Tall in the Saddle...

The top image shows the full sky map as seen by Planck – the CMB temperature fluctuations on large scales are more extreme on the right side of the sky. The bottom image illustrates our bubble universe being born from the larger universe (depicted only by the blue background). Our bubble forms at time T0 and expands outwards, as shown with red rings at times T1, T2 and T3. In the 3D space–time, this expansion forms a "bubble wall" that acts as the starting point (t0) of our universe. Subsequent times (t1, t2, t3) are defined on hypersurfaces above the bubble wall. The yellow lines show the trajectories of constant positions in space inside this open universe. (Courtesy: ESA and the Planck Collaboration/Alan Stonebraker, Phys. Rev.Lett.)

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.

Physics World: Is the universe saddle shaped?