ObjectivesObjectives• SummarizeSummarize the big bang theory. the big bang theory.

• List List evidence for the big bang theory.evidence for the big bang theory.

• ExplainExplain how Hubble’s discoveries lead to an how Hubble’s discoveries lead to an understanding that the universe is expanding.understanding that the universe is expanding.

• DescribeDescribe the three main types of galaxies. the three main types of galaxies.

• ExplainExplain how a quasar differs from a typical galaxy. how a quasar differs from a typical galaxy.

• DescribeDescribe characteristics of the universe in terms characteristics of the universe in terms of distance, and organizationof distance, and organization

The Big Bang TheoryThe Big Bang Theory• Astronomy: Astronomy: study of the universestudy of the universe• Cosmology: Cosmology: the study of the origin and evolution of the the study of the origin and evolution of the

universe universe • Big bang theory: Big bang theory: the theory that all matter and energy in the theory that all matter and energy in

the universe was once compressed into a small volume, the universe was once compressed into a small volume, called a Singularity, at an extremely high temperaturecalled a Singularity, at an extremely high temperature

• Exploded approximately 15 billion years agoExploded approximately 15 billion years ago• Began expanding in all directions and coolingBegan expanding in all directions and cooling

• All matter and space in the universe resulted from the Big All matter and space in the universe resulted from the Big BangBang

• By the mid-20th century, almost all astronomers and By the mid-20th century, almost all astronomers and cosmologists accepted the big bang theory.cosmologists accepted the big bang theory.

• The Big Bang

Timeline of the Big BangTimeline of the Big Bang

After the Big BangAfter the Big Bang Gravity pulls matter into small clustersGravity pulls matter into small clusters Clusters of matter become huge clouds of dust Clusters of matter become huge clouds of dust

and gasand gas Clouds become solar systems and galaxiesClouds become solar systems and galaxies We will discuss how these form in more detail We will discuss how these form in more detail

later in the courselater in the course

Raisin Bread Analogy of an Raisin Bread Analogy of an Expanding UniverseExpanding Universe

Think loaf of unbaked raisin bread. The Think loaf of unbaked raisin bread. The raisins are galaxies and the dough is raisins are galaxies and the dough is spacespace

Expanding UniverseExpanding Universe

Every object in the universe is moving away from Every object in the universe is moving away from the center. the center.

Think of waves spreading out from where you drop Think of waves spreading out from where you drop a pebble into a pond.a pebble into a pond.

Just like the waves furthest from the center….Just like the waves furthest from the center….

Expanding UniverseExpanding Universe …….objects near the very edge of the universe are the oldest.objects near the very edge of the universe are the oldest The most distant known objects in the universe are over 12 The most distant known objects in the universe are over 12

billion light years from the Earth!billion light years from the Earth! Wait a minute….this all sounds great, but how do we Wait a minute….this all sounds great, but how do we

KNOW these galaxies and other objects are moving away KNOW these galaxies and other objects are moving away from the center?from the center?

Big Bang Evidence #1Big Bang Evidence #1• We KNOW the universe is expanding thanks to our ability We KNOW the universe is expanding thanks to our ability

to detect electromagnetic radiation. to detect electromagnetic radiation.

• The types of radiation emitted by galaxies moving towards The types of radiation emitted by galaxies moving towards Earth are nearest the blue light end of the spectrum Earth are nearest the blue light end of the spectrum (shorter wavelength)(shorter wavelength)

• The types of radiation emitted by galaxies moving away The types of radiation emitted by galaxies moving away from Earth are nearest the red light end of the spectrum from Earth are nearest the red light end of the spectrum (longer wavelength)(longer wavelength)

• We know which are moving toward and away thanks to the We know which are moving toward and away thanks to the DOPPLER EFFECTDOPPLER EFFECT

Doppler EffectDoppler Effect• Change in the Change in the

wavelength of light wavelength of light emitted by an object due emitted by an object due to its motion to its motion

• Movement toward Movement toward “squeezes” the wavelength“squeezes” the wavelength

• Shorter wavelengthShorter wavelength• Light shifted toward the Light shifted toward the

blue end of the spectrum blue end of the spectrum

• Movement away stretches Movement away stretches the wavelength the wavelength

• Longer wavelengthLonger wavelength• Light shifted toward the Light shifted toward the

red end of spectrumred end of spectrum

Big Bang Evidence #1: Big Bang Evidence #1: Red ShiftRed Shift

In 1929, Edwin In 1929, Edwin Hubble found that the Hubble found that the most distant galaxies most distant galaxies consistently showed consistently showed the greatest red shift the greatest red shift and thus were moving and thus were moving away from Earth the away from Earth the fastest.fastest.

