Transcript: In cosmology galaxies are used as markers of expanding space. In addition to the overall cosmic expansion gravity has caused matter to clump or cluster on many different scales. In addition to the gravitational attraction that caused galaxies to collapse and form in the first place gravity has also caused galaxies to aggregate together or congregate in regions of space. The distribution of galaxies in three dimensions in the universe overall is called large scale structure.

Transcript: Most of the techniques for measuring distance in astronomy depend on well understood properties of stars or entire galaxies. However, if astronomers believe they’ve measured the current expansion rate of the universe then redshift itself can be used to indicate distance. Redshift is defined as the fractional wavelength shift caused by the expansion of the universe on the light of a galaxy. At low redshift it’s also equal to the recession velocity divided by the velocity of...

Transcript: When a massive star dies the supernova that results can rival an entire galaxy in brightness, so it can be seen to a very large distance, a billion lightyears or more. When a single massive star dies it does not do so in a well regulated way, but the supernovae that result from a binary system are standard bombs that can be used as distance indicators. Basically the mass from the companion spoons slowly on to a white dwarf until it pushes it over the Chandrasekhar limit, 1.4...

Transcript: A clever technique for measuring the distance of galaxies that applies to galaxies with smooth light distributions like ellipticals is called surface brightness fluctuations. The idea of surface brightness is that it is the flux per unit area in a galaxy. The important thing to realize is that the observed flux of a galaxy goes down as the distance squared, inverse square law, but in any fixed area of a detector like a CCD detector the number of stars enclosed by a pixel of the...

Transcript: One of the best distance indicators for elliptical galaxies is the Faber-Jackson relation, named after the two astronomers who discovered it. The Faber-Jackson relation relates the range of stellar velocities or their velocity dispersion in the nucleus in an elliptical galaxy with the size of the galaxy. The width of the stellar absorption features from a spectrum of the elliptical galaxy is used to give the velocity dispersion, and the size of the galaxy comes from an image. ...

Transcript: An important distance indicator for spiral galaxies is called the Tully-Fisher relation, named after the two astronomers that discovered it. Observationally the luminosity of spiral galaxies is correlated with their rotation speed of their gas disks. There’s a physical basis for this distance indicator because the rotation of the gas disk is an indicator of mass, and if the mass to light ratio of the spiral galaxy is relatively constant then it’s also an indicator of luminosity....

Transcript: Beyond a distance of about twenty megaparsecs, or sixty or seventy million lightyears, it becomes difficult to use individual stars as distance indicators. Cepheid variables are hopelessly blurred in the summed light of billions of stars in the distant galaxy, and even supernovae, which indeed can be seen above the light of an individual galaxy, may be imbedded in dusty regions. In addition, there are multiple types of supernovae, and with out high quality spectroscopic...

Transcript: In the nearby universe astronomers primarily use stars as distance indicators. Cepheid variables which were classically used by Hubble to demonstrate the distance to the nebulae and the universal expansion are still used. As luminous stars with well understood physics they can be found locally in the Milky Way where their distances are tethered by parallax and main sequence fitting, but they are bright enough to be observed out to distances of twenty megaparsecs or over fifty...

Transcript: The Hubble constant sets the current expansion rate of the universe and gives an indication of its size and age. The best currently measured value of the Hubble constant comes from a heroic project done with the Hubble Space Telescope over a number of years. The Hubble Space Telescope project was based on observations of Cepheid variables as the distance indicator because they represent well understood physics that can be applied across large distances in space. Several dozen...

Transcript: If we assume that the cosmic expansion has been uniform we can use the current value of the Hubble constant to get an estimate of the edge of the universe. Velocity is the same as distance times time, and the Hubble relation states that the velocity is equal to the Hubble constant times the distance. Using these two relations we can get that the age of the universe is equal to one divided by the Hubble constant, the reciprocal of the Hubble constant. Substituting the value of...

Transcript: For galaxies velocity is proportional to distance, and the constant in proportionality in this linear relationship is called the Hubble constant given by the large letter H and the subscript zero. It’s defined therefore as velocity divided by distance, and in astronomers units it has units of kilometers per second per megaparsec. Modern measurements place the best value of the Hubble constant at around H0 of seventy kilometers per second per megaparsec. This means, for example,...

Transcript: The Hubble relation, a linear relationship between recession velocity of galaxies and their distance from the Milky Way, is a cornerstone of modern cosmology. It’s sometimes called the Hubble law, but it’s not a law of physics in the way that Newton’s laws are laws of physics. There’s nothing in physics that says that the universe should behave this way. Hubble relation is a purely observational result, but it’s a major clue on how the interpret the universe we live in. The...

