a journey through space and time

7/31/2019 A Journey Through Space and Time

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A Journey Through Cosmic Time & Space

Credit: Illustration: Karl Tate, based on a photo of Galaxy M74 (NASA, ESA, and

Hubble Heritage Collaboration) and the engraving "Awakening of the Pilgrim" from

"The Atmosphere: Popular Meteorology" by Camille Flammarion, 1888

Our journey toward understanding the nature of our universe began thousands of

years ago and had its roots in religion and philosophy. Around 2,300 years ago,

careful observers in the Mediterranean deduced that the Earth must be round andmust orbit the sun. With no way for these early theories to be proved correct,


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however, they could not stand against the more flattering notion that the Earth

was at the center of everything and that the cosmos existed to support human life

and destiny. When Italian astronomer Galileo Galilei invented the astronomical

telescope some 1,900 years later, it was finally possible to make precise

observations about the planets and stars. A science of the structure and history of

the entire universe, called "cosmology," emerged.

Overview: Space and Time

Credit: Hubble Space Telescope Science Institute

Our current understanding of the history of the universe is visualized above, with

time running from left to right. We think that immediately after its creation at the

time of the Big Bang, the universe expanded dramatically an event calledinflation. Our Earth formed when the universe was around 9.2 billion years old. The


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expansion of the universe continues today and is accelerating. In this series of

infographics, we will first look at the structure of the universe at larger and larger

scales and find out a little about how we came to our current understanding of it.

In the second part of our sequence, we will begin with the Big Bang and move

forward in time to see how the universe has evolved to the present day.

You Are Here The Earth is round

Credit: Earth image: NASA; Eratosthenes portrait: unknown artist

Our first stop is the planet we call home. The knowledge that the Earth is shaped

like a ball is actually quite old. About 2,500 years ago, Greek travelers reported

that different constellations were visible in the sky when one went far to the north

or south. Keen observers also would have noticed that during an eclipse of themoon, the shadow cast by the Earth has a round edge. A few centuries later, the


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scholar Eratosthenes estimated the size of the Earth by noting the difference

between the lengths of shadows cast by the sun in locations a few hundred miles

apart. By assuming that the sun was so far away that its rays of light were parallel,

Eratosthenes could use simple geometry to calculate the circumference of the

Earth. It is not known how accurate his measurement was, but it may have been

off from the true figure by no more than a few percentage points.

Scale 2: Inner Solar System Earth is a planet

Credit: Karl Tate, SPACE.com; Johannes Kepler portrait: unknown artist


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Now we pull back to see the Earth in the context of the inner solar system. Early

ideas about the movements of the sun, Earth and planets were derived from

theological, astrological and philosophical notions of how God must have ordered

the world. Polish astronomer Nicolaus Copernicus caused an uproar in the mid-

1500s by suggesting the Earth moved around the sun and not, as leaders of

Christianity taught, the sun around the Earth. For centuries the planets were

thought to move because they were embedded in nested "crystal spheres" that

rotated around a central point. However, it was noted in the 16th century that

comets moved in such a way that would crash them through those crystal spheres.

Replacing the spheres was the idea of "epicycles," circles superimposed on circles,

mathematically influencing each other to result in the observed planetary motions.

Finally, in 1609, German mathematician Johannes Kepler published his theories of

planetary motion, which established that bodies in our solar system move in orbits

shaped like ovals rather than circles.

Scale 3: Solar System The planets are worlds


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Credit: Karl Tate, SPACE.com; Galileo Galilei: portrait by Ottavio Leoni

From the earliest eras of human pre-history, the entire universe was thought to

encompass only the elements visible to the naked eye: Earth, its moon and sun,

five points of light that moved and were called "planets," plus a distant sphere

upon which the stars and the glowing band of the Milky Way were embedded.Theories of astrology and, later, astronomy were devised to explain the

movements of these celestial objects, but their true nature could only be guessed

at. When in 1609 the Italian astronomer Galileo finally trained a crude telescope

on the heavens, he discovered that the planets were other worlds. Several of these

worlds were found to have moons of their own. With the aid of the telescope,

previously unknown planets were discovered in our solar system: Uranus in 1781

and Neptune in 1846. With the telescope it became possible to study smaller

bodies such as comets and asteroids, and also the stars and nebulas on the distant

celestial sphere.


