The Big Bang Theory Basic Idea There was a time when the whole universe was together Called the big bang Label it t = 0 It is impossible to answer which galaxies are “really” moving Probably meaningless to ask “where” Probably everywhere The Universe began as a giant explosion It has been expanding and cooling every since We will label early times by when they happened and what the temperature was

The Temperature of the Universe The early universe was filled with high energy light Early on, the Universe was very dense Everything interacted with everything It was in thermal equilibrium with nearby objects As the universe expanded light wavelengths got stretched This is just another way of thinking about red shift As the universe expands, it cools This light is still with us – it is just very cold It is now microwaves

The Cosmic Background Radiation We can see the “light” left over from the Big Bang Radio telescopes, later spacecraft It is almost perfectly thermal Wilkinson Microwave Anisotropy Probe Planck Observatory

Same Temperature in All Directions? Same Temperature in all directions Almost Slightly hotter in one direction Because of our motion This can be subtracted Slightly hotter in plane of galaxy This can also be subtracted There are small variations that remain A few parts per million More about this later

Outline of History of Universe TimeTempEvents s10 31 KPlanck Era/Beginning? s10 29 KBeginning of Inflation s10 29 KEnd of Inflation, Grand Unification 2  s10 15 KElectroweak breaking 1.5 s10 10 KProton/Neutron freezeout 200 s10 9 KNucleosynthesis 380,000 y4000 KRecombination 400 My30 KFirst Structure 13.7 Gy2.73 KToday The matter era

Recombination Early on, there were nuclei, electrons, and photons Lots of photons! Free electrons scatter light efficiently Universe is opaque It was so hot atoms rarely formed Any that did form were destroyed by high energy photons At t = 380,000 y, universe cooled to 4000 K Cool enough for atoms to form Universe becomes transparent – CMBR forms t = 380 ky T = 4000 K e-e- p+p+       Plank Era Inflation GUT Electroweak p/n freezeout Nucleosynthesis Recombination First structure Today

The Cosmic Microwave Background Planck data March 21, 2013

First Structure Forms The universe is definitely not uniform today Dense spots, less dense spots But at t = 380,000 yr, it was nearly so We think the tiny variations in the density grew over time: More dense spots: gravity draws things together Less dense spots: become voids By 400 Myr, these density fluctuations were big enough to make globular cluster scale t = 400 My T = 30 K Plank Era Inflation GUT Electroweak p/n freezeout Nucleosynthesis Recombination First structure Today

First Structure Forms  Today Globular cluster size clouds form First stars form Small objects merge to make small galaxies Galaxies gather to make clusters Large galaxies form from mergers Superclusters form t = 400 My – 13.7 Gy T = 30 K – 2.7 K Plank Era Inflation GUT Electroweak p/n freezeout Nucleosynthesis Recombination First structure Today

Three different methods help us learn the order and structure of the universe: 1. Studying White Dwarf Supernovae distances 2. Studying how large scale structure grew 3. Studying fluctuations in the Cosmic Microwave Background The three methods give very consistent results:  m  0.30,  e  0.70 What the Evidence Tells Us

The four forces of nature Gravity Holds the Solar System together Electromagnetic Holds atoms together Strong Nuclear (Nuclear) Holds the nucleus together Weak Nuclear Radioactive decay

The Weak Force The weak force can convert protons to neutrons, and vice versa proton + electron  neutron + neutrino Weak force is weak (slow) today, because energies were low It is stronger at higher energies In the early universe, temperature hotter, they were faster Also, lots of electrons and neutrinos around Protons and neutrons were in equilibrium e-e- p+p+ n0n0 n0n0 p+p+ e-e-

Proton/Neutron freezeout At high temperatures, equal parts neutrons & protons At about 1.5 s, or K, protons, which are lighter, are favored as the temperature falls At the same time, the weak reaction slows down, “freezes out” Locked in at about seven protons for every neutron e-e- p+p+ n0n0 p+p+ e-e- t = 1.5 s T = K p+p+ p+p+ n0n0 n0n0 n0n0 e-e- e-e- e-e- p+p+ e-e- p+p+ e-e- p+p+ Plank Era Inflation GUT Electroweak p/n freezeout Nucleosynthesis Recombination First structure Today

