Hubble Tension

Dark energy is the name of something that is responsible for the accelerated expansion of the universe. We don't know what it is, but we are observing the effects of this mysterious factor. The existence of the dark energy is a challenge for physics. Current microphysics – models of the particle interaction – do not require its existence. In particular, there is no space for the dark energy in the Standard Model of particle physics. However, observations of the Cosmic Microwave Background (CMB), Supernovae Ia (SN Ia), and the growth rate of the Large Structure show convincingly that the expansion of the Universe accelerates during the last 7 Gyr instead of being decelerated by the gravity. Usually, gravity causes gravitational attraction, so the medium responsible for accelerated expansion of the Universe must have strange properties. There is a way to get antigravity effect: Einstein introduced a mathematical term – cosmological constant – to his equations, and if this constant is interpreted as a material source, it has positive energy density and negative pressure, and this negative pressure term wins in the global energy budget. However, the fact that there is something like that, and actually dominates now the dynamics of the Universe, came as a surprise. The name – dark energy – for this medium helps us to see the challenge we face in astronomy and physics.

Current observational are simply consistent with the Einstein’s cosmological constant, i.e. the dark energy seems to be uniform in space and time. Numerous models of possible additional forces or fields are being developed by theoreticians, but there would be a way to differentiate between various options if astronomers can reliably measure a departure from a single universal constant. With increasing accuracy, some tension between the measurements of the Universe parameters starts to appear, either real or just due to underestimation of the systematic errors, known as Hubble tension. For example, the local measurements of the Hubble constant (e.g. H0 = 73.24±1.74, Riess et al. 2016) are not quite consistent with the constraints from the microwave background (e.g. H0 = 66.93±0.62 km/s/Mps from Adam et al. 2016). Thus the known methods are being continuously refined, and new probes are being searched for, since the most important issue at this stage are systematic errors, the most difficult to estimate in any single method.