1 Universe! The Big Bang Cosmology
2 History of the Universe
3 The Universe In the beginning the Universe began in a Big Bang - a large primordial explosion Bya. The primordial Universe was: Extremely hot, very young, very small, and with very few forces & particles. The Universe Today is: Extremely cold, very old, exceedingly large, and with many forces & particles.
4 Cosmology The study of the cosmos.This is a story of the evolution of matter and radiation and their interactions cast in the form of a scientific theory. The physical evolution of the universe is not over!
5 Origins There have been many beliefs about the beginning of our Universe. There have also been many hypothesis. In the early 1900s, the Steady State theory was the reigning theory. It has since been replaced by the Big Bang theory.
6 General Relativity In 1915, Albert Einstein published his paper on General Relativity. By the early 1920s, there was evidence to support this theory as a correct description of gravity. What was some of the evidence in support of General Relativity?
7 General Relativity Massive objects change theshape of space-time in their vicinity. Objects then follow the shortest path along curved space-time. This appears to us as gravity.
8 Einstein Field EquationEinstein realized that, with General Relativity, he could write one equation that described the behavior of the whole Universe!! Rµυ – ½ Rgµυ (+ ??) = 8πGTµυ “R” is related to the curvature, or shape, of space-time, T is related to the contents (mass and energy) of the Universe, and G is Newton’s constant of Universal Gravitation. We’ll discuss the ?? later.
9 The Rise of the Big Bang Model1922 – Alexander Friedman, a Russian mathematician, challenged Einstein’s equations that described our universe. Einstein’s equations described a steady state universe, but the equations were unstable. Based on the equations, if anything was “nudged,” the universe started expanding or contracting.
10 Consequence of General RelativityThere are only two classes of solutions to the field equation: either the Universe is contracting, or it is expanding. This was shown by the physicist, Alexander Friedmann. Einstein found both options dissatisfying, and added a term to his equation to “fix” the “problem.” Rµυ – ½ Rgµυ (+ ??) = 8πGTµυ
11 Consequence of General RelativityIn one set of solutions, the Universe is expanding. An outward push would have sent it initially doing so. If there is enough matter and energy, their gravity will stop the expansion and cause the Universe to contract. If there is not enough, their gravity will slow down the expansion, but never be able to stop it.
12 The Rise of the Big Bang Model1927 – Georges Lemaître, a Belgian cleric, proposed that the expansion of the universe could be traced back to a “super atom” and that there might be leftover radiation from an initial explosion out there to find. If the Universe is now expanding, running time backwards (which you can do with math, but probably not in reality) showed that galaxies should be packed tighter together in the past. The idea of the Big Bang was first proposed by Lemaître, and it was developed further by Gamow, Alpher, and Herman.
13 The Rise of the Big Bang Model1929 – Edwin Hubble finds evidence of an expanding universe! (Data to follow…) The term “Big Bang” was meant to be derogatory to the idea, but it stuck regardless. (See Fred Hoyle and the Steady State Theory.)
14 The Big Bang Model At the beginning of time, elementary nuclear particles and radiation exploded from an infinitely hot, infinitely dense state with initially zero dimension. (A singularity!)
15 What we don’t know Anything before t = 0 (the big bang)We can’t model t = 0, the moment of the Big Bang. We can’t model t = 0 to t = seconds. (Probably no matter, just radiation, and one unified force.)
16 What it isn’t The Big Bang is not analogous to a bomb exploding and sending fragments of matter into surrounding space. There was no matter or space at the time of the Big Bang.
17 Understanding the Model – The Balloon UniverseImagine that you are the creator of a balloon Universe! All of the creatures that live in your Universe live on the skin of the balloon. Make your Universe small. (Blow it up only a little.) Notice that the balloon Universe is finite but unbounded. There is a set amount of area and there is no edge. Walk in any direction and you will end up back at the starting point.
18 Understanding the Model – The Balloon UniverseArea in the balloon Universe is created while expanding; it doesn’t pre-exist. Draw places (stars, galaxies, etc.) on your Universe. Be careful not to pop it. Choose one of these places to live and mark it. Take note of your neighbors. Specifically look at a neighbor that is close to you and one farther away. Get an estimate on how far each is from your home.
