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Planck reveals big bang's fireball in hi-def

Posted: 21 March 2013

Planck CMB
Planck's highly detailed view of the temperature fluctuations in the cosmic microwave background, represented if false colour. See the full resolution image here (2 MB). Image: ESA and the Planck Collaboration.

The cosmic microwave background - the cooling fireball of the big bang - has been observed in greater detail than ever before by the European Space Agency's Planck spacecraft. In the process some models of the Universe's birth and subsequent evolution have been refined and its age measured more accurately, but some new mysteries have also revealed themselves.

The cosmic microwave background (CMB) is radiation that pervades the entire Universe, having been emitted when the Universe cooled to below 2,700 degrees Celsius some 380,000 years after the big bang. At this point, atomic nuclei were able to soak up all the free electrons that swarmed around the Universe and photons, no longer with anything to scatter off, rushed out into the Universe at large. As the Universe has expanded, the wavelength of these photons has stretched into cooler microwave realms. Today, that microwave radiation is just 2.725 degrees above absolute zero.

It has been a long road towards understanding the CMB. Discovered in 1965 by Nobel laureates Robert Wilson and Arno Penzias of Bell Laboratories, New Jersey, our first images of the CMB didn't come until 1992, when NASA's Cosmic Background Explorer (COBE) satellite presented a smeared, false colour all-sky map showing that the CMB was not totally uniform. Instead it has temperature variations amounting to just 25 millionths of a degree, but these variations or 'anisotropies' as they are described in the jargon are areas of slightly more or slightly less dense matter in the very early Universe, and became the seeds for the great chains and clusters of galaxies that we see today.

NASA's Wilkinson Microwave Anisotropy Probe (WMAP) followed, releasing its first data in 2003 and providing a breathtakingly clear view of the CMB, determining the matter-energy content of the Universe, the rate at which it is expanding and even the Universe's age.

Now Planck, which was launched in 2009, has taken WMAP's view of the CMB and turned it into high definition and, by doing so, is allowing scientists to constrain their various theories of inflation, the big bang and the content of the Universe to far greater precision.

"Planck has produced the most precise picture of the CMB ever obtained," said Professor George Efstathiou of the University of Cambridge at an ESA press conference in Paris, today. "It's a goldmine of information. If you are wondering what the importance of this is, some cosmologists might have given up their children to get their hands on this map!"

The percentages of ordinary matter, dark matter and dark energy have changed slightly as a result of Planck's more accurate measurements. Image: ESA and the Planck Collaboration.

Planck has accurately determined the age of the Universe to be 13.82 billion years - slightly older than WMAP's measurement of 13.77 billion years. It has also found that 4.9 percent of the Universe is made up of ordinary matter, up 0.3 percent on WMAP's measurement. Meanwhile, there is slightly less dark energy than previously thought, 68.3 percent compared to WMAP's 71 percent, and a little more dark matter, 26.8 percent as opposed to 24 percent.

The ripples in the CMB grew along with the expansion of the Universe to form chains of galaxy clusters. By measuring the size of the ripples in the CMB compared to the size of the galaxy clusters today, it's possible to get an accurate handle on how fast the Universe is expanding, known as the Hubble Constant. WMAP measured it to be 69.32 kilometres per second per megaparsec, while Planck has refined that to 67.15 kilometres per second per megaparsec. What that means is that every million parsecs (where one parsec is 3.26 light years, so every 3,260,000 million light years) of space expands by 6,715 metres each second.

This image shows the Cosmic Microwave Background as seen by ESA's Planck satellite (upper right half) and by its predecessor, NASA's Wilkinson Microwave Anisotropy Probe (lower left half). With greater resolution and sensitivity over nine frequency channels, Planck has delivered the most precise image so far of the Cosmic Microwave Background. Image: ESA and the Planck Collaboration; NASA / WMAP Science Team.

"The conclusion is that standard cosmology is an extremely good match to the Planck data," says Efstathiou, adding that there was nothing in the data that damaged inflationary theory, which is the idea that in the first fractions of a second after the big bang the Universe inflated in size momentarily faster than the speed of light. "Inflationary scientists should be very happy."

However, there's always a 'but', and Planck's new findings are no different. "Because we have such good data we should also look critically at what doesn't fit [with it]," says Efstathiou. For example, at large angular scales on the sky the anisotropies are weaker than expected compared to the results on small scales. "On small scales the data fits perfectly, but there may be potential new physics on the largest scales."

Planck detected anomalies in the CMB. One is a large, unexplained cold spot (circled) while the other is an asymmetry in the size of the temperature fluctuations north of the ecliptic compared to the south of the ecliptic, as indicated by the white line. Image: ESA and the Planck Collaboration.

Efstathiou referred to several puzzling findings in the CMB data that had first been hinted at by WMAP, but which Planck has conclusively shown to be real. First is a giant cold spot in the CMB that shouldn't exist if the Universe is truly isotropic and homogenous, i.e. sporting a fairly uniform distribution of matter in all directions on the largest scales. Secondly, and even more puzzling, is an unusual asymmetry in the CMB that the amplitude of the anisotropies are larger on one side of the sky than the other. "This implies a preferred direction in space," says Efstathiou. "Even more unusual is that it is lined up with the ecliptic. Why characteristics of the CMB should relate to our Solar System is not understood."

There are further opportunities to solve the mysteries with two more data sets from Planck to be released, one in 2014 and a final one in early 2015. These will also include polarisation data, caused by the signature of gravitational waves at the very dawn of time. This first release is based on the first 15 months of observations and Planck will continue to observe until around November this year when its coolant runs out.

"This is the beginning of a new journey," says Planck's Project Scientist, Jan Tauber. "We expect that our continued analysis of Planck data will help shed light on these conundrums."