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Doing Science

Unit Six: Investigating the Universe
Doing Science

1. The Universe Through Time

Is the universe evolving? Current observations indicate that the universe began with a "big bang." Everything in the universe—stars, galaxies, dust and gas—was compressed in a super-hot, super-dense microscopic dot that expanded into the vast universe we see today. Theories about the expansion suggest that stars and galaxies must have had a beginning and that the objects found in the universe have changed through time. Can we observe those changes? To answer that question, let's look at the distribution in time of two types of objects in the universe—galaxies and quasars.

You will be using data gathered by the Sloan Digital Sky Survey (SDSS), an ongoing project that is mapping most of the sky and creating a catalog of the objects it finds.

  1. First, go to the SDSS web site: If you click on DR5 at upper left, you will reach a page that has a map that reveals the current area of the sky covered by the survey. That will be useful later.
  2. Under SkyServer Tools select Search. From the Search menu, select Search Form and then launch the Search Form tool. This will bring up the form you will use to collect your data.
  3. For Show me, select first Galaxies, and for in the region select around. This will change the form to show boxes for right ascension (ra) and declination (dec) of a point in the sky that you choose from the map noted above, and radius—the size of the circle the program will search around the point you select.

    The units are degrees for ra and dec, not hours, and arcminutes for the radius. You will need to experiment a little bit with the radius to get a reasonable statistical sample. Select only objects with spectra under for, and the number of objects and data.

  4. Then click on Generate Query and Submit Query to SkyServer. That will produce a list of galaxies observed in the area you chose.
  5. Repeat the search in the same area for quasars (under Show me) to get a list of quasars.

You now have lists of galaxies and quasars that include values of z—the cosmological redshift and a measure of the distance and lookback time to when the galaxy or quasar existed. Bin your data (break up the data into intervals of z such as 0 to 0.1, 0.1 to 0.2, etc.) and make separate graphs for galaxies and quasars that show the number of objects as a function of z. Repeat for two or three other areas in the sky.

Your Report:

  1. Collect your graphs and data and produce a Power Point or hard-copy report with an explanation of what conclusions can be made from the data.
  2. Describe the distribution of galaxies and quasars in time. (i.e., in terms of z).
  3. What does your data indicate about the evolution of the universe?
  4. Are there significant differences in different directions? To help you think about your results, how would the graphs look if the universe were in a Steady State, that is, non-evolving. (NOTE: The Steady State theory was an important idea about the nature of the universe in the mid-twentieth century. Search the Internet for more information.)