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1446 Introductory Astronomy II. Chapter 18B Cosmology II R. S. Rubins Fall 2011. INTRODUCTION 1.
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1446 Introductory Astronomy II Chapter 18B Cosmology II R. S. Rubins Fall 2011
INTRODUCTION 1 • The universe was brought into being in a less than fully formed state, but was gifted with capacity to transform itself from unformed matter into a truly marvelous array of structure and life-forms. Augustine (5th Century) • The most beautiful thing we can experience is the mysterious. It is the source of all true art and all science. • He to whom this emotion is a stranger, who can no longer pause to wonder and stand rapt in awe, is as good as dead: his eyes are closed. Albert Einstein (1879 – 1955)
The Temperature Ripples in the CMB • The symbol Q ≈, which gives a magnitude of about ±(3 x 10–5)K for the temperature ripples in the 3K cosmic microwave background, is a crucial indicator of the future development of the universe.
The Effect of the Size of Q • If Q were appreciably larger than 10–5, regions far larger than our galaxies would have formed early in the history of the universe. • Stars would not have formed, and the galaxies would have collapsed into gigantic black holes. • If Q were appreciably smaller, the formation of stars and galaxies, would have been much slower and less efficient. • If Q were less than 10–6, the universe would have remained forever dark and featureless. • After, Martin Rees, in Just Six Numbers (1999).
Evidence for the Big Bang Story • In addition to Hubble’s redshift measurements, which lead to the Big Banghypothesis, much more indirect evidence has been found, which is listed below. • 1. The remarkably precise fit of the COBE data for the CMB to thermal (blackbody) radiation at 2.73 K. • 2. No object has been found with a helium concentration of less than 23%, since there is no reverse process to the production of He from H. • 3. The current distribution of galaxies is consistent with the temperature ripples in the 3K cosmic microwave background. • 4. The abundances of deuterium and lithium nuclei produced in the Big Bang are in agreement with theoretical estimates.
From Electron-Quark Soup to the Elements Generations of stars must have died for us to be born (Brian Cox, 2011).
Chronology of the Universe 1 • Big Bang (t = 0) At the instant of the big bang, it is thought that i. mass, energy, space and time came into existence; ii. the universe was a point (or singularity). Since time came into existence only after the Big Bang, the concept of “before” the Big Bang is meaningless. • The Planck Epoch( t < 1043 s) All the forces of today were unified as a single force, but at the Planck instant (1043 s), the gravitational force “separated” from the other fundamental forces. Heavy particles (quarks) and light particles (electrons) existed on an equal footing.
Chronology of the Universe 3 • Grand Unification Era(from 1043 s to 1035 s) • During the GUT (grand unified theory) era, the strong nuclear, weak nuclear, and EM theories were unified. • Separation of the Strong Nuclear and Electroweak Forces(t 1035 s) At this instant, the strong nuclear force separated from the weak nuclear and EM forces, which together are known as the electroweak force. The popular hypothesis (Guth,1980) is that a large amount of energy was released, which caused a short-lived but enormously rapid expansion of the universe, in a process known as inflation.
Chronology of the Universe 4 • Inflationary Epoch(from about 1035 s to 1032 s) The concept of inflation, proposed by Alan Guth in 1980, is that, for in extremely brief time-period of about 1032 s, the radius of the universe increased by an immense factor, possibly of the order of 1050. • Two “problems” solved by inflation were • i. thehorizon problem,which refers to the observation that distant parts of the universe, between which signals cannot pass, are extremely close in temperature, • ii. theflatness problem, whichis that space appears to be flat, not curved, as might be expected. • The measurements of the Cosmic Microwave Background (CMB) and galaxy distributions observed today agree with the predictions of inflation
Two-dimensional Representations of Space • Spherical space would make distant objects look larger, while the reverse is true for hyperbolic space. • Observations indicate that space is “flat”.
Inflation Solves the “Flatness” Problem • Two-dimensional representation of how a greatly expanded curved surface appears flat.
Cosmic Inflation • There is currently no direct evidence supporting inflation, although it explains both the horizonand flatnessproblems.
Chronology of the Universe 5 • Break-up of the electroweak force(t 1012 s, T 1015 K) At this instant, the universe consisted of an electron-quark soup, and the four fundamental forces became essentially what they are today.