19 Understanding the Model – The Balloon UniverseNow, while watching your home and neighbors (if possible), gradually expand your Universe. What happened? How did your neighbors move? Did your neighbors move away at the same rate? Not sure? Try again! Move to another place. Does the same thing happen? Is there a center to the Universe?
20 The Expanding UniverseOn large scales, galaxies are moving apart, with velocity proportional to distance. It’s not galaxies moving through space. Space is expanding, carrying the galaxies along! The galaxies themselves are not expanding!
21 Expanding Space Analogy:Analogy: A loaf of raisin bread where the dough is rising and expanding, taking the raisins with it.
22 Finite, But Without Edge?2-dimensional analogy: Surface of a sphere: Surface is finite, but has no edge. For a creature living on the sphere, having no sense of the third dimension, there’s no center (on the sphere!): All points are equal. Alternative: Any point on the surface can be defined as the center of a coordinate system.
23 The Necessity of a Big BangIf galaxies are moving away from each other with a speed proportional to distance, there must have been a beginning, when everything was concentrated in one single point: The Big Bang! Time ?
24 Understanding the Model – The Balloon UniverseNote the receding galaxies (yellow) and the photons that are red-shifting.
25 Understanding the Model – The Balloon UniverseSo, how does the balloon Universe end? Possibilities: Open Universe: Forever Exapanding (Big Chill) Closed Universe: Collapsing (Big Crunch) It coasts to a halt and remains in equilibrium forever.
26 The Big Bang and Scientific MethodAssumptions Rules Experiments Predictions Measurements At the beginning of time, elementary nuclear particles and radiation exploded from an infinitely hot, infinitely dense state with initially zero dimension.
27 Assumptions The assumptions come from 2 dilemmas:We do not occupy a privileged position in space. We do not understand the reason for our existence in purely scientific terms, partly because the early Universe was chaotic. To get around these, we assume 2 basic things: The universe is homogeneous. The universe is isotropic.
28 Homogeneous and IsotropicAssumptions Homogeneous and Isotropic Isotropic - On average, the Universe looks the same in any direction. This refers to the attributes of space itself, not its contents. No direction or location can be distinguished from any other by experiment or observation. That is, the Universe has no geometric center, and the rate of expansion of the Universe is the same everywhere. Homogeneous - Matter and energy are spread uniformly throughout space. There are no large voids, or bunching. The same types of structures and the same elemental composition are found in all directions. EXAMPLE: fruitcake versus Neapolitan ice cream.
29 Homogeneous and IsotropicAssumptions If you assume our universe is homogeneous and isotropic, then our local sample is not too different from other points in space in terms of general structure, density, and composition. These assumptions lead to the simplest model of the Universe, and they assure that the laws of physics apply universally.
30 The Anthropic PrincipleAssumptions The Anthropic Principle augments these minimal assumptions to rationalize our existence in the Universe. This assumption asserts that the very existence of human beings explains certain characteristics of the Universe that would otherwise remain mysterious.
31 The Anthropic PrincipleAssumptions Example: X + 3 = 5 where 5 is human life! This means if X = 2, we get life. If X is anything but 2, we don’t. So, because humans are here, we know X = 2. Or… X = 2, so humans are here. Circular Reasoning!
32 The Big Bang and Scientific MethodThe universe is homogeneous and isotropic. Rules Anthropic Principle At the beginning of time, elementary nuclear particles and radiation exploded from an infinitely hot, infinitely dense state with initially zero dimension. Predictions Measurements Experiments
33 Rules Rules The General Theory of Relativity, which describes interactions between matter, radiation, and gravity around very large bodies such as stars and black holes. It does not work at the instant of the Big Bang (t = 0), in infinite gravity fields (supermassive black holes), or at the atomic level. Quantum mechanics must therefore be included in cosmological models to describe interactions between fundamental particles where gravity is negligible. A Unified Field Theory is being sought to merge these distinct treatments into a more comprehensive theory.
34 The Big Bang and Scientific MethodThe universe is homogeneous and isotropic. The General Theory of Relativity and Quantum Mechanics. Anthropic Principle At the beginning of time, elementary nuclear particles and radiation exploded from an infinitely hot, infinitely dense state with initially zero dimension. Predictions Measurements Experiments
35 The Big Bang and Scientific MethodThe universe is homogeneous and isotropic. The General Theory of Relativity and Quantum Mechanics. Anthropic Principle At the beginning of time, elementary nuclear particles and radiation exploded from an infinitely hot, infinitely dense state with initially zero dimension. Predictions Measurements The experiment is running!
