26 September 2011

Is the universe expanding asymmetrically?

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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.
Is the universe expanding asymmetrically?

This image shows the two hemispheres of a spherical mapping of the cosmic microwave background. New research examining the velocities of Type Ia supernovae suggests faster expansion in the northern hemisphere of the universe, challenging the cosmological principle that the universe is expanding uniformly in all directions. Credit: WMAP/NASA

GOSFORD: 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 it is assumed to have 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 assumes 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.

Undermining the cosmological principle?

These new studies – if verified – would mean that the laws of physics might vary depending on where you are in the universe, which would make it extremely difficult to fully understand the evolution of the universe and its origins.

For instance, if the laws of physics were different elsewhere, it would mean everything we understand about nature would be restricted to our small portion of the universe and the rest of the cosmos would remain enigmatic.

Both teams ran a hemisphere analysis, comparing supernovae velocities in the northern hemisphere to those in the southern hemisphere. These hemispheres were defined using the Milky Way’s galactic orbital plane as a reference equator.

The analysis determined a preferred axis of anisotropy in the northern hemisphere. This suggests that a part of the northern sky represents a part of the universe that is expanding outwards with a greater acceleration than elsewhere.

Instead of an expanding universe as a perfectly round, spherical bubble, it would be more like an egg-shaped or asymmetric expanse, meaning the visual end to our observable universe would exist at different lengths depending on the directional target.

Statistical analysis

Both teams have stated that the statistical analysis does not necessarily correspond with significant results, but strengthen their findings by appealing to other anomalies in cosmic microwave background (CMB) data.

In isolation the findings are not statistically significant but by putting all these anomalies together, somehow a consolidated significance emerges that wasn’t apparent in isolation. In other words, each small element of the studies are not significant on their own but taken together with many other elements a significant result does indeed emerge.

Other researchers, such as cosmologist John Webb from the University of New South Wales in Sydney, are working on similar problems. “There are several independent observations which hint at large-scale departures from isotropy. The possibility therefore remains that the cosmological principle is only approximately correct.”

It’s too soon to tell if these results are definitive. Other researchers have varied opinions as to what could be the origin of the anisotropy, or the property of being directionally dependent.

Tamara Davis is the one of the lead cosmologist on the Australian WiggleZ Dark Energy Survey team and remains reserved: “It does seem suspicious that the alignment is approximately perpendicular to the Milky Way and so it may well be an observational effect that we don’t quite understand.

“Furthermore the results are consistent with the dark energy model, which has recently been confirmed by our team, but are inconsistent with the early seeds of fluctuation of the universe,” she commented.

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