I intend this lecture to be an overview of the big bang model of the universe from the standpoint of the observational motivation.

Transcription

1 I intend this lecture to be an overview of the big bang model of the universe from the standpoint of the observational motivation. Nowadays, the big bang model is overwhelmingly preferred over other alternatives; Why is that? I want to explain why I am going to focus so intently on one idea and not give serious attention to any alternative.

2 Five Pieces of Evidence for the Big Bang 1. Expanding universe. 2. Universe has an age. 3. Universe is evolving. 4. Microwave background radiation. 5. Abundance of elements.

3 1. Expanding Universe Galaxies are observed to be flying apart from one another. More distant galaxies are moving away from us faster. If one does the thought experiment of reversing time and considers the galaxies to be moving together then they all merge together in ~12 billion years. This observation is consistent with the idea that the galaxies emerged from a common place roughly that long ago. (Car travels at 60 mph and is now 120 miles from home. How long since it started?)

4 Five pieces of evidence for a big Bang 2. Universe has an age We have a good understanding of how stars work and can calculate how old they are at various stages of their development. Galaxies are aggregates of stars. Throughout the universe we find star populations of all possible ages up to a certain maximum. The oldest stars are estimated to have ages of ~14 billion years. We do not find anything older and infer that the universe is roughly this old. There is a small inconsistency between the ages mentioned in (1) and (2) but a variety of explanations are possible within the Big Bang framework such details will be discussed later.

5 3. Universe is evolving. Light travels at 300,000 km/s so that light we receive from more distant objects was emitted long ago. As we look outward, we are looking back in time. What we find is that things that are farther away are different from things nearby. Extremely distant galaxies are red because they have large velocities away from us due to the expansion of the universe, hence have large doppler redshifts. - more galaxies with active nuclei. - proportion of blue galaxies (young, hot stars) is greater at large distances; nearby galaxies tend to be intrinsically redder (dominantly composed of old, cool stars). - more interacting and irregular galaxies at very large distances. These observations are compatible with the idea that we are looking out, and back, to the time when galaxies were just forming and the stars in them were young.

6 4. Microwave Background Radiation. If we run the clock of time backward, then at earlier times temperatures and densities were higher. At the earliest moments, these densities and temperature were extreme. The black body radiation from this hot material would have redshifted with the flying apart of the universe. It was anticipated that the remnant of this radiation should still be around but now shifted to very low energies. This radiation has been detected and peaks at ~1mm ~ 3 o K. Not only does this measurement provide a strong consistency check with the idea that the universe was once very hot, but it also provides detailed quantitative information about the energy density of the universe. This information can be used to build very specific models. This radiation is not entirely smooth and the irregularities in it provide information relevant to the formation of structure in the universe.

7 5. Abundance of Elements. In the Big Bang model, the primordial soup consisted of only the most basic nuclear particles and all the chemical abundances were build up with time. A distinction is made between the light elements (hydrogen, helium, lithium, beryllium, boron) and the rest of the elements in the periodic table. There is now a reasonable understanding of how the heavy elements formed in stars, a little at a time, over the long history of the cosmos. By contrast, the production of the light elements, for the most part, would have occurred during the Big Bang. Consistent with this idea, the abundances of the light elements are approximately the same everywhere we look while the abundances of the heavier elements depend very much on environment. A specific triumph of the Big Bang concept is the prediction, in accordance with observations, that ~25% of the mass in normal particles would be Helium and almost all the rest would be Hydrogen. Detailed measurements of rare species like Deuterium and Lithium provide strong constraints on the details of the Big Bang model.