ORIGIN AND EVOLUTION OF THE UNIVERSE AND SOLAR SYSTEM

INTRODUCTION

     The most accepted theory for the origin of the Universe is the Big Bang Theory, which basically states that about 13 to 15 billion years ago (astronomers now estimate  an age of 13.7 billion years for the Universe) an infinitely dense point of pure energy "exploded" and expanded outward to eventually evolve into our present Universe.  The Universe was initially extremely hot (about 100 billion k), but as expansion continued the Universe cooled, matter formed and clumped together by gravity, and stars, planets, and galaxies formed.

     Our Solar System started forming about 5 billion years ago.  According to the Solar Nebula Theory, our Solar System formed as the result of the contraction of a diffuse, gaseous nebular cloud (consisting primarily of hydrogen and helium, but with some dust containing heavier elements).  As the nebular cloud contracted (perhaps initiated by a nearby supernova explosion of a star) it flattened into a disk shaped structure with 90% of the material going to the center (by gravity) to eventual form the sun.  Eddy currents in the disk attracted matter that would eventually condense and coalesce to form the planets.  Our Solar System was pretty much as we see it today by about 4.6 billion years ago.
 

THE COSMIC BACKGROUND RADIATION

     Arno Penzias and Robert Wilson received the Noble Prize in Physics in 1978 for their discovery of the cosmic background radiation.  In 1965, as astronomers with Bell Telephone Laboratories in New Jersey, they discovered a background radiation coming from outside our solar system that corresponded to a constant temperature of 2.7 kelvin (or 2.7 ^oC above absolute zero).  They postulated that this 2.7 kelvin temperature is the relic of the initial high temperature of the early Universe.

Wilson and Penzias with their historic horned antenna at Crawford Hill, N.J.  Wilson is on the left and Penzias on the right.  (From: Cosmology - Penzias and Wilson's Discovery is One of the Century's Key Advances at http://www.bell-labs.com/project/feature/archives/cosmology/)

     This cosmic background radiation has a maximum wavelength of about 2 mm (in the microwave range of the electromagnetic spectrum).  The radiation has all the characteristics of radiant heat and is the same kind of radiant heat as emitted by an object with a temperature of 2.7 ^oC above absolute zero.  So the initial extremely hot radiant heat from the primordial fireball (of the Big Bang) has cooled down to 2.7 k, and will eventually cool to absolute 0.  This is direct evidence of the "Big Bang".
 

THE BIG BANG THEORY

     According to the Big Bang Theory, the Universe was initially concentrated into an extremely dense region billions of times smaller than a proton.  The initial core exploded about 13 to 20 billion years ago (although most astronomers think this happened about 13 to 15 billion years ago) and sent energy (and eventually matter) expanding outward in all directions.

     There are two main lines of support for the Big Bang Theory and an expanding Universe (these are the main lines of support, there are several others):

• 1) Distant galaxies are moving away from Earth at high speed and the velocities of recession increase with distance from Earth.  Thus, the Universe is expanding.  This was first discovered by Edwin Hubble (1929).
• Edwin Hubble
• From:  Part 6: Red Shift at http://www.arachnoid.com/sky/redshift.html
• He noted that the spectral lines (wavelengths of light) of the receding galaxies are shifted toward the red end of the spectrum, thus a shift towards longer wavelengths.  We say that the light is redshifted.  This redshift of the light from receding galaxies is caused by the Doppler Effect.
• 2) The Cosmic Background Radiation permeating the Universe as discovered by Penzias and Wilson (as discussed previously).

• Doppler Effect: The apparent shift of the wavelength of waves caused by movement of the source of the waves relative to an observer.  The Doppler Effect is commonly observed by most people when moving vehicles approach and pass.  The sound waves from the moving vehicle are shifted towards shorter wavelength as the vehicle approaches an observer (the observer hears a higher pitched sound) and is shifted towards longer wavelengths as the vehicle moves away from an observer (the observer hears a lower pitched sound).  A source of light that is moving towards an observer will have an apparent shift of the wavelengths to shorter wavelengths (blueshifted), where as a source of light moving away from an observer will appear to have the wavelengths shifted to longer wavelengths ( redshifted).
• 
• From:  Doppler Effect at http://www.ncsa.uiuc.edu/Cyberia/Bima/doppler.html
• Red Shift of Stars and Galaxies:  Light coming from receding stars or galaxies appears to be more red, that is, the wavelengths are shifted towards the red end of the spectrum.  If the stars or galaxies were moving towards us, the wavelengths would be shifted towards the shorter wavelengths and the light would appear more blue.
• Hydrogen Spectra:  The wavelengths of hydrogen spectra from receding galaxies are shifted to progressively longer wavelengths for galaxies at larger and larger distances from Earth.
• The velocity of recession of galaxies from each other is proportional to the distance between them, thus farther galaxies are receding with higher velocities.
• 
• From:  Expanding Universe at http://bustard.phys.nd.edu/Phys171/lectures/expandu.html
• We can compare the expansion of the Universe to expanding raisins in a loaf of rising (and baking) raisin bread.
• Based on the idea of an expanding Universe and the rate of recession of galaxies from each other, astronomers can use the expansion rate to calculate how long ago the galaxies were together at a single point.
• The estimate works out to 13 to 20 billion years (depending on the value of the Hubble constant).  Most astronomers believe the Universe is about 13 to 15 billion years old.  Has the recessional velocity been constant with time?  What is the gravitational effect? How much matter is in the Universe?  What is "Dark Matter" and how much of it is in the Universe?

