In recent years observations of the light curves from Type 1a supernova have determined that not only is the Universe continuing to expand, but that it is also undergoing a period of accelerated expansion, which began around 5 billion years ago. In reality this means that the second derivative of the scale factor R(t) is positive, or that the first derivate is increasing with time. This was unexpected, since instead it was thought that the expansion would slow down over time due to the gravity from all of the matter in the Universe. This can be accounted for by the adoption of a positive value of the cosmological constant, equivalent to the presence of a positive vacuum energy, otherwise referred to as dark energy.
Current measurements of the Hubble parameter give two different values for the expansion rate. The first is from Type 1a supernova data as so called standard candles and the latest value is observed to be
73.24 +/-1.74 (km/s)/Mpc
The Second is from measurements of the Cosmic Microwave Background (CMB) and the latest value is observed to be
67.4+/-0.5(km/s)/Mpc
Taking into account the uncertainties of these values, current measurements are predicting a ~5-11% variability on the value of the Hubble parameter.
The Interstellar Research Centre is pursuing a particular idea that may explain the source and nature of dark energy.
References:
Riess A. G, Macri L. M, Hoffman S. L, A 2.4% Determination of the Local Value of the Hubble Constant, The Astrophysical Journal, 826, 1, 21 July 2016.
Aghanim N, Akrami Y, Ashdown M et al., Planck 2019 Results. VI. Cosmological Parameters arxiV.org/abs/1807.06209, 17 July 2018.