Transcript: The problem with using helium 3, helium 4, or lithium 7 for testing cosmic nucleosynthesis in the big bang model is that all three of these are created in stars, and lithium can be destroyed in stars. So it’s difficult to estimate which of the nuclear abundances are primordial, originate in the universe itself, and which are caused by stars. As an alternative technique to the helium abundance astronomers have looked at the deuterium abundance, and they’ve measured it in the...

Transcript: The best traditional test of cosmic nucleosynthesis and the predictions of the big bang model is the helium abundance in the universe. In the model of the big bang the predicted helium abundance is relatively insensitive to the cosmic density, so astronomers need a measurement accurate to three decimal places to make a good test of the model. This is a difficult observation. It’s best made by looking at unevolved stellar systems where the material has not been processed over...

Transcript: In the big bang model it’s possible to calculate the abundance of light elements created by fusion reactions in the early phases of the expansion. This is called cosmic nucleosynthesis. The calculations depend on only one fundamental parameter of the universe which is the density of protons and neutrons. Although dark matter dominates normal matter, dark matter interacts so weakly that it’s not relevant for the calculations of nucleosynthesis in the early universe. These...

Transcript: The cosmic abundance of light elements is a primary piece of evidence in favor of the big bang model. Stars are fusion factories, and main sequence stars create helium from hydrogen by the fusion process in their cores. However, a careful accounting of stellar fusion over the history of the universe shows that it’s impossible to create twenty-five percent of the mass of the universe in the form of helium as observed. In fact in the 1940s George Gamow, Russian theorist,...

Transcript: What is the physical meaning of the cosmological constant? Astronomers do not know and physicists are puzzled too, but they’ve both come up with the idea of dark energy. In physics the vacuum is not totally empty. Particles and anti-particles can be created from the vacuum, and the ground state of any particle in the quantum theory has finite energy. Thus there’s a possibility that energy and positive pressure to cause expansion can exist in the vacuum of space. ...

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...

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...

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...

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...

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...

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. ...

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...

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...

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...

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...

Transcript: Many cultures around the world have creation myths based on the idea of cycles and time. Buddhist and Hindu legends measure the birth, death, and rebirth of the universe in units of four trillion years, a day in the life of Brahma. The Greeks Stoics imagined that the universe was created from fire, only to be destroyed by fire, and so on in endless cycles. Cycles of time also appear in the religions of the Mayans and the Aztecs. The Judeo-Christian religions set the stage for...

Transcript: Present day observations of the universe lead to speculation about its origin. There’s an extension of the cosmological principle called the perfect cosmological principal which holds that the universe is unchanging both in time and space, that the universe is not only the same in all directions and at all distances from the Milky Way but has been unchanging in time as well. This idea leads to the steady state model. The galaxies are all moving apart. In the steady state model...

Transcript: A very basic assumption about the universe which forms the basis for modern cosmology is called the cosmological principle, that the universe is isotropic and homogeneous. Isotropic means the same in all directions. This means that in any direction we look we tend to see the same structures and numbers of galaxies, and that is in fact confirmed by observation. It also means that the Hubble expansion is the same in every direction we look, that the expansion rate is smooth and...

Transcript: It’s extremely difficult to conceptualize curved three dimensional space, so astronomers use analogies. The analogies are useful as long as we recognize that they all have limitations. The true situation is only described exactly by the equations and the mathematics of Einstein’s Theory of Relativity. The expanding universe and the big bang are not like an explosion with the galaxies flying apart like fragments or shrapnel through air because there is no air. Space itself is...

Transcript: According to the theory of general relativity, the universe and the space we live in may actually have a shape, and the shape need not be the flat infinite space described by Euclidean geometry. Infinite space will be flat, but curved space could be finite or infinite, bounded or unbounded. Consider for example the surface area of a sphere. A sphere is a curved surface. Its area is finite, four pi r squared, and yet it’s an unbounded surface. You can travel endlessly on the...

Transcript: It’s important to realize that the cosmological redshift, usually given by the symbol z, is not the same as a Doppler shift. The Doppler shift is a relative shift in the wavelength of waves passing through a medium. The cosmological redshift in the universe is caused by the expansion of space itself everywhere in the universe. The best analogy is a balloon with beads glued to the surface. It’s a two dimensional analogy for a three dimensional situation. The beads on the...

Transcript: When the equations of General Relativity are applied to the situation of the entire universe the solutions tend to be solutions of motion, either expansion or contraction. However, when Einstein visited Mt. Wilson Observatory in 1921 he asked the astronomers what their best view of the state of the universe was. Remember that this was before the discovery of the extragalactic distances to the nebulae and before Hubble’s discovery of the expansion. So the astronomers told...

Transcript: The ideas of geometry were invented by Euclid two thousand years ago, but in the nineteenth century Russian mathematicians and a few German mathematicians invented the mathematics of curved spaces. They also invented mathematics that would apply to geometries in more than three dimensions. When invented these mathematics were utterly hypothetical. No one had any idea that they might apply to the universe itself. In general these are called non-Euclidean geometries. There are...

Transcript: The modern idea of cosmology is based on Einstein’s theory of General Relativity. Einstein realized in a thought experiment that acceleration due to gravity could not be distinguished from acceleration due to any other force. He also knew that mass and energy were equivalent according to E = mc2. The two ideas combine to lead to the fact that mass can bend light. In Newton’s idea of an infinite universe space and time flow smoothly, and never bend, and are linear at all...

Transcript: The solution to Olber’s paradox is subtle and involves three different concepts. The first is the redshift of light that occurs due to the expansion of the universe. This means that the light from distant objects is redshifted or reduced in energy, so as we move further out from the Earth or the Milky Way the light from successively larger and larger distances is reduced relative to the inverse square law of light. So the light does not pile up infinitely. Second, there is a...