4,000 Years of Cosmology4,000 Years of Cosmology

(The Abbreviated Version)(The Abbreviated Version)

What is Cosmology?What is Cosmology?

The study of the The study of the Universe as a wholeUniverse as a whole

From the start of From the start of time until now...time until now...

And until the end And until the end of time.of time.

Before people could understand the history of the Universe, Before people could understand the history of the Universe, they had to know its geography...they had to know its geography...

• 66thth century BC, philosopher and century BC, philosopher and travelertraveler

• brought back to Greece brought back to Greece knowledge and records of the knowledge and records of the Babylonians and Egyptians Babylonians and Egyptians

• first ideas that lay between the first ideas that lay between the mythologies and true sciencemythologies and true science

• Note: he believed the Earth was flat and floated on water like a log

Thales

lived from 384 to 322 BClived from 384 to 322 BC

3 experimental proofs that the 3 experimental proofs that the Earth was roundEarth was round

Believed the Earth was the center Believed the Earth was the center of the Universe and the Sun, of the Universe and the Sun, Moon, planets, and a sphere Moon, planets, and a sphere containing all the "fixed" stars containing all the "fixed" stars revolved around itrevolved around it

Aristotle

What’s Popular is Not Always Right, What’s Popular is Not Always Right, and What’s Right is Not Always Popularand What’s Right is Not Always Popular

Most Greeks believed Aristotle and his Most Greeks believed Aristotle and his ideasideas

The exception (a century later) ... The exception (a century later) ... Aristarchus, who thought the Earth moved Aristarchus, who thought the Earth moved around the Sunaround the Sun

- he found very few supporters for his - he found very few supporters for his ideas ideas

- often overlooked in the history of - often overlooked in the history of heliocentric ideasheliocentric ideas

HipparchusHipparchus 100s BC, compiled a star catalog100s BC, compiled a star catalog giving a magnitude or comparative brightness scalegiving a magnitude or comparative brightness scale calculated the Moon's distance from the Earthcalculated the Moon's distance from the Earth

Claudius PtolemaeusClaudius Ptolemaeus

(better known as Ptolemy of Alexandria)(better known as Ptolemy of Alexandria) came along 300 years latercame along 300 years later first astronomer to make truly scientific first astronomer to make truly scientific

maps of the heavensmaps of the heavens

What’s Popular is Not Always Right, and What’s Popular is Not Always Right, and What’s Right is Not Always PopularWhat’s Right is Not Always Popular

The Ideas of PtolemyThe Ideas of Ptolemy

• Believed that the Sun, Moon, and planets circled the EarthBelieved that the Sun, Moon, and planets circled the Earth

• Planets sometimes appear to travel backward? Planets sometimes appear to travel backward? -each moved in a small circle (epicycle) -each moved in a small circle (epicycle) that was orbiting Earth that was orbiting Earth

• Summarized the theory in Summarized the theory in

the the Almagest Almagest

-written in the 140 AD-written in the 140 AD

-theory was believed for -theory was believed for

about 1500 yearsabout 1500 years

Copernican TheoryCopernican Theory

Polish monk Nicolaus Polish monk Nicolaus Copernicus says “Sun is the Copernicus says “Sun is the center.”center.”

The Church, however, says The Church, however, says Earth was centralEarth was central– Copernicus' theory was Copernicus' theory was

heresy! heresy! – He did not publish it until He did not publish it until

the day he died the day he died

(On the Revolution of the (On the Revolution of the Celestial Spheres)Celestial Spheres)

Two events made the "Copernican revolution" inevitable:

• Brahe's precise observations of the sky • Galileo's use of the telescope.

Tycho BraheTycho Brahe

• Danish astronomerDanish astronomer

• In 1572, saw brilliant “new In 1572, saw brilliant “new star” in the sky in the star” in the sky in the constellation Cassiopeia; constellation Cassiopeia; supernova! supernova!

• In 1604, a second supernova was observedIn 1604, a second supernova was observed

• These discoveries undermined the cornerstone These discoveries undermined the cornerstone of the of the Ptolemaic system, i.e., outermost Ptolemaic system, i.e., outermost sphere contained all the stars and it was sphere contained all the stars and it was unchangingunchanging

New “Eyes” on the SkiesNew “Eyes” on the Skies

• In 1609, Italian scientist Galileo Galilei In 1609, Italian scientist Galileo Galilei heard about the spyglass, made one for heard about the spyglass, made one for himself, and turned it on the heavens himself, and turned it on the heavens

• One of the first discoveries was four moons One of the first discoveries was four moons circling Jupitercircling Jupiter

• He revealed a miniature version of He revealed a miniature version of Copernicus' solar system, with the moons Copernicus' solar system, with the moons moving around the planet in simple, circular moving around the planet in simple, circular orbitsorbits

• The Catholic Church later forced Galileo to The Catholic Church later forced Galileo to recant his ideas and live out his life under recant his ideas and live out his life under house arrest, but his discoveries had already house arrest, but his discoveries had already changed the face of astronomychanged the face of astronomy

Saving FaceSaving Face

• British genius Isaac Newton unwittingly gave the Church a British genius Isaac Newton unwittingly gave the Church a "way out”"way out”

• Newton put mathematics to observationsNewton put mathematics to observations

- the equations to describe the relationships - the equations to describe the relationships

observed for motion, and by gravity observed for motion, and by gravity

- applied his equations to Kepler's - applied his equations to Kepler's

observations (it fit almost perfectly)observations (it fit almost perfectly)

• The conclusions were indisputable: Copernicus had been right The conclusions were indisputable: Copernicus had been right all along (actually all along (actually Aristarchus had been!) had been!)

