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23. The Big Bang, Inflation, and General Cosmology 2

23. The Big Bang, Inflation, and General Cosmology 2

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Fine Tuning in Cosmology

Transcript: One of the very difficult things to explain in standard cosmology is the flatness of space. Remember that the early universe was small and dense, and space-time was actually curved and knotted in the quantum era so the current flatness of space is an unusual condition. In standard cosmologies with no vacuum energy, flat space corresponds to a critical energy density, and in the expansion dynamics of the universe it’s an equal amount of energy in kinetic energy of the expansion and in the potential energy of all the gravity in the universe. The best analogy for this strange situation is to imagine throwing something up into the air. If you give it some particular random speed it’s likely to fall back down to your feet, but if you give it the very special velocity of the escape velocity, the object will just leave the gravity of the Earth. If you give it much larger than the escape velocity, the object will sail off into space and travel forever. The universe itself is poised between re-collapse and endless expansion. The flat space condition is a very particular condition corresponding to equal amounts of potential and kinetic energy. It’s as if the odds of throwing something up into the air and having it at exactly the escape velocity were very small. The fine tuning of the universe is the closeness of the space-time shape to being flat. The fact that space-time is this close to being flat now means that the initial parameters of the big bang expansion must have been very finely tuned early in the universe.

1min

28 Jul 2011

Rank #1

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Size of the Observable Universe

Transcript: The fact that the early universe had a phase of superluminal or faster than light expansion means that it is not trivial to calculate the size of the observable universe. In other words the following simple idea does not work. The universe is eleven or twelve billion years old, so we might imagine that it must be eleven or twelve billion lightyears across. That’s not true because the early expansion was so rapid. Galaxies early in the expansion were being carried apart faster than the speed of light. Later on they were moving apart less rapidly, and eventually the light became able to reach us. So depending exactly on the cosmological model, the size of the observable universe is twenty-five to thirty billion lightyears across. The observable universe represents the horizon, the distance we can see or the distance within which light has had time to reach us in the history of the universe and the cosmic expansion. Given that we think that the universe is now accelerating, the future of the observable universe is very interesting. Acceleration, if it continues, will gradually take galaxies out of our horizon and away from being visible. So over long periods of time the accelerating universe will remove objects from our view. Over a time scale of many billions of years there will actually be less and less to study. Observational astronomy will not last forever. In the end, the acceleration will cause the only galaxies to be visible to be those that are bound to the Milky Way itself.

1min

28 Jul 2011

Rank #2

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Observable Universe

Transcript: Astronomers make a distinction between the physical universe, all that there is, and the observable universe, all that we can see. The distinction comes about because early in the cosmic expansion any two points in space were separating at faster than the velocity of light. This sounds like a violation of relativity. Special relativity however, which says that the velocity of light is the maximum speed for any signal applies, only to local reference frames. General relativity is the theory that governs the cosmic expansion, and general relativity places no speed limit on the expansion of the big bang. The time when points were moving apart faster than the velocity of light was a time about five billion years ago when the universe was forty percent of its current size or redshift of 1.25. Back much further, three hundred thousand years after the big bang at the time of the microwave background radiation, points in space were moving apart at forty times the velocity of light. Thus there are regions of physical space that we have never seen in the history of the universe, implying that the physical universe is potentially much larger than the observable universe.

1min

28 Jul 2011

Rank #3

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Space - Time

Transcript: Events take place in both space and time. This is fairly obvious if we consider some every day examples. To make an appointment you have to be at the right place and the right time. The two ways you might miss an appointment are by going to the right place but on the wrong day or by going on the right day but going to the wrong place. You might for instance be in a baseball game trying to catch a fly ball. To be in the position and the time to catch the fly ball is a decision in both time and space. If you miss the fly ball it could be because you got to the right place but too late, or perhaps you got there on time but were slightly to the wrong side of where the ball landed. These trivial examples illustrate that time and space are connected. In physics and astronomy it’s routine to talk about space-time as connected entities whether we’re discussing the behavior of microscopic particles in the physics lab or objects in the universe. In the theory of relativity, time and space are formally coupled and linked mathematically. The distortions of relativity caused by motions close to the speed of light apply equally to space and time.

1min

28 Jul 2011

Rank #4

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The Arrow of Time

Transcript: There may be a profound connection between entropy in the universe and the arrow of time, the pervasive since that time moves only in one direction. Remember that the microscopic laws of physics have no arrow of time. However, in any statistical system of particles there’s a tendency for it to move to its most probable state. Entropy is disorder or chaos, but it’s also related to the number of possible states in a system. Imagine the shuffling of a deck of cards as an analogy for the evolution of the universe. Our universe has a relatively low entropy or low degree of disorder, especially compared to a situation where the universe was filled with black holes; the entropy would be millions of times higher. This corresponds to a highly ordered deck of cards. As the deck is shuffled, objects interact in the universe. The order becomes less. The entropy increases. The arrow of time is obvious in this. Interactions will always tend to produce more disorder. However, if the disorder was high to start with, a high degree of entropy at the beginning of the universe, a well shuffled deck of cards, then as the deck is shuffled again there is no change as the deck is shuffled over and over. So high entropy initial state produces no sense of the arrow of time.

