The Big Bang

Attendance Quiz

Are you here today?

(a) yes

(b) no

(c) Big Bang! Big Crunch! Nestle’s Crunch! Mmmm…

Here!

Final Exam

• The final exam is Thursday, 6/9, from 11:30am to 1:30pm (2 hours), in this room; please arrive early!

• The final exam will be comprehensive, i.e., it will cover all the material you have studied this quarter

• It will be multiple choice, so make sure to bring a 100-question (2-sided) scantron to class!

Homework reminder

• For homework, complete the ranking task “Stellar Evolution and Lookback Time” on course website

Reselling Your Clicker

• Reminder: you can resell your clicker to the bookstore after this class

• You may also be able to sell it someone who will use it another class

Today’s Topics

• The Beginning of Time• The Big Bang

• Cosmic Background Radiation• The Helium Problem• Big Bang Helium Nucleosynthesis• Inflation

• Discussion/Questions

The Big Bang • Since the universe is expanding, and the galaxies are moving farther apart with

time, then at some earlier time, they must have been closer together• If we imaging running the “movie” of the universe back in time far enough, the

universe would become denser and hotter (Big Crunch movie)

The Big Bang • As we run the “movie” further

back, the density and temperature keep increasing (note axis scales!)

• At high enough temperatures (> 3000 K), before stars and galaxies formed, the universe would be ionized (~400,000 yrs)

• Thus, the very early universe was a hot, dense, fireball of interacting particles and photons

• The expansion of the universe from this early, hot phase is called The Big Bang - (movie)

• The Big Bang is not an explosion -it is just the time at which the universe started expanding

The Helium Problem• We have seen how stars of various

masses form all the elements heavier than hydrogen, from helium to uranium, through nuclear fusion and supernovae

• However, if one calculates the rate of helium production over the age of the universe by all the stars, it produces an abundance of helium of only ~10% by mass

• Observations of stars and galaxies show, however, that ~24% of all the mass of stars is helium

• Furthermore, there is almost no correlation between oxygen and helium abundance, suggesting they come from different places

• Where did this helium come from?

Big Bang Nucleosynthesis of Helium• Where else was the temperature high

enough to drive nuclear fusion?• During the Big Bang!• During the first three minutes of the

universe, the temperature was greater than 109 K, hot enough to fuse H → He

• What determined the actual amount of helium formed? Why wasn’t all the hydrogen fused into helium?

• Very early, things were too hot for any fusion to stick—when a proton and neutron fused into deuterium, it was immediately destroyed by a gamma ray

• Above 1011 K, there was so much energy available, protons and neutrons converted back and forth freely (even though mn > mp) so that Nn ~ Np

n + e+ ↔ p + νe

n + νe ↔ p + e−

Big Bang Nucleosynthesis of Helium• Below 1011 K, the mass difference (in

energy units via E = mc2) was too large to convert p → n, so neutrons started decaying into protons

n → p + e− + νe

• At the temperature where deuterium could form (and not be destroyed), about 1010 K, the neutrons and protons quickly formed deuterium which then combined to form 4He

• At the time (and temperature) that this happened, we can calculate (from the neutron to proton conversion rate), that there were ~7 protons for each neutron, a ratio of 14:2

• Since all the available neutrons were incorporated into 4He nuclei, this leads to 12 protons for each 4He nucleus, a mass ratio of 12:4 or 75:25, exactly as observed!

Big Bang Nucleosynthesis• Even more detailed calculations,

which include other elements, such as D, 3He, and 7Li, are all consistent with Big Bang predictions, and further predict that only 4.6% of the mass-energy in the universe is ordinary matter

• Recall that current measurements suggest that this is correct, with the remaining ~95% made up of dark (non-ordinary) matter and dark energy

The Early Universe• The average energy of particles in the early universe depends on their temperature• As long as the matter is ionized, the photons of light will interact strongly with the

matter, and will have the same average energy as the particles• After the universe cools below the ionization temperature, the photons of light

flow freely through the universe filling it with light (Interactive Figure 23.6)

The Early Universe• As the universe expands from this time forward, this radiation is stretched by the

expansion of the universe to longer wavelengths (Interactive Figure 20.24)• Since this is thermal radiation, this is equivalent to the radiation cooling

(Wein’s Law) — longer wavelength = lower temperature• Following the expansion of the universe from 400,000 years to today, astronomers

as far back as the 1950s, had predicted that, if the Big Bang had occurred, the universe should be filled with thermal radiation of about 3 K temperature

Cosmic Background Radiation• In 1965, two physicists at Bell Labs were

looking for sources of interference with microwave communications

• After identifying all other sources of radiation, they found a leftover source of “noise” coming equally from all directions

• They considered this “noise” to be an embarrassment, and buried their results at the end of a long paper on their work

• In fact, they had discovered the radiation left over from the Big Bang — the Cosmic Background Radiation

• More detailed observations of this radiation agree very precisely with thermal radiation of T = 2.73 K in exceptionally good agreement with the predictions of the Big Bang

Penzias and Wilson

COBE satellite

Wilkinson Microwave Anisotropy Probe (WMAP)

COBE satellite

ΔTT~ 10−5

Wilkinson Microwave Anisotropy Probe (WMAP)

COBE satellite

Contents of the Universe• WMAP’s results confirmed that only 4.6% of the matter in the Universe is

regular matter while 23% is made up of dark matter. • The remaining 72% is dark energy.

The Big Bang and Inflation• What caused the universe to be so close to exactly flat (Ω = 1)?• Also, the relative lack of structure in the cosmic background radiation is a puzzle

because at the time the CBR was scattered for the last time (when the universe was 400,000 years old), widely separated parts of the universe would not have had time to “communicate” and hence equalize their temperatures

The Big Bang and Inflation• The relative lack of structure in the cosmic background radiation is a

puzzle because at the time the CBR was scattered for the last time (when the universe was 400,000 years old), widely separated parts of the universe would not have had time to “communicate” and hence equalize their temperatures

Note  the  timescale!

Inflation• However, in 1981, Alan Guth proposed

that in the first 10−35 sec of the universe, as it cooled, an initially unified set of three of the four forces of nature (EM, weak nuclear, strong nuclear), known as the Grand Unified Theory (GUT) force, split into the strong nuclear force and the electroweak force

• This split released an enormous amount of energy (like the latent heat of a phase transition) causing a rapid expansion (by a factor of 1030) in the size of the universe, called inflation

• During this expansion, regions that had been in close contact were carried to their present distant separation, explaining the uniform temperature of the CBR (Interactive Figure 23.14)

Triumphs of the Big Bang and InflationTogether, the Big Bang and Inflation explain a large number of facts1. Existence and uniformity of CBR2. 4He abundance of 25% by mass3. The universe is 13.7 billion years old4. The total mass-energy of the universe is

almost exactly the critical density of the universe

5. The density of “ordinary” matter (protons, neutrons, electrons) is 4.6% of the total mass-energy of the universe

6. The total matter density (ordinary and dark matter) is 28% of the total mass-energy

7. There exists another form of mass-energy, dark energy, which accounts for 72% of the mass-energy of the universe

Exam review

• Take out your exam scantrons• We will spend the rest of class allowing you to

review your exams and ask me questions about them• You can also ask me any questions you like about

the tutorials or anything else we have covered in class