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Scale 4: Nearest Stars The stars are suns

Credit: Diagram of local stars: Karl Tate, based on public domain data plot;

Friedrich Bessel portrait: Christian Albrecht Jensen

In the 17th century, the invention of the telescope by Galileo and the discovery of

the laws of motion by Kepler prompted the realization that stars were just like the

sun, all obeying the same laws of physics. In the 19th century, spectroscopy the

study of the wavelengths of light that are emitted by objects made it possible to

investigate the gases that stars are made of.

Scientists also figured out in the 19th century how to measure the distances to

stars. When an object is viewed from different vantage points, the object appears

to shift relative to the more distant background. The shift is called "parallax." As

the Earth orbits the sun, it provides a changing vantage point for observing the

stars. Since the stars are so much more distant than objects in our own solar

system, the parallax shift is very small and hard to measure. The Germanmathematician and astronomer Friedrich Bessel was the first to successfully

measure the parallax of the star 61 Cygni and estimated its distance from Earth to

be 10.4 light-years. (Later estimates adjusted this distance to 11.4 light-years.)


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Scale 5: Our Arm of the Galaxy The sun orbits in a galaxy held together

by dark matter

Credit: Milky Way Galaxy map: Robert Hurt; Fritz Zwicky photo via University of

Virginia Dept. of Astronomy

The layout of our galaxy is difficult to figure out from our vantage point, which is

embedded in it. By studying the shapes of distant galaxies and carefully

measuring the objects that we see in our own galaxy, we have inferred that ours is

a barred-spiral galaxy. A central bar-shaped core composed of stars (and harboring

an extremely large black hole) is surrounded by spiraling arms, also formed of

stars as well as gas and dust. We are located in a spur, or branch, that stretches

between major spiral arms. The exact configuration of spiral arms is still debated

by astronomers, but a recent survey found that our Milky Way galaxy has two

major arms, which branch out into four arms toward the outside.


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The spiral arms of our galaxy are thought to be a kind of density wave that travels

around the flat disk. Material bunches up, and stars are formed along the arms.

Everything in the galaxy orbits around its center, and the arms are not solid

structures. Our solar system travels into and out of the spiral arms as it orbits.

While studying the rotation of galaxies, it was noted that they do not rotate as wewould expect them to based on the gravitational pull of the matter we can see.

Swiss astronomer Fritz Zwicky suggested in 1934 that there must be a large

amount of invisible, or "dark," matter present, making spiral galaxies more

massive than they appear. Since that time astrophysicists have searched for this

dark matter, often speculating that it might consist of exotic particles unlike

anything we know on Earth. Current estimates show that our universe is mostly

composed of unknown forms of dark matter and dark energy, with familiar atoms

being only a tiny fraction of the total.


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Scale 6: Milky Way Galaxy Galaxies are made of stars

Credit: Milky Way Galaxy map: Robert Hurt; Edwin Hubble photo via NASA

The Milky Way, a faint ribbon of light that spans the sky, has been known

throughout history. Its true nature was not discovered until the 17th century, when

Galileo Galilei studied the Milky Way with a telescope and determined that the

ribbon was composed of a multitude of stars. Small fuzzy patches of light can be

seen in the sky; these were called nebulae. By the 18th century it was speculated

that the Milky Way was a huge system of stars bound together by gravity, but the

nature of the nebulae remained unknown. They could have been small clouds of

gas within the Milky Way, or perhaps they were external to our galaxy. It could not

be proved whether or not the Milky Way constituted the entire universe.

Using the newly constructed 100-inch telescope at Mount WIlson Observatory in

California, American astronomer Edwin Hubble studied stars called Cepheids,

which brighten and dim in a pattern related to their intrinsic brightness, makingthem suitable for use as a yardstick in estimating cosmic distances. In a 1925

paper, Hubble concluded that some of the nebulae were external to the Milky Way,

and were giant galaxies in their own right, revealing a universe much larger than

our own home galaxy


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Scale 7: Local Supercluster of Galaxies Massive organization

Credit: Diagram: Karl Tate based on NASA illustration; Brent Tully photo via

Institute for Astronomy, University of Hawaii

It was first noticed in the latter half of the 19th century that there is a large groupof nebulae in the constellation Virgo. Later it was discovered that these nebulae

are separate galaxies external to our Milky Way. One hundred years later,

astronomers speculated that the apparent alignment of these galaxies might

indicate a higher level of cosmic structure, variously dubbed a "metagalaxy" or

"supercluster." In 1982 astronomer R. Brent Tully published an analysis of the

distances to the supercluster member galaxies, showing that they were indeed

part of a larger organization. The distances were determined by noting the redshift

of the spectra of light from the galaxies. (See "TIME ZERO: THE BIG BANG" for a

fuller explanation of redshift.)


