Two precision studies of the remnants of the Big Bang are almost in agreement about what they have found... almost.
In the 2000s, two space probes set out to study the relics of an ancient chaos in exacting detail. Their subject was a tremendous smattering of matter and radiation across trillions of light-years, possibly all the way across the universe. Scientists believe these relics hold the complicated clues to our universe’s origins, why the laws of nature are what they are, even why the hundreds of billions of galaxies are where they are now.
The chaos itself, whose residue interests us, was caused by an event popularly called the Big Bang, and its immediate aftermath. In 2001, the Wilkinson Microwave Anisotropy Probe (WMAP) was launched by NASA; in 2009, the European Space Agency launched the Planck space-probe. While both probes studied the relic entity and produced agreeing results of the bigger picture, they do have smaller inconsistencies between them – inconsistencies physicists think need resolving because of the very-high sensitivities the instruments boast of.
Even if one of them is proved right and other wrong, our knowledge of the universe’s origins, pieced together since a monumental discovery in the 1960s, would change.
The story began in 1964 at the Bell Telephone Laboratory in New Jersey, where a radio antenna was being readied for a specific task: to listen to radio-waves being emanated by the Milky Way galaxy. The two astronomers who were going to make the observations, Arno Penzias and Robert Wilson, decided to start with a short wavelength of 7.35 cm. This was being done so they could check for static noise being generated by the antenna itself, before moving on to more precise readings. However, they were in for a pleasant surprise.
Cooking up heavier elements
They were able to discern a faint excessive signal of microwave radiation at 7.35 cm. No matter which way they turned the antenna, the signal persisted, meaning that it was coming in from all parts of the sky, not just specific areas. Because they’d known that pigeons had roosted in the past around the antenna’s receivers, they cleaned out those areas… but the eerie signal kept on. Penzias and Wilson used the signals strength to calculate the temperature of the objects that could be producing it – it was found to be about 3.5 K (–269.65 degrees Celsius).
They didn’t know what to make of it.
Around the same time, in March 1965, an astrophysicist named P.J.E. Peebles from Princeton University was trying to understand a strange anomaly. In the few minutes succeeding the Big Bang, the universe should have resembled a massive cauldron, with its ingredients being quickly cooked into bigger and bigger lumps of heavier elements. Today, however, fully three-quarters of matter in the universe is hydrogen. Where are the heavier elements?
Peebles surmised that there also must have been a lot of short-wavelength radiation that blasted heavier atoms apart as soon as they formed, preventing the mass-cooking of hydrogen into heavier nuclei of the metals. Further, his calculations showed that the radiation should have survived to this day, leaving the universe with a low but prevailing temperature of around 10 K. To see if he was right, Peebles and his colleagues, Robert Dicke, P.G. Roll and D.T. Wilkinson, started to set up an antenna to look for the signal from this radiation…
… when Dicke received a called from Penzias. Together, the five of them published a pair of papers in the Astrophysical Journal speculating on the implications of their finding. It was July 1965, and the start of a fascinating quest.
In the decades since, physicists have been able to piece together the story of how this radiation could have originated, what it has to do with the Big Bang, and how a rapidly expanding universe’s signatures could have impinged on it. One of the first probes launched to study this relic radiation – called the cosmic microwave background (CMB) radiation – was the Cosmic Background Explorer, in 1989. This also marked the rise of cosmology, the study of the universe’s life.
In the moments immediately succeeding the Big Bang, the universe was seething hot and dense – protons, neutrons, electrons and photons were scattered about, their energy making them restless enough to resist entrapment into atoms. After 10-32 seconds, the volume of space started to expand rapidly – a period called the inflationary epoch which lasted for one microsecond but left the universe at least (hold your breath) 1 million trillion trillion trillion trillion trillion trillion times more voluminous.
After around 380,000 years, the temperature had dropped to about 3,000 K, and the first atoms formed. Because the electrons and radiation had existed in an equilibrium until then, the formation of atoms meant electrons were being used up, leaving the radiation to ‘move around’ and expand freely.
Thus formed the CMB radiation.
