Astronomy - Stat eof the Art - Cosmology


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Astronomy - State of the Art is a course covering the hottest topics in astronomy. In this section, the properties of the whole universe are covered, including Hubble expansion, the age and size, the big bang, and dark energy.

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Astronomy - Stat eof the Art - Cosmology

  1. 1. Science is Seeing Space
  2. 2. A Scale Model Set the Earth to the size of a walnut, or a 1:10,000,000 scale model = • The Moon is a pea at arm’s length • The Sun is a 3 m ball 100 m away • Neptune is another pea 2 km away • The nearest star is 50,000 km away
  3. 3. • The Moon is a seconds walk away • The Sun is 8 minutes walk away • 10 hours to walk the Solar System • A year to walk to the nearest stars And at this scale, light is reduced to very slow walking speed. There’s no way information in the universe can travel any faster
  4. 4. Reduce the scale by a factor of 100,000,000 • The Solar System is a grain of sand • The distance between stars is 10 m • The Milky Way is the size of the U.S. • The MW has 100,000,000,000 stars
  5. 5. Now reduce by another factor of 100,000,000 • The Milky Way is the size of a plate • The nearest galaxy is 10 m away • The universe is the size of the U.S. • Billions of galaxies within this space
  6. 6. How Empty? A one-inch cube of the air you’re breathing holds 1020 atoms in it The average density of the universe is 1022 times lower, about 1 atom per cubic meter
  7. 7. For an idea of the true density of the universe, imagine the 1-inch cube of air stretched upwards …all the way to the Andomeda galaxy (M31)
  8. 8. Olbers Paradox
  9. 9. “The night sky is dark.” This statement is Olbers’ paradox, after astronomer who discussed the subject in 1823, before galaxies were discovered. Why is the darkness of the night sky paradoxical? In an infinite universe with infinite number of stars, a mathematical paradox arises.
  10. 10. In a large enough forest, every line of sight ends at a tree. In a large enough universe, every sight line ends in a star. The sky should not be dark.
  11. 11. The surface area of a spherical shell: Area = 4 π r2 The volume of the spherical shell: Volume ≈ area × thickness ≈ 4 π r2 t How many stars are in the shell? Number = volume × n = 4 π r2 t n The flux from a single star: 2 r4 L Flux   The flux from all the shell’s stars: Total flux = Number of stars × flux per star 2 2 r4 L ntr4fluxTotal    Total flux of shell = t × n × L
  12. 12. Total amount of light is independent of r, the distance of the shell from us! Total flux from shell = t × n × L # of stars per cubic parsec luminosity of single star Number of stars goes up at the same rate as the light from each goes down. 1. The expanding universe – light from distant objects is reduced in energy by the cosmological redshift. 2. The finite age of the universe – light from distant galaxies has not had time to fill the voids of space. The Paradox Explained:
  13. 13. Old Light
  14. 14. Lookback Time If the speed of light were infinite, light from everywhere in the universe would reach us at exactly the same time and we would see the entire universe as it is now. But it is not, so we see distant regions as they were in the past.
  15. 15. How can we know what the universe was like in the past? • Light travels at a finite speed (300,000 km/s). • Thus, we see objects as they were in the past: The farther away we look in distance, the further back we look in time. Destination Light travel time Moon 1 second Sun 8 minutes Sirius 8 years Andromeda (M31) 2.5 million years
  16. 16. We know of an ancient radiation That haunts dismembered constellations, A faintly glimmering radio station. While Frank Sinatra sings Stormy Weather, The flies and spiders get along together, Cobwebs fall on an old skipping record. Beyond the suns that guard this roof, Beyond your flowers of flaming truths, Beyond your latest ad campaigns, An old man sits collecting stamps In a room all filled with Chinese lamps. He saves what others throw away. He says that he'll be rich some day. We know of an ancient radiation That haunts dismembered constellations, A faintly glimmering radio station. It’s 50 years since the first SETI transmissions. But the outer shell of our accidental radio messages, containing early work of Frank Sinatra, has swept over the planets around thousands of stars.
  17. 17. Example: This image shows the Milky Ways’ neighbor, the Andromeda Galaxy (or M31) as it looked about 2.5 million years ago. Question: When will be able to see what it looks like now? M31, Andromeda
  18. 18. This galaxy is 100 million light years away. We see it not as it is now but as it was 100 million years ago. Distant light is old. Measurements of this star show that is it 10 million years old. It is a type of star that lives a total of 50 million years and dies as a supernova. Let’s see how the light and information actually travel
  19. 19. 100 million light years • It has taken 100 million years for the galaxy light to reach us. As we see it, the bright star within the galaxy is 10 million years old and will have a total lifetime of 50 million years. Has the supernova event already occurred and if so, when?
