The Big Bang TheoryBasic 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

0 2.725 KT

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

Time Temp Events10-43 s 1031 K Planck Era/Beginning?10-39 s 1029 K Beginning of Inflation10-37 s 1029 K End of Inflation, Grand Unification210-11 s 1015 K Electroweak breaking1.5 s 1010 K Proton/Neutron freezeout200 s 109 K Nucleosynthesis380,000 y 4000 K Recombination400 My 30 K First Structure13.7 Gy 2.73 K Today

The matter era

Recombination•Early on, there were nuclei,electrons, and photons•Lots of photons!

•Free electrons scatter lightefficiently•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 kyT = 4000 Ke-

p+

•Plank Era•Inflation•GUT•Electroweak•p/n freezeout•Nucleosynthesis•Recombination•First structure•Today

The Cosmic Microwave BackgroundPlanck 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 densitygrew over time:

•More dense spots: gravity draws things together•Less dense spots: become voids

•By 400 Myr, these density fluctuations werebig enough to make globular cluster scale

t = 400 MyT = 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 GyT = 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 distances2. Studying how large scale structure grew3. Studying fluctuations in the Cosmic Microwave BackgroundThe 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-p+

n0

n0

p+ e-

Proton/Neutron freezeout•At high temperatures, equal parts neutrons &protons

•At about 1.5 s, or 1010 K, protons, which are lighter, are favored as the temperature falls•At the same time, the weak reaction slowsdown, “freezes out”•Locked in at about seven protons for every neutron

e-

p+ n0

p+e-

t = 1.5 sT = 1010 K

p+

p+n0

n0

n0e-

e-e-

p+ e-

p+

e-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, 109 K, temperature is coldenough 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 (2H, 3He, 6Li, 7Li)

p+

p+

t = 200 sT = 109 K

p+

p+n0

p+

p+

p+

p+

p+p+

p+

n0

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 4He

•2H, 3He, 6Li, 7Li•All other atoms are made in stars

•Comparison with observations shows atoms = 0.046

•Yet another confirmation for Big Bang

Electroweak Unification•At low energy/temperatures, electric forcesare much stronger than weak forces•But weak forces get stronger as energyincreases•At 1015 K, these forces are equal strength•Theory and experiment says they are reallypart of a unified theory – electroweak theory

Speculated but unproven:•Dark matter may be created here•We should be able to test this experimentallyin the next couple of years at current colliders

t = 210-11 sT = 1015 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 1015 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 < 10-11 sT > 1015 K

•Plank Era•Inflation•GUT•Electroweak•p/n freezeout•Nucleosynthesis•Recombination•First structure•Today

Grand Unificationt = 10-37 sT = 1029 K

•Plank Era•Inflation•GUT•Electroweak•p/n freezeout•Nucleosynthesis•Recombination•First structure•Today

•Above 1015 K, the strong, and electromagnetic forces have different strengths•Theory says they should change as we go to higher energies•Likely that at highenergy these forcesbecome “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 densityfluctuations 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 earlierIf the universe went through a period of rapidexponential growth, then:•Places that are currently far apart startedclose together•Solves the horizon problem

•A universe that is not flat becomes very flat•Solves the flatness problem

•Small quantum fluctuations in universegrow to cause perturbations on large scales

t = 10-39 - 10-37 sT = 1029 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, thecurvature 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 1031 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 = 10-43 sT = 1031 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 < 10-43 sT ~ 1031 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