- 4061 - DARK SKIES - why is it not filled with starlight? When we look at dark skies we see all those stars sending us their light. Why does all that light not fill up the sky like our Sun? I know they are far away but all that light should eventuall reach us.
---------------------------- 4061 - DARK SKIES - why is it not filled with starlight?
- This was a question that first was answered
by Heinrich Olbers in the German city of Bremen on May 7, 1823. He
wrote an answer in an article that left his name in history. After the deaths
of his wife and daughter, Dr. Olbers had recently given up his work as an
opthalmologist to devote himself to his nocturnal passions: the stars, the
moon, meteorites and comets.
-
- Like many of his peers, Olbers trained
himself in astronomy. He gained a solid reputation in the academic world and
spent long nights observing the sky from the observatory on the second floor of
his house.
-
- On that morning, Olbers had come to a
strange conclusion: based on all that was known about the universe at that
time, the night sky should not have been dark. In fact, the entire heavens
should have been glowing as brightly as the sun.
-
- Olbers was not the first to note this paradox.
But his name is the one we attach to it today. The enigma of the night sky's
darkness has echoed down the centuries from Olbers and the poet Edgar Allan Poe
to 20th-century astronomers and space probes today.
-
- Like many of his contemporaries, Olbers
followed Isaac Newton and René Descartes in believing the universe was
infinite. If the universe were finite
and static, the force of gravity should draw all the stars together at a
central point. But if the universe stretched on forever, gravitational forces
would on average be balanced in all directions.
-
- But Olbers realized this model of the cosmos
was inconsistent with observations. In a limitless universe filled with an
infinite number of stars, wherever we look at night our gaze should land on the
surface of a star, in much the same way as every line of sight in a forest ends
at a tree.
-
- This is the problem Olbers raised in his
paper of May 7, 1823: the cosmological model of the time suggested every point
in the sky should be as bright as the surface of the sun. There should be no
“night”.
-
- Olbers proposed a solution: the light from
more distant stars was absorbed by dust or other material floating in space.
The English astronomer John Herschel later pointed out this couldn't be right,
because anything absorbing that much light would eventually heat up enough to
glow.
-
- When Olbers died on March 2, 1840, at the
age of 81, the riddle we know today as Olber's paradox was unsolved.
-
- Eight years later, on the other side of the
Atlantic Ocean, poet and writer Edgar Allan Poe thought he had found an answer.
On February 3, 1848, he gave a public lecture about his ideas to 60 people at
the New York Society Library.
-
- Veering between metaphysics and science,
Poe argued the cosmos had emerged from a single state of matter
("Oneness") that fragmented and dispersed under the action of a
repulsive force.
-
- This meant the universe was a finite sphere
of matter. If the finite universe is populated by a sufficiently small number
of stars, then we won't see one in every direction we look. The night can be
dark again.
-
- Even if we assume the universe is infinite,
if it began at some point in the past then the time taken by light to reach us
would limit the size of the amount of the universe we can see. This travel time
would create a horizon beyond which distant stars would remain inaccessible.
-
- Poe's audience at the New York Society
Library did not give him the rapturous reception he had hoped for. Later the same
year, he published his theories in the prose poem Eureka, which was little
circulated.
-
- The following year, on October 7, 1849, Poe
died at the age of 40. It would be more than a century before scientists
confirmed his intuitions about the enigma of the dark night sky.
-
- In the first half of the 20th century many
new theories of the cosmos were developed, spurred on by Einstein's theory of
general relativity, which explained gravity, space and time in new ways. In the
second half of the century, these cosmological theories began to be tested with
observations.
-
- In 1963, British astronomer Peter Scheuer
argued that cosmology was based on only "two and a half facts": (1) the night sky is dark, which had been
known for some time. (2) galaxies are
moving away from each other, as shown by Hubble's observations published in
1929. (2.5) the content of the universe
is probably evolving as cosmic time unfolds.
-
- Strong controversies on the interpretation
of facts 2 and 2.5 agitated the scientific community in the 1950s and 1960s.
Was the universe essentially stationary, or had it begun in an enormous
explosion, “a Big Bang”?
-
- British cosmologist Edward Harrison
resolved the conflict in 1964. He showed that the main factor determining the
brightness of the night sky is actually the finite age of the stars. The number of stars in the observable
universe is extremely large, but it is finite. This limited number, each
burning for a limited time, spread over a gigantic volume, lets darkness
manifest itself between the stars.
-
- Harrison later realized this solution had
already been proposed not only by Edgar Allan Poe, but by British physicist
Lord Kelvin in 1901. Observations in the
1980s confirmed the resolution proposed by Poe, Kelvin and Harrison. Olber's
paradox had finally been put to rest.
-
- Or perhaps not quite. Viewed from a
different angle, there is another resolution to the paradox: the night sky is
not actually so dark after all.
-
- After the discovery of the expansion of the
universe in the late 1920s, scientists realized the universe could have started
off extremely compact, dense and hot. This is the "hot Big Bang"
model we have today.
-
- One core prediction of this model is the
existence of "fossil light" released in the cosmic dawn. This fossil
light should be observable today, but, not with the naked eye, as the expanding
universe would have shifted it to longer wavelengths.
-
- This radiation, called the cosmic microwave
background, was detected in 1964. Now measured with exquisite accuracy, the
cosmic background radiation is the most common light in the universe.
-
- We now know the cosmos is also illuminated
by a second, much fainter background light, produced by galaxies as they form
and evolve. This light is referred to as the cosmic ultraviolet, optical and
infrared background.
-
- So we can also answer Olber's paradox by
saying the sky is not dark, but faintly glimmers with the dim relic radiation
of all that has been over the finite lifetime of the universe.
-
- In 2023, Olber's paradox has evolved into a
rich field of research. We carry out
ever-more precise measurements of the brightness of the night sky, and simulate
the stars of the cosmos with supercomputers. We can now determine the number of
stars in the sky with great accuracy.
-
- Nevertheless, puzzles remain. Last year the
New Horizons space probe, out beyond the orbit of Pluto and away from the dust
of the inner Solar System, found the sky is twice as bright as we expected it
to be. And , so the question of the
darkness of the sky lives on, crossing ages and cultures.
-
-
June 20, 2023
DARK SKIES -
why is it not filled with starlight? 4061
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