  20. 20. Nearby “Now” “Long Ago” Galaxies LOOKBACK TIME “Recent” Stars “Ancient” Universe
  21. 21. The Contents
  22. 22. Scattered in a universe 46 billion light years across Galaxies
  23. 23. The Milky Way is typical with 400 billion stars Stars
  24. 24. Almost all the simple elements hydrogen and helium Atoms 1080
  25. 25. A billion photons for every particle Photons 1089
  26. 26. Mediocrity We therefore live on an: • Average planet around • An average star in an • Average galaxy in a • Very large universe Copernicus
  27. 27. © 2005 Pearson Education Inc., publishing as Addison-Wesley Composition of the Universe Dark matter: 22% Dark energy: 73% Universal Pie
  28. 28. Us WE’RE NOT SPECIAL Universe
  29. 29. Cosmology
  30. 30. Cosmological Principle Homogeneous means the same, on average, at any location. This wall extended is homogeneous but it’s not the same in any direction. Isotropic means the same, on average, in any direction. This pattern is isotropic as viewed from the center but it’s not homogeneous.
  31. 31. Cosmological Principle Cosmology involves the assumption that the universe is homogeneous on large scales and isotropic. Isotropy is verified by galaxy surveys in different directions, but the assumption of Homogeneity can’t be tested because we are forced to look back in time as we look out in space.
  32. 32. Laws of Physics There’s also an assumption that the laws of physics are invariant across time and space, which is very difficult to test experimentally. An even deeper assumption is the rationality of nature, that there are laws of physics, and that causality holds.
  33. 33. Testing for Variation One recent experiment looked for subtle changes in atomic transitions between light in the lab and light from a distant quasars. The physical constant at issue is the fine structure constant. Change was seen but the significance level was low.
  34. 34. Cosmology: What We Know 1. Redshift – it’s a cosmic expansion of space-time, not a Doppler shift. 2. The microwave background radiation (CMB), a signal that is close to perfectly thermal, at temp of 2.726K. 3C 273
  35. 35. © 2005 Pearson Education Inc., publishing as Addison-Wesley 3. Deuterium and Helium synthesized (much higher temperatures in the past). Hubble Ultra Deep Field HST•ACS 4. Galaxies in past look younger (smaller and more irregular).
  36. 36. Cosmological Parameters • A variety of observations affirm the presence of dark matter on large scales, exceeding baryons. • Supernovae show accelerating expansion, attributed to dark energy, i.e. repulsive vacuum. • CMB observations show that space curvature is small and confirm the age of 13.7 Gyrs. • These observations all agree with a model where cosmic expansion continues forever.
  37. 37. Microwave Background
  38. 38. Cosmic Microwave Background • The universe is immersed in a sea of radiation. • 380,000 years after the big bang, the universe had cooled enough for free electrons to become bound into atoms of Hydrogen and Helium • Without electrons to scatter them, photons were able to travel unhindered throughout the universe • The universe became transparent • The temperature of the universe was 3,000 K at this time, similar to a stellar photosphere • It has expanded by a factor of 1000 since then, reducing the temperature to 3000/1000 = 3 Kelvin
  39. 39. The Infant Universe
  40. 40. A Thousand Times Smaller
  41. 41. The CMB photons have been stretched by the cosmological redshift, and have the highest redshift of anything we can observe (z = 1000). When we look at the CMB, we look at the surface of the glowing “fog” that filled the entire early universe!
  42. 42. The Big Bang is Everywhere
  43. 43. There is evidence for expansion, and the universe was hotter and denser in the distant past. The microwave background has been shown to be a signal at cosmological distance, not a local signal. Hundred of thousands of big bang photons permeate every breath you take: the big bang is everywhere. The big bang theory naturally explains the microwaves, which are very hard to explain any other way. Microwave Background
  44. 44. 1% of the specks on any TV tuned between stations are interactions with the big bang
  45. 45. 1% of the specks on any TV tuned between stations are interactions with the big bang
  46. 46. Big Bang
  47. 47. • A good scientific model should always make predictions which can be verified. • The big bang model makes two predictions which have been verified since the 1960s: • the existence and characteristics of the cosmic microwave background (CMB) • the expected Helium abundance in the Universe • The model predictions agree with all current observations. There is much indirect evidence the universe was smaller and hotter in the past Evidence
  48. 48. Space is Flat
  49. 49. The Flatness Problem For the matter density now to be so close to critical, the boundary between eternal expansion and a subsequent collapse, requires extraordinary fine-tuning in the early universe on a special value. For values of the matter density much different from the present value, the universe would either have collapsed and lived a much shorter time or expanded rapidly enough for no stars and galaxies .
