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  • We are stardust, or so they say.

  • Long ago, giant exploding stars spread clouds of cosmic dust.

  • Some of the dust coalesced into our solar system.

  • Over time, on this planet, Nature put it to use in an increasingly complex experiment

  • called life.

  • We've been using components of the dust cloud - iron, aluminum, and silicon -

  • to look back out into space.

  • What we've learned has only deepened the mystery of our place in the universe.

  • Scientists and philosophers alike want to know: Is life a fluke,

  • a lucky ro ll of cosmic dice?

  • Or is the universe somehow fine-tuned to allow life to arise, and flourish

  • throughout the cosmos?

  • We are living in an age of precision cosmology. With new generations of giant telescopes on

  • land, and specialized instruments in space,

  • astronomers are probing the forces that shaped our universe.

  • They are documenting its behavior on the largest of scales

  • going back to the first micro-moments of its existence.

  • They are charting its evolution hundreds of billions of years into the future.

  • For all we have learned about the universe as a whole, profound questions remain:

  • what does it all add up to? How do you explain this rise of complex life, intelligent life?

  • Some scientists point to the finely tuned interactions of gravity, electromagnetism,

  • and the strong and weak nuclear forces within the atom.

  • If the strength of any these forces had varied only slightly, the universe might after

  • have flown apart and dissipated moments it was born.

  • Or gravity might have reined it in, forming one giant black hole.

  • In either case, the universe would have no galaxies, stars, planets, or life.

  • Was the basic layout of physical laws a coincidence? Or was it somehow meant to be?

  • One answer, from what's often known as the Anthropic principle, holds that the universe

  • is uniquely suited to spawn life.

  • To some, this implies that the universe is imbued with purpose. We, who are able to observe

  • the universe and document its workings, are proof that this is so.

  • But that conclusion, which can't be falsified, is more of a religious or philosophical concept

  • than a scientific one.

  • Consider the broad arc of cosmic history. Trillions upon trillions of years from now,

  • all the stars in the universe will slowly burn out. Much of the matter we see in galaxies

  • will gradually drift in toward their centers and be swallowed by black holes.

  • All will go dark, eventually fading into the dark reaches of infinity.

  • Overall, the evolution of life, and intelligent life, is a minor event, probably confined

  • to a relatively narrow window of time.

  • There's another way to ask the question. Are the processes that give rise to life central

  • to the way the universe works?

  • In search of answers, we'll explore the origin of two of life's most important components:

  • water and dust.

  • Our story begins in the earliest moments of time, when the universe was awash

  • in hydrogen gas.

  • Gravity drew it into denser and denser regions, where the earliest stars were born.

  • Generation after generation of stars formed and died.

  • In the larger ones, fusion produced the elements carbon and oxygen.

  • They became, by mass, the third and fourth most abundant elements in the universe.

  • Heavier elements followed, including those created in the violent death of large stars.

  • This animation depicts a supernova spotted by Chinese astronomers in the year 1054 AD.

  • The star was utterly destroyed, except for a spinning ultra-dense core the size of a

  • city, a neutron star, and an expanding cloud of gas and dust.

  • Astronomers have been poring over the Crab nebula, still growing at a rate of a thousand

  • kilometers a second.

  • What they found is that the filaments of matter that roared out of the blast contain large

  • volumes of dust, an array of mostly carbon or silicate compounds that absorb visible

  • light.

  • These solid particles are crucial for the formation of solar systems. Within the Crab

  • nebula, there is enough dust to make some 30 to 40,000 Earths.

  • Over time, and after countless stellar explosions, dust has collected in dense pockets throughout

  • our galaxy.

  • But dust grains can be found throughout intergalactic space as well.

  • Some might have come from the earliest supernovas.

  • There is another source as well.

  • Since the 1960s, astronomers have been studying bright beacons of light that shine from the

  • centers of distant galaxies.

  • A quasar's power source, they've found, is a black hole that has grown to millions, even

  • billions, of times the mass of our own sun.

  • The thinking is that magnetic fields leap off a disk of matter that's spiraling into

  • the black hole, drawn by its extreme gravity. These fields channel a portion of the inflowing

  • matter out into powerful particle beams.

  • The jet is part of a larger rush of matter away from the black hole.

  • You can see it in a large spiral galaxy called NGC 3783, 30 million light years from earth.

  • Astronomers used the Very Large Telescope array in Chile's Atacama Desert to peer into

  • the core of this galaxy to study the environment of a supermassive black hole.

  • From a disk of matter flowing into the black hole, intense radiation had created a dusty

  • wind that is moving up and away from the black hole.

  • The source of the dust is likely generations of giant stars that lived and died in the

  • galaxy's central region.

  • Black hole winds are now thought to have had a major impact on the universe at large.

  • You can see it in a simulation of early cosmic evolution.

  • It starts 12 million years after time zero, and evolves within a volume 350 million light

  • years across.

  • Not everything is known, including the nature of a dominant substance known as dark matter.

  • Gravity draws matter into filaments, where it heats up, forming a vast luminous web.

  • In time, tens of thousands of galaxies take shape.

  • The action heats up where filaments meet.

  • Giant bubbles of hot gas form and expand outward, pushing well beyond the central galaxies.

  • This happens when galaxies merge, sending streams of matter into black holes that lie

  • in their centers. These monsters, in turn, blast inflowing matter out in jets and winds.

  • This seeds the wider universe with dust and gas.

  • In our galaxy, dust has collected in star-forming regions, like the Orion Nebula, 1500 light

  • years from Earth.

  • At its heart is a star cluster called the Trapezium, a brilliant formation first discovered

  • by Galileo. It pumps out a wind of ultraviolet radiation that clears the surrounding region

  • and makes the gas glow.

