1. First Million Years Today’s Lecture: First Million Years History of the Universe Particle Era Nucleosynthesis Atoms CMB Anisotropies Problems with Standard Big Bang/ Inflation Homework 9: due today Homework 10: due Tuesday, April 29 Help Session: Thursday., May 1, 1:30 pm LRGT 1033 Final Exam: Friday, May 2, 1:30 pm, HASB 138 Reading for today and next lecture: Chapter 21

2. We have no direct knowledge of the conditions in the Universe before about 380,000 years after the Big Bang. However, we can use the laws of physics to extrapolate the conditions at much earlier times. The Universe in these early times was much denser and hotter – a regime also of interest to particle physicists. Observational Evidence for the Big Bang: Expansion of the Universe Cosmic Microwave Background (CMB)

3. We can readily extrapolate back to a time of about 10 -6 seconds (one millionth of a second) after the Big Bang. At this time T ~10 13 °K (10 trillion °K). The average particle energy and photon energy was ~3x10 9 eV or 3 GeV. At 1 second, temperature was nearly 10 billion degrees Now Backwards in Time At 10 -6 second, temperature was nearly 10 trillion degrees

4. Particle Era At time 10 -6 seconds, the Universe was composed of elementary particles (quarks and leptons) and radiation and they interacted in a way not possible today. Particles in the “Standard Model” are all detected. Each quark and lepton have their anti-particle. The Gauge Bosons are the force carriers (EM, weak and strong).

5. Spontaneous creation and annihilation of particles such as quarks/anti-quarks and electrons/positrons was possible. Two very energetic gamma ray photons could produce a particle and its anti-particle (need energy ≥ the rest mass energy), and particles and their anti-particles could meet and mutually annihilate to form two energetic gamma ray photons. Examples: e + + e - → + + → e + + e -

6. As the universe cooled to below 10 13 K, quarks could be confined in groups of 3 to make baryons. Of the baryons, only the proton and neutron are long lasting. However, the neutron is unstable and beta decays with a half-life of 611 s (n → p + e - + anti-ν e ) Soon after, radiation could no longer create protons/anti-protons or neutrons/anti-neutrons (rest mass energy is 940 MeV more than the energy of the gamma-rays). If there were equal numbers of particles and their anti-particles, as the universe cooled, they would all annihilate – have no matter !!!!

7. Early in the universe, there developed a slight excess of matter over anti-matter (the laws of physics are slightly different for matter as opposed to anti-matter) but this is poorly understood. For every billion anti-protons there was 1 billion and 1 protons. The slight excess of matter over anti-matter makes up all of the ordinary matter in the present-day universe. Note that after annihilation, there was roughly 1 billion photons for every proton (Universe dominated by radiation).

8. However, still possible for neutrons and protons to transform back and forth via the weak interaction, equal number of each. p + e - → n + ν e, and ν e + n → e - + p As the temperature fell below 10 10 °K (time about 1 second after Big Bang), reactions favored the formation of protons, since neutrons are slightly more massive than protons (take energy to form). At end of this era, protons out numbered neutrons 6 to 1. The neutrons would all have decayed unless some new process intervened.

9. Proton Neutron Deuterium Hydrogen-3 Helium Era of Nucleosynthesis For a few minutes the universe was hot enough (10 8 -10 9 °K) to allow for fusion reactions to synthesize the light elements. Since there are free neutrons, first reaction: p + n → 2 H proceeds rapidly because there is no Coulomb barrier). However, nuclei that formed were rapidly broken apart by the numerous high energy photons.

10. Proton Neutron Deuterium Hydrogen-3 Helium At a time about 100 s after Big Bang, nuclei formed could survive. By this time the proton/neutron ratio had increased to 7 due to neutron decay. Since Universe is rapidly cooling (density much lower than in the interior of stars), no possibility for helium fusion. Fusion could continue by adding protons or neutrons, however no stable element of atomic number 5 and 8. Form H and 4 He and very small amounts of 2 H, 3 He, 7 Li, and 8 Be.

11. Abundance of the Elements Thus, the first four minutes of the Big Bang determined the abundance of the elements in the universe. Remember that the ratio of protons to neutrons was 7. Nearly all of the neutrons are fused into 4 He. Thus, by number the ratio of hydrogen to helium is 12, but by mass, the universe was about 3/4 hydrogen and 1/4 helium. Abundances consistent with the oldest stars in the universe.

12. The amount of deuterium made in Big Bang nucleosynthesis is very sensitive to the density of ordinary matter. The measured abundance of deuterium of about 1deuterium atom per 40,000 hydrogen atoms, suggests that the density of ordinary matter (baryons) is only about 4% of the critical density !!

13. The early universe was composed primarily of Hydrogen (H) and Helium ( 4 He), with small amounts of Deuterium ( 2 H) and 3 He and yet smaller amounts Lithium ( 7 Li) and Beryllium ( 7 Be).

14. Photons outnumber baryons a billion to one. The mass/energy density of the Universe is dominated by photons. The density of baryons in the Universe goes as the scale factor R(t) -3. However, photons are stretched as the Universe expands, so the photon energy density goes as R(t) -4. Eventually, the mass/energy density of the Universe becomes matter dominated (time of a few thousand years).