From:  Mysteries of Deep Space - History of the Universe Timeline at http://www.pbs.org/deepspace/timeline/index.html (At the site containing this graphic timeline each stage is a link to a discussion of what was happening at that time)

     Astrophysicists can theoretically go back to a fraction of a second (10^-43seconds) following the "Big Bang".  Beyond this the Universe consisted of pure energy and the four major forces of nature that we recognize were not distinct entities (they may have been unified).  Those four major forces of nature are 1) the gravitational force, 2) the electromagnetic force, 3) the strong nuclear force (holds nuclear particles together), and 4) the weak nuclear force (involved in radioactive decay).  At the birth of the Universe the temperature and density were extremely high and only pure energy could exist.
 

• Within the first second after the "Big Bang":

• The four forces of nature separated out.
• The Universe inflated rapidly.
• Quarks and electrons started forming, as did antiquarks and antielectrons (antimatter).
• Matter and antimatter collided and annihilated each other, but fortunately the Universe was asymmetrical and had more matter than antimatter.  Thus a slight excess of matter was left over, which makes up our present Universe.
• Quarks formed protons and neutrons.
• By three minutes after the "Big Bang":
• The temperature had cooled to about 1 billion kelvin.
• Protons and neutrons fuse to form hydrogen and helium nuclei.
• By about 300,000 years after the "Big Bang":
• The temperature had cooled to about 3000 kelvin.
• Electrons combined with nuclei to form complete atoms of hydrogen and helium.
• Photons separated from matter and light burst forth for the first time.
• By about 200 to 300 million years after the "Big Bang":
• Matter began collecting into clouds of various sizes that eventually collapsed due to gravity to form stars and galaxies.  In other words, the Universe became "clumpy".
• 

Birth of Stars and Galaxies - 300 million years

"Gravity amplifies slight irregularities in the density of the primordial gas. Even as the universe continues to expand rapidly, pockets of gas become more and more dense. Stars ignite within these pockets, and groups of stars become the earliest galaxies. This point is still perhaps 12 to 15 billion years before the present. The Hubble Space Telescope recently captured some of the earliest galaxies ever viewed. They appear as tiny blue dots in the Hubble Deep Field, the image on the left."
From:  Mysteries of Deep Space - History of the Universe Timeline at http://www.pbs.org/deepspace/timeline/index.html
 

• The early Universe consisted of 100% hydrogen and helium.  Today the Universe has about 98% hydrogen and helium.

     After stars began forming, heavier elements formed in the cores of stars by a process known as nucleosynthesis.  Stars are born when hydrogen in the core starts fusing into helium.  Low mass stars (about the size of our Sun or a little more massive) will start fusing helium into carbon when they use up the hydrogen in the core.  When the core has be converted to carbon, these low mass stars cannot generate enough pressure by gravitational collapse (the core will collapse because of gravity if no energy is pushing out due to fusion) to fuse carbon into higher elements.  They will collapse to the size of the Earth and become extremely hot (a white dwarf), but they will never fuse again so will eventually cool off to become a black dwarf (a burned out chunk of carbon).  High mass stars can generate enough pressure to fuse carbon into heavier elements.  However, as these stars successively fuse elements higher than carbon, a problem occurs when the core is fused into iron.  Iron absorbs the energy of gravitational collapse and will not fuse into higher elements.  This results in rapid collapse (implosion) of the star and a rapid rebound (i.e., the star explodes).   The explosion of a massive star is called a supernova.  During a supernova explosion nuclear particles are rammed together at extremely high velocities.  This results in further fusion to form all the elements heavier than iron.

The Crab Nebula (supernova remnant). From:  The Crab nebula (M1) at http://www.aao.gov.au/images.html/captions/crab.html

     As a result of the explosion of stars, hydrogen, helium, and the heavier elements are returned to space to form clouds of dust and gases (nebula) that can then collapse to form additional stars.  Our Sun is a second or third generation star that has abundant heavy elements (although it still is mostly hydrogen and helium).
 

