The precise picture delivered by Planck conforms most spectacularly to the expectations of the Standard Model of the universe, but the evidence of anomalous features also indicates that something fundamental may be missing from the standard framework. By R. RAMACHANDRAN
THE universe and its evolution, as cosmologists have tried to describe, particularly after the 1980s, by the so-called Standard Model of Cosmology (SMC) is indeed a fairly good description. In the main, the SMC, built on Albert Einstein’s theory of gravity based on general relativity, Edwin Hubble’s discovery of an expanding universe with its implication that the universe came into existence following a “Big Bang”, the discovery of the free-streaming relic radiation from the early hot universe, known as the Cosmic Microwave Background (CMB), which bathes the entire universe nearly uniformly, and the idea of an inflationary epoch when the universe underwent a rapid exponential expansion within fractions of a second after its birth, is on firm grounds nearly in all its key aspects.
This is the upshot of the new, best-resolution all-sky map of the CMB, the earliest light of the universe emitted about 380,000 years after the Big Bang that we can detect today, based on data that the Planck satellite of the European Space Agency (ESA) gathered over 15.5 months in its nine different microwave frequency bands. The new results were announced on March 21. This affirmation of the SMC was only to be expected because Planck was not the first space-based high-resolution measurement of the CMB and the basic correctness of the SMC was already evident from earlier, but coarser, measurements. But, significantly, while the ESA’s press release was titled “Planck reveals an almost perfect universe”, its detailed information release was titled “Simple, but challenging”. This was because Planck data have revealed some unexpected features in the large scale, which the SMC that does remarkably well in the smaller scales would find hard to accommodate, and it will be a challenge for physicists to come up with theories that can explain them.
So what is this standard model of cosmology? The SMC is based on the assumption that on the very large scale, the universe is homogeneous and isotropic, which is known as the Cosmological Principle. That is, the properties of the universe are similar at every point and there is no preferred direction in space. But on the smaller scale, we know that this is not true—there are stars, galaxies and clusters and superclusters of galaxies.
The understanding, according to the SMC, is that, while the CMB is smooth and uniform in the large scale, the rich structure on relatively small scales is the result of tiny random quantum fluctuations that were embedded in the CMB during cosmic inflation. Inflation is the extremely short period of accelerated expansion immediately after the Big Bang—which lasted only 10 to 10 seconds, much quicker than the blinking of an eye, when the size of the universe ballooned by an enormous factor of 10!
These tiny fluctuations, typically of the order of 1 part in 100,000, on an otherwise almost uniform density of plasma (ionised matter comprising mainly protons, neutrons and electrons) of the very early universe, were amplified to cosmologically large scales during inflation. Denser regions grew increasingly denser owing to gravity to give the highly rich structure of the universe that we see around us today, with different densities in the different large regions in the sky. The primordial seeds of this are hidden in the temperature variations in the CMB. These variations, which show up as splotches in a high-resolution CMB map, such as that of Planck, reveal the imprints of the sound waves (density oscillations) in the plasma on the photons when electrons and nuclei recombined and the matter and radiation decoupled.
The relationship of the temperature fluctuations in the CMB and the density fluctuations in the plasma during the inflationary epoch can be understood as follows: if photon was in the denser region of space, it would have spent more energy to break away from the gravitational attraction of the region, thus becoming colder than the average energy of the emerging photons, and vice versa. To understand this evolution of the matter distribution in the universe, from an almost homogeneous state to a highly substructured one of today, requires an enormous amount of information about the universe’s history and the nature of its different components.
Scientists try to reconstruct this evolution by taking snapshots of the universe at different epochs in its history. For a snapshot of the present, a survey of the galaxies that populate the neighbourhood universe is done. Observations of distant galaxies give the picture of earlier cosmological times. The CMB pattern is thus the snapshot of the earliest observable shell of the universe, 380,000 years after the Big Bang when photons broke free of all matter and began to propagate freely through the universe.
The SMC is the result of knowledge derived from a number of different astronomical observations based on entirely different physical processes. Indeed, in spite of the highly intricate substructure of the universe at relatively smaller scales, the universe appears remarkably homogeneous and isotropic on the very large scale. The observed data sets, based on entirely different astrophysical processes and sources, over different scales and in different regions of the sky, have agreed extremely well within the framework of the SMC built on its two pillars: homogeneity and isotropy.
However, for the data to strictly conform to theory, cosmologists have had to postulate, besides the idea of rapid cosmic inflation itself, two additional ingredients, direct evidence for both of which are yet to be seen: dark matter, which does not interact with light and only weakly interacts with visible matter, mainly through gravity; and, dark energy, a mysterious component that permeates the universe whose negative (repulsive) pressure acting against gravity is pushing the universe apart. Both dark matter and dark energy are required for the universe’s mass-energy balance. Dark matter is invisible and can be detected only by its gravitational effect on normal matter. Dark energy is considered responsible for the accelerated expansion of the universe evident in the present epoch. Both these concepts now form an integral part of the SMC.
