Post on 22-Dec-2015
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
tim
e
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 the Universe started out very dense and very smooth (the `Big dense and very smooth (the `Big Bang’).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