The Instant After Zero Hour
The work of LMU physicist Viatcheslav Mukhanov probes the immediate aftermath of the Big Bang, illuminating crucial aspects of theories of the origin of the Universe. And he is sure that forthcoming data from the Planck Mission will prove him right.
Sometimes the world can seem quite simple – when complex calculations suddenly yield a unique solution. In Viatcheslav Mukhanov’s case the answer is 0.96, and is his predicted value for the spectral index, an important parameter in cosmology. Mukhanov is an acknowledged expert in Theoretical Quantum Cosmology. He uses intricate equations and mathematical operations to explain how the Universe came into being and why it expanded at an enormous rate for a split second immediately after its emergence. In a conversation in his office at LMU’s Arnold Sommerfeld Center for Theoretical Physics, during which he outlined his theory, Professor Mukhanov repeatedly returned to this number, drawing a circle around it or underlining it. “It is an experimentally verifiable value,” he says. “The number is anything but trivial.”
To understand why, one must delve into the subject to learn something about the birth of the Universe, and about a theory proposed by Mukhanov back in 1981 as part of a model of the Big Bang. That theory concerns so-called quantum fluctuations and the inflation phase – a postulated burst of exceedingly rapid expansion directly after the Big Bang – and it is once again in focus. For in January 2015 the complete set of observations of the cosmic microwave background radiation (CMB) made by the European Space Agency’s Planck Space Telescope will be released. The Planck Project involves scientists from over 300 institutions, and has measured the intensity distribution of the CMB with unprecedented accuracy. The CMB constitutes the afterglow of the Big Bang, and the Planck data essentially provide a high-resolution photograph of the young (~300,000-year-old) Universe. And the value of the CMB’s spectral index that fits this picture best, according to a study of the Planck data published in 2013, is 0.9585±0.007. “That agrees precisely with my calculated value,” Mukhanov points out. “I couldn’t hope for a better confirmation of my theory.”
A phase of extremely rapid expansion
Mukhanov had intuited in 1981 not only why the Universe is so strikingly uniform on the largest scales, but also how the deviations from uniformity that gave rise to galaxies, stars and planets could have originated. Together with the American Alan Guth, and his own compatriots Andrei Linde (now based in Stanford) and Alexei Starobinski, Mukhanov’s name can thus be numbered among the fathers of the theory of cosmic inflation. Its central tenet is that the very early Universe underwent a phase of extremely rapid expansion, growing by a factor of 1025 (1 followed by 25 zeroes) within a minuscule fraction of a second, before subsequently expanding at a less frenzied pace to its present extent over the next 13 billion years.
Mukhanov’s work actually focuses less on inflation than on the notion of quantum fluctuations, without which inflation cannot work. Only the inclusion of quantum fluctuations can account for certain fundamental features of the observable Universe, with its stars, galaxies and black holes. The idea occurred to him, Mukhanov says, as he pored over the effects of Heisenberg’s uncertainty principle, which states that the position and momentum of a particle cannot be simultaneously determined with arbitrary precision. Mukhanov realized that quanta in the early Universe must have been subject to such fluctuations.
“Everything is incipiently present in the embryonic Universe”
Under normal conditions their effects are negligible, he says, but thanks to the speed of expansion during the inflationary epoch, these energy fluctuations were embedded in the very structure of space, and thus left their mark on the CMB. Over time, the microscopic quantum fluctuations became the seeds of macroscopic density variations. Without such a mechanism, whose nature and magnitude Mukhanov has precisely characterized, the inflation theory could not account for the distribution of matter we observe in the Universe today. Moreover, Mukhanov’s theory shows that their variability conforms to Gaussian statistics, which for physicists is a non-trivial insight. His model also stands as one of the most spectacular demonstrations of the validity of the laws of quantum mechanics on the largest cosmological scales. “Only as a result of quantum fluctuations during the inflationary phase could large-scale density differences arise,” says Mukhanov. “For 300,000 years these ‘embryonic galaxies’ were in a frozen state, so to speak. Only then did the first galaxies slowly begin to form. The very earliest instants are crucial for an understanding of all that followed. Everything is incipiently present in the embryonic Universe.”
