WE ARE ALL DESIGNIST NOW:

The goal of modern cosmology is to explain the evolution of the Universe from its inception to the present time using our limited understanding of its composition and physical laws. This is even harder than it sounds! The first difficulty materialized in 1929 when initial estimates of the present expansion rate -- known as the Hubble constant or H0 -- rewound to the Big Bang singularity implied that the Universe was younger than the age estimated for the Earth and Sun. In retrospect, both figures were well off the mark, but there has been tremendous progress in the meantime. The measurement of H0 improved from 10% uncertainty at the start of the 2000s to less than 2% by 2019. In the past few years, reduced uncertainties from both the cosmic microwave background (CMB) -- the afterglow of the Big Bang -- and local Universe measurements have revealed an underlying discrepancy that is growing harder to ignore.

Scientists now have a 'standard model of cosmology', called ΛCDM (lambda cold dark matter), economically crafted from six free parameters and a number of well-tested ansatzes. The model characterizes a wide range of phenomena including the accelerating expansion, structure formation, primordial nucleosynthesis, flat geometry of spacetime, fluctuations of the Big Bang afterglow and the first combination of baryons into atoms. Remarkably, dark components (matter and energy) account for 95% of the Universe, as described by ΛCDM, their presence robustly inferred from their gravitational effects. Yet despite the success in better understanding our Universe confirmed by a wealth of precise measurements, in the past few years there has been growing evidence that the expansion of the Universe is still exceeding our predictions. [...]

If the Universe fails this crucial end-to-end test (it surely hasn't yet passed), what might this tell us? It is tempting to think we may be seeing evidence of some 'new physics' in the cosmos. Indeed, a large number of theoretical solutions have been proposed and are reviewed in ref.10. For example, if we lived near the middle of a vast and deep void in the large-scale structure of the Universe, this could cause excessive, local expansion. However, the odds of a void this large occurring by chance is incredibly low. Calculations show that it exceeds 10σ (ref.11) and is also strongly ruled out empirically by the lack of evidence of any end to the void from SNe Ia at greater distances12. Dark energy with an equation of state lower than vacuum energy could produce stronger acceleration and explain the discrepancy, but this possibility is disfavoured by other intermediate-redshift measurements.

Greater success in explaining the H0 measurement discrepancies has been achieved by altering the composition of the Universe shortly before the emergence of the CMB. An additional component in ΛCDM, such as a new neutrino or scalar field (the latter called early dark energy or EDE), could have increased the early expansion, decreased the sound horizon of primordial fluctuations and raised the predicted value of H0 depending on the approach used, to 70-73 km s−1 Mpc−1 (ref.10) in plausible agreement with the local value. New particles tend to create new conflicts with the CMB, whereas EDE is claimed to improve agreement with the CMB. A criticism of EDE is that its scales must be finely tuned, although the same may be said of the other two episodes of dark energy (inflation and present acceleration). This raises the question of whether apparent episodes of such anomalous expansion are common or even related

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