Science

Why is the sky dark at night? The 200-year history of a question that transformed our understanding of the Universe

Published

1 year ago

June 19, 2023

As dawn rose over the German city of Bremen on May 7 1823, Heinrich Olbers put the finishing touches to 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.

Finite light in an infinite Universe

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 realised 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.

In an infinite forest, every line of sight leads to a tree trunk. In an infinite Universe, is the same true for stars?
PXHere, CC BY

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.

A poet’s intuition

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.

Two and a half facts

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”:

fact 1: the night sky is dark, which had been known for some time
fact 2: galaxies are moving away from each other, as shown by Hubble’s observations published in 1929
fact 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? Supporters of both sides conceded, however, they needed to explain the darkness of the night sky.

The lifetime of stars

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 realised 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.

Fossil light

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 realised 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.

When seen via microwave radiation, the sky is dominated by our Milky Way galaxy. But behind it we can see the fainter glow of the cosmic microwave background.
ESA, HFI & LFI consortia, CC BY

This radiation – 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.

New answers, new questions

In 2023, Olber’s paradox has evolved into a rich field of research. In our own work, 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.

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News

Here’s how Helene and other storms dumped a whopping 40 trillion gallons of rain on the South

Published

1 month ago

September 30, 2024

Anne Joseph

More than 40 trillion gallons of rain drenched the Southeast United States in the last week from Hurricane Helene and a run-of-the-mill rainstorm that sloshed in ahead of it — an unheard of amount of water that has stunned experts.

That’s enough to fill the Dallas Cowboys’ stadium 51,000 times, or Lake Tahoe just once. If it was concentrated just on the state of North Carolina that much water would be 3.5 feet deep (more than 1 meter). It’s enough to fill more than 60 million Olympic-size swimming pools.

“That’s an astronomical amount of precipitation,” said Ed Clark, head of the National Oceanic and Atmospheric Administration’s National Water Center in Tuscaloosa, Alabama. “I have not seen something in my 25 years of working at the weather service that is this geographically large of an extent and the sheer volume of water that fell from the sky.”

The flood damage from the rain is apocalyptic, meteorologists said. More than 100 people are dead, according to officials.

Private meteorologist Ryan Maue, a former NOAA chief scientist, calculated the amount of rain, using precipitation measurements made in 2.5-mile-by-2.5 mile grids as measured by satellites and ground observations. He came up with 40 trillion gallons through Sunday for the eastern United States, with 20 trillion gallons of that hitting just Georgia, Tennessee, the Carolinas and Florida from Hurricane Helene.

Clark did the calculations independently and said the 40 trillion gallon figure (151 trillion liters) is about right and, if anything, conservative. Maue said maybe 1 to 2 trillion more gallons of rain had fallen, much if it in Virginia, since his calculations.

Clark, who spends much of his work on issues of shrinking western water supplies, said to put the amount of rain in perspective, it’s more than twice the combined amount of water stored by two key Colorado River basin reservoirs: Lake Powell and Lake Mead.

Several meteorologists said this was a combination of two, maybe three storm systems. Before Helene struck, rain had fallen heavily for days because a low pressure system had “cut off” from the jet stream — which moves weather systems along west to east — and stalled over the Southeast. That funneled plenty of warm water from the Gulf of Mexico. And a storm that fell just short of named status parked along North Carolina’s Atlantic coast, dumping as much as 20 inches of rain, said North Carolina state climatologist Kathie Dello.

Then add Helene, one of the largest storms in the last couple decades and one that held plenty of rain because it was young and moved fast before it hit the Appalachians, said University of Albany hurricane expert Kristen Corbosiero.

“It was not just a perfect storm, but it was a combination of multiple storms that that led to the enormous amount of rain,” Maue said. “That collected at high elevation, we’re talking 3,000 to 6000 feet. And when you drop trillions of gallons on a mountain, that has to go down.”

The fact that these storms hit the mountains made everything worse, and not just because of runoff. The interaction between the mountains and the storm systems wrings more moisture out of the air, Clark, Maue and Corbosiero said.

North Carolina weather officials said their top measurement total was 31.33 inches in the tiny town of Busick. Mount Mitchell also got more than 2 feet of rainfall.

Before 2017’s Hurricane Harvey, “I said to our colleagues, you know, I never thought in my career that we would measure rainfall in feet,” Clark said. “And after Harvey, Florence, the more isolated events in eastern Kentucky, portions of South Dakota. We’re seeing events year in and year out where we are measuring rainfall in feet.”

Storms are getting wetter as the climate change s, said Corbosiero and Dello. A basic law of physics says the air holds nearly 4% more moisture for every degree Fahrenheit warmer (7% for every degree Celsius) and the world has warmed more than 2 degrees (1.2 degrees Celsius) since pre-industrial times.

Corbosiero said meteorologists are vigorously debating how much of Helene is due to worsening climate change and how much is random.

For Dello, the “fingerprints of climate change” were clear.

“We’ve seen tropical storm impacts in western North Carolina. But these storms are wetter and these storms are warmer. And there would have been a time when a tropical storm would have been heading toward North Carolina and would have caused some rain and some damage, but not apocalyptic destruction. ”

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Follow AP’s climate coverage at https://apnews.com/hub/climate

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Follow Seth Borenstein on Twitter at @borenbears

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Associated Press climate and environmental coverage receives support from several private foundations. See more about AP’s climate initiative here. The AP is solely responsible for all content.