15. Era of Atoms For first 380,000 years, the universe was too hot for electrons to become permanently attached to the atomic nuclei. Universe a hot plasma of hydrogen nuclei, helium nuclei, and free electrons. Photons interact readily with electrons, so they could only travel a short distance before being absorbed – Universe opaque. As the universe cooled below 3000°K the electrons combined with the nuclei (time of recombination) forming neutral atoms. After recombination the universe became transparent to light. Before, universe opaque to light After, universe transparent to light

16. As you look out in space and back in time, you can see back to the era when the universe was opaque. Radiation was initially blackbody emission at a temperature of about 3000 °K. We see now the light redshifted by a factor of about 1000 - the cosmic microwave background.

17. Acoustic Waves Before recombination, radiation, protons and electrons behaved as a single gas. Disturbances in the gas density propagated as acoustic or sound waves in the early universe (these were very small perturbations in the density). In the compressed regions the gas is heated and in the rarefied regions it is cooled. At the time of recombination, the radiation is free to travel unhindered through space, and the pattern of hot and cold spots is frozen into the CMB. The fundamental acoustic wave sets the largest scale of these fluctuations, corresponding to a size of about 150 Mpc.

18. Fi Illustration of the acoustic waves that were generated at a very early epoch in the universe – evolution of baryons, photons, and dark matter.

19. These acoustic waves set a maximum size scale, the observed angular scale now of the fluctuations in the CMB tells us about the geometry of space-time.

20. The WMAP and Planck results shown below provide a measure frequency (or power) of fluctuations versus angular scale. The peak at the largest angular scale (about 1 degree) is the fundamental acoustic wave and the smaller scale peaks the overtone waves.

21. Convincing evidence that we live in a flat universe, Ω = 1, to better than 1%. The secondary peaks tell us about baryon content, find that it is 4.5% of the critical density of Universe, Ω B = 0.045, consistent with nucleosynthesis. The fluctuations versus angular scale for different geometries of space. The location of the first peak corresponds to the angular scale of the largest fluctuations and depends on whether the Universe is closed (positive curvature), flat, or open (negative curvature).

22. Inflation What we have just described is called the standard Big Bang model. However, it has two “fine-tuning” problems: At time of recombination, the average density and temperature of the universe were remarkable similar everywhere. (1) Why is the large scale structure of CMB so uniform (called the horizon problem) ?

23. When we look in opposite directions on the sky, the CMB is virtually identical. Yet, these regions of space could never have interacted (not causally connected), so why are they exactly the same ? Would expect measurable variations.

24. (2) Why is the universe so Flat ? The geometry of the universe depends on the average density. However, nothing in the Big Bang theory suggests what the density should be. Why isn’t the density 1,000 times higher or lower (or a billion times for that matter) than the critical density. For the Universe to be so close to flat today, at a time 10 -9 seconds after the Big Bang, the density had to be fine-tuned to about one part in 10 25 !!!!

25. Alan Guth in 1981 was trying to solve the problem of the absence of magnetic monopoles. Suggested inflation and recognized it might solve the fine tuning problems. First consider the forces in nature. The “standard model” explains the weak, electromagnetic and strong forces in terms of “exchange particles” or carriers that account for the action of a force. Gravity not part of the standard model, and is currently described by general relativity.

26. Unification of Forces – Are all forces a manifestation of one larger force? Electroweak interaction is a unified description of the electromagnetic and weak forces (developed late 1960s). At today's energies, they appear quite different, however, at very high energies (>100 GeV), they merge as a single force – symmetry between the forces. We have only some ideas (goes beyond the “standard model”) on how the strong force is unified with the electro-weak force often called Grant Unified Theories or super-symmetric models.

27. A result of symmetry breaking as the grand unified force splits into the electroweak and strong forces is that the universe is left in a metastable state (false vacuum). In quantum physics, the vacuum is the state of lowest energy. As the universe transitions to the true vacuum, enormous energy was released that caused rapid expansion. This is much like a phase transition, where latent heat is liberated. In less than 10 -30 seconds, the universe expanded by a factor of order ~10 40. When the true vacuum was reached, inflation stopped.

28. The release of this “vacuum” energy causes the universe to inflate. At the end of inflation, the universe continues its evolution as in the standard Big Bang. March 2014, BICEP2 collaboration announced the detection of B-mode polarization in CMB – strong evidence for inflation and gravitational waves.

29. The Horizon Problem Note that inflation takes regions of space originally in causal contact and inflates them so they are not in causal contact today. The regions are so identical, because they were originally equalized by their close contact.

30. The Flatness Problem Inflation solves the flatness problem. The inflation expands space, and like a balloon, as it expands its surface becomes less and less curved. As the universe expands, regions of space become flatter and flatter. The enormous inflation of space would essentially eliminate any initial curvature within the observable universe.

31. The Vacuum: The vacuum of quantum mechanics is not the uninteresting empty space of classical physics. The Heisenberg Uncertainty Principle results in the quantum vacuum being a sea of continuously appearing and disappearing particles (particle and anti-particle pairs). These represent minute density fluctuations. The presence of these “virtual” particles has observable effects (Lamb shift and Casimir effect). Inflation blows these minute density fluctuations to macroscopic sizes and in fact produces just the correct spectrum of fluctuations needed for the growth of structure in the universe

32. Added Bonus for Inflation Inflation takes quantum fluctuations in the very early universe and expands from to macroscopic scales. These macroscopic scale fluctuations grow are the origin of the acoustic waves and. These fluctuations are the origin of all structure seen in the universe today.