- 3903 - BLACK HOLES - are more active than thought? Black holes at galactic centers blast out 10 times more light than previously thought. Blackholes in the hearts of galaxies are usually shrouded in dust. Black holes residing at the center of galaxies produce way more energy than scientists previously thought.
------------ 3903 - BLACK HOLES - are more active than thought?
-
Black holes residing at the center of galaxies are active galactic
nuclei that devour huge amounts of matter, releasing massive amounts of energy
in the form of electromagnetic radiation in the process.
-
- Scientists
have known that these black-hole-powered active galactic centers produce more
radiation than all the stars combined that inhabit the galaxies that surround
them. Now astronomers have found that the amount of this radiation, or light,
that these central black holes exude, could be about ten times larger than
previously estimated.
-
- The
researchers reached this conclusion by calculating how much of the ultraviolet
light coming from these black holes gets absorbed by the dust that surrounds
them. It was a known fact that this dust reduces the amount of radiation from
the active galactic nuclei that astronomers can detect.
-
- A team from
the University of California Santa Cruz managed to answer this question by
analyzing in detail the dimming effect of the dust surrounding one of the
brightest known active galactic black holes, one found inside the galaxy known
as NGC 5548 located some 245 million light-years away from Earth.
-
- When there
are intervening small particles along our line of sight, this makes things
behind them look dimmer. We see this at
sunset on any clear day when the sun looks fainter and it does not burn your
eyes.
-
- The dimming
of the evening sun and the reddening of the solar disk both have the same
cause. The same effect applies to the distant galactic centers, which appear
redder than they actually are. In the case of the sun, however, astronomers can
easily compare the dimmed light with the actual wavelength composition of the
sun's radiation and its intensity.
-
- To do the
same for distant galaxies is much more complicated, as estimates of the
intensity of the various wavelength components of undimmed radiation from
distant active galactic nuclei are mostly based on theoretical predictions.
-
- In the new
study, they used seven different indicators to estimate the amount of dust that
obscures the center of NGC 5548, and found them all to be in good agreement.
The dimming they found was considerable when looking out at NGC 5548, over ten
times more significant than what astronomers experience due to the dust in our
own galaxy, the Milky Way..
-
- What happens
at the center of a black hole? All of
the possibilities are very weird. 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 starZ”, 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).
-
- All the
material that has ever fallen into the black hole gets compressed into a ball
not much bigger than this. Perfectly microscopic, but definitely not infinitely
tiny.
-
- This
resistance to continued compression eventually forces the material to
un-collapse ( explode), making black holes only temporary objects. But because
of the extreme time dilation effects around black holes, from our perspective
in the outside universe it takes billions, even trillions, of years before they
go boom.
-
- 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. It
sounds like sci-fi, but it's real: 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. A black hole's event horizon is
its point of no return the boundary beyond which nothing, not even light, can
escape.
-
- However,
recent observations of merging black holes with gravitational wave detectors
have potentially ruled out the existence of gravastars, because merging
gravastars will give a different signal than merging black holes, and outfits
like LIGO (the Laser Interferometer Gravitational-Wave Observatory) and Virgo
are getting more and more examples by the day.
- Planck
stars and gravastars may have awesome names, but the reality of their existence
is in doubt. So maybe there's a more mundane explanation for singularities, one
that's based on a more nuanced and realistic view of black holes in our
universe.
-
- 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.
-
- One
challenge: the interiors of rotating black holes are catastrophically
unstable. The problem with rotating
black holes is that, well, they rotate. 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 toward 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.
-
- The
formation of an inner horizon sows the seeds for the destruction of the black
hole. But rotating black holes certainly exist in our universe, so that tells
us that our math is wrong and something funky is going on.
-
What's really happening inside a black hole? We don't
know. The origin of dark energy has been
perplexing scientists for decades. When
astronomers discovered that the universe is expanding at an accelerating rate,
they theorized that some force must be pushing things farther apart and
overcoming gravity, which should be slowing things down. That force was
suggested to be dark energy, but no one has ever figured out from where it
comes.
-
- Black holes
acquire mass in two ways: accretion of gas and mergers with other black holes.
But in studying nine billion years of black hole evolution in dormant giant
elliptical galaxies, the researchers discovered that the older black holes are
much larger than they should be based on those two methods of growth.
-
- That means
there must be another way these black holes are acquiring mass. Researchers suggest
the answer is dark energy in the form of vacuum energy, a kind of energy
included in spacetime itself that pushes the universe further apart,
accelerating the expansion.
-
- If the
theory holds, then this is going to revolutionize the whole of cosmology,
because at last we've got a solution for the origin of dark energy that's been
perplexing cosmologists and theoretical physicists for more than 20 years
-
- The idea
that black holes are a source of dark energy isn't new. In fact, it's part of Einstein's
theory of general relativity. But this is the first time astronomers have
obtained observational evidence to support the theory.
-
- There's
evidence the typical black hole solutions don't work for you on a long, long
timescale, and we have the first proposed astrophysical source for dark energy.
-
March 5, 2023 BLACK
HOLES - are more active than thought? 3903
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