1

Cosmologyin the 21st Century

Chapter 15:Cosmology

• Cosmology attempts to answer big questions:– What was the ____ of the Universe?– What is _____ of the Universe?– Is the Universe infinite or limited?

• The Universe means ________: all matter, all energy,and even all of space.

• Thus there is no “edge” or “wall” to the universe• A wall or edge divides inside from outside, but there is

nothing “outside” the universe!

Olbers’s Paradox

So, why is the sky dark at night?

In the 1820’s Heinrich Oblers noted thatif the universe is infinite, then every lineof sight should end on the surface of astar eventually.

The night sky should be asbright as the surface of stars!

Everywhere you lookyou see a tree/star

Solution to Olbers’s ParadoxWhy is the sky dark at night?

If the universe had a beginning, then we can only see lightfrom galaxies that has had time to travel to us since thebeginning of the universe.

•The speed of light is ___ _____

•The age of the universe is ____________

We can only see part of the universe:

The ___________ Universe

Isotropy

• No matter where we look, the Universelooks roughly _________

• No preferred or special ________• The Universe is “isotropic”

Homogeneity

• No matter where we are, the Universecontains ___________

• No preferred or special _______• The Universe is “homogeneous”

2

Isotropic + Homogeneous= Cosmological Principle

• The Cosmological Principle states thatany observer in the Universe shouldsee the same basic features

• This reinforces the no edge, no center• This applies on a _______ scale

Expansion of the Universe

• Almost all galaxies are moving away from us,and more distant galaxies are moving awayfaster (________ Law).

• We can explain this with an ________ universemodel

The Expanding UniverseSpace itself is a significant part of the universe.

In the universe, ______ is expanding, carryingthe galaxies along!

Expansion of the Universe• Just because everything’s moving away from us doesn’t

mean we’re at the ______!• Remember: No special _______• Expansion looks the same ________, no matter where

you are in the universe

v = H0 x d

d = v / H0

Expansion of the UniverseLecture Tutorial: Page 133

• Work with a partner or two• Read directions and answer all questions

carefully. Take time to understand it now!• Discuss each question and come to a

consensus answer you all agree on beforemoving on to the next question.

• If you get stuck, ask another group for help.• If you get really stuck, raise your hand and I

will come around.

Cosmology Overview (so far):

• The Universe: Everything• Observable Universe: Everything we

can ____• The Universe has no special locations

– No _____– If no ______, then no ______

3

The Big Bang• Galaxies are moving _____ from us now.• If we play the movie backward, we would see them

getting closer• This initial “________” of space outward is called

The Big Bang

Not an explosion which occurred in ___ place: it was__________! The Universe was just smaller backthen.

The Age of the Universe• The Big Bang gives us a _____: a point when we

can start keeping track of time

• This tells us that the Universe isn’t _______ old; ithas an age!

• The Hubble Constant H0 measures how ____ theUniverse is expanding

The Age of the UniverseUsing various techniques, astronomers have

determined the Hubble Constant:

H0 = ____ km/s / Mpc

Using this value of the HubbleConstant it is possible todetermine the age of theUniverse:

The Universe is about: ____ ______ years old.

Cosmology Overview (so far):

• Oblers’s Paradox: The sky is dark at night, sothe Universe must have a _________

• Hubble Expansion: All galaxies are movingaway from us, so they must have all startedout at ___ _______

• Beginning + Expansion = ___ _____!

Predictions of the Big Bang Theory

• Like any good theory, the Big Bang Theory must makepredictions which are confirmed by observation.

1. It predicts a _______ __________ __________ (CMB)

2. It predicts small ________ in this CMB

3. It predicts the _________ of light elements (Hydrogen,Helium…)

Cosmic Microwave Background• The early Universe was very ___, ionized, and opaque.• As the Universe expanded, it ______.• 400,000 years after the Big Bang, it had cooled to 3000 K• Electrons joined protons to form the first _______ ____

•The universe was no longeropaque, and the light could____ _______.

