Planck reveals an almost perfect Universe
Planck measures the Universe - detailed all-sky map of the cosmic background radiation confirms standard cosmological model but also finds deviations
On 21 March 2013 the Planck collaboration presented its first all-sky map of the cosmic microwave background radiation, which impressively confirms the standard model of cosmology and determines its parameters more accurately than ever before. At the same time, the researchers also found significant anomalies and inhomogeneities indicating that some aspects of the "standard model" are not yet understood.
The all-sky map released now is based on the first 15.5 months of observations with the Planck space telescope, a mission of the European Space Agency (ESA), and shows the oldest light in the universe. This was emitted when the universe was only 380,000 years old and became transparent for the first time after the Big Bang. The "primordial soup" of protons, electrons and photons cooled gradually, allowing neutral hydrogen atoms to form and the light to escape. As the universe continued to expand and to cool, this radiation was shifted to longer wavelengths, so that it is received today as the cosmic microwave background (CMB) at a temperature of about 2.7 Kelvin.
Tiny temperature fluctuations in this CMB map reflect smallest density fluctuations in the early universe. "The Planck CMB map provides us with an extremely detailed picture of the very early universe," said Simon White, Co-Investigator in the Planck Collaboration and director at the Max Planck Institute for Astrophysics (MPA), who helped to establish the standard model of cosmology in the 1980s by analysing the evolution of structure in the universe. “All the structures that we see today grew from tiny density fluctuations shortly after the Big Bang.”
Planck was designed to measure these fluctuations across the whole sky with greater resolution and sensitivity than ever before, allowing scientists to determine the composition and evolution of the universe from its birth to the present day.
"The Planck data fit extremely well with the standard model of cosmology," says Torsten Enßlin of the MPA, who is managing Germany's participation in the Planck mission. "The cosmological parameters have been refined with Planck more accurately than ever, and our analysis passed all tests against various other astronomical observations with flying colours."
The analysis of the Planck data show that normal matter, making up galaxies, stars and also Earth, contribute only about 4.9% to the mass and energy density of the universe. About 26.8% is dark matter, which interacts only through its gravitational effect – contributing far more than previously assumed. Dark energy, the mysterious component that causes the universe to expand ever faster, on the other hand, accounts for only 68.3%, less than expected.
Finally, the Planck data also set a new value for the rate at which the Universe is expanding today, known as the Hubble constant. At 67.15 km/s/Mpc, this is significantly less than the current standard value in astronomy. The data imply that the age of the Universe is 13.82 billion years.
However, because the precision of Planck’s map is so high, it also revealed some peculiar unexplained features, which cannot easily be reconciled with the standard model. One of the most surprising findings is that the fluctuations in the CMB temperatures at large angular scales are not as strong as expected from the smaller scale structure revealed by Planck. Another is an asymmetry in the average temperatures on opposite hemispheres of the sky. This runs counter to the prediction made by the standard model that the Universe should be broadly similar in any direction we look. Furthermore, a cold spot extends over a patch of sky that is much larger than expected. This data could point to an extension of the standard model or even new theories. “But even if we do not yet understand these anomalies, we can eliminate the possibility that they are due to foreground effects,” says Torsten Enßlin. “The ‘cold spot’, in particular, has been known for quite a while and could well be a statistical fluctuation.”
The MPA scientists have been involved in software development even from before the beginning of the mission, to process the data and remove foreground emission from objects such as galaxies, quasars, and even our own Milky Way. By now, their work focuses on analysing information from the cosmic microwave background radiation and trying to better understand our universe.
One aspect, amongst many others, is the discovery and measurement of galaxy clusters by the Sunyaev-Zel'dovich effect. The SZ effect is a characteristic signature imprinted by galaxy clusters on the cosmic microwave background, when the light from the CMB passes through the cluster. Because of the different frequency bands available with Planck, the SZ effect can be used as a unique tool for detecting galaxy clusters.
Rashid Sunyaev, Co-Investigator in the Planck Collaboration and director at the Max Planck Institute for Astrophysics, together with Yakov Zel'dovich predicted not only the effect of galaxy clusters on the CMB but also the existence of the acoustic peaks in the CMB itself which Planck has now measured so precisely. He is excited by the Planck results: "When we developed our models of the CMB radiation more than 40 years ago, we thought of it mainly as a theoretical thought experiment. It is amazing that the measurements are now so detailed that it can even be used as tool to discover hundreds of new galaxy clusters that where unknown before. A great success for Planck!”
Planck scientists were even able to use this sample of clusters of galaxies to derive key parameters of the universe – a method that has been employed with CMB data for the first time. This is an additional a completely independent method from the way which uses the shape and amplitude of the acoustic peaks.