# Universe 8e Lecture Chapter 26 Cosmology

The Big Bang Olbers Paradox Why is the Night Sky Dark? Eventually line of sight lands on a tree if youre in a forest of infinite extent! Eventually line of sight lands on a star if youre in a

Universe of infinite extent! Olbers Paradox Why the Night Sky is Dark Any large, imaginary, spherical shell of radius R and thickness RR centered on the Earth encloses some # of stars. A 2nd shell of radius 2R and the same thickness RR encloses 4 x as many stars (the surface area

of a sphere is 4RR2) Each star at 2R is as bright as a star at R Therefore each shell of stars is equally bright as seen from Earth Oh my what happens as you add up the brightness contributed by all the shells?

An Expanding 1D Bugworld Colored bugs at rest on an line which is stretching at constant rate The spacing between bugs doubles every second. Purple and red bugs initially 10 units away from yellow bug

1 sec later 20 units away. Green and brown bugs initially 20 units away from yellow bug and 1 sec later 40 units away. Green and brown bugs, twice as far away, must be moving away from yellow twice as fast as red and purple! Bugworld Hubbles Law V = Ho D Smart bugs

construct a graph of recessional speed vs distance to other bugs They conclude bugs further way recede at faster speed Hubbles Law The Doppler Shift

The Expanding Universe Redshift (Speed) Proportional to Distance Hubbles Law: v = Ho D Or v = D / To Question

The Expansion of the Universe refers to the fact that 1) All matter in the Universe is moving through space away from the point where the The Big Bang occurred. 2) Matter in the Universe is continually appearing spontaneously and existing matter spreads outward to accommodate it. 3) The space between galaxies is expanding and so it appears as that they are moving away from each other.

4) Everything in the Universe is expanding and so everything always looks the same. Why Is the Universe Expanding? Observation of CMBR by Penzias and Wilson dealt Steady State Theory a Death Blow Arno Penzias & Robert Wilson Arno Penzias and Bob

Discoverers 3oK to Cosmic Wilson were of trying find the Microwave Background source of excess

noise in Radiation their antenna, where pigeons were roosting. They spent hours searching for and removing the pigeon dung still the noise remained

"Either weve seen "Thus, they looked for dung but found gold, which is just opposite of the experience of most of us." a pile of pigeon *& %&# or the creation of the universe.

Ivan Kaminowone of Penzias and Wilsons colleagues at Bell Labs, The COBE Space Telescope Question What is the Cosmic Microwave Background Radiation (CMBR)? 1. a uniform background of radiation from electrons spiraling in weak intergalactic magnetic fields.

2. radiation from a very tenuous, ionized gas that fills the universe equally in all directions. 3. radiation that generates the radon gas that trickles up from the Earth and accumulates in the basements of many houses, causing a real health-hazard. 4. radiation left over from the Big Bang, after the universe expanded and cooled. 5. an almost-uniform background radiation from distant, unresolved, overlapping galaxies.

Lookback Time z a measure of redshift and therefore distance from us If you observe a galaxy 9 billion light years away from Earthyou see it as it was 9 billion years ago because it took light 9 billion years to get here! Thus when you look at things far away, you are also looking back in time! History of the Universe Run clock backwardssee what happens

after Big Bang 13.8 billion years ago Use known laws of physics to calculate conditions of universe at each earlier time Works well except for times before the Planck time when the universe was less than 10-43 seconds old T<10-43 secneed a quantum theory of gravitywhich still eludes us! Letting Time run backwards Universe gets increasingly dense

Universe gets increasing hot How long before its density and temperature reach infinite values . galaxies, stars, etc piled up on top of each other? Hubbles Law tells us v = Ho D So To = D/v = 1/Ho = 13.8 billion years! The Universe Began 13.8 Gya With the Big Bang and It is Expanding Temperature of Universe vs Time

Origin of Particles Vacuum Fluctuations Inflation Generates Real Particles Inflation Must have Happened Radiation EraUniverse Filled With High Energy s 10-35 Seconds After Big Bang

Symmetry Breaks as Different Forces Freeze Out During Cooling of Universe Primordial Nucleosynthesis When the temperature of the Universe drops down to about 109 oK, nuclear fusion reactions produce H2 (deuterium)

He3 (helium-3) H3 (tritium) He4 (helium-4) Other fusion reactions (not shown) produce small amounts of Li6 (lithium-6) Li7 (lithium-7) Be7 (berylium-7) Why Werent Elements More Massive than He (and

some Li and Be) Created During the Big Bang? Fusion of elements heavier than hydrogen could not began until the temperature of the Universe was < 109 K. High energy s would quickly break deuterium apart. Universe was expanding and cooling fusion of heavier elements requires higher densities and temperatures to overcome electrical repulsion between nuclei of greater electrical charge! Free neutrons (those not bound up inside nuclei) decaycaused by the weak interaction! Their - life is about 10 minutes! n p e e

