When astronomers talk about the ‘shape’ of the Universe, they are talking about something more specific than whether it is a sphere or a cube, for example. The theory of General Relativity allows three shapes, or geometries, for the Universe. These are ‘flat’, ‘closed’ or ‘open’.

It’s difficult to envisage these shapes in terms of the Universe, but they can be compared to a sheet of paper (flat), a sphere (closed) or a saddle (open).

The shape of the Universe determines whether it will expand forever (or eventually collapse) and whether it’s finite or infinite. Which shape it is, depends on its total density and its rate of expansion.

The most convenient way of determining the shape of the Universe is to use the cosmic microwave background (CMB), the relic afterglow of the Big Bang. Small spatial variations in the temperature of this faint light are produced by sound waves moving through the early Universe.

The actual size of these hot or cold spots can be computed accurately and then compared to their measured size. This is like doing a vast trigonometry measurement across the entire Universe and revealing the geometry of space.

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Over the past few decades, astronomers have measured the temperature fluctuations in the CMB very accurately. The results have shown to a high degree of accuracy that the density of the Universe is such that it expands in every direction without any positive or negative curvature, in other words, the Universe is ‘flat’. This flat Universe is a major component of the standard cosmological model.

It should be pointed out that a flat Universe is not universally accepted. Some studies have shown that other measurements, such as the amount of gravitational lensing – how much the CMB is distorted by the gravity of matter in its path – is more consistent with a closed Universe.

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Asked by: Paul Reed, Lincoln

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