1min

28 Jul 2011

Rank #5

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Fate of the Universe

Transcript: In standard big bang models the curvature of the universe and its fate are related. In a high density universe where the density of matter exceeds the critical density, the universe will reach a maximum size and then re-collapse. The strong force of gravity causes the expansion to be overcome. The result is a reversal of the big bang called the big crunch. Our universe appears to have insufficient matter to match the critical density and so will likely expand forever. Extra evidence for this is the existence of vacuum energy or cosmological constant. The fate of our universe is therefore going to be the big chill. In this outcome, the cycle of star birth and death eventually is broken within galaxies as all the gas is used up. After about ten to the thirteen years, ten trillion years, most of galaxies will just contain white dwarfs, black holes, and other dark or dim stellar remnants. Galaxies essentially will switch off slowly with time. After ten to the thirty-five years fundamental particles will decay, and after ten to the power eighty years, although the number is uncertain, black holes within galaxies, even at their nuclei, will evaporate. The eventual result is a uniform sea of electrons, positrons, neutrinos, and low energy photons.

1min

28 Jul 2011

Rank #6

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Dark Matter Density

Transcript: The big bang model cannot be properly described without a measurement of the dark matter density in the universe. Observations of individual galaxies, either their rotation curves or velocity dispersions, and observations of clusters of galaxies show that ninety to ninety-five percent of the matter in the universe is dark. We don’t yet know what it is, but it’s almost certainly in the form of microscopic particles not yet detected in labs on the Earth. A variety of techniques are used by astronomers to measure dark matter on different scales. They show that galaxies are imbedded in dark matter, that there’s a large amount of dark matter in clusters, and that even in the space between galaxies there is dark matter. The cosmic density of dark matter with the current best measurements is about thirty percent of the critical density; omega equals 0.3. Even with the baryonic material added, dark matter does not have sufficient density to close the universe or overcome the cosmic expansion.

1min

28 Jul 2011

Rank #7

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Expansion Rate

Transcript: The current expansion rate of the universe is given by the Hubble constant. Measurement of the Hubble constant was subject to an enormous effort over the past forty years culminating in the Hubble Space Telescope key project. In this huge collaboration involving hundreds of orbits with that precious facility, a series of measurements of the local universe involving Cepheid variables was used to derive the expansion rate. The answer was seventy kilometers per second per megaparsec with an error of only ten percent on that measurement. It’s still one of the most accurately measured numbers in cosmology. The expansion was shown to be isotropic, that is the same in different directions in the sky. The velocity of any galaxy predicted by the smooth expansion is called Hubble flow. However galaxies do not usually have exactly the velocity predicted by Hubble flow because gravitational interactions between galaxies cause them to depart from a smooth flow. The amount by which they depart is called a peculiar velocity. The average amount of peculiar velocities of galaxies in the local universe is about a hundred to a hundred and fifty kilometers per second.

1min

28 Jul 2011

Rank #8

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Structure Formation

Transcript: Throughout most of the history of the universe structures have been forming under the action of gravity. Remember that the visible parts of galaxies just represent the tip of an iceberg of mostly dark matter. The way in which structure forms depends on the detailed properties of dark matter. If dark matter is cold, which to a physicist means that the particles that form the dark matter were not relativistic at the time of recombination when neutral hydrogen occurred and the microwave background photons were released, then structure forms in what’s called a bottom up way. Galaxies form first, then clusters, then even larger structures. On the other hand, if dark matter is hot, which means it was moving relativistically at the time of recombination, then structure forms in what’s called a top down way. Structure on galaxy scales is erased, and the largest structures form first. Astronomers believe there’s good evidence that the universe shows structure consistent with cold dark matter and not hot dark matter. Hot dark matter particles produce too much structure on large scales, more than is observed. Also there are no good candidates for a hot dark matter particle. Neutrinos have a tiny amount of mass but are insufficient in their mass to account for cosmic structure.

1min

28 Jul 2011

Rank #9

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Decoupling

Transcript: As we trace events in the universe back toward the big bang, the first important epoch reached is a time about three hundred thousand years after the big bang. The universe by this time has cooled to a temperature of about three thousand Kelvin like the photosphere of a cool star. The density of matter and radiation has become low enough that photons no longer routinely interact with the charged particles that are there, protons and electrons. This has two fundamental consequences for the universe. First, the photons start to travel freely without interacting with matter. This is the era of decoupling. Photons decouple from matter, and it’s the era at which we see the cosmic microwave background radiation. Those photons have been flowing freely through the universe for eleven billion years since. The universe becomes transparent at this point. The second consequence is that protons and electrons combine to form stable neutral hydrogen atoms. This is called recombination. The two physical processes are connected.

1min

28 Jul 2011

Rank #10