The geometry of the universe is "open" or negatively curved like a saddle, according to a new model proposed by researchers in Europe who have studied anomalies in the cosmic microwave background radiation. The anomalies were first detected by NASA's Wilkinson Microwave Anisotropy Probe (WMAP) in 2004 and were confirmed earlier this year by the European Space Agency's Planck space mission.

Cosmologists believe that when the universe was very young – a mere 10–35 s after the Big Bang – it underwent a period of extremely rapid expansion known as "inflation". About 380,000 years after the Big Bang, the cosmic microwave background (CMB) – the thermal remnant of the Big Bang – came into being. Physicists had expected the temperature of the CMB to be the same everywhere but for almost 10 years, evidence of a puzzling CMB anomaly has grown. It is becoming clear that the experimentally observed temperature fluctuations in the two hemispheres of the sky are slightly different. This means that the density of matter and energy seems to vary more strongly on one side of the sky than on the other.

Large-scale asymmetry

When first spotted by WMAP, this "hemispheric asymmetry" was met with doubt until the Planck mission independently confirmed it. The observations show that while the average temperature is the same in both hemispheres, the fluctuations are about 10% larger on one side compared with the other. While the statistical significance of the anomaly is debatable, the fact that both WMAP and Planck have detected it means that it needs to be thoroughly investigated.

"There seems to be a preferred direction in space...such that the hot spots are hotter and the cold spots are colder on one side of the sky. While it might be a statistical fluke, there might also be something more going on," says Andrew Liddle, a physicist at the University of Edinburgh. Liddle explains that the CMB dataset is a complex one and that "the eye gets drawn to one unusual thing and you focus on it...so the anomalies and our observations of them have many caveats".

In 2008 a team of researchers from the California Institute of Technology in the US came up with a physical model that could explain the existence of the asymmetry in terms of a very large-scale variation in the density of the universe that is observable on a particular distance scale – one which is slightly larger than the size of the observable universe.

Inflationary theories

The team's model works by using a slightly modified version of the current theory for inflation – this assumes that inflation was caused by quantum fluctuations or a quantum scalar field known as the "inflaton". Instead, the modified theory includes an additional scalar field that comes into play in the form of the "curvaton". In this case, the inflaton would control the density parameter for the expanding universe and ensure that it remains homogenous, while the curvaton generates curvature perturbations. It is these perturbations that explain the CMB asymmetry. The problem with this theory was that the researchers had no explanation for where the curvaton fluctuation would arise from.

Now, Liddle and his colleague Marina Cortês at the Lawrence Berkeley National Laboratory in the US have published a paper where they say curvaton fluctuation could be intrinsically linked to the geometry of our universe. In particular, they assume that our universe could have an "open" or negative geometry. There are three possible geometries for the universe – open, closed or flat – that occur depending on the density of the universe. In a flat universe, the density is exactly equal to the critical density – the average density of matter required for the universe to just halt its expansion – and so its geometry would be like a sheet of paper and infinite in its extent. An open universe, on the other hand, would mean the density of the universe is less than the critical density. It can be visualized as having a saddle-like negative geometry, where parallel lines would diverge.

Curved horizons

Current observational evidence suggests that ours is a flat universe. "But the measurements still allow for a universe where the density is one-third of the critical density and the universe is still within 1% of being flat," explains Liddle. This is the crux of the researchers' argument: it may be possible that the universe appears flat but is really curved with a characteristic radius on a very large scale. This "superhorizon curvature radius" determines the wavelength of the asymmetry-generating curvaton fluctuation. This radius does extend beyond our observable horizon but by no more than an order of magnitude. "So, if the universe is within 1% of being flat, then the curvature scale is three times as big as the observable scale, but there could be some physical processes related to it that could be measured," according to Liddle.

Pop the bubble

The researchers then point out that their curvaton fluctuations could pop up in another set of "open inflation" theories, first proposed in the 1990s, that suggest that our observable universe forms like a bubble in a larger universe. In this theory our bubble universe is born thanks to a quantum-tunnelling event from a low-energy state and is trapped in what Liddle describes as a "false vacuum state" (click on figure above). The walls of such a bubble would expand at a velocity approaching that of light. "So, on the inside it would look to us as if we were in an open universe that is homogenous and isotropic," says Liddle, further explaining that inside the bubble, the concept of time is different from outside. "The amount of inflation inside the bubble would determine how 'flat' it will be...Will it be dominated by dark matter?...Will it suffer a heat death?"

There may be many other such bubble universes within the larger universe, but our bubble would almost never interact with them and neither would we be able to see out of our "opaque bubble" explains Liddle. But, the initial event that induced the birth of our bubble universe would also cause fluctuations in the bubble wall, which in turn imprint themselves on the curvaton fluctuations.

Liddle and Cortês are clear that their theory is currently "highly speculative" and that current data might even rule it out. But Liddle feels that data from the Planck mission (more will be released next year) and new data from the upcoming Euclid mission might test their model. While the researchers will never be able to probe the larger universe, they might successfully measure the geometry of our bubble universe and show its "openness" in the years to come.

The research is published in Physical Review Letters.