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Scale 8: Walls, Filaments and Voids The largest structures in space

Credit: Credit: 2dF Galaxy Redshift Survey, Anglo-Australian Observatory; Margaret

Geller photo via Harvard University Dept. of Astronomy

The largest structures that we know of are the galactic filaments also called

supercluster complexes that surround vast voids in space. The galaxies in a

filament are bound together by gravity. When the first of these structures was

discovered by Margaret Geller and John Huchra in 1989, it was dubbed "the Great

Wall." A much larger structure, the "Sloan Great Wall," was discovered in 2003 by J.

Richard Gott III and Mario Juri?.

Current research into the large-scale structure of the universe utilizes data

gathered by redshift surveys such as the Sloan Digital Sky Survey. These efforts


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use digital camera sensors to photograph regions of the sky, capturing millions of

distant objects along with the data needed to map them in 3-D space.

Scale 9: The Observable Universe The farthest we can see

Credit: Simulation of observable universe: Karl Tate, SPACE.com; Alan Guth photovia Brookhaven National Laboratory

The observable universe is everything that we can detect. It is a sphere 93 billion

light-years in diameter, centered on Earth. We cannot perceive the entire universe

at once, due to the slowness of the speed of light compared with the vast scale of

the universe. As we look out into space, we see objects as they were at earlier and

earlier times in history. Also, because of the accelerating expansion of the

universe, distant objects are much farther away than their age would have us

think. For example, the edge of the observable universe is estimated to be about

46 billion light-years away, even though the universe itself is only 13.7 billionyears old.


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The true extent of the universe is unknown. It could be much bigger than the

observable universe perhaps even infinite in size. However, light from the most-

distant regions would never be able to reach us; the space it must pass through is

simply expanding too fast.

Our current picture of the observable universe owes a lot to American physicistAlan Guth, who in the 1980s worked out how a universe resembling our own might

have emerged from the Big Bang event which created it. Next, we will reset the

clock to time zero and see how the universe evolved from its beginning to today.

Time Zero: The Big Bang 13,750,000,000 years ago

Credit: Karl Tate, SPACE.com

In the early 20th century, Belgian astronomer and Catholic priest Georges

Lemaitre calculated that the universe is expanding. By mathematically running the

expansion backward, he theorized that everything in the universe once must havebeen compacted into a small, dense object, which he called "the primeval atom."


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This atom exploded, an event that astronomer Fred Hoyle flippantly called "The Big

Bang." The expansion of the universe explains why the light from distant objects is

shifted toward the red end of the spectrum, a phenomenon called "redshift." Just

as the Doppler effect causes sound from moving vehicles to change pitch, redshift

causes light from moving stars to change color as its wavelength gets stretched by

expanding space. The farther an object is from Earth, the more the intervening

space has expanded, and the more the object's light will have been shifted toward

red.

American astronomer Edwin Hubble later proved with observations that redshift

was indeed related to distance, and the correlation is now known as Hubble's law.

Time 1: Inflation Earliest fraction of a second following the Big Bang

Credit: Map of Cosmic Microwave Background temperature fluctuations from

Wilkinson Microwave Anisotropy Probe (WMAP) data; Alan Guth photo via

Brookhaven National Laboratory


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Astronomers in the 1970s had a problem understanding the early universe. When

they probed deep space with radio telescopes, they discovered a faint background

glow of microwave radiation. Variations in the density of the microwave signal

were interpreted as variations in the density of matter in the early universe.

Surprisingly, the background glow of radiation was found to be uniform in every

direction. This seemed unreasonable; scientists expected to find regions of space

with different densities and temperatures, because these regions seemed too far

apart to have evolved together

American physicist Alan Guth proposed an explanation in 1980. He theorized that

in the tiny fraction of time just following the Big Bang, the universe underwent

extremely rapid expansion. In a flash, its volume increased by a factor of 10^78

(the number 10 followed by 78 zeroes). Almost immediately the universe cooled

slightly and the event, called "inflation," was over. The inflationary model explains

why the universe appears uniform in all directions: Everything in it evolved

together before inflation. It has other staggering implications, too: The part ofspace that we can see must be just a tiny patch in what must be a vast universe

that we can never directly detect.