Saving Face Saving Face

• It is the force of It is the force of gravitygravity which keeps all the planets which keeps all the planets in their elliptical orbits around the Sunin their elliptical orbits around the Sun

• Newton's Laws say that nothing is naturally at rest; Newton's Laws say that nothing is naturally at rest; all heavenly bodies should be constantly moving, with all heavenly bodies should be constantly moving, with no limits on space and timeno limits on space and time

• Newton believed that the Universe was eternal and Newton believed that the Universe was eternal and infinite; this made the Church happy infinite; this made the Church happy

• It was an uneasy reconciliation, but one that allowed It was an uneasy reconciliation, but one that allowed science to move forward unimpededscience to move forward unimpeded

Big BangBig Bang • In 1917, Albert Einstein proposed a description of the In 1917, Albert Einstein proposed a description of the Universe based on his Theory of General RelativityUniverse based on his Theory of General Relativity

• In Belgium, Georges Lemaitre came up with his own In Belgium, Georges Lemaitre came up with his own model when he heard that the Universe was expandingmodel when he heard that the Universe was expanding

• Universe began as a “primeval atom”Universe began as a “primeval atom”

• Hot and denseHot and dense

• Explosion caused expansion to startExplosion caused expansion to start

• Lemaitre can be called the “father of the big bang”Lemaitre can be called the “father of the big bang”

Note: He was an ordained priest before turning to cosmology.

Big Bang; As We Now Know ItBig Bang; As We Now Know It

• George Gamow, a Russian-American physicist (late George Gamow, a Russian-American physicist (late 1940s), conceived of Big Bang we know today 1940s), conceived of Big Bang we know today

• He and his colleagues proposed He and his colleagues proposed - elements created in very brief period early in - elements created in very brief period early in

expansion of Universeexpansion of Universe

- predicted an afterglow, still should be present - predicted an afterglow, still should be present arising from hot, early phasearising from hot, early phase

- largely forgotten until afterglow was - largely forgotten until afterglow was discovered in the 1960s discovered in the 1960s

Note: Gamow was also a popularizer of science, wrote poetry, and did research in genetics

George GamowGeorge Gamow1904 - 1968

Hey Look at Me… I’m Expanding! Hey Look at Me… I’m Expanding!

• Edwin Hubble in 1920’s; major breakthrough in understanding of Edwin Hubble in 1920’s; major breakthrough in understanding of Universe Universe

• For centuries, astronomers believed Milky Way entire UniverseFor centuries, astronomers believed Milky Way entire Universe

• In 1923, Hubble was among the first to In 1923, Hubble was among the first to

show that the fuzzy patches they saw in show that the fuzzy patches they saw in

the sky through their telescopes were the sky through their telescopes were

whole other galaxies, not parts of the whole other galaxies, not parts of the

Milky Way Milky Way

What Hubble SawWhat Hubble SawIn 1929, by looking at the spectra of these galaxies, he In 1929, by looking at the spectra of these galaxies, he concluded that they were speeding away from us - the concluded that they were speeding away from us - the Universe was expanding!Universe was expanding!

QuickTime™ and aGIF decompressor

are needed to see this picture.

Edwin HubbleEdwin Hubble

Note: Before turning to astronomy, Hubble was a lawyer.

1889 - 1953

Why Do We Believe the Big Why Do We Believe the Big Bang Theory?Bang Theory?

• Expansion of the Universe • Origin of the cosmic background radiation • Nucleosynthesis of the light elements • Formation of galaxies and large-scale structure

Observations, Observations, Observations!Observations, Observations, Observations!