36 Measurements There are 3 pieces of data that give us information on the origin of the universe: Galactic Redshift Cosmic Background Radiation The Cosmic Abundance of the Light Elements
37 Measurements In 1929, Edwin Hubble announced that almost all galaxies appeared to be moving away from us. This phenomenon was observed as a redshift of a galaxy's spectrum. He noticed that the further the galaxy, the greater the redshift.
38 Recession Velocity (km/s)Hubble’s Law Distant galaxies are flying away (= receding) from us with a speed proportional to distance Recession Velocity (km/s) Distance (Mpc)
39 You can calculate the slope of the line.Measurements You can calculate the slope of the line. Slope = Rise Run H = Velocity Distance v = Hd = Hd d t = H 1 t
40 Measurements There are 3 pieces of data that give us information on the origin of the universe: Galactic Redshift Data = Redshift of galaxies Interpretation = expanding universe Bonus = the age of the universe! Cosmic Background Radiation The Cosmic Abundance of the Light Elements
41 Measurements In 1927, Georges Lemaître, proposed that there might be leftover radiation from an initial explosion out there to find. In the 1940s, Robert Dicke suggests that his microwave radiometer could be used to search for the radiation. In 1960s, Wilson and Penzias discover the cosmic background radiation! Won Nobel Prize in 1978!
42 Background Radiation MeasurementsThe cosmic microwave background radiation is a remnant of the Big Bang and the fluctuations are the imprint of density contrast in the early universe. The density ripples are believed to have given rise to the structures that populate the universe today: clusters of galaxies and vast regions devoid of galaxies.
43 Planck Curves
44 CMB Details The temperature that corresponds to the intensities measured by Penzias and Wilson is 2.73 K or about 3 degrees above absolute zero The shape of intensities measured fit that of a blackbody curve.
45 Measurements There are 3 pieces of data that give us information on the origin of the universe: Galactic Redshift Data = Redshift of galaxies Interpretation = expanding universe Bonus = the age of the universe! Cosmic Background Radiation Data = Redshifted background radiation Interpretation = early universe was HOT The Cosmic Abundance of the Light Elements
46 Measurements THE OBSERVED COSMIC ABUNDANCES OF HE AND D.The Sun, a typical star, contains (by mass) 75% H, 24% He, and 1% for all the remaining elements in the Periodic Table. This is too much He to have been produced by stellar nucleosynthesis inside stars at T > 5x105 K. There is no plausible astrophysical source for its production other than the Big Bang itself. Light elements such as D and He could be synthesized in the Big Bang because of the very high density and temperature. According to current models of stellar evolution, approximately 96% of the He in the Universe today must have been made during the first minute or so after the Big Bang. The other 4% was made billions of years later inside stars.
47 Measurements There are 3 pieces of data that give us information on the origin of the universe: Galactic Redshift Data = Redshift of galaxies Interpretation = expanding universe Bonus = the age of the universe! Cosmic Background Radiation Data = Redshifted background radiation Interpretation = early universe was HOT The Cosmic Abundance of the Light Elements Data = Large amounts of light elements Interpretation = early universe was HOT and dense
48 The Big Bang and Scientific MethodThe universe is homogeneous and isotropic. The General Theory of Relativity and Quantum Mechanics. Anthropic Principle At the beginning of time, elementary nuclear particles and radiation exploded from an infinitely hot, infinitely dense state with initially zero dimension. 1. Redshift of Galaxies 2. Cosmic Background Radiation 3. Cosmic Abundances of the Light Elements Predictions The experiment is running!
49 The Shape of Space-timeThe amount of matter and energy in the Universe affects its overall curvature or shape. closed open flat
50 The Shape of Space-timeIt’s very difficult to visualize how 3 (or 4) dimensions can be bent, stretched, and squeezed. Let’s look at 2 dimensions instead.
51 The Shape of Space-timeThis is fine if space is not curved, but even on the surface of the Earth, we must account for curvature.