•The Big Bang Theory sayswe should still see this lighttoday.

IonizedH

NeutralH

4

Look-back Times

• We say a galaxy is 10million light years away

• The light from thisgalaxy has beentraveling for __ ______years to reach our eyes!

• Its look-back time is 10million years: we areseeing a 10-million-year-old picture of that galaxy

Looking at Distant ObjectsLecture-Tutorial: Page 131

• Work with a partner or two• Read directions and answer all questions

carefully. Take time to understand it now!• Discuss each question and come to a

consensus answer you all agree on beforemoving on to the next question.

• If you get stuck, ask another group for help.• If you get really stuck, raise your hand and I

will come around.

Look-back Times

• By observing more and more distantobjects, we can look further and furtherinto the _____

• This is because the speed of light is _____• Also means we can observe _____ from

the Big Bang: The Cosmic MicrowaveBackground (CMB)

Looking Back Towardsthe Early Universe

The more distant the objects weobserve, the further back into the past

of the universe we are looking.

CMB

Cosmic Microwave Radiation

• The wavelength of this light from the Big Bang would havestretched with the expansion of the universe.

• It should have a temperature of about 3º K.• It would now have a wavelength in the ________ region

The Fate of the Universe

• It’s expanding now… will it keep going?• The universe has three possible fates:

– The current expansion could continueforever (Big _______)

– The expansion could halt, and reverse (Big_______)

– Or, it could stop expanding and become______

5

The Fate of The Universe• The true density divided by the critical density is called “Omega” Ω =

ρtrue / ρcrit.• If Ω >1, the universe will collapse. (“______” Universe)• If Ω <1, the universe will expand forever. (“_____” Universe)• If Ω =1, the universe’s expansion will gradually slow (“____” Universe)

Shape of the Universe•“Open”, “Closed” and “Flat”universes have different “shapes”.

•How do we learn which one we’reliving in?

•The key is _______!

Closed surface Flat surface Open surface(positive curvature) (zero curvature) (negative curvature)

CMB and Density• The CMB is mostly smooth, but has small

variations about 1 degree in size• Spots are the right size for a _____ universe

– Open universe -> smaller spots– Closed universe -> larger spots

Cosmology Review

• How do we know the universe has a finiteage?

• How do we know there must be a Big Bang?• What is the Cosmic Microwave Background?

What are its basic properties?• What predictions are made by Big Bang

theory?• What is the shape and fate of the Universe?

The Solar System• One star• Planets• Asteroids

• Comets• Kuiper Belt

Objects (KBOs)

PlanetsThe planets of the solar system fall into three categories:

• Terrestrial Planets (those like _____)– Mercury– Venus– Earth & Moon– Mars

• Jovian Planets (those like _____)– Jupiter– Saturn– Uranus– Neptune

• “_____ Planets”, including Pluto, and Eris & Ceres

6

Terrestrial PlanetsTerrestrial comes from Latin “terra”meaning “earth”

Mercury, Venus, Earth, and Mars havesimilar features:

• Rocky _______• Thin (or nonexistent) ___________• Dense metal cores• Craters• _____ moons

Jovian Planets (not gas giants!)• Better name: _____ giants (Jupiter & Saturn) or

___ giants (Uranus & Neptune)• ______ mass (about 15x-300x mass of Earth)• ____ density (1/3rd to 1/7th of Earth)• Lots of moons, ring systems

What exactly is a planet?

1. It orbits the Sun2. Has enough mass so that it is round3. It has “______ ___ _____________”

• Pluto & Ceres satisfy #1 and #2, but not#3!