Primordial Nucleosynthesis At high temperatures, too hot for nuclei to fuse At about 200 s, 10 9 K, temperature is cold enough for neutrons to stick to protons Quickly thereafter, Helium nuclei are built up Universe ends up (by mass) 25% He, 75% H Small amounts of other stuff ( 2 H, 3 He, 6 Li, 7 Li) p+p+ p+p+ t = 200 s T = 10 9 K p+p+ p+p+ n0n0 p+p+ p+p+ p+p+ p+p+ p+p+ p+p+ p+p+ n0n0 p+p+ p+p+ p+p+  Plank Era Inflation GUT Electroweak p/n freezeout Nucleosynthesis Recombination First structure Today

Primordial Nucleosynthesis Hydrogen and Helium formed in first few minutes

Primordial Nucleosynthesis Fraction of isotopes depends on how much ordinary matter there is A few isotopes besides 4 He 2 H, 3 He, 6 Li, 7 Li All other atoms are made in stars Comparison with observations shows  atoms = Yet another confirmation for Big Bang

Electroweak Unification At low energy/temperatures, electric forces are much stronger than weak forces But weak forces get stronger as energy increases At K, these forces are equal strength Theory and experiment says they are really part of a unified theory – electroweak theory Speculated but unproven: Dark matter may be created here We should be able to test this experimentally in the next couple of years at current colliders t = 2  s T = K Plank Era Inflation GUT Electroweak p/n freezeout Nucleosynthesis Recombination First structure Today

Speculation vs. Reality Using current colliders, we can see effects up to a temperature of about K Above this energy, we have no experimental evidence As we work our way to earlier times/higher temperatures, we are speculating From now on, we grow increasingly uncertain of our conclusions t < s T > K Plank Era Inflation GUT Electroweak p/n freezeout Nucleosynthesis Recombination First structure Today

Grand Unification t = s T = K Plank Era Inflation GUT Electroweak p/n freezeout Nucleosynthesis Recombination First structure Today Above K, the strong, and electromagnetic forces have different strengths Theory says they should change as we go to higher energies Likely that at high energy these forces become “unified” into a single force This is called “Grand Unification” Speculative, unproven Could be time when all ordinary matter formed

Unsolved Problems in Cosmology Why did the universe start so uniform? Things that are far apart look similar The horizon problem Why is  so close to 1? The flatness problem What is the origin of the density fluctuations we see? Where did all the matter come from? What is the nature of the dark matter? What is the nature of the dark energy? Inflation may solve these Plank Era Inflation GUT Electroweak p/n freezeout Nucleosynthesis Recombination First structure Today

Inflation The universe is currently undergoing a (very slow) exponential growth Maybe it did so earlier If the universe went through a period of rapid exponential growth, then: Places that are currently far apart started close together Solves the horizon problem A universe that is not flat becomes very flat Solves the flatness problem Small quantum fluctuations in universe grow to cause perturbations on large scales t = s T = K Plank Era Inflation GUT Electroweak p/n freezeout Nucleosynthesis Recombination First structure Today

How Inflation Solves Flatness: The universe started off very curved, like a small sphere As the universe grows, the curvature gets much smaller

Graphic from WMAP

The Cosmic Microwave Background These fluctuations may be signatures of inflation

The Planck Era At the grand unified scale, gravity is weaker than the other forces But getting stronger as energy increases! At a temperature of about K, it is as strong as the others Maybe all forces are unified! This scale is called the “Planck Scale” A theory unifying all forces is called a “Theory of Everything” We don’t know what it would look like We have lots of candidates String theory, Loop quantum gravity, etc. t = s T = K Plank Era Inflation GUT Electroweak p/n freezeout Nucleosynthesis Recombination First structure Today

What Came Before the Big Bang? No one knows One possibility: Eternal/Chaotic Inflation Inflation went on forever, and is still going on One small pocket escaped and became “our universe” Other pockets escaped and became others My guess: Space and Time was created in the big bang Time becomes quantum uncertain “Before” becomes meaningless Plank Era Inflation GUT Electroweak p/n freezeout Nucleosynthesis Recombination First structure Today t < s T ~ K

What is the Nature of Dark Matter? We don’t know, but there are always kooks who try to guess

What is the Nature of Dark Energy? We don’t know, but there are always kooks who try to guess