Big Bang Evidence #2: Big Bang Evidence #2: Cosmic Background Cosmic Background

RadiationRadiation Cosmic Background Radiation: Cosmic Background Radiation:

backgroundbackground radiationradiation left behind after the left behind after the Big Bang, which has now cooled to Big Bang, which has now cooled to approximately 270°C below zero.approximately 270°C below zero.

• Maps of cosmic background radiation over Maps of cosmic background radiation over the whole sky show ripples.the whole sky show ripples.

• Like the waves which spread out from Like the waves which spread out from dropping a pebble into water, these cosmic dropping a pebble into water, these cosmic background radiation ripples indicate the background radiation ripples indicate the history of the formation of the universe.history of the formation of the universe.

What Happens Next?What Happens Next? Big Bang theory leads to two possible futures for Big Bang theory leads to two possible futures for

the Universe:the Universe:1.1. Open UniverseOpen Universe2.2. Closed UniverseClosed Universe

Open Universe: The EndOpen Universe: The End The universe will continue to expand. The universe will continue to expand.

All stars will die as the last of their energy is All stars will die as the last of their energy is released. released.

Eventually, all matter will be reduced to single Eventually, all matter will be reduced to single atoms moving away from each other. There will atoms moving away from each other. There will be nothing left. be nothing left.

The end.The end.

Closed Universe: Closed Universe: CollapseCollapse

Gravitational attraction between objects will cause the Gravitational attraction between objects will cause the expansion of the universe to slow and, eventually, expansion of the universe to slow and, eventually, come to a halt. come to a halt.

Gravity will then begin to pull everything back to the Gravity will then begin to pull everything back to the center of the universe. center of the universe.

Eventually, all matter and energy will again condense Eventually, all matter and energy will again condense into a single point. into a single point.

Another Big Bang will occur and the formation of the Another Big Bang will occur and the formation of the universe will begin all over again.universe will begin all over again.

GalaxiesGalaxies• Galaxy: Galaxy: a collection of stars, a collection of stars,

dust, and gas bound together dust, and gas bound together by gravityby gravity

• Galaxies are the major building Galaxies are the major building blocks of the universe. blocks of the universe. Astronomers estimate that the Astronomers estimate that the universe contains hundreds of universe contains hundreds of billions of galaxies.billions of galaxies.

• That’s more than That’s more than 100,000,000,000 galaxies!100,000,000,000 galaxies!

• A typical galaxy, such as the A typical galaxy, such as the Milky Way (that’s our galaxy), Milky Way (that’s our galaxy), has a diameter of has a diameter of approximately 100,000 light-approximately 100,000 light-years and may contain more years and may contain more than 200 billion stars.than 200 billion stars.

Looking Back in TimeLooking Back in Time• Remember it takes time for Remember it takes time for

light to reach uslight to reach us• travels at 300,000 km/stravels at 300,000 km/s• So we see things “as they So we see things “as they

were” some time agowere” some time ago• The farther away, the The farther away, the

further back in time we are further back in time we are lookinglooking• 1 billion LY means 1 billion LY means

looking 1 billion years looking 1 billion years back in timeback in time

• So, when we see the So, when we see the stars, we are actually stars, we are actually looking at light which looking at light which was emitted long ago.was emitted long ago.

Spiral GalaxiesSpiral Galaxies

Galaxies are Galaxies are classified by shape classified by shape into three main into three main types.types.

Spiral galaxySpiral galaxy : has : has a nucleus of bright a nucleus of bright stars and flattened stars and flattened arms, which spiral arms, which spiral around the nucleus.around the nucleus.

Elliptical GalaxiesElliptical Galaxies Elliptical galaxy:Elliptical galaxy:

has various has various shapes, are shapes, are extremely bright in extremely bright in the center, and do the center, and do not have spiral not have spiral arms.arms.

Irregular GalaxiesIrregular Galaxies Irregular galaxy:Irregular galaxy:

has no particular has no particular shape, and is fairly shape, and is fairly rich in dust and rich in dust and gas.gas.

The Milky WayThe Milky Way The galaxy in which we The galaxy in which we

live, the Milky Way, is a live, the Milky Way, is a spiral galaxy in which the spiral galaxy in which the sun is one of hundreds of sun is one of hundreds of billions of stars.billions of stars.

Two irregular galaxies, the Two irregular galaxies, the Large Magellanic Cloud Large Magellanic Cloud and Small Magellanic and Small Magellanic Cloud, are our closest Cloud, are our closest neighbors.neighbors.

These three galaxies are These three galaxies are called the called the Local GroupLocal Group..

Galaxies

Features of the Milky Way• Large, spiral galaxy• The distance across is about 100,000 light years

(ly)• The thickness at the nucleus (bulge) is about

10,000 light years• Sun is in outer edge of a spiral arm & more than

26,000 ly away from the center of the galaxy• Contains more than 1 trillion stars