Satellite prepares to go super-cold


Feb 4, 2007

Some 380,000 years after the Big Bang, matter and radiation are said to have "decoupled" Matter went on to form stars and galaxies; the radiation spread out and cooled. The radiation - the CMB - now shines at radio wavelengths, at a frigid -270.45C

Cobe showed the CMB's profile to follow a predicted distribution - a so-called blackbody curve. Cobe mapped tiny temperature fluctuations (mottled colours, top) in the CMB. These fluctuations (anisotropy) correspond to the early distribution of matter

The data informs scientists about the age, geometry and fate of the cosmos. Images from the more advanced WMap probe show finer detail (bottom). The Planck telescope is expected to obtain the best resolution images yet

George Smoot wants to know the answers to some big questions.

"I want to know how the Universe came into being, how it developed and what its future might be," he says.

The US Nobel Laureate has spent a large part of his career investigating the Cosmic Microwave Background (CMB) - the "first light" released after the Big Bang.

Scientists like Professor Smoot study this remnant radiation from the birth of the Universe in the hope of answering some of these questions.

That dim afterglow, which fills the entire sky today, carries a wealth of information about the cosmos in its infancy.

"When we look back to the past, we can see everything between the past and now, and that allows us to project to the future," Professor Smoot says.

The researcher, from the Lawrence Berkeley National Laboratory (LBNL) in California, shared last year's Nobel Prize in Physics with collaborator John Mather for work on the Cosmic Background Explorer (Cobe) mission.

The Cobe satellite detected subtle temperature variations in the CMB which pointed to the ripples in density that gave rise to the first stars and galaxies.

Superior precision

Another US spacecraft, the Wilkinson Microwave Anisotropy Probe (WMap), was launched in 2001.

It has helped constrain estimates for the age of the Universe, shed important light on its composition and has shown that its shape is very close to flat.

In the middle of 2008, the third satellite to investigate the CMB will blast off on a mission to address similar issues - but from a fresh perspective.

The European Space Agency's (Esa) Planck satellite, named after the great German physicist Max Planck, will map the background sea of microwaves with unparalleled precision.

Professor Smoot was talking at a news conference to mark the approaching end of Planck's physical "integration" as a satellite at the aerospace firm Alcatel Alenia Space's facility in Cannes, France.

"We are proposing that this mission creates a recipe that will allow us to better understand the Universe," said Jan Tauber, Planck's project scientist.

Professor Smoot added: "With Planck, we are exploiting the relic radiation from the Big Bang. That's difficult to do unless you have contrast."

Accepted theory

Contrast is a key strength of Planck. It will look at nine wavelength bands in the electromagnetic spectrum - the range of all possible radiation from gamma rays through to visible light and radio waves.

The CMB has a well defined "signature" in this spectrum.

"We can use the nine bands to distinguish the CMB from the other foreground signals in the sky," said the LBNL scientist.

Planck's superior sensitivity, angular resolution and frequency range should open up some hazy areas in our understanding of the cosmos.

Few scientists today would question the broad details of the Big Bang model, in which the Universe was born in a hot, dense fireball that gradually expanded and cooled.

But some fundamental details about the nature and evolution of the Universe are missing.

For example, only 4% of the Universe is made up of ordinary matter that we can see. The rest is dark energy (73%) and dark matter (23%). Their influence can be detected indirectly, but scientists do not understand their nature.

Planck could provide new information on dark energy, but it will depend on what this mysterious quantity is.

This hypothetical form of energy has negative pressure and permeates all of space. Scientists have proposed two main ideas for what dark energy might be.

Dynamic field

One is the so-called cosmological constant, originally put forward by Albert Einstein. This represents a constant energy density which fills space homogeneously.

The second idea is that dark energy is a dynamic field, called quintessence, whose energy density varies in time and space.

"If [dark energy] is the cosmological constant, then Planck doesn't provide that much extra information," Professor Smoot explained.

"But if it's some kind of complicated dark energy, like quintessence, then it's quite possible that Planck will provide some key details."

Researchers want to confirm whether the very early cosmos underwent a short phase of exponential expansion, called inflation.

"Inflation makes our model of the Universe fit in one piece, but it is still quite a mysterious thing," said Jan Tauber.

"We want to understand whether it actually happened and, if so, what made it happen."

In addition, scientists want to understand why we live at a time when the expansion of the Universe is accelerating again.

"We have this model in which the Universe flares out. Then it slows down again, allowing structures to form," said George Smoot.

"Now, we have this flaring in which the age of structure formation is coming to an end and the expansion of the Universe is speeding up again."

Magic numbers

Cosmologists have built their models of the Universe on a dozen or so "magic numbers" that explain its large-scale properties.

Planck will be able to measure these "cosmological parameters" to a very high degree of accuracy in order to select the model which best fits the Universe around us.

Planck consists of a telescope and science instruments placed on top of an octagonal service module. The baffle which surrounds them prevents light from the Sun and Moon interfering with detection of the microwave radiation.

In order to achieve its scientific objectives, Planck's detectors have to operate at very low and stable temperatures. The spacecraft is therefore equipped with a system to cool these detectors to temperatures close to absolute zero (-273.15C), the theoretical state of zero heat energy.

"It is an extremely elaborate cryogenic system," said Thomas Passvogel, Planck programme manager at Esa.

"I think it's the first time a space mission has gone down to those low temperatures to provide detectors of that sensitivity in continuous operation."

The spacecraft is due to lift off on an Ariane 5 rocket from the Kourou spaceport in French Guiana on 31 July 2008.

The Ariane rocket will carry Planck and another mission, the Herschel infrared telescope, into space simultaneously.

With a 3.5m mirror, Herschel will be the orbiting telescope with the largest mirror ever deployed in space. Together, Planck and Herschel will survey the cold Universe.

But instead of looking for the formation of the Universe, Herschel's primary mission will be to see the formation of stars and galaxies.