A relatively small number of parameters suffice to characterise the SMC: the density of ordinary matter, dark matter and dark energy, the rate of expansion of the universe at the present epoch (known as the Hubble constant), the geometry of the universe (closed, like a sphere; or open, say, like a saddle; or flat), and the relative amount of the primordial fluctuations that were embedded on different scales and their amplitudes. Different values of these parameters give a different distribution of structures in the universe, and, projecting back in time, a correspondingly different pattern of fluctuations imprinted on the CMB. The importance of studying the CMB in the greatest detail possible essentially arises from this fact.
There were two important space missions to measure the CMB in detail that preceded Planck. In 1989, the American National Aeronautics and Space Administration (NASA) launched the Cosmic Background Explorer (COBE), which provided us with the first precision map of the CMB (Frontline, July 17, 1992). Among its key discoveries was that the CMB, averaged across the whole sky, conformed very accurately to a “black body” (or pure thermal) radiation at a temperature of 2.73 Kelvin. At the same time, the temperature also shows tiny fluctuations of the order of 1 part in 100,000 across the sky. These findings fetched John Mather and George Smoot the 2006 Physics Nobel (Frontline, December 15, 2006). NASA followed this up with the Wilkinson Microwave Anisotropy Probe (WMAP) satellite launched in 2001 to give an all-sky CMB survey with even better resolution to map these temperature fluctuations in greater detail (Frontline, September 14, 2001). These observations, together with the observations by ground-based radio telescopes such as the Atacama Cosmology Telescope (ACT) in Chile and the South Pole Telescope (SPT) and balloon-based observations such as BOOMERanG and MAXIMA, have provided a fairly good picture of the CMB which, in all its key details, could be explained fairly well by the simple SMC. But a few questions remained to be resolved, which required more accurate and sensitive measurement of the CMB than the WMAP.
Enter Planck. It was launched on May 14, 2009, with the primary objective to map these fluctuations across the whole sky with greater resolution and sensitivity than ever before and it is continuing to scan the sky (Frontline, July 3, 2009). By analysing the distribution pattern of these fluctuations, virtually all important quantities that describe the universe soon after it formed, the composition and evolution of the universe through billions of years can be determined. This, in turn, would determine all the important quantities necessary to characterise the SMC that describes the observable universe completely and accurately.
To achieve this, Planck observed the CMB in nine wavelength bands (1 cm - 0.3 mm), ranging from microwaves to the very far infrared (with two different instruments operating in 30-70 GHz and 100-857 GHz frequency bands respectively). Planck’s detectors are cooled to temperatures very close to the absolute zero. Otherwise, thermal radiation of the instruments themselves will spoil the measurements. To measure the CMB temperature at every point, Planck observes every point an average of 1,000 times.
Analysing Planck data
Planck released its first all-sky uncorrected image in 2010. A major painstaking exercise in extracting the new results, which contain the real information from the observed CMB data, was the removal of all possible contamination due to foreground sources that lie between the instrument and the universe’s first light— emission by other galaxies as well as interstellar dust and gas and other intervening matter in our own galaxy. Only then can the CMB data be fully analysed and cosmological models compared.
The significance of the new Planck measurements is that they have not only greatly refined our knowledge about the universe but constitute the most sensitive and precise measurements of the CMB possible. While COBE measured the CMB down to an angular resolution of about 7 degrees only, the WMAP improved it to half-a-degree resolution and Planck three times better (5-10 arc minutes). In terms of the CMB temperature at different points of the sky, Planck’s sensitivity is such that it can detect variations up to a millionth of a degree, which is 10 times better than the WMAP (Figure 1). According to the ESA, Planck’s instruments are so sensitive that they are limited not by the instrument’s limitations but by fundamental quantum astrophysical effects themselves. “In other words,” as the ESA’s overview of Planck says, “it will be impossible to take better images of this radiation than those obtained from Planck.”
Planck data have been found to conform most spectacularly to the expectations of the simple SMC, and on the basis of these, scientists have been able to extract much more accurate values of some of the key parameters of the SMC. These suggest that the universe is expanding more slowly than previously thought; it is 13.8 billion years old, 100 million years older than the earlier estimate. The new value for the Hubble Constant, which gives the rate of expansion of the universe, is 67.15 km/s/megaparsec +_ 1.2 km/s/mpc, which is significantly less than the current estimate of around 72 km/s/mpc derived from NASA’s space telescopes, Spitzer and Hubble, using a different technique. (A megaparsec, or mpc, is about three million light years.)