Chronology of the Universe 6 • Formation of Nucleons(t 10– 5 to10– 4 s, T 1012 K) The lower temperatures finally allow quarks to stick together, forming protons and neutrons (and their antiparticles), with 1 neutron for every 10 protons. • Particles and antiparticles annihilate(t 1 s, T 1010 K) After about 1 s,particles and their corresponding antiparticles annihilate, producing EM radiation. • To explain our universe, which appears to be constructed of matter, rather than antimatter, we must assume a slight excess of particles over antiparticles. • After annihilation, just the particles remained.
Why More Matter than Antimatter? A mathematically perfect universe might be expected to have equal quantities of matter and antimatter; e.g. electrons and positrons. • If this were the case, matter and antimatter would be totally annihilated, so that stars and planets could not have existed. • In 1977, Steven Weinberg suggested that the universe could be divided into regions of matter and antimatter, but X rays, produced at the boundaries of these regions were not found. • However, in 2010, Guennodi Borisov of Lancaster University found that the very short-lived B-meson, which oscillates between matter and antimatter, spends more time as matter, thus providing a natural mechanism for dominance of matter. • This process is related to an idea first suggested by the great Soviet physicist, Andrei Sakharov.
Chronology of the Universe 7 • Fusion of 2H and 4He(t 1 min, T 109 K) • Much of the heavy hydrogen (2H) nuclei and helium nuclei now present in the universe, and a trace of Li, were created at this time. An additional 3% He and all heavier elements have since been created by stellar fusion and supernovae. According to current ideas, about fifteen 2H nuclei were created for every million 1H nuclei at this time, although 2006 measurements on dust grains have indicated that there may have been far more 2H than previously thought.
Chronology of the Universe 8 • Epoch of Decoupling(t ≈ 400,000 y, T ≈3000 K) Up to this time, the universe contained an electron-photon soup, in which the electrons scattered the EM radiation in all directions, making the universe an opaque fog. When the temperature dropped below 3000 K, at a time determined to be 380,000 years, electrons and nuclei combined to form the first H and He atoms. Only those photons with wavelengths corresponding to Bohr transitionsinteracted with atoms, so that matter no longer absorbed EM radiation, and the universe became transparent.. The EM radiation, then at 3000 K, has now cooled to become the 3K Cosmic Microwave Background (CMB) radiation.
Timeline 1 The timeline shown below includes the synthesis of two hypothetical dark matter candidates, axions and neutralinos.
Timeline 2 • In 2009, the most distant star yet observed was deduced to be at about 13.1 ly away, when it exploded. • Measured with the NASA Swift -ray telescope, it is deduced to have exploded about 630 million years after the Big Bang.
A Mini Bang Collision at RHIC In the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, thousands of particles stream from the collision of two gold nuclei at 99.99% the speed of light. These collisions simulate the first few microseconds of the Big Bang.
1800 scientists from 164 institutions in 35 countries collaborate on this project. Beams of protons travel 200 m underground around a path about 15 km long. ATLAS Collaboration at CERN, Geneva 2
The ATLAS Experiment UTA’s Dr. Kausik De was named the US ATLAS Operations Coordinator.
Stellar Birth Rates 1 Following the dark ages, star formation is thought to have begun quickly, and has steadily tapered off, as the amount of interstellar hydrogen has decreased.
Structure Formation with Dark Matter 1 • Current calculations indicate that without (exotic) dark matter, gravity is insufficient to hold galaxies together. • Since about 85% of the matter in the universe appears to be in the form of dark matter, which interacts only through gravity, the formation of galaxies must depend on dark matter clumping together in spherical blobs, known as (galactic) halos. • In the early universe, the normal matter does not clump, because it interacts with the EM radiation, and is too hot to form stars. • Only after about 200 million years has the universe cooled sufficiently for the first stars, and later, galaxies to form near the centers of the dark matter halos.
Dark Matter Stabilizes Galactic Structures Current calculations indicate that without (exotic) dark matter, gravity is insufficient to hold galaxies together. The Millenium Simulation Project indicates that dark matter sculpts the universe into a web of galaxies.
The Search for Dark Matter 1 • Since about 85% of the mass found in the universe is thought to be in the form of dark matter, which interacts with gravity, but not with EM radiation, a major effort is underway to find its source. • There is much indirect evidence for the existence of dark matter, but we still await direct evidence. • A 2008 study which combined observations made with the Chandra X-ray telescope and the Hubble space telescope on a gigantic collision between two galactic clusters about 5.7 billion ly away • The X-ray telescope showed a normal matter signal from the very hot gases produced by the collisions, while the Hubble telescope mapped the dark matter from the gravitational lensing of light from more distant galaxies.