Apart from Starobinski’s prediction of the properties of the gravitational waves produced during the inflationary phase in the early Universe, most of the theoretical work done on the phenomenon of inflation in the 1980s, which is still regarded as fundamental, either failed to yield any physically verifiable predictions or their forecasts have already been ruled out by the initial analysis of the Planck data. Gravitational waves are generated whenever processes involving large masses and high energies are underway. An inflationary phase of expansion in the early Universe would certainly qualify as such and, given sufficiently sensitive instruments, the gravitational waves it would have generated should be detectable. Mukhanov takes the view that a worthwhile theory must lead to experimentally verifiable predictions. In this respect, he says, his thinking was strongly influenced by the great minds of Russian atomic physics, such as Andrei Sakharov and Yakov Zel'dovich or Nobel Laureate Vitaly Ginzburg. He learned from their example how to think for himself, while studying under their tutelage or later collaborating with them. But they also taught him the essence of physics, he adds. “In the last analysis, everything that is inaccessible to experimental tests is not physics,” he affirms. “It is either natural philosophy or it is theology, and is of no interest to me.”
Progress in cosmology was long hampered by a lack of empirical observational data, which physicists could work with. “That is why cosmology remained so speculative for so long,” Mukhanov says. With the launch of the COBE satellite in 1993, the balloon-borne Boomerang telescope in 1997, and later on the WMAP and Planck satellites that situation has now been transformed. With these instruments, the intensity or polarization of the electromagnetic waves that make up the CMB could be measured with ever increasing precision. It is now possible to distinguish fine structure within the CMB at various scales, and this is revealing in ever greater detail what went on in the earliest stages of the Universe’s existence. Indeed, this is the only means we have of reconstructing what the embryonic stages of our Universe really looked like, and what particles and forces, some perhaps long extinct, played the leading roles in its embryonic phase. As ever more sensitive instrumentation becomes available, the better cosmologists can understand how particles and larger-scale structures formed in the early Universe. Other projects using ground-based radiotelescope arrays, such as that sited in Chile’s Atacama Desert, will further enrich this picture.
Dust within the Milky Way
The features of greatest interest can only be seen if minute differences in temperature or position can be resolved. The intensity of the CMB across the sky is extremely homogeneous, varying from its average temperature of around 2.7 Kelvin by only tiny fractions of a degree. Nevertheless, observations with high angular resolution can detect these minute variations between neighboring patches of sky The detectors on Planck can measure differences of 2 microKelvin. Planck can also register the presence of dust in our own galaxy. That might seem rather prosaic, but it provides a vital control for other experiments, including BICEP2. In March 2014, the BICEP2 group announced sensational results that made headlines even in the popular press. The BICEP2 radiotelescope at the Amundsen-Scott Station in the Antarctic uses microwave antennas to measure the polarization of the CMB. Analysis of these data led the team, headed by American physicist John Kovac, to believe – and announce – that they had detected the imprint of primordial gravitational waves on the CMB, signals from the Big Bang which imparted a subtle rotation to the polarization of the CBM. In their analysis published in “Physical Review Letters”, however, the authors admitted that dust within the Milky Way could perhaps account for the pattern they had seen. Since then, the Planck Consortium has reported that its data support the latter view. Hence the signal detected in the BICEP2 data may be attributable entirely to galactic dust.
At a conference in Moscow in June 2014 Mukhanov had asserted that BICEP2 and Planck could not both be right. This issue at least now seems settled. “Gravitational waves may well be there,” he asserts, “but our instruments are not yet good enough to pick them up.” Irrespective of whether or not a real detection of primordial gravitational waves confirms the inflation theory, Mukhanov believes that the Planck data, and results from other experiments, prove that quantum fluctuations were an integral part of the very early history of the Universe. Indeed, he says, essentially all of the alternative models that attempt to explain what happened after the Big Bang also make use of quantum fluctuations. Hubert Filser, Translation: Paul Hardy