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Cosmic Microwave Background Primodial Nucleosynthesis The Early Universe

SPECIAL END OF SEMESTER LECTURE. Cosmic Microwave Background Primodial Nucleosynthesis The Early Universe. The Cosmic Microwave Background (CMB). Observational discovery of the CMB The hot big bang model What can we learn from the CMB?.

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Cosmic Microwave Background Primodial Nucleosynthesis The Early Universe

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  1. SPECIAL END OF SEMESTER LECTURE • Cosmic Microwave Background • Primodial Nucleosynthesis • The Early Universe

  2. The Cosmic Microwave Background (CMB) • Observational discovery of the CMB • The hot big bang model • What can we learn from the CMB?

  3. I : THE OBSERVATIONAL DISCOVERY OF THE COSMIC MICROWAVE BACKGROUND Penzias & Wilson (Bell-Labs)

  4. Arno Penzias & Robert Wilson (1964) • Attempted to study radio emissions from our Galaxy using sensitive antenna built at Bell-Labs • Needed to characterize and eliminate all sources of noise • They never could get rid of a certain noise source… noise had a characteristic temperature of about 3 K. • They figured out that the noise was coming from the sky, and was approximately the same in all directions…

  5. II : THE HOT BIG BANG MODEL • Penzias & Wilson had discovered radiation left over from the early universe… • The hot big bang model… • Developed by George Gamov • He suggested that the universe started off in an extremely hot state. • As the Universe expands, the energy within the universe is spread over in increasing volume of space… • Thus the Universe cools down as it expands

  6. From Black Body Radiation Eq. (4.5) Energy Density E~T4 But E=(# photons/L3)hn =(# photons/L3)(hc/l) ~1/R4 T~1/R T(t)=T(to) [R(to)/R(t)] = T(to) (1+z) If T= 2.7 K today at z=1 it was 5.4 K

  7. Why did they suggest this model? • If the early Universe was hot (full of energy), a lot of features of the current universe could be explained… • Could explain where the matter that we see around us came from (baryogenesis occurred well within first second) • Could explain the observed ratio of elements (nucleosynthesis occurred within first few minutes) • Predicted that there should be left over radiation in the present Universe…

  8. A brief look at the stages of the Universe’s life… • Most crude description… • t=0: The Big Bang • For first 400,000ys, universe is an expanding “soup” of tightly coupled radiation and matter • After 400,000yrs, radiation and matter “decouple”. They are free to flow and expand independently of each other. • This is the left over radiation that we see now as the CMB…

  9. III : THE COSMIC BACKGROUND EXPLORER (COBE)

  10. The COBE mission • Built by NASA-Goddard Space Flight Center • Launched Nov. 1989 • Purpose was to survey infra-red and microwave emission across the whole sky. • Primary purpose – to characterize the CMB. • Had a number of instruments on it: • FIRAS (Fair infra-red absolute spectrophotometer) • DMR (Differential Microwave Radiometer) • DIRBE (Diffuse Infrared background Experiment)

  11. Our Galaxy observed by the DIRBE instrument on COBE

  12. Back to the CMB: DMR map of the microwave sky…

  13. Map of the microwave sky (frequency of 50GHz)… • We’re looking at the CMB • The map is very uniform. • Means that the CMB is extremely isotropic (i.e. the same in every direction we look) • Supports the idea that the Universe is isotropic (one of the basic cosmological principles). • In fact, if we measure the universe to be isotropic, and we’re not located at a special place in the Universe, we can also deduce that the Universe is homogeneous!

  14. The spectrum of the CMB (FIRAS)

  15. Spectrum has precisely the shape predicted by the theory… • So-called “Blackbody” spectrum • Characteristic temperature of 2.728K

  16. FLUCTUATIONS

  17. Left with a random pattern of fluctuations in the CMB… correspond to temperature differences of 30 millionths of a Kelvin

  18. What are these fluctuations… • The early universe was very close to being perfectly homogeneous • But, there were small deviations from homogeneity… some regions were a tiny bit colder and some were a tiny bit hotter. • When matter and radiation decoupled, this pattern of fluctuations was frozen into the radiation field. • We see this nowadays as fluctuations in the CMB.

  19. Why are the fluctuations important? • Before decoupling, fluctuations in the radiation field also meant fluctuations in the mass density • After decoupling, these small fluctuations in density can get amplified (slightly dense regions get denser and denser due to gravity). • These growing fluctuations eventually collapse to give galaxies and galaxy clusters. • So, by studying these fluctuations, we are looking at the “seeds” that grow to become galaxies, stars, planets, people, dogs, cats etc.

  20. The geometry of the Universe… • From the cosmic microwave background… • We know how far apart these “blobs” should be on average. • Can use this knowledge to make a giant triangle.

  21. us Distance to decouping epoch

  22. Result: • The universe is flat • In terms of density, =1 to better than 1% • How do we reconcile this with our direct measurement of the density?

