1

Cosmology Overview (so far):

• The Universe: Everything• Observable Universe: Everything we

can see• The Universe has no special locations

– No edges– If no edges, then no center

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

getting closer• This initial “explosion” of matter outward is called

The Big Bang

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

Cosmology Overview (so far):

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

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

• Beginning + Expansion = Big Bang!

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

can start keeping track of time

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

• The Hubble Constant H0 measures how fast theUniverse is expanding

The Age of the UniverseUsing various techniques, astronomers have

determined the Hubble Constant:

H0 = 70 km/s / Mpc

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

The Universe is about: 14 billion years old.

Predictions of the Big Bang Theory

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

• The Big Bang Theory makes three such predictions:

1. It predicts a Cosmic Microwave Background (CMB)

2. It predicts small variations in this CMB

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

2

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

•The universe was no longeropaque, and the light couldfly freely.

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

IonizedH

NeutralH

Look-back Times

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

• This is because the speed of light is limited• Also means we can observe light 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 Background

• 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 microwave region

Cosmic Microwave Background

• In the 1960’s, two young researchers (Penzias andWilson) were calibrating a horn antenna(This is a type of telescope sensitive to microwaves)

• They found a static “hiss” which was not instrumental noise.

They found a bird’s nestin their antenna….

And removed it.

Cosmic Microwave Background• To their surprise, the static remained.• It came from every part of the sky.

– So it could not be from our solar system

• It had a “black body” spectrum with atemperature of about 3º K.

They had discovered theCosmic MicrowaveBackground predicted by theBig Bang Theory

They won the Nobel Prize inPhysics in 1987!

3

Early Universe LightSupplemental Lecture Tutorial

• 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.

Primordial Nucleosynthesis• The Universe is made up of:

– Hydrogen (H) & Helium (He)– Lithium (Li) & Deuterium– Heavier elements like Carbon, (C), Nitrogen (N), Oxygen (O)

• We know that the heavy elements (C,N,O) aremade inside stars by nuclear fusion.

• Stars can also make He, but not as much as isfound in the universe…

• Where did the “Primordial” He, H, Li comefrom?

Primordial Nucleosynthesis

• A long time ago, the universe was:– Smaller– Denser– Hotter– Like an ionized gas

Primordial Nucleosynthesis

• Amount of each element createddepends on the density of the earlyuniverse

• In order to get the abundances oflithium and deuterium that we observetoday, the universe had to contain darkmatter

Hot vs. Cold Dark matter

• Temperature -> speed of particles andability to “clump together”

• Neutrinos would be a form of Hot DarkMatter

• But… most successful theories needCold Dark Matter

How has Dark Matter affectedgalaxy formation?

• Early universe is hotter -> more high-energy photons

• Regular matter must be able to cool offand clump up to create stars andgalaxies

• Dark matter can form clumps earlierthan regular matter

4

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 Freeze)

– The expansion could halt, and reverse (BigCrunch)

– Or, it could stop expanding and becomestable

The Fate of The Universe

• The expansion depends upon the gravity thateach galaxy feels from the rest of the universe

• If there is enough mass in the universe, then itwill have enough gravity to halt the expansion.

• The density (ρ) of the universe is its averagemass per volume.

• If the actual density is greater than a criticaldensity (ρcritical), then the universe will collapse.

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

Ω = ρtrue / ρcrit.• If Ω >1, then the universe will collapse. (“Closed” Universe)• If Ω <1, then the universe will expand forever. (“Open” Universe)• If Ω =1, then the universe will gradually slow its expansion (“Flat”

Universe)

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

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

•The key is density!

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 flat universe

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

Chapter 19: Life on OtherWorlds

5

Searching forLife:

• What does life look like here?• How did Earth get life?• Is Earth ordinary or extraordinary?

If Earth is ordinary, where is everyoneelse?

Life in the Universe

• The Earth formed about 4.5 billion years ago• The oceans formed about 4.1 billion years

ago• It appears that life arose very quickly on

Earth: 3.8 billion years ago• Perhaps life could easily form on other

planets as well….

Earliest Fossils

• Oldest fossils showthat bacteria-likeorganisms werepresent over 3.5billion years ago

• Carbon isotopeevidence pushesorigin of life to morethan 3.85 billionyears ago

Laboratory Experiments• Miller-Urey experiment

(and more recentexperiments) showsthat building blocks oflife form easily andspontaneously underconditions of earlyEarth.

• Building blocks oflife… but no life yet!

Extremophiles• We find life almost everywhere on Earth!