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Science

‘Big Sam’: Paleontologists unearth giant skull of Pachyrhinosaurus in Alberta

Published

2 months ago

September 25, 2024

Anne Joseph

It’s a dinosaur that roamed Alberta’s badlands more than 70 million years ago, sporting a big, bumpy, bony head the size of a baby elephant.

On Wednesday, paleontologists near Grande Prairie pulled its 272-kilogram skull from the ground.

They call it “Big Sam.”

The adult Pachyrhinosaurus is the second plant-eating dinosaur to be unearthed from a dense bonebed belonging to a herd that died together on the edge of a valley that now sits 450 kilometres northwest of Edmonton.

It didn’t die alone.

“We have hundreds of juvenile bones in the bonebed, so we know that there are many babies and some adults among all of the big adults,” Emily Bamforth, a paleontologist with the nearby Philip J. Currie Dinosaur Museum, said in an interview on the way to the dig site.

She described the horned Pachyrhinosaurus as “the smaller, older cousin of the triceratops.”

“This species of dinosaur is endemic to the Grand Prairie area, so it’s found here and nowhere else in the world. They are … kind of about the size of an Indian elephant and a rhino,” she added.

The head alone, she said, is about the size of a baby elephant.

The discovery was a long time coming.

The bonebed was first discovered by a high school teacher out for a walk about 50 years ago. It took the teacher a decade to get anyone from southern Alberta to come to take a look.

“At the time, sort of in the ’70s and ’80s, paleontology in northern Alberta was virtually unknown,” said Bamforth.

When paleontogists eventually got to the site, Bamforth said, they learned “it’s actually one of the densest dinosaur bonebeds in North America.”

“It contains about 100 to 300 bones per square metre,” she said.

Paleontologists have been at the site sporadically ever since, combing through bones belonging to turtles, dinosaurs and lizards. Sixteen years ago, they discovered a large skull of an approximately 30-year-old Pachyrhinosaurus, which is now at the museum.

About a year ago, they found the second adult: Big Sam.

Bamforth said both dinosaurs are believed to have been the elders in the herd.

“Their distinguishing feature is that, instead of having a horn on their nose like a triceratops, they had this big, bony bump called a boss. And they have big, bony bumps over their eyes as well,” she said.

“It makes them look a little strange. It’s the one dinosaur that if you find it, it’s the only possible thing it can be.”

The genders of the two adults are unknown.

Bamforth said the extraction was difficult because Big Sam was intertwined in a cluster of about 300 other bones.

The skull was found upside down, “as if the animal was lying on its back,” but was well preserved, she said.

She said the excavation process involved putting plaster on the skull and wooden planks around if for stability. From there, it was lifted out — very carefully — with a crane, and was to be shipped on a trolley to the museum for study.

“I have extracted skulls in the past. This is probably the biggest one I’ve ever done though,” said Bamforth.

“It’s pretty exciting.”

This report by The Canadian Press was first published Sept. 25, 2024.

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News

The ancient jar smashed by a 4-year-old is back on display at an Israeli museum after repair

Published

2 months ago

September 11, 2024

Anne Joseph

TEL AVIV, Israel (AP) — A rare Bronze-Era jar accidentally smashed by a 4-year-old visiting a museum was back on display Wednesday after restoration experts were able to carefully piece the artifact back together.

Last month, a family from northern Israel was visiting the museum when their youngest son tipped over the jar, which smashed into pieces.

Alex Geller, the boy’s father, said his son — the youngest of three — is exceptionally curious, and that the moment he heard the crash, “please let that not be my child” was the first thought that raced through his head.

The jar has been on display at the Hecht Museum in Haifa for 35 years. It was one of the only containers of its size and from that period still complete when it was discovered.

The Bronze Age jar is one of many artifacts exhibited out in the open, part of the Hecht Museum’s vision of letting visitors explore history without glass barriers, said Inbal Rivlin, the director of the museum, which is associated with Haifa University in northern Israel.

It was likely used to hold wine or oil, and dates back to between 2200 and 1500 B.C.

Rivlin and the museum decided to turn the moment, which captured international attention, into a teaching moment, inviting the Geller family back for a special visit and hands-on activity to illustrate the restoration process.

Rivlin added that the incident provided a welcome distraction from the ongoing war in Gaza. “Well, he’s just a kid. So I think that somehow it touches the heart of the people in Israel and around the world,“ said Rivlin.

Roee Shafir, a restoration expert at the museum, said the repairs would be fairly simple, as the pieces were from a single, complete jar. Archaeologists often face the more daunting task of sifting through piles of shards from multiple objects and trying to piece them together.

Experts used 3D technology, hi-resolution videos, and special glue to painstakingly reconstruct the large jar.

Less than two weeks after it broke, the jar went back on display at the museum. The gluing process left small hairline cracks, and a few pieces are missing, but the jar’s impressive size remains.

The only noticeable difference in the exhibit was a new sign reading “please don’t touch.”

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