Solving the Hubble tension with a local void

Talk for my department on how cosmology is currently in a crisis due to the Hubble tension, the observation that the expansion rate measured locally is 10% higher than predicted in ΛCDM with parameters calibrated to fit the CMB anisotropies. Observations of the galaxy luminosity function in different redshift ranges show that we are living in a significant underdensity out to at least 300 Mpc (Keenan+ 2013). I previously showed that this KBC void is incompatible with ΛCDM, but it can plausibly induce outflows sufficient to inflate the locally measured expansion rate by about 10% (Haslbauer+ 2020). This detailed study used a semi-analytic MOND cosmological model as a guide to what might happen if structure formation is faster than expected in ΛCDM, as would be needed to form the KBC void. A generic consequence is that peculiar velocities would also be faster than in ΛCDM. It has recently become possible to test this thanks to observations of the bulk flow, the average velocity of matter in a sphere of fixed radius centred on our location (Watkins+ 2023). Plotting the bulk flow against the considered radius leads to a bulk flow curve, which however disagrees significantly with ΛCDM expectations independently of the local expansion rate. I found the predicted bulk flow curve in my previously published model using techniques similar to those used by observers, with Watkins acting as referee. Out of six combinations of void density profile and our location within the void, two were found to yield a good match to the observed bulk flow curve. These results suggest that structure grows faster than expected in ΛCDM on scales of tens to hundreds of Mpc. This could be a promising new approach to cosmology given that the Hubble tension is observationally confirmed by JWST and cannot easily be solved by modifying physics prior to recombination.

Conversation article about my work:

Slides:

Paper (open access):

[anindya]

I started following you after reading your last paper about MOND. It was amazing.

[deathwarmedover]

I'm also intrigued by Kate Brown at Case finding that the possibility is MOND is responsible for "Planet Nine" effects in the Kuiper Belt.