Multiverse Musings - Testing the Copernican Principle, Part 2
Wednesday, November 26th, 2008
History disfavors any theory placing Earth in a geometrically special location. Early scientists, such as Ptolemy of ancient Greece, thought Earth resided at the center of the solar system. However, geocentric cosmology eventually gave way to heliocentrism most notably associated with Nicolas Copernicus. Since Copernicus’s time extensive observations have demonstrated that the Sun does not reside at the center of the Milky Way Galaxy (MWG). Nor does the MWG reside at the center of the Local Group of galaxies or the universe. Scientists refer to the fact that Earth is not in a central, specially favored position as the Copernican Principle.
Although this view provides a foundation of cosmological research, scientists don’t simply accept the Copernican Principle. They continue to test it.
One area of research particularly suited to testing is the universe’s mysterious dark energy. The first need to invoke dark energy to explain features of the universe arose as astronomers tried to understand observations of distant Type Ia supernovae. The supernovae appeared dimmer than expected and the simplest explanation was that dark energy was causing the expansion of the universe to accelerate. However, dark energy is not the only explanation.
The same supernovae data would arise if the MWG resides at the center of a large region (something similar in size to the observable universe) with a lower density than that of the surrounding regions. However, placing the solar system (located within the MWG) at the center of such a special region clearly violates the Copernican Principle. Nevertheless, scientists do not simply reject the low density region, also called the void model. They seek to test its validity.
In a previous TNRTB I highlighted one test of the void model that used the cosmic microwave background. Now scientists have developed another test using supernovae data. Reseachers started by characterizing void density profiles that could explain the supernovae data. Then they modeled in detail how the supernovae data would appear with a much larger sample than currently exists. They found that with a sufficient sample of supernovae data from a specified distance, the void model produced different results when compared to dark energy models. Observations over the next few years should definitively tell which model is correct.
If dark energy models prevail, cosmologists will continue to face the great challenge of trying to understand what it is and why it exhibits such extraordinary fine-tuning in order for this universe to support life. If the void models prevail, a guiding scientific principle will need revision. Either way, exciting times lay ahead.
If you would like to see a question about the multiverse addressed in this forum, send it to multiverse@reasons.org.