ORIGIN AND HISTORY OF THE SOLAR SYSTEM

     Any theory to explain the origin and evolution of the solar system must explain the following observations about the solar system:

• All planets revolve around the Sun on or near the plane of the ecliptic  (exceptions: Pluto about 17 degrees off plane of ecliptic and Mercury about 7 degrees off the plane of the ecliptic).  Pluto is thought to have been a moon of Neptune, but possibly escaped the gravitational pull of Neptune and took up its own orbit around the Sun.  Pluto's composition is similar to the moons of the Jovian planets.
• All the planets (except Venus and Uranus) and most of the moons in our solar system rotate on their axis in a counterclockwise direction (as viewed from above the north pole of the Sun and looking down on the solar system).   Venus rotates slowly to the right, possibly due to a catastrophic collision early in the evolution of the solar system.  Uranus is tilted on its side with the north pole pointing towards the Sun, again possibly due to a catastrophic collision early in the evolution of the solar system.
• All the planets revolve around the Sun in a counterclockwise direction.
• All the planets (except Pluto and Uranus) have a rotational axis that is near perpendicular to the plane of the ecliptic.
• The difference in chemical composition and characteristics of the inner Terrestrial planets as compared to the outer Jovian planets.  The Terrestrial planets have high densities (about 4 to 5.5 g/cm³), no or few moons, and are primarily composed of metals and silicate rocks.  The Jovian planets have low densities (about 0.7 to 1.76 g/cm³), many moons, and are primarily composed of gases (hydrogen, helium, methane, ammonia, and water vapor) and frozen compounds rich in hydrogen.
• The slow rotation of the Sun.
• Asteroids, comets, and interplanetary debris and dust must be explained.
• The relatively slow axial rotation of the Sun must be explained.

• About 5 billion years ago a portion of a large nebular cloud of interstellar material in a spiral arm of the Milky Way Galaxy began to collapse due to gravity.  The initiation of gravitational collapse may have been due to a pressure wave from a nearby supernova explosion (which pushed matter closer together in the cloud and caused a stronger gravitational force within the cloud - remember Newton's inverse square law of gravitation).
• As the interstellar material collapsed it began to rotate in a counterclockwise direction with 90% of the material collapsing to the center of rotation.  Due to the increasing rate of spin as the material collapsed (due to conservation of angular momentum), the material flattened into a disk shaped structure (a solar nebula).
• As the solar nebula continued to concentrate material in the center and the disk, the embryonic Sun formed with a rotating, turbulent cloud of material around it.
• Gases and solids began to condense in localized turbulent eddies to form particles that then collided because of gravitational attraction to form planetesimals  (from a few meters to a few tens of meter in size).  Dust from the original interstellar material may have served as nuclei for condensation.
• Planetesimals collided with each other to accrete into protoplanets and eventually into the planets we know today.  This process to form the planets probably took about a million years or so.
• The inner (Terrestrial) planets, being near the evolving Sun (which was growing very hot due to gravitational collapse) could only form from condensation of metals and silicates (refractory elements) due to the high temperature.  So not much gas could accumulate in the inner solar system due to the high temperature.
• The outer (Jovian) planets, which formed far from the embryonic Sun, in the outer much cooler region of the solar system were formed from condensation of metals, silicates, and gases (because all could condense at the cooler temperatures).  However, because gases (primarily hydrogen and helium) were much more abundant in the original interstellar material than refractory elements, the outer planets are primarily gaseous giants, but do have rocky, metallic cores.
• Eventually the core of the embryonic Sun grew so hot and was under such high pressure that hydrogen was fused into helium.  This provided an outward pressure from the core that halted the gravitational collapse.  Thus our modern Sun came into existence as a nuclear fusion furnace.  When our Sun "turned on", solar  radiation and particles (protons, neutrons, and electrons) began streaming outward from the Sun into the solar system.  This is referred to as the solar wind.  The solar wind blew any additional gas and dust out of the inner part of the solar system (most being captured by the Jovian planets).  (Our Sun has been fusing hydrogen into helium for about 5 billion years and has enough hydrogen to continue this process for about another 5 billion years.  When the hydrogen is used up in the core, our Sun will start to die..........another story for another time)
• Planetisimals between Mars and Jupiter was affected by the gravitational fields of the two planets (particularly Jupiter) and could not accrete into a planet, thus the asteroid belt was formed.
• Icy planetesimals that formed in the outer solar system were attracted to the large Jovian planets, if they missed collision with them they were gravitational flung into a spherical region surrounding the solar system known as the Oort Cloud.  These icy planetesimals are incipient comets and perhaps are occasionally pulled into the inner solar system as another star comes close to our Sun (maybe we have a sister star that revolves around our Sun).  Comets often hit planets and moons.
• A small amount of dust and gases still remain in interplanetary space that was never accreted into larger bodies.  Collisions of asteroids with each other generate fragmentary material (meteoroids) that move out of the asteroid belt, sometimes impacting planets and moons as meteorite collisions.
• The slow rotation of the Sun on its axis is due to the magnetic field lines of the Sun interacting with ionized gases that were in the solar nebula.  This is referred to as magnetic braking.
• The formation of our solar system was complete by about 4.6 billion years ago.