  50. 50. Horizons The Earth has a horizon: we can’t see past it because of the Earth’s curved surface. A black hole has an event horizon: we can’t see into it because photons can’t escape. The universe has a cosmological horizon: beyond it photons haven’t had time to reach us in 13.7 billion years since the BB.
  51. 51. Light travels simply and directly in the local universe.. But early expansion of the universe carried any two points away from each other faster than light speed. Distant galaxies are only just becoming visible now. PHYSICAL UNIVERSE >> OBSERVABLE UNIVERSE!
  52. 52. The Smoothness Problem What COBE and other satellites saw was an extremely smooth radiation, with the temperature not varying by more than 1 part in 1000 in any direction in space. Heat or energy must travel for temperatures to equalize. The radiation from the universe is much to uniform to understand because patches a degree apart were expanding faster than light.
  53. 53. Cosmic Inflation The standard big bang has trouble explaining why the universe is as smooth and flat as it is, leading to the idea of an epoch of extremely rapid inflation, just 10-35 sec after the big bang. The mechanism is unclear but probably associated with the Grand Unified theories that seek to unite all the forces except gravity.
  54. 54. Status of Inflation Inflation makes the universe flat and smooth (by design!), and it implies vast region of space that are beyond view. It has tentative support from CMB satellite data.
  55. 55. Combining CMB with other data, a key characteristic of inflation is observed, deviation from pure power law of equal power on all spatial scales (a tilted power spectrum). The other key prediction involves the strength of gravity waves.
  56. 56. © 2005 Pearson Education Inc., publishing as Addison-Wesley There is evidence for expansion, and the universe was hotter and denser in the distant past. The microwave background and the helium abundance cannot easily be explained in any other way. There are hundred of thousands of big bang photons in every breath you take: the big bang is all around us. It is a theory, but a theory with a web of evidence to support it. The theory is mute about the cause of the cause. Status of the Big Bang
  57. 57. Early Universe
  58. 58. History of the Universe
  59. 59. Planck Era (t < 10 –43 sec) • This era, the “first instant”, lasted for 10 –43 sec. • Because we are as yet unable to link… • quantum mechanics (our theory of the very small) • general relativity (our theory of the very large) • We are powerless to describe what happened in this era. • 10 –43 sec after the big bang is as far back as our current science will allow us to go.
  60. 60. The big bang was extraordinary― the instantaneous creation of all of space and time, containing energy to drive the expansion and enough matter for 100 billion galaxies. The initial state was so compact that it can only be described by a theory that unites gravity and the quantum world. We do not have such a theory at present. The big bang can be thought of as a quantum event, originating from very chaotic space-time in which the other quantum fluctuations might have led to other parallel universes.
  61. 61. An image of quantum fluctuations blown up to the size of the universe
  62. 62. LHC 10-12 s
  63. 63. Particle Era (t < 0.001 sec; T > 1012 K) • Particles were as numerous as photons. • At 10–6 sec old, the universe cooled to 1012 K. • p+ , p , n, and n could no longer be created from two photons • the remaining particles and antiparticles annihilated each other into pure radiation • slight imbalance in number of p and n allowed a small residue of matter to remain • e and e+ are still being created from photons. – until t = 4 sec; T > 5 x 109 K
  64. 64. Nucleosynthesis Era (t < 3 min; T > 109 K) • During this era, p+ and n started fusing… • but new nuclei were also torn apart by the high temperatures • When the universe was 5 minutes old… • it had cooled to 1010 K and the fusion stopped • Afterwards, the baryonic matter leftover in the universe was: • 75% Hydrogen nuclei (i.e. individual protons) • 25% Helium nuclei • trace amounts of Deuterium (H isotope), Helium-3, and Lithium nuclei
  65. 65. Cosmic Helium Abundance • In the Era of Nucleosynthesis • number of p+ and n roughly equal as long as T > 1011 K • below 1011 K, proton-to-neutron reactions no longer occur • neutrons still decay into protons • protons begin to outnumber neutrons • At T < 1010 K, He, Deuterium, and Li remain stable – At this time, big bang model predicts a 7-to-1 p+ -to-n ratio. • For every 2 n and 2 p+ which fused into a Helium nucleus… • there are 12 p+ • Model predicts a 3-to-1 H:He • This what we observe: • minimum of 25% He in all galaxies
  66. 66. Era of Nuclei (3 min < t < 380,000 yr) • The universe was a hot plasma of H & He nuclei and electrons. • photons bounced from electron to electron, not traveling very far • the universe was opaque (like inside a cloud) • When the universe was 380,000 yrs old… • it had cooled to a temperature of 3,000 K • electrons combined with nuclei to form stable atoms of H and He • the photons were free to stream across the universe • the universe became transparent
  67. 67. Era of Atoms (380,000 < t < 109 yr) • The universe was filled with atomic gas. • referred to as the “Cosmic Dark Ages” • Density enhancements in the gas and gravitational attraction by dark matter… • eventually forming proto-galactic clouds • the first star formation lights up the universe • the first heavy elements are created • closely followed by the formation of galaxies
  68. 68. Present and Future
  69. 69. Age of the Universe The age in a linear expansion model is 1/H0. Dark matter causes deceleration then dark energy caused speeding up so the net result is still close to 1/H0. The age in a model is 13.7 Gyr, with an error of only 3-4%.
