The Cosmic Microwave Background (CMB), beyond doubt, has been one of the most important and rich sources of information in modern cosmology. Through studies of the primary CMB temperature and polarisation anisotropies, cosmologist were able to establish the standard ΛCDM concordance model, which accurately describes the properties of the Universe over a vast range of physical scales in both space and time. These investigations furthermore provided invaluable insights into fundamental laws of physics at energy scales inaccessible by any other means. As such, cosmological studies have long become one of the most important drivers of advances in modern physics.
These unprecedented successes in observational and theoretical cosmology beg the question what is there left to be done? The answer is a roaring we have only started to scratch the surface. While arguably all valuable information has indeed been extracted from the primary CMB temperature anisotropies (those unique wave patterns traveling through the photon-baryon fluid since the Big Bang), there is still a lot to be learned from CMB polarisation signals. In addition, novel complementary information can be extracted by measuring the energy spectrum of the CMB (the rainbow equivalent of the microwave sky). These are the main observables for the future and accessing this information is one of the main targets for the proposed Voyage 2050 concept. This will allow us to study the physics of the primordial Universe, shedding new light on inflation and the cosmic thermal history. We will furthermore be able to advance several frontiers of modern particle physics, collecting more clues about the mysterious dark matter and dark energy components as well as probing the properties of neutrinos (e.g., their number and mass).
But the above only describes a small fraction of what there still is to be learnt within the proposed Voyage 2050 program. We are entering an exciting new era in terms of precision and angular resolution of CMB space missions, exceeding the capabilities of ongoing and planned experiments many-fold. The small imprints of structures in the Universe through gravitational lensing and scattering effects now define the next targets. This will open an immensely rich new window in astrophysics and cosmology, allowing us to ask important questions about the growth of structures, feedback processes and large-scale velocity fields in the Universe. Numerous cluster of galaxies will be detected through the so-call Sunyaev-Zeldovich effect, allowing us to study the physical processes acting in the clusters atmosphere but also what their role is in cosmology in great detail. This will deliver immensely rich legacy data for many cross correlation studies with X-ray and optical surveys.
Beyond CMB bolometry, microwave spectroscopy is now coming into the focus of attention. The experimental requirements necessitate the use of microwave spectrometers to be able to perform reliable separation of the multiple components. This provides unique new opportunities for studies of galaxies and their assembly in the Universe. We can furthermore extract novel information about the large scale Universe through microwave line intensity mapping. In combination, this will deliver incredibly rich datasets, including many high-redshift sources to facilitate further cross correlation studies. With microwave spectro-polarimetry, we will furthermore be able to study the properties the gas and magnetic fields in our own galaxy. The proposed mission thus uniquely joins the common goals and experimental requirements for a wide range of science cases in an optimal synergistic way.
In the sections below, we now highlight some of the main opportunities in more detail. We can only touch the surface and refer to the associated ESA Voyage 2050 White Papers for more details.