• Pluto: other Kuiper Belt Objects• Ceres: other Asteroids

Solar System Today(Not to Scale)

Inner Planets, Orbits to Scale

Inner Planets, Sizes to Scale

Top Row: Earth, VenusBottom Row: Mars, Mercury, the Moon

7

Planets, Sizes to Scale• Top Row: Jupiter & Saturn• Middle Row: Uranus & Neptune• Bottom Row: Earth, Venus, Mars,

Mercury, & the Moon

Planets with the Sun,Sizes to Scale

Early SolarSystem

• The young Sun probably had a disk ofgas & dust: the _____ ________

• Planets formed out of this disk: smallcores grow through accretion

• Temperature is _____ as you get closerto the center (where the Sun is!)

Differentiation in the SolarNebula

• Material formsclumpsaccording to______

• Only high-densityelements canform clumps at____temperatures

Cooler Temperatures

Step 1: Condensation• Gas → Dust → Grains or

Particles

• Small stuff grows fast thisway!

• At high temperatures, only_____ elements cancondense

8

Step 2: Accretion

• Particles sticking together

• Planetesimals (usually about 1 km wide)

• Different composition depending onwhere they formed in the solar nebula

Step 3: From Planetesimals toProtoplanets

• Head-on collisions or same-directioncollisions?

• Largest planetesimals grow fastest

• Can grow via gravitational collapse when itreaches 10-15 times the Earth’s mass

Step 4: Clearing the Nebula

• Sun turns on: ______!

• Two effects:– Radiation pressure from fusion– Solar wind

• Small dust grains and gas atoms getpushed out

Predictions from the SolarNebula Hypothesis

• Planet orbits should fall roughly in oneplane

• Orbit and spin directions should bemostly the same

• Planets should have roughly the sameage as their star

• Evidence of collisions!

Temperature and Formation ofOur Solar System LectureTutorial: Pages 103-104

• Work with a partner or two• Read directions and answer all questions

carefully. Take time to understand it now!• Discuss each question and come to a

consensus answer you all agree on beforemoving on to the next question.

• If you get stuck, ask another group for help.• If you get really stuck, raise your hand and I

will come around.

Extrasolar Planets

• The first Extrasolar planets were discoveredin the 1990s.

• These planets were very different from theplanets in our solar system– ____ orbital periods– Highly _________ orbits

• Most are about the size of ______– MJupiter = 300 MEarth

9

51 Pegasi: A “Hot Jupiter” Planet

Temp:1,500 K

51 Peg. Orbit

Exoplanet Discoveries

• Over 200 Planets Discovered (7% of stars)• 20 Multiple-Planet Systems• 10 Transiting Planets• 1 Possibly habitable planet• _______ planets are more numerous.

55 Cancri1 Jupiter-like planet at 5 AU (d)1 Jupiter-like planet at 0.1 AU (b)2 Saturn-like planets at 0.25 and 0.8

AU (c & f)1 Neptune-like planet at 0.03 AU (e)

Saturn-like planetat 0.8 AU (f) is inthe habitable or“Goldilocks”zone!

Life on moons?

Planet Search Methods

• Doppler Effect– Detect wobble toward or away from us

• Astrometry– Detect side-to-side wobble

• Transits– Search for “eclipses” as the planet passes in

front of the star.

We’ve never actually ____ the planets!

Doppler Method• Planets orbiting other stars “tug” on their star a bit• The star wobbles when the planet tugs on it• We can detect that wobble by measuring Doppler

shifts

Doppler Method• Measuring light from the ____, not the

_____!• Use the motion of the star to infer the

presence of a planet

10

Astrometry• Like the Doppler method, astrometry

searches for wobbling stars.

• The Doppler Effect measures starscoming toward or moving away fromus.

• Astrometry measures stars wobblingside to side.

Wobble of our own Sun -->

Eclipsing Planets

A planet might pass right in front of its star,Making a small _______, or transit.

Transit MethodA few planets havenow beendiscovered using theTransit Method.

If a planet happensto transit its hoststar:

We can learn thesize of the planet

We might see theplanet’s___________.

Selection Effects:

• Stars with the most massive planets willwobble more

• Their Doppler shifts will be larger,easier to measure

• Our method “selects” the most massiveplanets