On the tracks of Big Bang

Already the start of the mission was very promising: Following 10 years of preparation, the Planck collaboration, which includes a team at the Max Planck Institute for Astrophysics, observed a launch right out of the textbook on 14. May 2009. As the satellite reached its operating position some 1.5 million kilometres outside the Earth’s atmosphere in summer 2009, right on schedule the sensitive instruments had been cooled to their working temperature of in some cases only 0.1 degrees above absolute zero (minus 273.15 degree Celsius).

This means that they are able not only to observe the 2.7 Kelvin emission of the very early universe, right after the Big Bang, but also to produce precise maps of its tiny temperature variations of just a few millionths of a degree. These temperature variations are the first indicators of all observable structure in the universe, stars, galaxies and galaxy clusters. Even though Planck can only look back to a time about 380 000 years after the Big Bang, from its data the scientists glean insights into the first few fractions of a second, when the cosmic structures were seeded, some 14 billion years ago.

Planck’s aim is the measurement of these temperature fluctuations with unprecedented accuracy. To this end, Planck scans the sky in nine frequencies, ranging from high-frequency radio waves at 30 gigahertz (GHz) to the far infrared with 857 GHz. The scientists need this broad frequency range as Planck observes not only the primordial emission but also noise from galaxies. This interfering signal, however, has a different spectral distribution, which can be identified, measured and subtracted due to the multi-frequency measurements with Planck. And while this foreground signal is an annoyance to cosmologists who want to look back to the cosmic nursery, as by-product it provides valuable information to galaxy researchers.

The largest part of this foreground light comes from our own galaxy, the Milky Way. As we are inside the galactic disk, we see the interstellar medium all around us, either due to the thermal radiation of dust clouds at high frequencies or due to the radio emission of electrons moving nearly with the speed of light in the galactic magnetic field.

One of the mission’s other tasks is to measure the Sunyaev-Zeldovich effect. This phenomenon was predicted in 1969 by Rashid Sunyaev, now Director at the Max Planck Institute for Astrophysics, and Yakov Borisovich Zeldovich. The electrons present in the hot, ionised intergalactic gas of a galaxy cluster can scatter photons of the cosmic background radiation. In this process energy is transferred from the electrons to the photons, whose frequency increases correspondingly – there is a shift in the relative number of low-energy and higher-energy photons compared to the original Planck spectrum. This means that galaxy clusters become visible as a “shadow” in front of the uniform spectrum of the cosmic background.

(TE / HOR)