The WMAP’s measurements had provided fairly good data on the proportions of the different constituents of the universe. Now, on the basis of Planck data, those values have got significantly refined. The data show that there is less dark energy and more matter, both normal and dark, in the universe than previously estimated. According to Planck, dark matter content is 26.8 per cent instead of the 24 per cent estimated earlier, normal matter is 4.9 per cent instead of 4.6 per cent and dark energy has dropped to 68.3 per cent from 71.4 per cent (Figure 2).
While this most detailed and accurate picture of the CMB has confirmed once more that the relatively simple standard model describes the universe amazingly well, but with refined key cosmological parameters, the high-precision data have also revealed some curious features and subtle anomalies in the CMB that cannot be explained easily. These, according to scientists, will require revisiting some of the fundamental assumptions made in the SMC and may even require bringing in some new physics. Earlier measurements, too, had indicated the presence of some of the anomalous features but needed to be confirmed with greater confidence in the measurements. These have now been determined with such great accuracy that they can no longer be wished away.
“The picture delivered by Planck,” Jan Tauber, Planck project scientist at the ESA, commented when the new results were released, “is so precise that we can use it to scrutinise in painstaking detail all possible models for the origin and evolution of the cosmos. After this close examination, the SMC is still standing tall, but at the same time, evidence of anomalous features in the CMB is more serious than previously thought, suggesting that something fundamental may be missing from the standard framework.”
By analysing the Planck data, it has been possible to set very tight constraints on the parameters that characterise the SMC, which include the relative amount of primordial fluctuations on different scales. This is equivalent to measuring correlations of the CMB temperature across different angular scales in the sky. Cosmologists use the SMC to predict such correlation. Predictions of the SMC match the Planck data amazingly well at small angles. But for larger angles (separated by more than 6), which means points farther apart in the sky, the temperatures seem to be more correlated than the SMC would predict (Figure 3).
This is one of the most surprising findings of Planck. The signals of fluctuations in the CMB temperatures at large angular scales are about 10 per cent weaker than predictions. This has provided for the first time evidence that the distribution of primordial fluctuations was not the same at all scales. Since these fluctuations were generated during cosmic inflation, this finding can test within an extremely short time the validity of many models that describe the dynamics of this accelerated expansion of the universe.
While this was the first ever detection of such an anomaly in the CMB, Planck also confirmed a number of other anomalies which the WMAP data too had suggested, but the evidence was not strong enough to unequivocally rule them out as artefacts of data processing or foreground emissions. One of them is the asymmetry in the average temperatures on opposite hemispheres of the sky. This runs counter to the basic isotropy assumption of the SMC that the universe should be broadly similar in all directions. The other is the presence of the so-called “cold spot” (Figure 4). One of the low temperature spots in the CMB extends over a patch of the sky that is much larger than expected. These findings are perhaps an indication that the SMC may not be adequate to describe the universe at all scales. In arriving at some of these important conclusions on anomalies there has been a significant Indian contribution. Tarun Souradeep, Sanjit Mitra and the former’s graduate student, Aditya Rotti, from the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, were involved in the statistical analysis of the data by using the formalism called BipoSH, which they had developed to confirm the anomalies that had been found. The IUCAA group’s work involved the removal of artefacts arising from the complex instrumental response as well as in the search for subtle violations of the cosmological principle of isotropy. The former was critical for the accurate estimation of the key cosmological results from the Planck data. The Indian team will continue to contribute towards the next major release of Planck results in 2014, which will include the full data set, in particular CMB polarisation data, and further refinement in the analyses.
One way to explain the anomalies is to give up isotropy as a fundamental tenet for building a model of the universe; that is, the universe is not the same in all directions on a larger scale than we can observe. One class of models that are based on such a theoretical framework is called Bianchi models. In such a scenario, the light rays from the CMB may have taken a more complicated route because of the complex geometry of space-time in such models, resulting in some of the unusual patterns observed by Planck, according to some scientists.
Indeed, says Krzystof M. Gorski of the Jet Propulsion Laboratory (JPL), California Institute of Technology, “When we take into account the large-scale anisotropy described by the Bianchi models in the analysis of Planck data, several anomalies are simultaneously reduced by a significant amount. However, it is not possible to merge this very specific anisotropic scenario with the SMC that holds very well on ‘local’ scales.”
So cosmology after Planck stands at an interesting crossroads. On the one hand, the SMC is still the best way to describe the CMB data although it includes components that still lack even a sound theoretical understanding, such as dark matter, dark energy and inflation. And, on the other, there are these anomalies that point to something fundamental missing in the foundations of cosmology.