[tt] Overturning Copernicus, eliminating dark energy

[Brian Atkins]

http://arstechnica.com/news.ars/post/20080929-overturning-copernicus-may-resolve-dark-energy.html

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.
Further reading:

     * Physical Review Letters, 2008. Upcoming (arXiv PDF)
       http://arxiv.org/PS_cache/arxiv/pdf/0807/0807.1443v1.pdf

-- 
Brian Atkins
Singularity Institute for Artificial Intelligence
http://www.singinst.org/