Lecture 36:Lecture 36:The Expanding UniverseThe Expanding Universe

ReviewReview the ‘zoo’ of observed galaxies and the ‘zoo’ of observed galaxies and

the Hubble Sequencethe Hubble Sequence the distance ladderthe distance ladder redshiftredshift Hubble’s Law and the expansion of Hubble’s Law and the expansion of

the Universethe Universe

Hubble SequenceHubble Sequence

Trends along the Hubble Trends along the Hubble SequenceSequence

red blueold stars young stars gas poor gas richno star formation lots of star formation

EllipticalLenticular Spiral Irregular

The distance ladderThe distance ladder planetsplanets

radar rangingradar ranging nearby starsnearby stars

parallaxparallax Milky WayMilky Way

main sequence fittingmain sequence fitting nearby galaxiesnearby galaxies

Cepheid variable starsCepheid variable stars distant galaxiesdistant galaxies

white dwarf supernovaewhite dwarf supernovae Tully-Fisher relationTully-Fisher relation

Tully-Fisher Tully-Fisher RelationRelation

for spiral galaxiesfor spiral galaxies relationship relationship

between rotation between rotation velocity and velocity and luminosityluminosity

measuring recession measuring recession velocityvelocity

reminder: Doppler formulareminder: Doppler formula

redshift = z = (observed-rest)/rest

Hubble’s LawHubble’s Law

distance

rece

ssio

n ve

loci

ty

Hubble’s Law (modern Hubble’s Law (modern version)version)

Hubble’s Law FormulaHubble’s Law Formula

v = H0 rrecession velocity = constant times distance

units of H0: km/s/Mpc

best estimates: H0 is between 65 km/s/Mpc and 79 km/s/Mpc

Example: Using Hubble’s Example: Using Hubble’s LawLaw

The K line of singly ionized calcium The K line of singly ionized calcium has a wavelength of 393.3 nm when has a wavelength of 393.3 nm when measured in the laboratory. In the measured in the laboratory. In the giant elliptical galaxy NGC4889, this giant elliptical galaxy NGC4889, this line is observed to be at 401.8 nm.line is observed to be at 401.8 nm. what is the redshift of this galaxy?what is the redshift of this galaxy? what is its recession velocity?what is its recession velocity? how far away is it?how far away is it?

The expanding UniverseThe expanding Universe Hubble’s Law implies that most Hubble’s Law implies that most

galaxies are moving away from us galaxies are moving away from us and, the farther away they are, the and, the farther away they are, the

faster they are moving away from faster they are moving away from usus

The center of the The center of the Universe?Universe?

does this mean that we are at the does this mean that we are at the center of the Universe?center of the Universe?

no – an observer in any other no – an observer in any other galaxy would see the same thinggalaxy would see the same thing

Cosmological RedshiftCosmological Redshift

The Cosmological PrincipleThe Cosmological Principle

on large scales, the on large scales, the Universe is Universe is homogeneoushomogeneous (uniform) and (uniform) and isotropicisotropic (same in all directions)(same in all directions)

Is everything expanding?Is everything expanding?

the expansion of space-time acts the expansion of space-time acts like a pressurelike a pressure

where the force of gravity is where the force of gravity is stronger than the expansion stronger than the expansion pressure, a gravitationally bound pressure, a gravitationally bound object is formedobject is formed

for example, the stars in our for example, the stars in our Galaxy are bound together by Galaxy are bound together by gravity, so it does not expand.gravity, so it does not expand.

The Age of the UniverseThe Age of the Universe

beginning of time

now

time

r

v = r/t = H0 r

space

t = 1/H0

The Cosmological HorizonThe Cosmological Horizon the distance that light can travel in the distance that light can travel in

the age of the Universe is called the age of the Universe is called our our cosmological horizoncosmological horizon

we cannot receive any information we cannot receive any information from beyond our horizonfrom beyond our horizon

rrhorizonhorizon = c/H = c/H00 = 4300 Mpc = 4300 Mpc (assuming H(assuming H00 = 70 km/s/Mpc) = 70 km/s/Mpc)

Lookback timeLookback time time it takes for time it takes for

light to travel from light to travel from an object to usan object to us

directly related to directly related to redshift of object – redshift of object – more sensible more sensible measure of measure of distancedistance

Coma ClusterComa Cluster

Hercules Cluster (200 Hercules Cluster (200 Mpc)Mpc)

Two Million GalaxiesTwo Million Galaxies

Large Scale Large Scale StructureStructure

Galaxy Formation and Galaxy Formation and EvolutionEvolution

How do galaxies form, and how do How do galaxies form, and how do they change over time?they change over time?

Why do we see so many different Why do we see so many different kinds of galaxies? Are their kinds of galaxies? Are their differences a result of ‘nature’ or differences a result of ‘nature’ or ‘nurture’?‘nurture’?

How do the properties of galaxies How do the properties of galaxies depend on their environment?depend on their environment?

let’s start from the let’s start from the beginning…beginning…

the Universe started out very dense the Universe started out very dense and very smooth (the `Big Bang’).and very smooth (the `Big Bang’).

there were small lumps caused by there were small lumps caused by quantum fluctuationsquantum fluctuations

as the Universe expanded, these as the Universe expanded, these lumps grew larger and denser lumps grew larger and denser because of the force of gravity because of the force of gravity

expanding

collapsing