover what other properties these exotic objects might have, and what they
might tell us about black holes.
- Astronomers have uncovered more than 400
previously hidden black holes feeding on stars and dust in the center of
galaxies. It appears that many of the new black holes, discovered using NASA's
Chandra X-ray Observatory, remained unknown until now because they are buried
beneath cocoons of dust.
-
- Supermassive black holes, which can be
millions of times heavier than the sun, live in the center of almost every
galaxy in the universe. These colossal objects produce bright beams of energy
as they feed on gas, dust, and stars in their immediate vicinity, creating what
are known as Active Galactic Nuclei (AGN).
-
- AGN are particularly bright in the X-ray
portion of the electromagnetic spectrum.
But certain objects have been spotted giving off tons of X-rays without
the specific optical signatures associated with AGN, "X-ray bright
optically normal galaxies" or "XBONGs."
-
- The researchers identified 820 XBONGs
located between 550 million and 7.8 billion light-years from Earth, the largest
such sample ever built. One possibility
is that Chandra is seeing extremely distant clusters of galaxies, which would
shine bright in X-rays but lack the characteristic optical signature
identifying them as AGN. This could explain around 20% of the remaining XBONGs.
-
- The final 30% are galaxies whose optical
light is particularly powerful, bright enough to wash out the optical AGN
signature, which could happen when such galaxies are particularly far away.
-
- The “singularity” at the center of a black
hole is the ultimate no man's land: a place where matter is compressed down to
an infinitely tiny point, and all conceptions of time and space completely
break down. And it doesn't really exist. Something has to replace the
singularity, but we're not exactly sure what.
-
- It could be that deep inside a black hole,
matter doesn't get squished down to an infinitely tiny point. Instead, there
could be a smallest possible configuration of matter, the tiniest possible
pocket of volume. This is called a “Planck star”, and it's a theoretical
possibility envisioned by loop quantum gravity, which is itself a highly
hypothetical proposal for creating a quantum version of gravity.
-
- In the world of loop quantum gravity,
space and time are quantized, the universe around us is composed of tiny
discrete chunks, but at such an incredibly tiny scale that our movements appear
smooth and continuous.
-
- This theoretical chunkiness of space-time
provides two benefits. One, it takes the dream of quantum mechanics to its
ultimate conclusion, explaining gravity in a natural way. And two, it makes it
impossible for singularities to form inside black holes.
-
- As matter squishes down under the immense
gravitational weight of a collapsing star, it meets resistance. The
discreteness of space-time prevents matter from reaching anything smaller than
the Planck length (around 1.68 times 10^-35 meters). Perfectly microscopic, but definitely not
infinitely tiny.
-
- Another attempt to eradicate the
singularity, one that doesn't rely on untested theories of quantum gravity, is
known as the “gravastar”. The
difference between a black hole and a gravastar is that instead of a
singularity, the gravastar is filled with dark energy.
-
- Dark energy is a substance that permeates
space-time, causing it to expand outward. Dark energy is currently in operation
in the larger cosmos, causing our entire universe to accelerate in its
expansion. As matter falls onto a
gravastar, it isn't able to actually penetrate the event horizon (due to all
that dark energy on the inside) and therefore just hangs out on the surface.
But outside that surface, gravastars look and act like normal black holes.
-
-
- The idea of a single point of infinite
density comes from our conception of stationary, non-rotating, uncharged,
rather boring black holes. Real black holes are much more interesting
characters, especially when they spin.
-
- The spin of a rotating black hole stretches
the singularity into a ring. And according to the math of Einstein's theory of
general relativity, once you pass through the ring singularity, you enter a
wormhole and pop out through a white hole (the polar opposite of a black hole,
where nothing can enter and matter rushes out at the speed of light) into an
entirely new and exciting patch of the universe.
-
- The problem with rotating black holes is
the singularity, stretched into a ring, is rotating at such a fantastic pace
that it has incredible centrifugal force. And in general relativity, strong
enough centrifugal forces act like antigravity: they push, not pull.
-
- This creates a boundary inside the black
hole, called the “inner horizon”. Outside this region, radiation is falling
inward towards the singularity, compelled by the extreme gravitational pull.
But radiation is pushed by the antigravity near the ring singularity, and the
turning point is the inner horizon. If you were to encounter the inner horizon,
you would face a wall of infinitely energetic radiation, the entire past
history of the universe, blasted into your face in less than a blink of an eye.
-
- What's really happening inside a black hole?
We don't know — and the scary part is that we may never know.
-
January 27, 2022 3843
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--------------------- --- Friday, January 27, 2023 ---------------------------
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