Because any change in the way it formed, howsoever small, would have altered it in significant proportions as it expanded, it was essential to know how precisely the CMB was distributed throughout the universe. Between 1990 and 1993, the Cosmic Background Explorer’s results presented the first view of the CMB on a vast scale.
Map of the universe
The Explorer’s successor was the Wilkinson Microwave Anistropy Probe (WMAP), launched in 2001. It is named for D.T. Wilkinson. In 2003, the WMAP presented its first results, considered to be groundbreaking for their immense detail, laying the foundation for a model of the universe that cosmologists abide by: the Lambda-CDM model. CDM here stands for ‘cold dark matter’, which WMAP found made up about 24 per cent of the universe. Of the remaining, 71 per cent came from dark energy while the rest was ordinary matter. It also measured the Hubble constant – the rate of the universe’s expansion – to be about 67.8 km/s/Mpc (interpreted as an object 1 megaparsec away moving away from the observer at 67.8 km/s).
In 2009, another probe, called simply Planck, was launched to study the CMB, as well as to investigate other cosmological problems. Planck boasted of a resolution thrice as much as WMAP’s, and could make its observations in nine frequency bands – as opposed to the one band that Penzias and Wilson used or the five bands that WMAP used. In fact, in order to achieve the precision that it did, the entire spacecraft was maintained at a temperature of 0.1 K, making it the coldest object in space!
By July 2010, Planck had completed an all-sky survey. On March 21, 2013, the results of its CMB-study were published. They were found mostly to be in agreement with the Lambda-CDM model, and the WMAP results by extension: according to Planck, the universe was 26.8 per cent dark matter, 68.3 per cent dark energy, and 4.9 per cent ordinary matter. The Hubble constant was pegged at a little less than 67.8 km/s/Mpc.
One thing that the Explorer, WMAP and Planck had all found was this: the CMB radiation was not evenly spread throughout the universe, but consisted of fluctuating ripples spreading across vast distances. This observation, called anisotropy, was attributed to minor irregularities in the way the radiation must have been packed together, predating its expansion for almost 14 billion years – the age of the universe. The experimental observation of this anisotropy was hailed as a major breakthrough in 1992.
On the other hand, the Planck results didn’t fall in line with the WMAP results on some counts. Among the nine frequency bands that Planck had made its measurements in, the sixth band (217 GHz) was the source of concern. The data analysis team behind WMAP argued that with the exception of this band, all other bands agreed with the WMAP results.
It was an important problem because even though the inconsistencies were minor, Planck’s high precision meant that they could lead to significant alterations of our knowledge of the universe if they were true. For instance, it could place more weight on why Planck data differs from WMAP’s on the universe’s composition, or why Planck has found the universe to be expanding at a slightly slower rate than the WMAP found it to be. Perhaps, between the sensitivity of the two instruments, there might be unknown physical processes at work.
At the time (2013), the Planck team responded saying they do stand by their observations, and that the inconsistencies could be due to “the improved performance of the Planck data”. However, they did also agree to revisit their observations. Its outcome was released last week in the February 6 issue of Nature as a correspondence.
Jan Tauber, who leads the Planck science team, has concluded that the difference in values between the Planck and WMAP results are within one standard-deviation of each other. More importantly, he wrote that they differed because of “methodological variations between the respective analyses rather than by systematic errors in the Planck data”. Tauber also stated that “the small, time-dependent systematic errors … have little impact on the Planck Collaboration’s cosmological results”.
That Tauber was able to say that the results’ deviation had little impact means the Lambda-CDM model can continue to be the ‘standard model’ of cosmology in its present form. Even though it might have been presumptuous to assume something paradigm-altering could have come out of this debate, cosmology is heavily reliant on precision-measurements of extremely small values. Even a seemingly trivial deviation would imply a relatively more consequential deviation of the value before the inflationary epoch.
For example, going by the Planck and WMAP maps, a tiny unevenness of energy in the pre-inflationary CMB could have snowballed into the gargantuan galactic clusters we see today. Calculating the other way, we could ask: is some region of space colder than the CMB because of an accumulation of dark energy?