Selections from 2019: Connecting the Universe’s Large and Small Scales


Editor’s note: In these last two weeks of 2019, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Implications of Symmetry and Pressure in Friedmann Cosmology. I. Formalism

Published August 2019

Main takeaway:

Two scientists at University of Hawai’i at Mānoa, Kevin Croker and Joel Weiner, have reexplored Friedmann’s equations — the set of equations that describe the expansion of the universe — under a more precisely specified set of fundamental assumptions. Using their revised formalism, they show that the universe’s growth rate can be influenced by the relatively small pressure contributions of compact objects left behind after a star’s death.

Why it’s interesting:

NS merger

According to the authors’ calculations, contributions from compact, relativistic objects like neutron stars, illustrated above, could affect the growth rate of the universe. [NASA/Goddard Space Flight Center/Dana Berry]

In previous studies, it’s been assumed that the universe’s matter is all alike and evenly distributed — an assumption that allows us to ignore the details of small structures like stars and galaxies when calculating the evolution of the universe as a whole. But Croker and Weiner’s calculations shows that the averaged contributions of massive, compact objects could affect the universe’s expansion rate after all — and in exchange, the universe’s evolution may affect the energy gain or loss of these compact objects over time. This work provides a new link between the small-scale structures and large-scale evolution of the universe.

What this work says about black holes:

Croker and Weiner’s model has an interesting side note: it has revived interest in an alternative picture of how we conceive of black holes. In the 1960s, Russian physicist Erast Gliner proposed that large stars would collapse into GEODEs — Generic Objects of Dark Energy — at the ends of their lifetimes. These objects would look like black holes from the outside, but on the inside, they would contain a bubble of dark energy instead of a singularity. Croker and Weiner have found evidence supporting Gliner’s hypothesis by demonstrating that if just a fraction of the oldest stars in our universe collapsed into GEODEs instead of black holes, the averaged contribution of these objects today would naturally produce the required uniform dark energy to produce the expansion of the universe we observe. In addition, collisions of GEODEs could naturally explain LIGO’s gravitational-wave observations.


K. S. Croker and J. L. Weiner 2019 ApJ 882 19. doi:10.3847/1538-4357/ab32da