Professor Nicola Vittorio, of the University of Rome ‘Tor Vergata’, chronicles past, current, and possible future breakthroughs in our understanding of the physics of the early Universe.
Cosmology is one of the most fascinating subjects of modern science. It addresses some fundamental questions such as: Where did we come from? In what kind of universe are we living? What will the fate of the Universe be? Although these questions appear to belong more to philosophy than to science, they have found rigorous scientific answers. Our ability to properly describe the Universe at the present time (an infinite distribution of galaxies), as well as at early times (a tiny fraction of seconds after the Big Bang), is remarkable. In fact, modern cosmology shifted over the years from the discovery phase to the current precision measurement era, providing, as a science, a unique test bench for the physics of the early Universe.
The cosmic microwave background
The discovery in 1965 of the cosmic microwave background (CMB) – the first and oldest light emitted when the Universe was 380,000 years old – by Nobel winners Arno Penzias and Robert Wilson provided the first and strong evidence for the so-called ‘hot Big Bang’ and constitutes by far one of the most powerful tools of experimental cosmology. In fact, the observations of the CMB today, 13.4 billion years after the Big Bang, provide a unique insight into how the Universe was at the beginning and how it evolved with time. This, in turn, sets limits on the models of particle physics at energies that will never be reached in the laboratory and provides a stress-test for new scenarios that go beyond the standard models of cosmology and particle physics.
Tiny variations in the mean CMB temperature – or intensity – with the observing directions (the so-called ‘CMB temperature anisotropies’) were first detected in 1992 by the DMR experiment onboard the NASA/COBE satellite. This detection provided the first and robust confirmation of the theories describing the formation and the evolution of the large-scale structure of the Universe based on the gravitational instability scenario pioneered in the 1940s by Evgeny Lifshitz.
The ‘concordance model’ of cosmology
More recently, the very precise measurements of the ESA/Planck mission have established a breakthrough in the comprehension of the current concordance model of cosmology, where only about 5% of the Universe constituents are formed by ordinary matter (protons and neutrons). The other 95% are a combination of dark matter (presumably still unknown weakly interacting massive particles) and dark energy (something conceptually very similar to the cosmological constant introduced by Albert Einstein roughly 100 years ago), in a rough proportion of 3:7. Dark energy dominates the cosmic dynamics today, and it is responsible for the present accelerated expansion of the Universe. The first evidence for such an accelerated phase was provided by Adam Riess and Samuel Perlmutter at the end of the 1990s and was further confirmed by the most recent data.
Despite these successes, the concordance model also suffers from the same known puzzles of the cosmological models proposed by Alexander Friedmann in the 20s of the last century. To resolve these puzzles, Alan Guth proposed at the beginning of the 1980s the so-called ‘cosmic inflation’. Interestingly enough, in the gravitational instability scenario, the properties of the galaxies and their spatial distribution, as observed ‘here and now’, are determined by the physics of the inflation, a period of accelerated expansion occurring in the very early Universe, just (10-36 seconds) after the conjectured Big Bang. In fact, inflation provides the only self- consistent mechanism able to explain the generation of the primordial ‘seeds’ out of which (via gravitational instability) all the observed cosmic structures have formed. Despite this, cosmic inflation was at the beginning considered to be a very interesting but speculative theoretical paradigm. However, the precise mapping of the CMB temperature anisotropy provided by the ESA/Planck satellite has lent credibility to this scenario and the case for cosmic inflation has strengthened significantly over the years (Fig. 1).
The conclusive evidence for the inflationary theory of the early Universe will be provided by a detection of a primordial gravitational wave background generated during inflation. This background imprints a unique pattern (the so-called ‘B-modes’) in the polarisation of the CMB. Therefore, the primary scientific exploitation of CMB B-mode data aims at a definitive probe of the inflation paradigm and at an estimate of the energy scale at which inflation occurs. In addition to this, the gravitational lensing of CMB photons due to the large-scale matter distribution will provide stringent information on the distribution of dark matter and, possibly, on the masses of neutrinos. The comparison between the neutrino properties inferred from cosmological measurement and those determined in laboratory experiments is likely to open a new window on our physical modelling of the micro- and macro-cosmos. Unfortunately, the amplitude of B-modes is model dependent. Thus, while the ESA/Planck observations of the weak polarised CMB signal (the so-called ‘E-modes’) have opened a new channel and a new era in the observations of the polarised microwave sky, the hunt for the elusive B-modes has only just started.
We can say that the great goal of modern cosmology is investigating the physics of the early Universe. This ambitious goal requires combined and synergic contributions from traditional astronomy, particle physics, and theoretical cosmology. These three research areas are progressively converging into a relatively new research field called astroparticle physics, which simultaneously addresses fundamental questions connected on the one hand with the elementary particles and their interactions and, on the other hand, with the formation and evolution of the large-scale structure of the Universe.
It is worth mentioning that the Astroparticle Physics European Consortium (APPEC) was founded in 2001 to promote co-operation among the members of the European scientific community. This was (and is) perfectly in line with the need and the effort to build a European research area (ERA). For this reason, APPEC presented the European Astroparticle Physics Strategy on 9 January 2018 in Brussels. The APPEC recommendations for the next decade address specific scientific issues around and updates on long-term scientific strategies, as well as societal issues like global collaboration, community building, gender balance, education, public outreach, and relations with industry. Among these recommendations there is the recognition that the ‘future CMB program sets the stage for a range of opportunities to link key themes together and provides a potential stepping-stone towards further fundamental discoveries’.
CMB B-mode detection
To discover the secrets hidden in the B-modes of the CMB, it is necessary to combine observations from space and from the ground. The ongoing and forthcoming ground-based CMB experiments use large detector arrays and reach high angular resolution. Space-borne CMB measurements are not limited by the atmosphere and can probe a wide frequency range to provide an effective foreground (mainly due to the diffuse emission of our own galaxy) subtraction to reveal the true holy grail: the primordial B-modes induced by the primordial gravitational wave background.
The last space experiment dedicated to CMB observations was the ESA/Planck mission. Following the completion of Planck and the forthcoming Planck Legacy release, European CMB researchers have continued, and will continue, to play leadership roles in a number of suborbital efforts, both in Europe and elsewhere (e.g. by collaborating with the ground-based
US-S4 programme). The European CMB community has also recognised the need and the urgency for a new CMB space mission aimed at CMB B-mode detection. There was quite a strong R&D effort in Europe in new technologies for the next-generation CMB experiments, and a proposal for the space-borne Core mission submitted to ESA has been, unfortunately, rejected. Therefore, LiteBIRD (Lite satellite for the studies of B-mode polarization and Inflation from cosmic background Radiation Detection) – a JAXA strategic large mission candidate now towards the end of its concept development – appeared to the European CMB community as a natural and logical continuation of these Europe-led efforts, aiming to disperse knowledge and skills acquired with the ESA/Planck satellite. So, the possible formal selection by JAXA of the LiteBIRD mission, expected towards the end of this year, seems to be an appointment that cannot be missed by the CMB European community.
Professor Nicola Vittorio
University of Rome ‘Tor Vergata’
www.nicolavittorio.euems to be an appointment that cannot be missed by the CMB European community.