  70. 70. Age of the Universe A cross-check comes from the age of the oldest star clusters in the halo or the oldest white dwarfs in the Milky Way disk. Another cross-check is the abundance of the decay products of heavy isotopes. These two test are not very accurate but they verify the big bang model and age.
  71. 71. Era of Galaxies ( t > 109 yr) • The first galaxies came into existence about 300 to 500 million years after the big bang. • This is the current era of the universe, it is mostly composed of large, cold vacuum.
  72. 72. Infinite Universe? The very early universe expanded so rapidly there are regions of space we’ve not seen.
  73. 73. Speed of Light Size Time The Universe The early universe expanded much faster than the speed of light, so there are objects and large regions of space we have never seen. Dark energy will remove more from view. This violates no law of physics since the cosmic expansion is governed by general relativity, which sets no limit on the speed of expanding space.
  74. 74. Dark matter binds galaxies and dark energy is currently driving the cosmic acceleration.
  75. 75. Fate of the Universe The early expansion was rapid, driven by intense radiation. It slowed down as the dark matter began to dominate, and more recently has begun to accelerate due to the relative growth of dark energy. With what we know now, it seems that it will expand forever.
  76. 76. Within the expanding and cooling universe, gravity forms stars and galaxies(and people)
  77. 77. Multiverse
  78. 78. Us Milky Way Earth THE UNIVERSE AND US Solar System Universe Multiverse?
  79. 79. Science is Seeing Life occurs in a range of scales that extends from galaxies to the atomic nucleus, as symbolized by the ancient symbol of the ouroboros, the snake that eats its own tail Cosmic Zoom
  80. 80. Beyond the Horizon The universe is bounded in time and not space. General relativity sets no speed limit to the expansion. As time goes by, ever more distant regions come into view. At z = 1.3, an object was moving away from us at c at the time the light was emitted. At z = 1000, two distant points were moving apart at ~60c at the time the radiation was emitted. Consequence of standard big bang: The physical universe is much larger than the observable universe, we are subject to a horizon. Consequence of the inflationary big bang: A microscopic region of space-time became our universe; the universe is a quantum entity.
  81. 81. Quantum fluctuations are a mechanism for multiple realizations of the universe …leading to the concept of the “multiverse”
  82. 82. More than just this… LEVEL 1: regions we can not see in big bang model LEVEL 2: many bubbles of space-time, unobservable by us, different properties LEVEL 3: indeterminacy, and quantum variation LEVEL 4: mathematical forms, multi-dimensional space-times, 10 preferred
  83. 83. String Theory Landscape Perhaps 10 different vacua 500 de Sitter expansion in these vacua create quantum fluctuations and provide the initial conditions for inflation. String theory provides context for the “multiverse”
  84. 84. Landscape of eternal inflation A. Linde
  85. 85. The Multiverse 1. The observed big bang is just one mediocre member of an (infinite) ensemble. 2. For a flat distribution of Pa priori (rV), the requirement that galaxies can form gives rV that is quite consistent with the observations. 3. If the scalar field that generates the potential couples only to gravity  anthropic ideas apply only to the cosmological constant. Our universe emerged from a quantum space-time foam at the Planck epoch. Other universes may have been spawned this way, with randomly different physical properties. Most of the universes would be inhospitable to life → anthropic principle.
  86. 86. Self-reproducing Inflationary Universe “Whenever a creature was faced with several possible courses of action, it took them all, thereby creating many distinct histories of the cosmos. Since there were many creatures and each was constantly faced with many possible courses, and the combinations of all their courses were innumerable, an infinity of distinct universes exfoliated from every moment of every temporal sequence.” O. Stapledon 1937, Star Maker
  87. 87. The ekpyrotic universe has a big bang, but it is not ever at an infinite temperature and density, and it is not the origin of all space-time… So our universe emerged from a collision of two 4D branes that are embedded in 5D space-time. The collision is the engine for expansion and matter creation.
  88. 88. Eternal Universe