Quark-gluon Plasma 0.001 second to 3 minutes after the Big Bang


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Credit: Graphic: Karl Tate based on image of data plot from collision of gold ions,

Brookhaven National Laboratory Relativistic Heavy Ion Collider

Following inflation, the cooling but still unimaginably hot universe experienced a

phase transition. Elementary particles were created from a form of matter called

quark-gluon plasma. A thousandth of a second following the Big Bang, vastamounts of matter and antimatter annihilated each other (leaving behind the

material that exists in the universe today). Within three minutes the temperature

of the universe dropped to about a billion degrees, and atoms could begin to form,

starting with the simplest elements: hydrogen and helium.

The quark-gluon plasma of the early universe is still theoretical and is thought to

be possible because of a theory called Quantum Chromodynamics. American

physicist Murray Gell-Mann was among the first to formulate this theory. The basic

nuclear particles protons and neutrons are thought to be made from still more-

fundamental particles called "quarks," which are never found traveling alone

except under very high temperatures like those that existed just after the Big

Bang. Physicists are trying to re-create on Earth the plasma that is thought to have

comprised the early universe; they are using particle accelerators to smash

subatomic particles together at high energy.


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Time 3: Dark Age 3 minutes to 379,000 years after the Big Bang

Credit: NASA, ESA

During this period, the early universe was hot and opaque. Starting at about

379,000 years after the Big Bang, the universe cooled enough so that light could

separate from matter and travel freely. In short, the universe became transparent.

Photo shows galaxy UDFy-38135539, one of the oldest and earliest galaxies yet

found, appearing just after the Dark Age at about 480 million years after the Big

Bang.


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Time 4: Violent Birth 150 million to 1 billion years after the Big Bang

Credit: Artist's conception of a quasar: NASA/ESA; Maarten Schmidt photo:

California Institute of Technology

In the 1960s Dutch astronomer Maarten Schmidt identified strange deep-space

objects, very bright in radio wavelengths, which he termed "quasi-stellar radio

sources." U.S. astrophysicist Hong-Yee Chiu named the phenomena "quasars."

Quasars had been picked up in the 1950s by large Earth-bound antennas called

radio telescopes. When Schmidt measured the quasars' distance by studying the

redshift of their spectrum, what he found was astonishing. The objects were

billions of light-years away, and therefore had to be incredibly bright to be

detected on Earth. Later study showed that the mysterious quasars were active

galaxies that had formed very early in the history of the universe. Gravitational

collapse had caused matter to coalesce, eventually forming giant black holes with

the mass of billions of suns. A black hole sits at the center of a quasar, collecting

matter and heating it to become high-temperature plasma that can be shot out

into huge jets traveling close to the speed of light.


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Time 5: The Solar System Forms 9 billion years after the Big Bang

Credit: Artist's conception of a young solar system: NASA/JPL-California Institute of

Technology; Albert Einstein photo via United States Library of Congress

The earliest stars formed when the universe was only 300 million years old. Theywere short-lived and supermassive, composed mostly of hydrogen and helium and

containing no metals. These first stars exploded into supernovas, and successive

generations were created from the remains of the earlier suns. Analysis of the

spectrum of the light from our sun shows that it is rich in metals, and therefore

could have been created only following many generations of stars.

The sun's power source was a mystery until German physicist Albert Einstein

worked out in 1905 that matter could be converted into energy, with his famous

equation E=mc^2. In 1920 British astrophysicist Sir Arthur Eddington suggested

that the sun might be powered by a nuclear fusion reactor, generating heat and

light energy by converting hydrogen into helium. Study of the spectrum of light


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from the sun and other stars led to a confirmation that nuclear fusion processes

created the atomic elements from which our world is composed.

Time: Now

Credit: NASA

Scientists have put together an impressive picture of the origin, history and nature

of our universe. However, we do not know everything there is to know. Many open

questions remain in the fields of physics and cosmology. For example:

What is dark matter, and does it actually exist?

Why does the universe's expansion seem to be accelerating?

What is the actual shape and size of the universe, and how many dimensions does

it have?


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What is the ultimate fate of the universe?

a journey through space and time

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