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Big Bang Nucleosynthesis Finally, Relative Abundances Are Sensitive to Density of Normal (Baryonic Matter)

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Big Bang Nucleosynthesis Finally, Relative Abundances Are Sensitive to Density of Normal (Baryonic Matter) Big Bang Nucleosynthesis Finally, relative abundances are sensitive to density of normal (baryonic matter) Thus Ωb,0 ~ 4%. So our universe Ωtotal ~1 with 70% in Dark Energy, 30% in matter but only 4% baryonic! Case for the Hot Big Bang • The Cosmic Microwave Background has an isotropic blackbody spectrum – it is extremely difficult to generate a blackbody background in other models • The observed abundances of the light isotopes are reasonably consistent with predictions – again, a hot initial state is the natural way to generate these • Many astrophysical populations (e.g. quasars) show strong evolution with redshift – this certainly argues against any Steady State models The Accelerating Universe Distant SNe appear too faint, must be further away than in a non-accelerating universe. Perlmutter et al. 2003 Riese 2000 Outstanding problems • Why is the CMB so isotropic? – horizon distance at last scattering << horizon distance now – why would causally disconnected regions have the same temperature to 1 part in 105? • Why is universe so flat? – if Ω is not 1, Ω evolves rapidly away from 1 in radiation or matter dominated universe – but CMB analysis shows Ω = 1 to high accuracy – so either Ω=1 (why?) or Ω is fine tuned to very nearly 1 • How do structures form? – if early universe is so very nearly uniform Astronomy 422 Lecture 22: Early Universe Key concepts: Problems with Hot Big Bang Inflation Announcements: April 26: Exam 3 April 28: Presentations begin Astro 422 Presentations: Thursday April 28: 9:30 – 9:50 _Isaiah Santistevan__________ 9:50 – 10:10 _Cameron Trapp____________ 10:10 – 10:30 _Jessica Lopez____________ Tuesday May 3: 9:30 – 9:50 __Chris Quintana____________ 9:50 – 10:10 __Austin Vaitkus___________ 10:10 – 10:30 __Kathryn Jackson__________ Thursday May 5: 9:30 – 9:50 _Montie Avery_______________ 9:50 – 10:10 _Andrea Tallbrother_________ 10:10 – 10:30 _Veronica Dike_____________ 10:30 – 10:50 _Kirtus Leyba________________________ Send me your preference. First come, first served. Main goals • With cosmology we aim to get a physical description of the universe. – What is the matter content? – What is the dynamics of the universe? • We have worked on a set of equations describing the universe, and we introduced a number of cosmological parameters. – Hubble parameter, scale factor, density parameter etc. • We would like to measure these parameters to get the theories right. • Eventually, we use the models to understand the origin and evolution of cosmic structure. Epochs • Friedmann's equation can be written as (Eq. 29.122): • At very early times, when R<<1, only the first two terms are important. • Radiation dominates until z~40000 • CMB released at z~1100 which is well into the matter dominated era. Planck units • GR expected to break down as Big Bang is approached • The estimate for onset of quantum mechanical effects on spacetime is when the de Broglie wavelength equals the Schwarzschild radius: • This defines what we call the Planck mass • We then define the Planck Length : • and the corresponding Planck time: • This corresponds to the earliest time that we can address by current physical theory. Our theories include a set of fundamental constants, and these are the limits of current theories. Horizon problem Start with the Horizon distance (recap) • Universe expands and ages, an observer at any point is able to see more distant objects (light has time to arrive) • As time increases, larger and larger regions of the universe come into causal contact with the observer • The proper distance to the farthest observable point at a time t is called the horizon distance, dh. – Two points separated by a distance > dh are not in causal contact – dh is the diameter of the largest causally connected region. Illustration: causal contact Insufficient time for signal to reach central observer Consider 3 locations in space, A, B & C. A&B are causally connected. A B C Particle horizon Causality problem: how can structures on large scale in the CMB know about each other? How did the temperature get so uniform? Flatness Friedmann's equation again implying during the matter-dominated era, we had • Then we achieve i.e., deviation of the total density parameter from unity grows with time. • Fine-tuning problem: example, if today At the Planck time, This is what we refer to as the flatness problem. Structure problem • Structure in the universe (galaxies, clusters of galaxies etc) came from inhomogeneities in the early universe. • We see those same inhomogeneities in the CMB maps • How was this coherence achieved? • How did the structures get there? • Why are they just the right magnitude and size to produce