METEORITES AND THE AGE OF THE EARTH

     Meteorites are thought to be primeval material left over from the formation of the solar system that has not evolved since formation.  Much of the meteorite bombardment of planets and moons in the early history of our solar system was from material that had not accreted into a planet or moon.  Many meteorites that hit planets and moons today are the pieces of asteroids that have collided and fragmented (asteroids have also not evolved much since the formation of the solar system).  (Note:  chunks of material moving through interplanetary space are called meteoroids; when a meteoroid enters an atmosphere of a planet and heats up it is called a meteor {a "shooting star"}, and when a meteoroid or meteor hits a planet or moon it is called a meteorite.)
 

• Stones - of all meteorites found on Earth, 93% of them are stones.  Stones are of two types: Chondrites and Achondrites.
• Chondrites are composed of olivine and pyroxene.  The olivine and pyroxene compose the chondrules, small spherical grains that represent the original rapid cooling and condensation to form the meteorite.  Most stoney meteorites are thse ordinary chondrites; however, some chondrites contain a small percentage of organic compounds, such as amino acids.  The chondrites that contain organic compounds are called carbonaceous chondrites.  The organic compounds found in these meteorites are not of biologic origin, but may represent precursors of biologic organisms.
• Achondrites contain no chondrules and have a composition much like terrestrial basalts.  They probably are pieces of large differentiated asteroids that have fragmented due to collision.
• Irons - make up about 6% of all meteorites found on Earth.  They are composed of large crystals of iron and nickel alloys and probably formed in the interior of large differentiated asteroids where relatively slow cooling took place.
• Stoney-irons - make up only about 1% of all meteorites that impact the Earth.  They are composed of about equal amounts of iron/nickel and silicates and probably represent broken fragments from the zone between the metallic and rocky portions of large differentiated asteroids.
• Another class of meteorites that are found on Earth (relatively rare) are chunks of rock that (based on their composition) represent surface material from the Moon or Mars.  These meteorites formed when Mars or the Moon were hit by meteorites with a glancing blow that shattered surface rock with such force as to knock some of the material into space, where it eventually hit the Earth.

DIFFERENTATION OF THE EARLY EARTH OR HOMOGENEOUS ACCRETION VERSUS INHOMOGENEOUS ACCRETION OF THE EARLY EARTH

• How did the Earth become stratified into layers of different densities and compositions?
• Homogeneous Accretion Model:  The Earth formed as a homogeneous mixture of silicates and metals, but due to meteorite bombardment, gravitational compression, and radioactive decay the Earth become molten soon after formation.  Once molten, the heavier metals (iron and nickel) settled to the core and was surrouned sequential by lighter and lighter silicate minerals.
• Inhomogeneous Accretion Model:  The layers of the Earth condensed sequentially (as cooling progressed) from the hot gaseous material of the inner solar system.
• At present, geologists and astronomers favor the homogeneous accretion model for the formation of the Earth's internal structure.

ORIGIN OF OUR MOON

• Our moon is thought to have formed early in the history of the solar system by the impact of a Mars sized planetesimal with the Earth some 4.6 to 4.4 billion years ago.  Material that was liquified and vaporized from the impact took up orbit around the Earth and eventually coalesced by gravity to form the moon."Half an Hour After the Giant Impact, based on computer modeling by A. Cameron, W. Benz, J. Melosh, and others. Copyright William K. Hartmann" From:  The Origin of the Moon at http://www.psi.edu/projects/moon/moon.html (GO TO THIS SITE AND READ ABOUT THE ORIGIN OF THE MOON).  
• The Moon's original crust cooled about 4.4 billion years ago.  Rocks sampled by the Apollo Missions from the lunar highlands (light colored areas on the moon) and thought to represent the original cooled crust of the Moon have been radiometrically dated at about 4.4 billion years.  The maria (dark areas on the Moon) formed later due to volcanic lava flows which filled in low places and date from 3.2 to 3.8 billion years old.

HERE IS A LINK THAT HAS A TREMENDOUS AMOUNT OF INFO. AND LINKS ABOUT THE PLANETS:  The Nine Planets at http://www.seds.org/billa/tnp/