In 1929, Edwin Hubble and Milton Humason reported on over a decade's worth of observations and concluded that galaxies further away from us were receding faster than those close to us—observations that suggested an expanding universe. In 1998, while making observations of Type Ia supernovae (SNe), a team of astronomers discovered that not only is the universe expanding, but the expansion is accelerating.

The common explanation for this acceleration is that some sort of exotic "dark energy" is acting on the universe. The current standard model of cosmology (ΛCDM) postulates that 74 percent of the universe's total mass-energy is comprised of this dark energy, while the remaining 26 percent is dominated by dark matter (with normal matter comprising less than five percent of the total). The existence of dark matter is supported by a wealth of experimental evidence, though its makeup is unknown. Dark energy, on the other hand, has precisely one piece of experimental evidence: the accelerating expansion of the universe.

Now, a paper set to be published in an upcoming edition of Physical Review Letters (arXiv pre-print available now) by a trio of Oxford astrophysicists suggests a different explanation for the accelerating expansion. In their proposal, dark energy does not exist at all and the supernovae data that led to scientists to propose it was improperly interpreted. In coming to this conclusion, however, the three researchers have to throw out a philosophical principle that has guided astronomy for over 450 years.

Challenging Copernicus

In 1543, Nicholas Copernicus' revolutionary tome De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres) was first printed. In the book, Copernicus put forth a heliocentric theory that eventually overthrew the Ptolemaic idea that the Earth stood at the center of the universe.

This single book produced modern astronomy and is credited with kick-starting the scientific revolution. In it, Copernicus (among other things) put forth the concept that the Earth is not unique and does not occupy any sort of special position within the solar system. A generalization of this principle—the earth does not occupy a favored position within the universe—has, along with Einstein's equations, become the founding assumption of modern cosmology.

In the upcoming PRL paper, the authors postulate that we are indeed in a special location within the universe, specifically, "near the centre of a void where the local matter density is low." This isn't exactly a small void—it would need to be on the order of the size of the visible universe to get the new model to work. Still, the earth would be near its center, which is a vaguely pre-Copernican notion and, as a side effect, the model does away with dark energy. Unfortunately, it also does away with the notion that we can infer universal properties from local observations.

To flesh out this idea, the authors worked out the equations that would describe this sort of universe. By examining the various properties predicted by the two universe models—the standard ΛCDM and the new smooth void model—they found that the two models would differ significantly in the region of the universe between a redshift of 0.5 and 0.1. The authors suggest that using a "Bayesian information criterion as a figure of merit," could help determine whether reality is better described by one model or the other.

For real world data, the authors use the information returned from the first-year SNe Legacy Survey, a survey that consisted of 115 distinct supernovae. Carrying out the Bayesian analysis to see how well each model fit the data obtained by the survey, the researchers found that neither of the two options was decisively favored, although the ΛCDM model was found to be slightly more accurate.

The authors note that current surveys focus on supernovae that existed at either a low redshift or very high redshift, not in the range where the authors predict large differences between the two models. Upcoming Joint Dark Energy Mission (JDEM) surveys are expected to examine over 2,000 supernovae with redshifts between 0.1 and 1.7. By simulating data in this redshift range, the team decided that JDEM data will eventually allow scientists to distinguish between the models through the Bayesian analysis.

This work represents a major departure from the currently accepted model of the universe, and it rejects a long-held tenet of astronomy and cosmology. Using the currently available data, the authors were unable to show that their model is closer to reality than the commonly accepted ΛCDM description of the universe. Perhaps one of the biggest repercussions of this work, if correct, would be that we could no longer rely on our local measurements to describe the universe as a whole.

New ideas



In addition to the concepts put forth, this paper illustrates nicely how science works. Pseudoscience often argues that controversial or contrarian papers and ideas never get published, suggesting that the "establishment" won't listen to new ideas. But this paper puts forth a completely novel idea, and in the process overturns a long held philosophical pillar of astronomy, and yet it is set to be published in one of the top-tier physics journals.

The main idea put forth here is vastly different from the generally-held consensus of how the universe works; however, as in all of science, every idea is tentative. Future surveys of supernovae may tell us whether or not we are special in relation to the rest of the universe.

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