the structures we see today? • How is it possible to have the same kind of inhomogeneities spread through the whole universe without causal contact between different parts of the early universe? – CMB is statistically the same in all directions – Galaxies that are formed are similar in properties on opposite sides of the universe The relic problem • Analogy: consider cooling of a liquid like water • Once liquid reaches freezing point: – freezing does not occur smoothly and uniformly – starts at certain locations, and the crystals starts growing – when crystals merge to form solid, there will be dislocations where individual crystals meet • The process of freezing is called a phase transition, that is when matter is changing from one phase to another. Example: dislocations in steel • This could produce exotic structures that we call topological effects – domain walls (2d sheet-like structures) – cosmic strings (1d string-like structures) – None of these have been seen in the observable universe (good limits from CMB data: strings would gravitationally lens the background) • GUTs also predict exotic particles produced in the early universe – magnetic monopoles – never detected, and don't reveal their presence in any observed phenomena • The absence of monopoles (and other relics predicted by particle physics theories) is called the relic problem. Summary of problems with the Hot Big Bang model: • Horizon problem – how could the CMB acquire a single temperature across the sky? – causality: how can structures in the CMB know about each other? • Flatness problem – how could the density parameter be fine-tuned such that today? • The relic problem – where are all the magnetic monopoles predicted? • Origin of structure – How could structure arise in this very smooth universe In principle, the purpose of inflation is to create a large, flat, homogenous universe. • Theory of cosmic inflation was first suggested by Alan Guth in 1982 • He postulated an inflationary epoch – very rapid, exponential expansion of the universe – occurs during the first 10-37-10-32 sec – During this time, the universe expanded by a factor of 1040-10100 • Inflation and the radius of the observable universe That would correspond to a drop in the density parameter time Start of End of Now Distant inflation inflation future Timeline: Does this rapid expansion imply a violation of relativity? No, because it is space itself that is expanding, R(t), rather than material particles moving apart at high speed in a fixed, stationary space. Solving cosmological problems with inflation • The flatness problem: – take any reasonably curved surface – expand it by enormous factor – after expansion it look flat locally – so inflation predicts a universe that is indistinguishable from being flat The horizon problem (without inflation) How does inflation solve the horizon problem? • Before inflation, the particle horizon is about 10-29m – determines causally connected region • After inflation, the particle horizon is about 1011 (up to 1070) m – normal expansion takes over, and expands by another factor 1022 before the epoch of last scattering surface – Thus causally connected regions have sizes of at least 1033m! • Since decoupling, the scale factor have increased by at least a factor of 1000 (since z=1100 at decoupling), so the causally connected radius would be 1036 m – Current horizon of the universe is about 1026 m. – Thus, the observable universe is a small part of the causally connected part of the early universe! How inflation solves the structure problem: • Initial inhomogeneities due to quantum fluctuations during inflationary epoch. • Virtual particle pairs that formed would be separated by inflationary expansion before they could annihilate, creating uneven densities. • Inhomogeneities were continually created, and then stretched to much larger scales, outside the horizon • This naturally gives a characteristic power spectrum for the inhomogeneities • Fluctuations created by inflation can only grow at much later times – the particle horizon must expand so that is larger than the size scale of that fluctuation – since the particle horizon increases with time, the smaller scale fluctuations grow first – clusters, voids, are results of quantum fluctuations originally occurring on tiny scales! How inflation solves the relic problem • Suppose you have exotic particles, or structures (cosmic strings, magnetic monopoles etc) created in the very early universe • They would become very diluted during the inflationary epoch, since the space expanded enormously – The probability that we see a relic exotic particle is then very small. • Hm, what about baryons then? Wouldn’t there be a very small probability of finding them as well? – No, because baryogenesis occurred after inflation, it was too hot before. Vacuum energy is converted to regular matter, including baryons, and radiation. Why did inflation happen? The answer
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