Recent examination of supernovae velocities suggests the universe may be expanding non-uniformly in its acceleration, which implies the laws of physics may vary throughout the cosmos. Physicists working with the Supernova Cosmology Project's Union2 data set have suggested that the expansion of the universe seems to display a preferred axis, meaning that the universe is expanding faster in one direction than any other.
This asymmetrical expansion is referred to as anisotropy, which is the property of being directionally dependent, and differs from isotropy, which implies identical properties in all directions.
The result is inconsistent with the standard cosmological model, which is based on the cosmological principle that requires the universe to be isotropic and homogeneous, namely: that is has the same underlying structure and principles operating throughout, and looks identical in every direction.
Challenging the isotropic understanding: Released in early 2010, the Union2 data set consists of 557 Type 1a supernovae - the brightest supernovae known, which result from the violent explosions of white dwarf stars at the end of their lives.
In late 2010, two cosmologists from the University of Ioannina in Greece published a challenge to the cosmological principle in the Journal of Cosmology and Astroparticle Physics, providing statistical evidence to support the notion of a preferred axis of expansion.
Earlier this month another such challenge was posted on the physics website arXiv by Rong-Gen Cai and Zhong-Liang Tuoy, cosmologists from the Institute of Theoretical Physics at the Chinese Academy of Sciences. Their paper also explores the non-uniform accelerated expansion of the cosmos.
"This present result is very interesting, as one of the key ideas underlying our understanding of the universe is that it is isotropic and should look the same in every direction," commented astrophysicist Geraint Lewis from the University of Sydney. "In the next few years, the numbers of distant supernova observations will increase and we will know for sure if this result is correct."
Current cosmological model: Since the discovery of cosmic acceleration in 1998, on the back of observations of supernovae type Ia (SNIa) being fainter than expected, these stellar explosions have become an important tool in determining the cosmological parameters and the rate of our Universe's expansion.
A standard cosmological model was formed by a general consensus of physicists and cosmologists, using the joint analysis with SNIa data in combination with other observations, such as large scale structure of the universe and the cosmic microwave background (CMB) radiation - the thermal radiation believed to fill our observable universe.
The current model suggests that the laws of physics are the same for all places in the universe - except in extreme places, such as the interior of a black hole - and the accelerated expansion of the universe is happening uniformly, adhering to isotropy and homogeneity, which is essentially the cosmological principle.