Description:

Abstract: The Hubble tension between measurements of H0 in the early universe vs. nearby can be resolved by a brief episode of dark energy contributing about 10% of cosmic energy density at redshift z ∼ 3500.  This can be tested a lower redshifts — for example, N-body simulations have recently shown that this Early Dark Energy scenario predicts earlier structure formation, e.g. 50% more clusters than ΛCDM at redshift z ∼ 1.  Galaxies were long thought to start as disks, but HST images show that most galaxies instead start prolate (pickle shaped). Galaxy simulations can explain this as a consequence of the filamentary nature of the ΛCDM dark matter distribution.  But comparisons between simulations and observations reveal other potential challenges, for example concerning long-lived massive star-forming clumps.  Earth may be a radioactively Goldilocks planet, with just the right amount of radiogenic heating by the long-lived radioactive elements thorium and uranium for plate tectonics and a magnetic field, both of which may be necessary for the evolution of complex life. The abundance of these r-process elements in stars and their planetary systems varies significantly since the neutron-star mergers that produce these radioactive elements are very infrequent.  A factor of 2 increase would have stopped Earth’s magnetic dynamo for hundreds of millions of years and also caused widespread vulcanism. A factor of 2 decrease could have stopped plate tectonics.  Radiogenic heating may thus be an important factor in rocky planet habitability.

Speaker: Joel Primack, Distinguished Professor of Physics Emeritus, University of California, Santa Cruz