  • In its midst, perhaps a thousand hot young stars are being born. Some are blasting out

  • jets of radiation that carve out ghostly red shapes in their surroundings.

  • Around young stars like these, enshrouded in dust, nature sets the stage for the alchemy

  • of life.

  • Within these solar nebulae, gravity can build an array of bodies, from rocks to asteroids

  • and planets.

  • But the deck is stacked against it. As small objects merge into larger ones, they can collide

  • with one another at high speed, and smash into smaller pieces.

  • If one does grow larger, friction with gas and dust can slow it down and send it spiraling

  • toward the star.

  • It takes a special set of circumstances to get the process going.

  • Astronomers saw it with a millimeter wave telescope called Alma. They used it to peer

  • into the light of one newly formed star 400 light years from earth.

  • They found an odd feature within the ring of dust surrounding the star.

  • It's a large vortex perhaps created by a planet or a companion star circulating through the

  • clouds.

  • The region acts like a trap, holding the dust grains in place and allowing them to form

  • larger objects.

  • If planets can form, their dusty environment may play a more direct roll in spawning life.

  • Some 40,000 tons of dust and rocks rain down onto the Earth each year, leftovers from the

  • birth of the solar system four to five billion years ago.

  • Interplanetary dust has long been known to carry organic, or carbon, compounds able to

  • survive the journey through Earth's atmosphere.

  • Scientists recently found something else, on the dust particles themselves.

  • Hydrogen ions, or protons, from the solar wind strike these particles.

  • When these protons interact with oxygen in silicate mineral grains, they form H20 molecules,

  • which then cling to the particles.

  • This process was documented in lunar dust grains picked up by Apollo astronauts. It

  • may be the source of water ice that has collected in the bottom of some lunar craters, and is

  • strewn about lunar landscapes.

  • In addition to watery dust raining down onto the earth, this may explain how comets and

  • asteroids were able to generate stores of water, and to bring them down to Earth.

  • This process happens one grain at a time, as solar particles race through clouds of

  • dust or strafe the surface of solid bodies.

  • It takes place in regions where stars are being born, where dust is prevalent and solar

  • winds are fierce.

  • That's only one source of water.

  • Out in the constellation of Perseus, about 750 light years from Earth, astronomers have

  • spotted a proto star called L1448-MM. It is shooting matter out of its poles in high-speed

  • jets containing oxygen and hydrogen.

  • When these molecules reach a cooler environment, they combined to form water, at a rate of

  • one hundred million times the flow of the Amazon River.

  • To get an idea of how much water the universe makes, check out a quasar known as APM08279+5255.

  • At 12 billion light years away, it's one of the most luminous objects known in our universe.

  • Astronomers were already amazed to find that it's spewing out large amounts of iron.

  • Now they have found that its center is literally saturated in water, about 140 trillion times

  • the amount of water found on Earth.

  • Given that water is created and spread so readily, it�s not surprising that it pervades

  • our own solar system. At its edges, Pluto and its moon Charon are lined with a thick

  • layer of frozen methane and water ice.

  • The blue planets Neptune and Uranus get their color from the methane clouds that make up

  • their frigid atmospheres.

  • Within, charged particles circulate in a sea of liquid water, producing electrical currents

  • and magnetic fields.

  • Within the folds of Saturn's rings are countless particles of ice.

  • Hovering just above them is the moon, Enceladus, with an icy surface that makes it one of the

  • brightest objects in the entire solar system.

  • The Cassini spacecraft detected jets of water ice shooting out of its south pole. Scientists

  • believe they are coming from an interior ocean, launched by the squeezing action of Saturn's

  • gravity.

  • The largest planet, Jupiter, harbors its own storehouse of water. The storms that drift

  • along its surface are the result of water vapor rising and falling like thunderstorms.

  • Beyond Jupiter's roiling atmosphere, lies a water world: Jupiter's second moon, Europa.

  • Its bright, smooth surface is crisscrossed by channels and grooves carved out by shifts

  • in an ocean of liquid water or slushy ice that lies below.

  • The faint emissions of water jets, illustrated here, were recently picked up by the Hubble

  • Space Telescope.

  • On our own moon, water ice is tucked into polar craters. There may well be additional

  • stores in rocks below the surface.

  • Mars, its surface etched by ancient dry riverbeds. If there's any water left, it too is most

  • likely hidden below the surface.

  • Then there's Venus. A thick carbon dioxide atmosphere has trapped enough solar energy

  • to turn it into the hottest planet in the solar system. There's no water here.

  • Earth, in contrast, is a water planet.

  • Vast interconnected bodies of water, the oceans, cover 71% of its surface.

  • Now scientists have detected whole oceans of water hidden in rocks deep inside the planet.

  • The story of dust and water is central to understanding how life arose on this planet.

  • As far as we know, these two components can be found throughout the galaxy.

  • One recent estimate, based on data from the planet-finding Kepler Space Telescope, says

  • our galaxy has around 8.8 billion stars with planets nearly the size of Earth, with surface

  • temperatures conducive to the development of life.

  • This means that in our galaxy, Nature has had as many as 40 billion chances to get life

  • started and evolve it from there.

  • But with Earth as our only data point, we know that this process must survive a host

  • of crushing threats.

  • Planet shattering impacts.

  • Massive volcanic eruptions.

  • Global ice ages.

  • So is the presence of life in our universe a fact guided more by destiny, or more by

  • chance?

  • We'll scan the heavens for evidence, a sign, or new ways to ask the question:

  • What does it all add up to?

  • 7

We are stardust, or so they say.

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B2 中高級 美國腔

宇宙之旅--生命。命運還是機會? (Cosmic Journeys - Life: Destiny or Chance?)

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    稲葉白兎 發佈於 2021 年 01 月 14 日
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