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...

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...

Transcript: Inflation is an extraordinary idea. What is the evidence in favor of it, and is it even possible to test something that happened so long ago? First, space is observed to be close to flat, and recent observations show that it’s extremely close to flat, within a few percent. This confirms a prediction of inflation but of course was one of the motivations for the idea of inflation in the first place, so it’s not really an independent test of the idea. Second, the fluctuations of...

Transcript: How does the idea of inflation explain some of the problems of the standard big bang model? First, by stretching space by an enormous degree, the universe is predicted to be flat. Imagine a tiny curved balloon that is rapidly inflated to a huge size. Someone observing one small piece of the balloon will see it to be flat or very close to flat. Flatness of space is a prediction of inflation. The same expansion also causes space to be smooth. Regions that we currently observe...

Transcript: In 1981, MIT physicist Alan Guth was looking for possible explanations for the smoothness and flatness of the universe when he came up with the idea of cosmic inflation. Inflationary cosmology is an adjustment to the standard big bang model wherein the universe went through a period of extremely rapid or exponential expansion at a time ten to the minus thirty-five to ten to the minus thirty-three seconds after the big bang; that’s a billion-billion-billion-billionth of a second...

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...

Transcript: The big bang model has considerable predictive and explanatory power, but there’s some fundamental questions about the nature of our universe that it leaves unanswered. Why is the universe so smooth and isotropic? The reason this question is important to ask comes from a consideration of the early expansion. In the early big bang, regions of space were separating at larger than the velocity of light, yet the microwave background radiation shows an almost perfect black body or...

Transcript: The big bang model provides a good description of the current universe which is large and cold and mostly empty. It implies that the early phase was hot and dense. The big bang model is supported by the observation of galaxy recession, by the light element abundance which is consistent with cosmic nucleosynthesis in the early hot big bang, and by the presence of cosmic microwave background radiation, smooth and isotropic suffusing space as leftover radiation from the big bang...

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...

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...

Transcript: Space-time diagrams are a way of graphically representing space and time. Remember that the universe we live in is made up of three dimensions of space and one of time, also called the space-time continuum. To represent this in a graphical form we have to collapse the three dimensions of space to one dimension; this becomes the x-axis. The other dimension is time, and that’s the y-axis. By convention the slope of a line at forty-five degrees corresponds to the fastest speed...

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...

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...

Transcript: The entropy of the universe is a measure of its disorder or chaos. If the laws of thermodynamics apply to the universe as a whole as they do to individual objects or systems within the universe, then the fate of the universe must be to increase in entropy. This is obvious in the case where the universe re-collapses in a big crunch because there all structures are erased in the heat death, but in the big chill it’s is not as obvious. However, entropy of the universe continues to...

Transcript: In the big bang model the universe does not necessarily have an edge in space, but it does have an edge in time. The age is measured to be about eleven or twelve billion years. There is a distance, therefore, beyond which light could not have reached us in the time since the big bang. This is called a cosmic horizon. We can’t see beyond it. The observable universe grows slightly everyday as light from more and more distant regions reaches us for the first time. Early in the...

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...

Transcript: It’s easy to forget, since the night sky is so dark, that the universe is filled with radiation. The cosmic microwave background represents photons from the big bang stretched by the expansion of space with their energy reduced to microwaves. There are vastly more photons in the microwave background radiation than there are particles in the universe, a hundred million photons for every hydrogen atom, so the total in the universe is ten to the power eighty-eight photons. What is...

Transcript: In general, light in the universe is a poor tracer of mass. Galaxies are bright markers of space, however, they poorly represent the distribution of dark matter. The different distribution of galaxies with respect to dark matter is called bias, and it’s a fundamental part of any theory of galaxy formation to explain how and why galaxies cluster the way they do and how it relates to the dark matter distribution. Even if galaxies do not trace the distribution of dark matter, they...

Transcript: There’s a basic problem in cosmology. Astronomers measure the light emitted by different objects, but they really want to detect or measure the mass. So the measurement of the mass is indirect. Some galaxies for example are so diffuse that their light level falls below the sky brightness, and so they are missing from galaxy catalogs yet they may contain much dark matter. The intergalactic medium is so diffuse that it cannot be seen in optical light. It is hot and should emit...

Transcript: Another way to look at the mass density of the universe is in terms of the cosmic mass to light ratio. Mass to light ratio is defined as the ratio of the mass, in solar units, to the luminosity, in solar units, so for the Sun by definition M over L is one. In general, low mass stars have mass to light ratios greater than one, and high mass stars have ratios less than one. We know that the stellar populations typical of normal galaxies give overall mass to light ratios in the...

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...

Transcript: There are two fundamentally different types of matter in the universe. Baryons is the term that refers to normal matter: the stuff that we’re made of; our world, the Earth, the Sun, most of the universe we’re familiar with. It’s the normal material of matter, protons, neutrons, and electrons, although electrons being nearly two thousand times lighter than protons or neutrons do not weigh much in the census. The baryon density is the cosmic density of all this material. To...

Transcript: In simple big bang models, that is models without a cosmological constant, the critical density of matter is the density needed to just overcome the expansion so that the universe continues to expand but at an ever decreasing rate. It’s defined mathematically as three times the Hubble constant squared divided by eight times pi times the gravitational constant. Since the critical density is proportional to the Hubble constant squared, this implies that a faster expansion rate...

Transcript: The big bang model predicts that the universe is evolving, that it was smaller, hotter, and denser in the past. Astronomers would like to test this basic prediction. One test involves spectral lines in distant quasi-stellar objects or quasars where the cosmic microwave background radiation excites the line. Observations and comparisons at different redshifts show that the temperature of the microwave background radiation, which is currently 2.7 Kelvin, was indeed hotter in the...

Transcript: Tracing the expansion rate back in time gives the age of the universe in a big bang model. Cosmological parameters, when they’re measured, can be used to predict the age of the universe. Based on the accurate measurements of the WMAP satellite, the age of the universe is between eleven and twelve billion years. Note that any universe where there is vacuum energy and a cosmological constant has an older age than a universe without such vacuum energy because the cosmological...