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Measuring Distances toGalaxies

• Too far for parallax!

• Standard Candles:– Cepheid Variables (for Local Group)– Type Ia Supernovae

• Redshifts

Type Ia Supernovae• These are another standard candle used to

measure distances to galaxies• Matter from large companion falls onto a white

dwarf, causing its mass to exceed 1.4 Msun• The resulting explosion is a Type Ia supernova.

Supernovae Types

Type Ia: Exploding White Dwarfin Binary

Type II: “ordinary” supernovaecaused by an explodingmassive stars

Supernovae are Good StandardCandles

• They are allthe samebrightness

• They can beseen at verylargedistances– (1000x

farther thanCepheids)

Supernova in galaxy NGC4526 (HST Image)

Galaxies in Motion• Motion of galaxies is measured using the Doppler effect.• Spectrum will be redshifted if it is moving away,

blueshifted if it is moving toward us.

Non-moving galaxy spectrum

Redshifted Spectrum

Galaxy Redshifts

• When we look at the spectrum of most galaxies,they are redshifted.

• This means they are moving away from us.

• Only a few exceptions: Andromeda is movingtowards the Milky Way (may collide with the MilkyWay… in 3 billion years)

Hubble’s Law• E. Hubble measured the distances to some of these

galaxies (using Cepheid variable stars)• He discovered that the redshift is proportional to the

galaxy’s distance.

•The relationship iscalled Hubble’s Law

Hubble’s Original Data

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Hubble Law

d = vr / H0

• d = distance to galaxy (Mpc)• vr = radial velocity of galaxy (km/s)• H0 = Hubble constant (70 km/s/Mpc)

Hubble Law Example: Find the distanceto a galaxy that has a radial velocity of

3,500 km/s.

d = vr / H0

1 Mpc is about 3 million light years!

Techniques for Measuring DistancesReview

1. Parallax

• Measure angle, p. D = 1/p

2. Cepheid method (standard candle)

• Measure Period, get luminosity

3. Type Ia Supernovae (standard candle)

4. Hubble’s Law

• Measure velocity Vr . Use: Vr = Ho x D

(nearbystars)

(nearestgalaxies)

(distantgalaxies)

(wholeuniverse!)

Cosmologyin the 21st Century

Chapter 15:

Cosmology

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

• The Universe means everything: all matter, allenergy, 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

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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 not infinite

•The age of the universe is notinfinite

We can only see part of the universe:

The Observable Universe

Isotropy

• No matter where we look, the Universelooks roughly the same

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

Homogeneity

• No matter where we are, the Universecontains the same stuff

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

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 large scale

Expansion of the Universe

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

• We can explain this with an expanding universemodel

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

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

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

mean we’re at the center!• Remember: No special locations• Expansion looks the same everywhere, no matter where

you are in the universe

v = H0 x d

d = v / H0

Expansion of the UniverseLecture Tutorial: Page 133-134• 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.

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 space outward is called

The Big Bang

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

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: 13.7 billion years old.

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 Cosmic Microwave Background (CMB)

2. It predicts small variations in this CMB

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

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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 couldmove freely.

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

IonizedH

NeutralH

Look-back Times

• We say a galaxy is 10million light years away

• The light from thisgalaxy has beentraveling for 10 millionyears to reach our eyes!

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

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 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 microwave 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 Freeze)

– The expansion could halt, and reverse (BigCrunch)

– Or, it could stop expanding and becomestable

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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. (“Closed” Universe)• If Ω <1, the universe will expand forever. (“Open” Universe)• If Ω =1, the universe’s expansion will gradually slow (“Flat”

Universe)

Chapter 20: Life on OtherWorlds

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….

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

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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

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

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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.

Elements and Habitability• Do we require heavy

elements (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!

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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

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?

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

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SETI• Listening: We can easily listen for signals from

ET• If the ETʼs are much more advanced than us,

they might share a wealth of knowledge.• Because radio waves have low energy, people

have searched using radio telescopes.• The first search was in 1960.

Now, several searches are underwayincluding one at Arecibo in PuertoRico

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