DEVIL PHYSICSTHE BADDEST CLASS ON
CAMPUSIB PHYSICS
TSOKOS OPTION E-6GALAXIES
Introductory Video:The Big Bang Theory
Objectives
Understand the Hubble classification scheme of galaxies and describe the structure of the Milky Way galaxy
State the Hubble law and solve problems using this law, v = Hd
State the meaning of the Hubble constant
Identify significant epochs in the life of the universe
Understand the term inflationary universe
An Introduction to Our Galaxy
Types of Galaxies Milky Way
Our galaxy About 200 billion stars
Our sun is one of them A spiral galaxy Diameter – 100,000 ly Thickness – 2000 ly Mass – 4 x 1041 kg Our solar system – 30,000 ly from center
Period is 225 million years
Types of Galaxies Milky Way
Types of Galaxies
Spiral Galaxies Milky Way is one Central disc – flattened nucleus, spiral arms,
halo of older faint stars Diameter – 6000 pc to 30,000 pc Mass – 109 to 1012 solar masses New stars formed mainly in the spiral arms Most common type, about 50%
Types of Galaxies
Spiral Galaxies Barred spirals
Bar of stars runs through central bulge and disc
Spirals start from ends of bar instead of disc
Types of Galaxies
Elliptical Spherical or ellipsoidal
shape Almost entirely older stars Almost no interstellar gas
Very little star formation activity
45% of all galaxies
Types of Galaxies
Irregular 5% of all galaxies No regular symmetry,
almost chaotic looking Intense star formation
activity Possibly caused by
collision of two regular galaxies
Star Clusters
Types of Galaxies
Local Group
Milky Way part of this cluster of 20 galaxies
Extends over 10,000,000 ly Collections of clusters are called
superclusters Our supercluster believed to be 15 x
106 pc across
Steady State Theory
Edwin Hubble and the Expanding Universe
Galactic Motion
Observed as early as 1914 All but the closest galaxies
appeared to be moving away at enormous speeds
Redshifted absorption lines (Doppler effect) Proportional to recession
speed Only applies line-of-sight
c
vz
z
0
0
Hubble’s Law Galaxies move away with speeds
proportional to their distance The more distant, the faster it moves
away H is the Hubble constant (slope of
graph) 72 km/s·MpcHdv
Hubble’s Law Implied that in the past, galaxies were
closer together Universe started as a point mass As space expanded, distance between
clumps of mass (galaxies) increased like two points on a balloon as it is inflated
Hubble’s Law Radius of the universe in arbitrary units
as a function of time
Hubble’s Law If we assume the expansion
to be constant, the inverse of H gives us the age of the universe – Hubble time ≈ 14 billion years
HT
Hv
d
Hd
v
Hdv
1
1
The Big Bang
Evolution of the Universe
Planck time – the earliest point we can find in which quantum gravitational effects are not dominant
From the fundamental constants h, G and c
sc
hGt p
435
10
Evolution of the Universe
Temperature estimated to be 1032 K The kinetic energy of whatever
particles that existed would be
GeVkTEk1910
2
3
Evolution of the Universe
Forces are unified: time = 10-43 s Strong nuclear and electroweak forces
unified Leptons indistinguishable from quarks and
turned into each other Strong nuclear force separates: time =
10-35 s Strong nuclear force decoupled from the
electroweak force Temperature falls to 1027 K
Evolution of the Universe
Inflation begins: time = 10-35 s Rapid period of expansion called
inflationary epoch Lasted no more than 10-32 s Size of the universe increased by factor of
1050
Forces separate: time = 10-12 s Temperature is about 1016 K Four fundamental forces (gravity,
electromagnetism, strong nuclear, and weak nuclear) behave as separate forces
Universe was 10-16 of its present size
Evolution of the Universe
Nucleons form: time = 10-2 s Temperature fallen to 1011 K Quarks bind together to form protons and
neutrons and their antiparticles Universe was 10-10 of its present size At t = 1s, T = 1010 K Protons, neutrons, electrons and their
antiparticles in thermal equilibrium
Evolution of the Universe
Nuclei form: time = 3 min Temperature fallen to 109 K Protons and neutrons start to combine to
form nuclei of light atoms – nucleosynthesis
Applying the laws of thermodynamic equilibrium, it can be determined that there were 14 protons for every 2 neutrons
Evolution of the Universe
Nuclei form: time = 3 min Two neutrons combine with two protons to
form a helium nucleus leaving 12 protons to form hydrogen nuclei
The mass of 12 hydrogen atoms is 12u and the mass of one helium atom is 4u
This ratio (75% H – 25% He) still exists today and is one of the strongest pieces of evidence for the Big Bang Theory
Evolution of the Universe
Nuclei form: time = 3 min This epoch lasted from t = 3 min until 104
years At the end, the temperature was 104 K The size of the universe was 10-4 of its
present size
Evolution of the Universe
Atoms form: time = 3 x 105 years Electrons join with protons to form neutral
atoms of hydrogen Called the period of recombination even
though they weren’t combined before Previously, photons bouncing off electrons
kept their energy in thermal equilibrium with radiation (photons)
From this point on, the universe is matter-dominated
Evolution of the Universe
First stars and galaxies form: time = 0.5 x 106 years Half a million years after the Big Bang Universe has cooled sufficiently for fusion
to occur Cold enough for fusion?
Our solar system formed just over a billion years after the Big Bang
Matter and Antimatter
Does it really matter? -- Yes Early universe had nearly equal
amounts of particles and antiparticles
Theory is that there was a very slight asymmetry in particle-antiparticle pairs 1 extra particle for every 109 pairs
Matter and Antimatter
Particles collided with antiparticles to create photons
Photons turned into particles and antiparticles
Below 1013 K, photons no longer reverted
Particle/antiparticle annihilation continued until only the fragment of extra particles remained to form matter
That formed the matter we have today
Summary Review Do you understand the Hubble
classification scheme of galaxies and describe the structure of the Milky Way galaxy?
Can you state the Hubble law and solve problems using this law, v = Hd?
Can you state the meaning of the Hubble constant?
Can you identify significant epochs in the life of the universe?
Do you understand the term inflationary universe?
QUESTIONS?
#1-19
Homework