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 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 – 10 9 to 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 10 6 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

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·Mpc

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

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

Evolution of the Universe  Temperature estimated to be K  The kinetic energy of whatever particles that existed would be

Evolution of the Universe  Forces are unified: time = s  Strong nuclear and electroweak forces unified  Leptons indistinguishable from quarks and turned into each other  Strong nuclear force separates: time = s  Strong nuclear force decoupled from the electroweak force  Temperature falls to K

Evolution of the Universe  Inflation begins: time = s  Rapid period of expansion called inflationary epoch  Lasted no more than s  Size of the universe increased by factor of  Forces separate: time = s  Temperature is about K  Four fundamental forces (gravity, electromagnetism, strong nuclear, and weak nuclear) behave as separate forces  Universe was of its present size

Evolution of the Universe  Nucleons form: time = s  Temperature fallen to K  Quarks bind together to form protons and neutrons and their antiparticles  Universe was of its present size  At t = 1s, T = K  Protons, neutrons, electrons and their antiparticles in thermal equilibrium

Evolution of the Universe  Nuclei form: time = 3 min  Temperature fallen to 10 9 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 10 4 years  At the end, the temperature was 10 4 K  The size of the universe was of its present size

Evolution of the Universe  Atoms form: time = 3 x 10 5 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 10 6 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 10 9 pairs

Matter and Antimatter  Particles collided with antiparticles to create photons  Photons turned into particles and antiparticles  Below 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?

#1-19 Homework