Planck reveals 'almost perfect' universe
Mar 21, 2013 21 comments
After more than two years of painstaking analysis, cosmologists working on the €700m Planck space mission have announced their first results. Speaking today at the headquarters of the European Space Agency (ESA) in Paris, the researchers have released the most precise measurement of the cosmic microwave background (CMB) radiation – a remnant of the Big Bang – to date.
The results revise downwards the proportion of the universe made up by dark energy from 74% to 68.3%, while dark matter accounts for 26.8% of the total (up from 22%) and ordinary matter 4.9% (up from 4%). Planck also reveals that the universe is some 80 million years older than thought, to put the age of the universe at 13.8 billion years old. Planck scientists also say there is no evidence from the data of an additional fourth type of neutrino, which had been hinted at by NASA's Wilkinson Microwave Anisotropy Probe (WMAP).
"The progress made in understanding the origin of the universe is an order of magnitude better compared with what has been done before," says ESA director general Jean-Jacques Dordain. "This [the data] is what they call perfect; but as scientists got much more than they expected, so it is almost perfect."
In July 2010 ESA released Planck's first all-sky survey of the CMB showing tiny temperature fluctuations thought to have been produced by the same irregularities in space that led to the formation of galaxies. However, ESA researchers deliberately scrambled the survey image that was released to the public while they spent the next two years carrying out a full scientific analysis. In the new results, released today, cosmologists have used some 15 months' worth of Planck data.
Probing the Big Bang
Launched by ESA in 2009, Planck uses two instruments to measure the CMB at frequencies between 27 GHz and 1 THz. It takes these measurements at a point in space that is some 1.5 million km further out from the Sun than the Earth. Known as Lagrange point L2, Planck hovers there, barely disturbed by stray signals from Earth and without needing to use much fuel to stay in position.
Cosmologists believe that the nascent universe underwent a period of extremely rapid growth – a period that began 10–35 s after the Big Bang – during which the universe is thought to have undergone enormous expansion called inflation.
The CMB was born about 380,000 years after the Big Bang, when primordial protons, neutrons and electrons formed neutral atoms that allowed photons to "decouple" and finally move freely. Photons could then suddenly travel unhindered through space, their wavelengths being stretched by the expansion of the universe to leave a haze of microwave radiation in every direction.
As with WMAP, the previous CMB space-based mission, Planck has found almost perfect agreement with inflationary models and the standard model of cosmology. Known as "lambda-CMD" (lambda cold dark matter), this model describes a flat, homogenous universe dominated by dark matter and dark energy. "There is little doubt that we have now uncovered a fundamental truth of the universe," says George Efstathiou of the University of Cambridge, speaking at the ESA press conference.
David Spergel, a theoretical astrophysicist from Princeton University who has worked on the WMAP data, told physicsworld.com that the Planck results are “a great triumph” for experiment and theory. “With even higher precision than WMAP, the [Planck] data fits the standard model,” he says.
There are, however, some hints of physics beyond the standard model of cosmology in the new Planck data. Efstathiou showed that fluctuations in the CMB temperatures at large angular scales do not match those predicted by the standard model, in addition to an asymmetry in the average temperatures on opposite hemispheres of the sky. Such deviations were hinted at by WMAP but were largely ignored because of doubts over their origin.
"Such features are not caused by galactic emission or instrumentation," says Efstathiou. "This is exotic physics – there seems to be some memory that has been retained on the largest scales from previous phases of the universe." One possible explanation for this is that the universe is not the same in all directions on a larger scale than we can observe.
Cosmologist Joanna Dunkley from the University of Oxford says that the large angular scale anomalies are “tantalising” and could point to new physics. “It needs some more thought about what kind of theoretical models could produce this sort of signal,” she says.
Another big aim of the Planck mission is to detect a so-far-unobserved type of polarization known as "B-modes", which date back to the period of inflation and are determined by the density of primordial gravitational waves. If such waves could be detected, they might tell us what mechanism generated them in the universe's first moments, what caused inflation, and even if there was something before the Big Bang. However, Efstathiou says that the Planck team has not yet exploited those data.
The Planck material released today represents only half the results expected to come from Planck over its lifetime.
The CMB was first discovered in 1964 by the US radio astronomers Arno Penzias and Robert Wilson, earning the pair the 1978 Nobel Prize for Physics. However, it was NASA's Cosmic Background Explorer (COBE) that set the field of cosmology alight in 1992, when it revealed that the CMB is not uniform but has slight variations that carry information about the early universe.
The launch of WMAP in 2001 and its study of the CMB proved to be huge vindication for the standard model of cosmology. A few years after launch, WMAP returned the first all-sky survey of the CMB and revealed the temperature of this background radiation in exquisite detail. In 2006, after three years of data-taking, the WMAP team measured the incredibly weak polarization signal of the photons, allowing cosmologists to infer how much the fluctuations are cuased by the distorting effects of matter and how much they are down to gravity waves in the infant universe. WMAP placed strong constraints on models of inflation, showing that the first stars formed when the universe was 400 million years old.
Some 30 papers based on Planck's findings will be released on arXiv tomorrow.
About the author
Michael Banks is news editor of Physics World