• Organisms that thrive in extremeenvironments:– Volcanoes (high temperatures)– Ice Caps (low temperatures)– Acidic environments– Salty environments– Dry environments

Is Life Possible Elsewhere?

• Life arose early and quickly on Earth• Complex organic molecules form easily

from ingredients and conditions on theyoung Earth

• Living organisms exist is extremeenvironments on Earth

6

Searches for Life on Mars

• Mars had liquid water in the distant past• Still has subsurface ice; possibly subsurfacewater near sources of volcanic heat.

Could there be life on Europaor other jovian moons?

Titan

• Surface too cold for liquid water (but deepunderground?)• Liquid ethane/methane on surface

Are “habitable planets”common?

Definition:A habitable world contains the basic

necessities for life as we know it,including liquid water.

• It does not necessarily have life.

Constraints on star systems:

1) Old enough to allow time for evolution (rulesout high-mass stars - 1%)

2) Need to have stable orbits (might rule outbinary/multiple star systems - 50%)

3) Size of “habitable zone”: region in which aplanet of the right size could have liquidwater on its surface.

Even so… billions of stars in the Milky Way seem at least to offer the possibility of habitable worlds.

The more massive the star, the largerthe habitable zone — higherprobability of a planet in this zone.

7

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 seen 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

Astrometry• Like the Doppler method,

astrometry searches forwobbling stars.

• The Doppler Effect measuresstars coming toward or movingaway from us.

• Astrometry measures starswobbling side to side.

Wobble of our own Sun -->

Eclipsing Planets

A planet might pass right in front of its star,Making a small eclipse, 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’satmosphere.

Spectral Signatures of Life

Earth

Venus

Mars

oxygen/ozone

8

Are Earth-like planets rare orcommon? Elements and Habitability

• Do we require heavyelements (Carbon, Iron,Calcium, Oxygen) inprecise proportions?

• Heavy elements aremore common in starsin the disk of the MilkyWay

• A galactic habitablezone?

Impacts and Habitability• Are large planets (like

Jupiter and Saturn)necessary to reducerate of impacts?

• If so, then Earth-likeplanets are restricted tostar systems withJupiter-like planets

Climate and Habitability• Are plate tectonics

necessary to keepthe climate of anEarth-like planetstable?

The Bottom Line

We don’t yet know how importantor negligible these concerns are.

Looking for Life:

• Any life:– bacteria, microbes, blue-

green algae

• Intelligent life:– beings that can build

telescopes and areinterested in talking to us

Probes to other planetsare very expensive, butradio signals are cheap!

9

SETI experiments look fordeliberate signals from E.T.

The Drake Equation

• In 1961 Astronomer Frank Drake tried to calculate thenumber of ET civilizations in our galaxy, N

• His calculation is known as the Drake Equation

The Drake EquationNumber of civilizations with whom we could

potentially communicate

NC = N* × fP × np × fHZ × fL × fI × FS

N* = # of stars in galaxyfp = fraction with planetsnp= number of planets per systemfHZ fraction with good conditions for lifefL = fraction with lifefI = fraction with intelligent lifeFS = Fraction of star’s (or galaxy’s) lifetime for

which a civilization exists

We do not know the values for theDrake Equation!

Are we “off the chart” smart?

• Humans havecomparativelylarge brains

• Does that meanour level ofintelligence isimprobably high?

The Drake Equation

• The final factor in the Drake Equation couldbe the most important: (L/LMW)

• LMW = lifetime of the Milky Way Galaxy (10billion years)

• L = lifetime of a technological civilization(50 years? 1000 years? 1 million years?)

What will it take for us to survive?

10

SETI

• We can search for ETI several ways

• Spaceships: Flying spaceships to other starswould take a long time, and be very expensive.

• Broadcasting: We could easily send signals toother stars using a radio telescope.

• These signals travel at the speed of light

Communicating with ET• November 16, 1974, astronomers sent

a message about people on Earthtoward the globular star cluster M13 inthe constellation Hercules.

• The message was intended to be easyto decode.

• M13 is 25,000 light years away

We won’t get ananswer for at least50,000 years!

M 13 Globular Cluster

SETI• Listening: We can easily listen for signals

from ET• Most searches are done in radio waves

because they are low-energy• The first search was started in 1960.

Now, several searches areunderway including one atArecibo in Puerto Rico

Fermi’s Paradox

• Plausible arguments suggest that civilizationsshould be common, for example:

• Even if only 1 in 1 million stars gets acivilization at some time ⇒ 100,000civilizations

• So why we haven’t we detected them?

Possible solutions to theparadox

Earth as seen from the edge of the Solar SystemVoyager 1 photo