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An exciting update to the story of the cosmological constant has been provided by the Boomerang experiment . This balloon-borne telescope measures the temperature of the Universe a mere 300,000 years after the Big Bang. This is a significant moment in the history of the Universe, because it is the time when its matter became cool enough for radiation to move freely, without constantly bumping into matter. Boomerang looks at what astronomers call the "surface of last scattering"- the last occasion when the heat radiation of the Big Bang interacted with matter, before setting off unimpeded on its journey to our telescopes.

This heat radiation appears to surround us very uniformly. In any direction we look, we eventually see far enough back in time to see this cooling "surface" of the Big Bang. But because this surface marks the last interaction of matter and radiation, the radiation carries traces of the tiny variations in the density of matter at that time. These are the fluctuations in density that will eventually grow into the huge structures, galaxies and clusters of galaxies, which we see today.

How does all this connect with the cosmological constant? The cosmological constant is a number that measures a mysterious property of so-called "empty space" - its definite gravitational effect. The amount of gravitating stuff in the Universe has a profound effect on its size and geometry, as explained in the main article.

It turns out that the size of the fluctuations that seed galaxies can be predicted with a lot of confidence; this is because the early Universe is just a hot gas, and the fluctuations in density are just like sound waves in a gas. In fact their size, in light years, say, is pretty much equal to the speed of sound times the age of the Universe at "last scattering". But Boomerang measures the size of these fluctuations as they appear on the sky, in angular measure; say degrees. In fact their typical size is about twice the size that the full moon appears to be from Earth.

Now if we know the angular size of something, and its actual size, we can measure its distance. We do this all the time; we know roughly how far away a hill is because the people on it look quite small. In curved space the sums are more difficult, but the principle is similar. The extra complication is that we have to make sure that the inferred distance to the "last scattering surface" fits in with its redshift, or age, which we know anyway. This means we have to choose the right curved space.

Now the punch line - at last! The conclusion is that space has to be "flat". This means it is just like the familiar space of high school geometry, where the area of a circle depends on the square of its radius, and parallel lines never meet. This is the only geometry in which the distance we measure to the last scattering, using the angular size, can match up with what we know about how long ago last scattering happened. Roughly, we have to have

distance = speed of light X how long ago

And when you put in the details, you find none of the fancy curved spaces, allowed by Einstein's gravitational theory, will fit the bill.

But, as the main article explains, whether space is curved or flat, and by how much, depends on how much gravitating stuff there is in the Universe. There isn't nearly enough ordinary mass in the Universe to make space flat. The balance must be made up by the puzzling gravitational effect of empty space - the cosmological constant.

In fact, the Boomerang result and the supernovae data agree that the cosmological constant, is much more important than matter in determining the curvature of space and the future of the Universe. In other words - nothing matters!