In trying to explain the origins of the Universe scientists have taken a step-by-step approach, and ESA’s Planck is currently at the top of the staircase. Several experiments, both on Earth and in space, are blazing the trail which will soon be travelled by Planck, on its search for the holy grail of cosmology: the set of ‘magic numbers’ which define our Universe. Planck, the ‘ultimate’ tool to find out how everything started, is the first European space mission to study the Big Bang and will be launched in 2007.

NASA’s MAP satellite, to be launched tomorrow, is one of Planck’s precursor experiments and ESA scientists hope that everything goes well with it. Like Planck, MAP will observe the Cosmic Microwave Background (CMB), the first light released after the Big Bang, which fills the entire sky today and carries a wealth of information about the infancy of the Universe. In a way, the CMB can be thought of as the ‘shock-wave’ of the Big Bang itself, a remnant that scientists can use to reconstruct the first moments of the Universe.

The mere discovery of this first light in 1964 gave strong support to the Big Bang model. And the first detection of ripples on its surface in 1992 – ripples that are the ‘imprints’ left in the light by the ancestors of today’s galaxies – triggered a huge number of follow-up experiments. Riding this wave of excitement during the past decade, many instruments have observed the CMB with increasing sensitivity and resolution.

In this sequence, ESA’s Planck will be at least ten times more sensitive than MAP.

As ESA’s Planck project scientist, Jan Tauber, explains, “It is useful to think of the pioneering COBE satellite, launched in 1989, as a ‘first generation’ experiment, MAP as a ‘second generation’ exploratory experiment, and Planck as the third and most sophisticated generation. The capabilities of MAP and Planck are actually very different. There is a progression, not competition.”

By observing the ‘first light’ after the Big Bang scientists will be able to distinguish between competing cosmological models. While hardly anyone today would question the broad outlines of the Big Bang model (i.e., that the Universe started as a very hot and dense fireball which gradually expanded and cooled), there are still big uncertainties concerning several key ‘details’. For instance, scientists are still rather puzzled about the nature of what could be a substantial percentage of the matter in the Universe – the so-called ‘dark matter’. And, they do not know what makes the Universe accelerate as it expands, as has been shown by recent measurements. In addition, it is still unclear whether or not the Universe went through a short phase of amazingly quick expansion – a period of so-called ‘inflation’ – in its very first moments of existence.

Physicists try to fill these holes in the overall picture by postulating different hypotheses and checking them against the data. These hypotheses, or ‘models’, are built according to a set of certain quantities – about a dozen ‘magic numbers’ referred to as the ‘cosmological parameters’ – that specify the large scale characteristics and evolution of the Universe. These parameters need to be determined with a high degree of accuracy, otherwise it becomes very difficult to select the model which best fits the Universe around us. The CMB data provide one of the cleanest and most straightforward ways to infer these cosmological parameters.

What makes MAP and Planck different

The main difference between Planck and MAP lies in the quality of the CMB data taken, and therefore, in the accuracy with which the cosmological parameters can be determined.

As Tauber explains, “The raw sensitivity of Planck is at least ten times better than MAP. In addition, the angular resolution of Planck is about twice as good as that of MAP, and its frequency coverage about ten times larger. All these things together allow Planck to measure many more quantities than MAP, and to measure them more accurately. This is crucial to ensure confidence in the results of the experiment. One of the problems with this game is that many different models (or sets of ‘magic numbers’) yield very similar signatures in the CMB. If one measures just a few quantities and the experimental error bars are large, then one cannot distinguish between different potential Universes. Planck’s ability to play this game is much higher than that of MAP.”

Planck is also the first European space mission to study the CMB. It will be launched by an Ariane 5 rocket from Kourou (French Guiana). The same launcher will carry ESA’s Herschel Space Observatory, the largest imaging space telescope ever built. Both satellites will separate after launch and proceed to different orbits around a virtual point in space known as ‘L2’, located 1.5 million km away from Earth ñ four times the distance from Earth to the Moon. Both spacecraft will be operated independently.

Planck is named after the German scientist Max Planck who was awarded the Nobel Prize for Physics in 1918.

For more information please contact:

Jan Tauber, Planck project scientist
Tel +31 71 5655342

ESA Science Communication Service
Tel: +31 71 5653223

The latest in the field… and prospective results from future CMB missions, such as ESA’s Planck

How old is the Universe?

Several international teams have committed their efforts in recent years to determining the age of the Universe. To do so, they needed to find the value of the so-called Hubble Constant, a number that describes the rate of expansion of the Universe – the expansion is due to the Big Bang itself. By finding out how fast the Universe is expanding now, scientists can estimate how long ago the process of expansion began – and thus determine the moment of ëbirth’.

To determine the value of the Hubble Constant scientists have to measure the distance to far away objects and then find out how fast these objects are moving away from us. The teams have used several independent methods to make these measurements, and have recently started to converge on a solution. The Universe seems to be between 13 and 14 billion years old.

Certain features in the Cosmic Microwave Background (CMB), the ‘first light’ released after the Big Bang, are closely related to the precise value of the Hubble Constant. Detailed observations of the CMB by missions such as ESA’s Planck will provide a more precise value for the age of the Universe.

Look! The Universe is accelerating!

The teams studying the age of the Universe also obtained an unexpected result: their measurements indicated that the rate of expansion of the Universe is actually increasing! It is as if an unknown force is pushing objects away from each other. This unknown force has been dubbed ‘dark energy’. Scientists are puzzled over this phenomenon, a complete mystery so far. Again, the answer to this puzzle may come from accurate observations of the CMB.

It’s flat! Has it ‘inflated’?

The most accurate observations of the CMB so far have been performed by instruments carried by balloons flying above most of the atmosphere (Boomerang and Maxima), and also by ground-based experiments (DASI and CBI). Their latest results were published in May 2001, and they all agree on one thing: the Universe is ‘flat’ or very close to flat. ‘Flat’ in this context refers to the geometry of the Universe: in a flat Universe two parallel lines will never meet, while in a curved one they will end up merging – like longitude lines on the Earth’s surface, which are parallel at the equator but meet at the poles.

The fact that the Universe is flat is rather puzzling. It can however be explained by accepting that during the first moments after the birth of the Universe, it went through a short period of incredibly rapid growth, dubbed ‘inflation’. Many scientists support the idea of ‘inflation’, especially after the latest CMB results, but there is no direct evidence that it actually happened.

Although it is usually impossible to prove a theory, one can always try to disprove it! ESA’s Planck satellite, as the most sophisticated CMB mission, will be able to uncover flaws in the theory – if there are any – and will otherwise be able to improve substantially our knowledge of what underlies the inflation phenomenon – the so-called ‘inflationary potential’.