Other TheoriesOther Theories

• • Steady State TheorySteady State Theory

• • Chronometric Theory - Chronometric Theory - incompatible with CMB observations,incompatible with CMB observations,incorrect source counts, predictsincorrect source counts, predictsredshift is quadratic function of distanceredshift is quadratic function of distance

• • Tired Light Theory - Tired Light Theory - no known mechanism to degrade photon no known mechanism to degrade photon energy, contradicts high redshift supernovaenergy, contradicts high redshift supernovaobservationsobservations

Steady State TheorySteady State Theory

• Universe had a beginning? Not appealing to all Universe had a beginning? Not appealing to all scientists scientists

• In 1948, Fred Hoyle, Hermann Bondi, and Tommy Gold, In 1948, Fred Hoyle, Hermann Bondi, and Tommy Gold, came up with Steady State Theorycame up with Steady State Theory

- Universe has no beginning and no end- Universe has no beginning and no end

-Although expanding, it stays in perfect balance - -Although expanding, it stays in perfect balance - like a pool kept full to overflowing by a trickle like a pool kept full to overflowing by a trickle from a faucet from a faucet

-The "faucet" is the continuous creation of matter -The "faucet" is the continuous creation of matter from energy from energy

Steady State Theory (cont)Steady State Theory (cont)

• They showed that only one atom per cubic mile of They showed that only one atom per cubic mile of space per hour was needed space per hour was needed

• Even this amount proved to be the theory's undoing Even this amount proved to be the theory's undoing

- It defied the laws of physics; It did not match - It defied the laws of physics; It did not match observationsobservations

Evidence Against the Steady State Evidence Against the Steady State TheoryTheory

• Background cosmic heat radiation discovered Background cosmic heat radiation discovered

• Amount of helium observed in the Universe exactly fit Amount of helium observed in the Universe exactly fit what was predicted by the big bang what was predicted by the big bang

• Radio astronomers found that galaxies were more Radio astronomers found that galaxies were more crowded together in the pastcrowded together in the past

- meaning the idea of an unchanging universe - meaning the idea of an unchanging universe was incorrectwas incorrect

Basking in The Afterglow Basking in The Afterglow

• In 1963/1965, physicists Arno Penzias and Robert Wilson started In 1963/1965, physicists Arno Penzias and Robert Wilson started to search for faint radio signals (actually microwaves) from the to search for faint radio signals (actually microwaves) from the outskirts of the Milky Way Galaxyoutskirts of the Milky Way Galaxy

- famous antenna located in Holmdel, New Jersey- famous antenna located in Holmdel, New Jersey

Penzias and WilsonPenzias and Wilson•Bedeviled by a persistent interference (a constant signal from Bedeviled by a persistent interference (a constant signal from all over the sky) equivalent to temperature of - 454 °F (-270 °C all over the sky) equivalent to temperature of - 454 °F (-270 °C or 3 Kelvin) or 3 Kelvin)

• They thought the signal was from pigeon droppings in the They thought the signal was from pigeon droppings in the telescope, but colleagues soon realized that they had found telescope, but colleagues soon realized that they had found the afterglow predicted by Gamowthe afterglow predicted by Gamow

Inflation Solves Some Inflation Solves Some Big Bang Problems Big Bang Problems

• In 1979, particle physicist Alan Guth In 1979, particle physicist Alan Guth

- Stanford Linear Accelerator Center - Stanford Linear Accelerator Center

- interested in the Grand Unification Theory of Forces- interested in the Grand Unification Theory of Forces

- calculations led to the idea of "cosmic inflation” (brief - calculations led to the idea of "cosmic inflation” (brief period of rapid expansion in the early Universe)period of rapid expansion in the early Universe)

• It explains why the Universe is so big and so smooth, why (at It explains why the Universe is so big and so smooth, why (at least) four different forces act in it today, and where the vast least) four different forces act in it today, and where the vast amounts of matter came from. amounts of matter came from.

• In 1998, Science magazine says "breakthrough of year" In 1998, Science magazine says "breakthrough of year"

Universe is expanding at an accelerating Universe is expanding at an accelerating rate!rate!

• For over ten years astronomers studying expansion of For over ten years astronomers studying expansion of Universe by measuring:Universe by measuring:

- Redshifts- Redshifts

- Brightness of Type Ia supernovae- Brightness of Type Ia supernovae

Accelerating Expansion?Accelerating Expansion?

• Note: Type Ia supernovae are stars that explode in Note: Type Ia supernovae are stars that explode in thermonuclear cataclysms brighter than entire galaxiesthermonuclear cataclysms brighter than entire galaxies

• Ideal "standard candles" to survey the Universe Ideal "standard candles" to survey the Universe

- Their light curves and spectra are all nearly alike and - Their light curves and spectra are all nearly alike and they are bright enough to be seen across billions of light-yearsthey are bright enough to be seen across billions of light-years

• By 1998, many of these supernovae analyzed in detail, leading By 1998, many of these supernovae analyzed in detail, leading scientists to conclude that the expansion of the Universe is not scientists to conclude that the expansion of the Universe is not slowing, but accelerating! slowing, but accelerating!

Accelerating ExpansionAccelerating Expansion

HST HST ObservationObservation

s s of Distant of Distant SupernovaSupernova

What Drives the Acceleration?What Drives the Acceleration?

• The supernova data strongly suggest The supernova data strongly suggest that Universe is filled with an unidentified that Universe is filled with an unidentified form of energy which is the cause of the form of energy which is the cause of the accelerating expansion accelerating expansion

• This is called "Dark Energy"This is called "Dark Energy"