  23. Primordial Nucleosynthesis • The structure of “normal” matter • Nucleosynthesis and the hot big bang • The density of baryonic matter in the Universe, B

  24. III : The accelerating Universe • Huge clue came from observations of Type-1a Supernovae (SN1a) • Very good “standard candles” • Can use them to measure relative distances very accurately

  25. Type 1A Supernovae

  26. What produces a SN1a? • Start off with a binary star system • One star comes to end of its life – forms a “white dwarf” (made of helium, or carbon/oxygen) • White Dwarf starts to pull matter off other star… this adds to mass of white dwarf (accretion) • White dwarfs have a maximum possible mass… the Chandrasekar Mass (1.4 MSun) • If accretion pushes White Dwarf over the Chandrasekar Mass, it starts to collapse.

  27. The results… • This program gives most accurate value for Hubble’s constant • H=65 km/s/Mpc • Find acceleration, not deceleration! • Very subtle, but really is there in the data! • Profound result!

  28. Dark Energy • There is an “energy” in the Universe that is making it accelerate • Call this “Dark Energy” • This makes up the rest of the gravitating energy in the Universe, and causes it to be flat! • Completely distinct from “Dark Matter” • Remember Einstein’s cosmological constant…? • Dark Energy has precisely the same effect as Einstein’s cosmological constant • So, he was probably right all along!

  29. I : SOME BACKGROUND: THE STRUCTURE OF MATTER • Atom is made up of… • Nucleus (very tiny but contains most off mass) • Electrons (orbit around the nucleus) • Atom held together by attraction between positively-charged nucleus and negatively-charged electrons.

  30. The nucleus is itself made up of: • Protons, p (positively charged) • Neutrons, n (neutral; no charge) • Collectively, these particles are known as baryons • p is slightly less massive than n (0.1% difference) • Protons and neutrons bound together by the strong nuclear force (exchange of “gluons”)

  31. Number of protons determines element: • Hydrogen – 1 proton • Helium – 2 protons • Lithium – 3 protons • Beryllium – 4 protons • Boron – 5 protons • Carbon – 6 proton • … • Number of neutrons determines the isotope… e.g., for hydrogen (1 proton), there are three isotopes • Normal Hydrogen (H or p) – no neutrons • Deuterium (d) – 1 neutron • Tritium (t) – 2 neutrons

  32. There’s one more level of complexity… not needed for this discussion, but generally useful to know: • Protons & Neutrons are made up of trios of quarks • Up quarks & Down quarks • Proton = 2 up quarks + 1 down quark • Neutron = 1 up quark + 2 down quarks • There are other kinds of quarks (strange, charm, top, bottom quarks) that make up more exotic types of particles…

  33. II : NUCLEOSYNTHESIS IN THE EARLY UNIVERSE • Nucleosynthesis: the production of different elements via nuclear reactions • Consider universe at t=180s • i.e. 3 minutes after big bang • Universe has cooled down to 1 billion K • Filled with • Photons (i.e. parcels of electromagnetic radiation) • Protons (p) • Neutrons (n) • Electrons (e) • [also Neutrinos, ghostly photon-like particles]

  34. Protons and Neutrons can fuse together to form deuterium (d) • p+n->d • But, deuterium is quite fragile…

  35. The first three minutes… • Before t=180s, Universe is hotter than 1 billion degrees. • High-T means that photons carry a lot of energy • Deuterium is destroyed by energetic photons as soon as it forms

  36. After the first 3 minutes… • But, after t=180s, Universe has cooled to the point where deuterium can survive • Deuterium formation is the first step in a whole sequence of nuclear reactions: • Helium-4 (4He) formation:

  37. An alternative pathway to Helium… • This last series of reactions also produces traces of left over “light” helium (3He)

  38. Further reactions can give Lithium (Li) • Reactions cannot proceed beyond Lithium due to the “stability gap”

  39. If this were all there was to it, then the final mixture of hydrogen & helium would be determined by initial number of p and n. • If equal number of p and n, everything would basically turn to 4He… Pairs of protons and pairs of neutrons would team up into stable Helium nuclei. • Would have small traces of other species • But we know that most of the universe is hydrogen… clearly there is something more interesting going on.

  40. Neutron decay • Free neutrons (i.e., neutrons that are not bound to anything else) are unstable! • Neutrons spontaneously and randomly turn in to protons • Half life for this occurrence is 15 mins (i.e., take a bunch of free neutrons… half of them will have decayed after 15 mins).

  41. While the nuclear reactions are proceeding, supply of “free” neutrons is decaying away. • So, speed at which nuclear reactions occur is crucial to final mix of elements • What factors determine the speed of nuclear reactions? • Density (affects chance of p/n hitting each other) • Temperature (affects how hard they hit) • Expansion rate of early universe (affects how quickly everything is cooling off).

  42. Full calculations are complex. We need to: • Work through all relevant nuclear reactions • Take account of decreasing density and decreasing temperature as Universe expands • Take account of neutron decay • Feed this into a computer… • Turns out that relative elemental abundances depend upon the quantity BH2 • Here, B is the density of the baryons relative to the critical density.

  43. From M.White’s webpage, UC Berkeley Bh2

  44. So, by measuring the abundance of H, D, 3He, 4He, and 7Li, we can • Test the consistency of the big bang model • Use the results to measure the quantity Bh2 • We can use the spectra of stars and nebulae to measure abundances. • Why is it not straightforward to then use these measurement to test the big bang theory and measure Bh2 ?

  45. Results • All things considered, we have Bh20.015. • If H0=65km/s/Mpc, • h=0.65 • B0.036 Bh2

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