52 The Shape of Space-timeHow does this affect our triangle? Now we know the shape and the fate of the Universe are not connected.
53 Shape and Geometry of the UniverseA 2-dimensional analogy: How can a 2-D creature investigate the geometry of the sphere? Finite Universe with no center or edge Measure curvature of its space! Flat surface (zero curvature) Open surface Closed surface Areas of all circles are equal to πr2 (negative curvature) (positive curvature) Areas of large circles are less than πr2 Areas of large circles are greater than πr2
54 Einstein Field Equation (again)Here’s the Einstein Field Equation again: Rµυ – ½ Rgµυ + Λgµυ = 8πGTµυ This time there is an extra term on the left side – the one with the “Λ” (pronounced “lambda”) in it. Remember that in Einstein’s original ideas, the mass in the Universe should provide gravity which would cause the Universe to contract (or slow its expansion). What should the Λ thing do to prevent the Universe from shrinking?
55 The Cosmological ConstantThe Λ thing is known as the “cosmological constant.” Einstein considered adding the cosmological constant to his field equations to be the “biggest blunder” of his life. Since we now know that the Universe is expanding, what effect would Λ have on the Universe? (Assume it behaves the same as your answer on the previous slide.)
56 Distant Supernovae Because supernovae are so luminous (more than 100 million – 108 or more – times the luminosity of the Sun), they can be seen from billions of light years away with large telescopes. Type Ia supernovae always come from carbon fusion in a white dwarf star – this makes them a good standard candle. Around 1990, two groups began trying to understand the distant universe by searching for faint, far-off Type Ia supernovae.
57 Distant Supernovae They reasoned that, because there is matter in the Universe, the expansion should slow down due to the gravity produced by that matter. As you look across great distances, you should find evidence for this slow down – distant Type Ia supernovae should be brighter because slower expansion should keep them closer.
58 Distant Supernovae The results they released in 1997 and 1998 showed the opposite. Type Ia supernovae were fainter than expected at larger distances. The only way this is possible is if the expansion of the Universe is speeding up or accelerating! A very suprising result!
59 Dark Energy This result was completely unexpected.There is still very little idea what causes this accelerated expansion, but whatever is driving it was given the name “dark energy” According to data that we have, dark energy would have become the dominant component of the Universe about 4 billion years ago – around z = 0.5. On the timeline of the Universe, this is a recent event. Nobel Prize in Physics for 2011 went to Saul Perlmutter, Brian Schmidt, and Adam Riess for discovering it.
60 Dark Energy When you throw an object in the air on Earth, gravity brings it back down to the surface. Even if you could throw the object faster than escape velocity (11 km/s), gravity would still slow it down as it moved away. Dark energy acts in such a way that you throw the object up, and then it starts to speed up as it heads away from you.
61 Dark Energy However, locally the shape of space-time can be affected by gravity. If the matter in a region is dense enough, dark energy will not act to stretch space-time. You will stay the same size, as will the Earth, the Sun, the Solar System, the galaxy, and any cluster of galaxies – all of these objects are held together by stronger forces. (For astronomical objects, it’s gravity). Dark energy will make the relatively empty space between galaxy clusters stretch faster and faster.
62 Strange Coincidence! Although we don’t know much about dark energy, we can measure its “strength” – how it will move galaxy clusters apart. Most recent observations show that it will behave exactly like the cosmological constant – the term, Λ, that Einstein put in his equation because he didn’t like the Universe changing size! (Einstein, most likely, would have been really upset by an expanding Universe that is growing faster and faster.)
63 Back to the Energy BudgetRecall: the contents of the Universe determine its “shape” or geometry. If the matter and energy add up to the right value, the geometry is flat. Using the data from the previous slides, it is very likely the Universe has a flat geometry.
64 The Big Bang and Scientific MethodThe universe is homogeneous and isotropic. The General Theory of Relativity and Quantum Mechanics. Anthropic Principle At the beginning of time, elementary nuclear particles and radiation exploded from an infinitely hot, infinitely dense state with initially zero dimension. 1. Redshift of Galaxies 2. Cosmic Background Radiation 3. Cosmic Abundances of the Light Elements. A Flat Accelerating Universe The experiment is running!
65 Will the Universe End That Way?Maybe...