Cosmology The study of the structure and evolution of the Universe as a whole. Seeks to answer questions such as: How big is the Universe? What shape is it? How old is it? How did it form? What will happen to it in the future?

Distribution of Galaxies Galaxies appear to be distributed throughout the Universe Galaxies clump together into clusters and superclusters. Distant galaxies appear to be moving away from us. We know this from the redshifts in the spectrum of galaxies. A map of one million galaxies

The Expansion of the Universe The motion of galaxies away from us is due to the expansion of the Universe. Like buttons on a balloon which when inflated all move away from one another, so too are all the galaxies moving apart. The buttons are not moving it is the balloon itself that carries the buttons away. Space itself is expanding and simply carrying the galaxies with it.

The Big Bang Since the Universe is expanding it must have been smaller in the past. At some point in the past the entire Universe must have been packed into some very small size. In 1927, Abbé George Lemaitre, a Belgian cosmologist and priest proposed that the Universe began as a “Primeval Atom” and estimated its age. To estimate the age you need to know the expansion rate – in other words Hubble’s Constant, H. H is 71 +/- 4 km/sec/Mpc giving an age to the Universe of 13.7 +/- 0.2 Billion years. A plot of the distance of galaxies versus their recessional velocity. The slope of the line is Hubble’s constant.

The First Few Moments Initially the Universe was incredibly hot. Matter and energy were freely transforming into one another through Einstein’s Mass-Energy relation. The mass formed were in matter anti-matter pairs. Matter and anti-matter destroy one another when they come into contact but just enough matter existed to survive. This remaining matter is what fills the Universe.

Composition of the Early Universe As the Universe cooled Hydrogen nuclei began to form. Conditions were still hot enough for fusion reactions to occur forming Helium. Nearly all elements heavier than Helium however were formed later by stars. We notice that the oldest stars have much less heavy elements than the Sun. This is further evidence for the Big Bang theory. After neutral atoms formed the radiation that filled the Universe was allowed to freely propagate.

Cosmic Microwave Background At the extremely high temperatures present in the first few moments after the Big Bang the Universe would be emitting very high energy light. With the expansion of the Universe we now can detect this light redshifted to much lower energies. This light is the Cosmic Microwave Background and is evidence in favor of the Big Bang theory. An image of very small variations in the Cosmic Microwave Background. These variations give us information on the early structure of the Universe.

The Fate of the Universe The Universe may expand forever (open) or it may collapse back in on itself (closed). Which occurs depends on the amount of mass in the Universe. With enough mass, the mutual gravitational attraction will slow the expansion and cause it to collapse. Astronomers do not see enough mass to cause this collapse to occur. There might be enough “Dark Matter” however. An HST picture showing a nearby galaxy and dozens of more distant galaxies.