Thus, after about 20 minutes or so, none were left to form heavier nuclei. Thus, the Big Bang left our Universe with only 90% hydrogen and 10% helium (by number of nuclei not weight) Primordial Nucleosynthesis History of Universe Pair production creates quarks, leptons

4 forces freeze out Heavy leptons decay into electrons, neutrinos Quarks make protons, neutrons Primordial Nucleosynthesis Atoms form matter & radiation decouple CMBR released Question

How does the Big Bang Theory resolve Olbers Paradox (Why the night sky is dark)? 1) The night sky isnt dark its loaded with bright stars! 2) Big Bang Theory indicates that the Universe began 13.8 Gya. Therefore, if you look further away than 13.8 Gya you see no light because nothing existed! 3) It demonstrates that the Universe is filled with nothing but dark matter. 4) It demonstrates that galaxies do not fill the

universe uniformly. Question What observational evidence supports the Big Bang Theory? 1) Hubbles Law remote clusters of galaxies are all observed to be receding from us and the further away they are, the faster they appear to be receding. 2) Bang Bang Theory correctly predicts the relative abundances of light elements (H, D, He, Be, Li)

in the Universe. 3) The existence, pervasiveness and relative isotropy of the 3o K CMBR. 4) Resolution of Olbers Paradox 5) All of the above. Key Ideas

The Big Bang Genesis of the Universe. The Universe began as an infinitely dense cosmic singularity that began its expansion in the event called the Big Bang, which marked the beginning of space and time. It is supported by an abundance of evidence: The Cosmic Microwave Background Radiation (CMBR): During the first 380,000 years, radiation and matter formed an opaque plasma called the primordial fireball. When its temperature fell below 3000 K, protons and electrons could combine to form hydrogen atoms and the universe became transparent. The CMBR, radiation that fills the current

Universe, whose temperature is nearly 3 K, is the greatly redshifted remnant of the radiation that filled the Universe as it existed about 380,000 years after the Big Bang. Key Ideas

Primordial Nucleosynthesis: The Big Bang Theory explains the relative abundance of the light elements hydrogen, deuterium, lithium, helium. (i) Helium could not have been produced until the expanding Universe cooled enough to eliminate most high-energy photons, which created a deuterium bottleneck by breaking down deuterons before they could combine to form helium. (ii) Fusion of heavier elements requires higher densities and temperatures to overcome electrical repulsion between nuclei

of greater electrical charge but the Universe was expanding and cooling! (iii) Free neutrons decay! Their lifetime is about 10 minutes! Thus, after about 20 minutes or so, none were left to form heavier nuclei. Key Ideas

The Expansion of the Universe: The Hubble law (the proportionality between redshift and distance of remote galaxies) is best explained by a Universe that exists in a space that is continuing to expand. The redshifts that we see from distant galaxies are caused by this expansion, not by the motions of galaxies through space. Olbers Paradox (why the night sky is dark) is best explained by an expanding Universe that had a beginning.

The observable universe extends about 13.7 billion light-years in every direction from the Earth. We cannot see objects beyond this distance because light from these objects has not had enough time to reach us. Key Ideas The Four Forces and Their Unification: Four basic

forces gravity, electromagnetism, the strong force, and the weak force explain all the interactions observed in the universe. Their existence is best explained by splitting off from a single unified force as the Universe expanded and cooled. Grand Unified Theories (GUTs) have succeeded in explaining three of the forces in terms of a splitting off from a single unified force as the Universe expanded and cooled. A super-grand unified theory (which still eludes theoretical physicists would also include gravity) would explain all four forces in terms of a single force that sprang

into existence during the first 10-43 seconds of the Big Bang the Planck time. Key Ideas Cosmic Inflation: A brief period of rapid expansion, called inflation, is thought to have occurred immediately after the Big Bang. During a tiny fraction of a second, the universe expanded to a size many times larger than it would have reached through its normal expansion rate.

Inflation explains why the universe is nearly flat and the 2.725-K microwave background is almost perfectly isotropic. Key Ideas

The Origin of Matter: Just after the inflationary epoch, the universe was filled with particles and antiparticles formed by pair production and with numerous high-energy photons formed by annihilation. A state of thermal equilibrium existed in this hot plasma. As the universe expanded, its temperature decreased. When the temperature fell below the threshold temperature required to produce each kind of particle, annihilation of

that kind of particle began to dominate over production. Matter was slightly more abundant than anti-matter it now makes up the Universe. The Matter that eventually emerged was mostly Hydrogen and Helium with traces of deuterium and lithium the so-called light elements. Key Ideas

Density Fluctuations and the Origin of Stars and Galaxies: The large-scale structure of the universe arose from primordial density fluctuations. The First Stars were much more massive and luminous than stars in the present-day universe. The material that they ejected into space seeded the cosmos for all later generations of stars.

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