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22. The Big Bang, Inflation, and General Cosmology

22. The Big Bang, Inflation, and General Cosmology

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Cosmological Constant

Transcript: Why is the universe accelerating, and how does this relate to the more standard cosmological idea that since the big bang the expansion rate has been decelerating due to the action of gravity on all the matter of universe? For the answer to this we have to go back to Einstein in the 1920s. Einstein solved the equations of General Relativity and realized that the solutions naturally indicated expansion or contraction. When told that the universe was static, Einstein added a term to the solution of his equations called the cosmological constant to suppress the natural expansion. Thus the cosmological constant represents something that acts opposite to gravity. Gravity is an attractive force; the cosmological constant represents something that is repulsive. In the standard model of the universe with a cosmological constant the big bang is followed by a period of deceleration due to all the matter in the universe. And then at some epoch several billion years ago the deceleration changes into an acceleration, and the rate of expansion increases. We are currently witnessing a phase of acceleration in the universe and its evidence that the term in gravity is balanced by another term, the cosmological constant.

1min

28 Jul 2011

Rank #1

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Evidence for Cosmic Acceleration

Transcript: The primary evidence that the universe is currently accelerating comes from distant supernovae. The observation is based on the fact that a standard cosmology filled with only radiation and matter, most of which is dark matter, predicts the brightness of distant objects. Supernovae at redshifts of about a half or greater are observed to be twenty or thirty percent fainter than expected in a standard cosmological model. The explanation is that they are more distant than anticipated and that this is caused by the acceleration over the time since the big bang. It’s worth questioning how good this evidence is based as it is on only one distance indicator. Astronomers have worked hard to measure local calibrators for the distance indicator of supernovae Type Ia, and they understand very well how energy comes out of supernovae. So it looks like supernovae are an excellent distance indicator. When observed at high redshift the supernova is imbedded in a distant galaxy, and the light from the galaxy is mixed with the light from the supernova. Very careful observations with the Hubble Space Telescope are required to dig out the distant supernovae. In some cases only the supernova is seen and not even the galaxy in which it’s imbedded because it’s so faint. These observations have been done with care, and it really does appear that supernovae are fainter than anticipated. Another possibility is that dust distributed through the universe might make supernovae fainter than expected. But dust would also make them redder, and there’s no that the distant supernovae are red due to dust. Thus their faintness looks like increased distance. The clinching evidence has not yet come. It will come when astronomers can look at distant enough supernovae to trace back in time to the phase of deceleration before it turned into acceleration. This is expected in the next decade.

1min

28 Jul 2011

Rank #2

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Accelerating Universe

Transcript: Standard cosmological models, where the universe is filled with matter and radiation, always have an expansion rate that slows down with time since the big bang, a deceleration. This is because gravity of all the matter in the universe acts to slow down the expansion rate, but in the late 1990s astronomers got a big surprise. Observations of distant supernovae indicated that the universe is currently in an acceleration phase. Here’s how the observation worked. Astronomers looked at distant Type Ia supernovae. These supernovae in binary systems indicate a situation where matter is spooned onto a white dwarf causing an explosion with a well regulated luminosity. These supernovae are excellent distance indicators. When observations with the Hubble Space Telescope and large ground based telescopes were used to look at the most distant supernovae, distances of about a gigaparsec or redshift of about a half, it was observed that their apparent brightness was twenty or thirty percent fainter than expected in a standard cosmological model. The implication was that the supernovae were further away than expected in a standard cosmological model, and the explanation was that the universe is accelerating placing them at a larger distance.

1min

28 Jul 2011

Rank #3

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Deacceleration Parameter

Transcript: Universe with matter in it will be continuously decelerating over time since the big bang. Cosmologists refer to this as the deceleration parameter, and it’s written as a little q with a subscript zero. In standard cosmologies q0 equals a half for a flat universe, less than a half for an open universe, and more than a half for a closed universe. The relationship is more complex in cosmologies with a cosmological constant. Measuring the deceleration depends on comparing the properties of distant objects like galaxies with nearby objects. This is difficult because of the problem of look-back time. When we look at distant objects we are looking at objects as they were and not as they are now, so when we compare objects that are distant or high redshift with nearby objects we are not comparing like with like. Unless astronomers can model and predict the rate of cosmic evolution of stellar systems like galaxies they can not do cosmological tests based on a comparison of distant objects with nearby objects. To this point it has been very difficult to conduct this type of cosmological test.

1min

28 Jul 2011

Rank #4

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Density Parameter

Transcript: Another fundamental quantity of the big bang model is the density parameter. It’s defined as the ratio of the mean density of the universe to the density just needed to overcome the cosmic expansion. The density parameter is denoted by the Greek symbol capital omega with a subscript zero. If omega equals one the universe is flat. If omega is less than one the universe is open, and if omega is greater than one the universe is closed. Unlike the deceleration the density parameter is a purely local measurement. All that’s required is to take a large volume of space, typically fifty to a hundred megaparsecs in distance from the Milky Way, add up all the matter that’s contained, luminous and dark, divide by the volume, and compare to the critical density. The answer gives the sense of whether the universe will expand forever. The best current measurements indicate that omega is about 0.3 or only one-third of the amount of density needed to overcome the expansion. Based on these measurements the universe will expand forever.