Transcript: The Hubble relation is a linear relationship between the distance of a galaxy from the Milky Way and its recession velocity or redshift. In this situation the more distant galaxies are moving away from the Milky Way faster. The Milky Way has no privileged position in this expansion because a viewer on another galaxy would see exactly the same thing. This relationship suggests that at some time in the distant past all the galaxies were in one place. We can trace the expansion...

Transcript: If all the galaxies are moving away from us and the more distant galaxies are moving away faster, does this not indicate that we are at the center of the universal expansion? No it does not because we observe a distant galaxy to be moving away from us; reverse the situation and an observer on that galaxy would observe us to be moving away from them. Draw dots on a balloon, and as you expand the balloon by inflating it you would see that the distance between any two dots on the...

Transcript: The cosmological interpretation of galaxy redshifts leads to the idea of the expanding universe. This is a sufficiently dramatic implication that astronomers have from time to time wondered whether it’s the correct interpretation. Occasionally objects of high and low redshift are seen close together on the sky or even with an apparent connection between them, and the implied luminosities of the highest redshift objects are extraordinary. For these and other reasons it’s worth...

Transcript: The redshift of galaxies is not caused by their motion with respect to a medium as in the example of the Doppler shift. It’s caused by the expansion of space itself. This is called the cosmological redshift. Imagine a balloon that you’re blowing up, and on it you’ve drawn a waving line to represent a light wave. As you inflate the balloon, a universal expansion, the wavelength of the wave on the balloon will increase, move to redder wavelengths. This example of a redshift...

Transcript: When Hubble first published his linear relationship between recession velocity and distance for galaxies he was cautious about interpreting it in terms of universal expansion. In fact it’s easy to be confused between two different types of redshift. One familiar type of shift is the Doppler shift. This is true for waves traveling in any medium where the velocity or Doppler shift depends on the velocity of the source of the waves relative to some stationary point in the medium. ...

Transcript: Lets imagine an expanding universe, one in which the distance between galaxies is increasing with time. It’s hard to imagine an expanding universe, but there are analogies that can help. Cut a rubber band and then stretch the rubber band with dots on the rubber bans to mark the galaxies, and as it stretches in one dimension the distance between all the dots will increase. This is a one dimensional analogy for cosmic expansion. Equally, galaxies can be marked on a two...

Transcript: Consider a universe of fixed size where the galaxies are milling around randomly. They might change their position over time, but the average distance between each galaxy and all of its neighbors does not change. If we observed these galaxies by measuring spectra and redshifts we would find on average that half of the galaxies had redshifts and half had blueshifts, and the result would not depend on the distance to the galaxies because the galaxies in any region of space are...

Transcript: Hubble had previously shown that many of the spiral nebulae were in fact distant systems of stars remote from the Milky Way. He then combined the distances he obtained from the Cepheid variable technique with Slipher’s redshifts and some that he measured himself to produce an amazing new result. In 1929 his study of galaxies showed that most galaxies had a redshift that was proportional to their distance from the Milky Way galaxy. This implied that galaxies were all moving away...

Transcript: In 1912 Vesto Slipher working at the Lowell Observatory began a project to observe spectra of spiral nebulae. He was working under the direction of Percival Lowell who became known later for his speculation about the canals on Mars. Slipher detected rotation in the nebulae that he studied. With the Doppler Effect he was able to show that some parts of the nebulae were moving towards us and other parts moving away from us indicating rotation, but in another version of the...

Transcript: When a large primordial gas cloud has a large amount of rotation or angular momentum the collapse will occur preferentially along the rotation axis leading to a disc-like formation. The disc will subsequently shear, and gas pockets will collapse within the disc to create star formation. This is the basic morphology of the disc of a spiral galaxy, but spiral galaxies are complex and have multiple stellar populations. Spirals have almost certainly formed throughout the history of...

Transcript: Some elliptical galaxies form relatively early in the universe, seven to eleven billion years ago. Giant gas clouds with a small amount of rotation began to collapse. Remember that most of the mass of these gas clouds was dark matter. As the gas collapses stars form from the gas and stay on their elliptical orbits rarely interacting. Thus the shape of the galaxy is frozen as an elliptical shape. With little gas left over for star formation the stars passively evolve becoming...

Transcript: Any successful theory of galaxy formation must explain certain fundamental differences between the types of galaxies. The bulges of spiral galaxies and elliptical galaxies contain mostly old, mostly red stars. They have a nearly spherical space distribution, and they are slowly rotating stellar systems. By contrast the discs of spiral galaxies are filled with mostly young stars, have a disc-like configuration, and are relatively rapidly rotating. The fact that things are not...

Transcript: The ubiquitous attractive force of gravity can cause galaxies to merge and even coalesce into a single object. Computer simulations have been used to show that when two equal size spiral galaxies merge they fling off tails of stars hundreds of thousands of lightyears into space. After several orbital times, however, the stars coalesce into a single object and relax into a more regular distribution. In other simulations after a series of major merges it’s possible to create an...