       Doing Science

## Unit Six: Investigating the Universe Doing Science

2. Supernovae, Gamma Ray Bursters, and the History of the Universe

Introduction:
From the development of general relativity by Albert Einstein in 1915 through the mid-1990s, models of the evolution of the universe have assumed a simple expansion of the universe driven by the force of gravity alone. However, in the last decade or so, astronomers searching for supernovae have found evidence of something in addition to gravity. Many recent discussions and articles about cosmology have been filled with allusions to dark energy, cosmological constants, vacuum fluctuations, and other exotic ideas based on theoretical physics. Will the universe ultimately collapse, quietly expand forever, or will everything—down to sub-atomic particles—be torn to bits in an eventual "Big Rip?" In this Doing Science, you will examine the data that are the basis of these new ideas.

Directions:
Find the most recent list of Type I supernovae. The best list in 2009 can be found at: http://braeburn.pha.jhu.edu/~ariess/R06/sn_sample. The highest redshift in this set is 1.755. A second set of objects has been studied recently with even higher redshifts—the Gamma Ray Bursters (GRBs), some with redshifts above 6. The best list if these objects can be fond in Table 6 of: http://arxiv.org/abs/astro-ph/0612285. Download the Full-text and scroll to table 6 near the end of the paper. Calculate cz and the Luminosity distance (DL) for each object. This is easily done using Excel or similar spreadsheet program. [Remember, the Distance Modulus, or DM is defined: DM = 5 log (DL/10 pcs)].

Plot the supernova and GBB samples on a Hubble Diagram with DL on the horizontal axis and cz on the vertical. Use different symbols for each. On the same diagram, plot curves representing the expansion of the universe assuming flat space (a good approximation) for at least three values of the Hubble constant Ho = 50, 70, and 100 km/s/Mpc. These curves can be calculated using:

DL = cz (1+z)/Ho

Carefully compare the distribution of the supernovae and GRBs with the model expansion curves. If the universe is expanding under the influence of gravity alone, the data should cluster along a single value of Ho. If the mean value of Ho changes over time, then something besides gravity is affecting the expansion.     