1min

28 Jul 2011

Rank #5

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Hubble Parameter

Transcript: Astronomers often talk about the Hubble constant, capital H with a subscript zero, which represents the local expansion rate of the universe measured with relatively nearby galaxies. When done with the Hubble Space Telescope, the local expansion rate was measured within about fifty to sixty million lightyears. This is a small fraction of the size of the observable universe and a small fraction of the look back time too, and the expansion rate has been constant over this time. However, any universe containing matter will have an expansion rate that decelerates with increasing time due to the attraction of galaxies acting on each other. They act to retard, or slow down, or decelerate the expansion. So if the expansion rate had been measured in the distant past, astronomers would find a higher expansion rate than they measure now. So the correct way to consider the Hubble parameter is as a parameter and not a constant. Its present day value is seventy kilometers per second per megaparsec, but the Hubble parameter has been decreasing in size ever since the big bang.

1min

28 Jul 2011

Rank #6

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Dynamics of Expansion

Transcript: General relativity makes a strong connection between the dynamics of the universal expansion, which is to say the rate of increase of the size with time, the density of matter, and the curvature of space itself. In an empty universe space is not curved. The size of the universe increases linearly forever at the same rate. The Hubble expansion is uniform and unchanging with time. If you have a universe with a certain amount of matter but still low density the size of the universe increases but at an ever slowing rate. In a critical density universe it’s defined as the universe where the scale of the universe increases but at an ever slowing rate where in an infinite time a maximum size will be reached. This is the case of flat space. And in a high density universe, density above the critical density, the radius of the universe initially increases at an ever slowing rate until a maximum size is reached, and then the size begins to decrease at an ever accelerating rate until it reaches zero size at sometime in the future. All of these possible destinies and densities of the universe can be measured, so in principle with observations of the present day universe we can predict its future evolution.

1min

28 Jul 2011

Rank #7

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Scale of the Universe

Transcript: The best way to think of the cosmological redshift z is in terms of the scale of the universe. We see regions near us as they are now, nothing has changed. That’s at redshift zero, z equals zero, but in general redshift is defined as the present day scale of the universe divided by the previous scale minus one. The universe was smaller in the past, so waves have been stretched out as they travel through cosmic time and space by the expansion of space itself. When we see light from an object at a redshift of one we are looking at light that was emitted when the universe was half its present size. When we see light from an object at a redshift three, we are looking at light emitted when the universe was a quarter of its present size. When we see light from the most distant objects at redshift six, that light was emitted when the universe was one-seventh of its present size. Cosmologists like to describe the universe in terms of a graph that plots the scale or scale factor of the universe verses cosmic time. Three characteristic curves can be seen on this kind of a diagram. In one, the open universe with negative curvature, the scale factor of the universe continues to increase although the line curves over indicating a deceleration. In the case of the flat universe the expansion continues and the scale increases, but it asymptotically heads to a maximum value of size. And in the closed case corresponding to positive space curvature, the universe reaches a maximum size, and then the curve reverses itself and falls back towards the origin.

1min

28 Jul 2011

Rank #8

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Evidence of the Big Bang

Transcript: The big bang theory sounds as fantastic as the creation myths of many of the world cultures. How could you, and I, and the Earth, and sun, and Milky Way, and billions of galaxies have emerged from a tiny dense dot of energy and matter. There are three primary pieces of evidence. First, galaxies are all taking part in a universal expansion which manifests as a linear relationship between redshift and distance. If this expansion is traced backward it points to a time billions of years ago when galaxies were all in the same place, and the universe was much smaller than it is now. Second, the abundance of the light elements, in particular, helium, lithium, and deuterium, cannot be explained by normal fusion processes in stars but can be explained by fusion in the universe itself when it was hot young and dense. Third, space is filled with relic radiation leftover from the early hot phase which has been redshifted over the subsequent billions of years to microwaves.

1min

28 Jul 2011

Rank #9

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Big Bang

Transcript: The man who first came up with the idea of the big bang was an unassuming Belgian priest called Georges Lemaitre. In 1929 he beat the giants of general relativity like Einstein to the punch by hypothesizing a universe derived from a cosmic singularity, “A day without a yesterday,” as he put it. This universe began infinitely small and infinitely curved and contained all matter and energy in a point, a singularity. Fred Hoyle, who supported the alternative theory of the steady state, disparaged this model and gave it the name big bang which stuck, and it’s used today to describe the current version of scientific belief in the origin of the universe. The big bang is based on the idea that the universe has not been the same but has evolved with time, and it’s also based on the idea of an origin event which forms a limit to our knowledge of the universe. Space and time both begin with the big bang.

1min

28 Jul 2011

Rank #10