The Search for Dark Matter 2 The mass of normal matter is deduced from the false-color X-ray emission signal , shown in red. The mass of dark matter is deduced indirectly from gravitational lensing, which produces the false-color signal, shown in blue. The results indicate that the ratio of dark to normal matter is roughly the same for distant and nearby galaxies.
Fate of the Universe pre-1998 • Only thebound universe has sufficient mass to reverse the velocity, resulting in the big crunch. • The unbound universe will expand for ever at a constant rate. • Themarginally bound will slow down, but never reverse.
The Accelerating Universe 1 • In 1998, independent measurements of Type Ia supernovaeat widely different distances, indicated that the rate of expansion of the universe is increasing with time. • This increase of the expansion rate suggests the presence unknown repulsive force, which is known as dark energy. • The 1998 measurements showed thatgravitationwas the dominant force in the universe only for the first few billion years. • The initial dominance of gravity is not surprising, since an attractive force was needed, early in the life of the universe, to produce planets, stars and galaxies from the gas clouds. • After about 10 billion years, dark energy became the dominant energy in the universe.
The Accelerating Universe 2 • 2006 Measurements on 23 supernovae dating back 8 to 10 billion years, made by a group led by Adam Reiss, have shown that the repulsive effects of dark energy have been in evidence for the last 9 billion years. • A dominant repulsive force would ultimately spread matter out smoothly everywhere. • As the universe continues to expand, the visible universe is expected to empty itself of matter. • As dark energy becomes more dominant, it should ultimately pull apart galaxies, stars and planets. • Major wide-field explorations, underway in Texas (HETDEX) and Hawaii (Pan-STARRS), should provide more substantial data on dark matter, dark energy, and the accelerating rate of universal expansion.
Fate of the Universe post-1998 (in green) An accelerating universe provided an answer to the age crisis; i.e. that the universe appeared to be younger than its oldest stars.
Dark Energy Dark energy is supposed to be distributed smoothly throughout space, while the expansion of space causes the density of matter to decrease with time. One result of this effect is to reduce the rate of star formation, as shown below.
Virtual Particles • The conventional view of a vacuum is that of Hero of Alexandria, who in the 1st Century, proposed that“air consists of atoms moving through a void ”. • However, quantum mechanics allows particle-antiparticle pairs to spontaneous appear and then annihilate in a very short time in otherwise empty space. • Thus, one of the strangest consequences of quantum mechanics is that a vacuum is filled with short-lived virtual particles, producing the vacuum energy. • While it appears that the vacuum energy could be responsible for the repulsive antigravity force known as dark energy, current calculations of the vacuum energy do not give a realistic size for the antigravity force.
Future of the Universe 1 • With time, the visible region (yellow) grows, but the universe (blue) grows faster, while gravity pulls the nearer galaxies together.
Future of the Universe 3 In 5 billion years, the Sun will have become a red giant, and the galaxy Andromeda will fill the night sky.
Future of the Universe 4 In a 100 billion y, the Earth’s remains will orbit the supergalaxy. In a 100 trillion years, lights out!
The Energy Pie • Current estimates lead to the strange conclusion that dark energy accounts for roughly 72% of the energy in the universe, with dark matter accounting for about 23%, and only about 4% attributable to “normal” matter and radiation.
Multiverses 1 • The extent of the observable universe, which is the distance of the furthest objects that we can currently observe, is believed to be about 42 billion ly (see Lookback Time, Chapter 18A). • Most astronomers believe that the universe extends beyond 42 billion ly, and may be infinitely large, although we have no way to prove this, so that this conjecture lies outside the realm of science. Single universe, with observable region in black.
Multiverses 2 • An alternative speculative idea, known as a Level 1 Multiverse, suggests that the space beyond our horizon is filled with perhaps an infinite number of similar universes, which are completely disconnected with each other and with our universe. • Even more speculative is a Level 2 Multiverse, in which the infinite number of universes have different laws of physics, so that the special values of the physical constants in our universe, which are just right for life, occur as a statistical possibility. Level 1 multiverse Level 2 multiverse
James Peebles Report Card, 2002 * *In 2005, the general theory of relativity was confirmed to a much higher precision through measurements made on a double-pulsar system, so perhaps that grade should be changed from A- to A.
