Are LIGO gravitational wave observations of merging black holes an illusion? In 1966, Erast Gliner, a young physicist at the Ioffe Physico-Technical Institute in Leningrad, proposed a hypothesis that very large stars should collapse into what could now be called Generic Objects of Dark Energy (GEODEs). GEODEs appear to be black holes when viewed from the outside, but unlike black holes, they contain dark energy instead of a singularity.
Fast forward to research in 2019 at the University of Hawaii at Manoa: physicists usually assume that a cosmologically large system, such as the universe, is insensitive to details of the small systems contained within it. Astrophysicist Kevin Croker and mathematician Joel Weiner have identified and corrected a subtle error that was made when applying Einstein’s equations to model the growth of the universe. They show that this assumption can fail for the compact objects that remain after the collapse and explosion of very large stars.
General Relativity Can Observably Connect Collapsed Stars
“For 80 years, we’ve generally operated under the assumption that the universe, in broad strokes, was not affected by the particular details of any small region,” said Croker. “It is now clear that general relativity can observably connect collapsed stars–regions the size of Honolulu–to the behavior of the universe as a whole, over a thousand billion billion times larger.”
New Observational Predictions
Croker and Weiner demonstrated that the growth rate of the universe can become sensitive to the averaged contribution of such compact objects. Likewise, the objects themselves can become linked to the growth of the universe, gaining or losing energy depending on the objects’ compositions. This result is significant since it reveals unexpected connections between cosmological and compact object physics, which in turn leads to many new observational predictions.
In the simulated image at the top of the page, the GEODE scenario does not change standard formation of structure in the Universe. Universe grows from left to right. Blue regions correspond to matter. GEODEs form in green regions and migrate into black regions. (University of Hawaii)
Formed from Gravitational Collapse of Massive Stars
The Event Horizon Telescope imaged the supermassive compact object below, called Powehi, at the center of galaxy M87. The supermassive compact object might actually be a GEODE, as the paper suggests. The Powehi GEODE, shown to scale, would be approximately 2/3 the radius of the dark region imaged by the Event Horizon Telescope. This is nearly the same size expected for a black hole. The region containing Dark Energy (green) is slightly larger than a black hole of the same mass. The properties of any crust (purple), if present, depend on the particular GEODE model. (EHT collaboration; NASA/CXC/Villanova University)
One consequence of this study is that the growth rate of the universe provides information about what happens to stars at the end of their lives. Astronomers typically assume that large stars form black holes when they die, but this is not the only possible outcome.
“GEODEs, Croker explained, “reduce to Kerr black holes on short timescales, and we expect gravitational collapse of massive stars to be their dominant formation channel. The additional contribution to dark energy density near the peak of cosmic star formation rate is expected to introduce a small correlation between the star formation rate and cosmological expansion rate. Such precision features may be accessible to next generation galaxy surveys such as the Dark Energy Spectroscopic Instrument (DESI).”
GEODEs Impact Expansion of the Universe
In 1998, two independent teams of astronomers discovered that the expansion of the Universe is accelerating, consistent with the presence of a uniform contribution of Dark Energy. It was not recognized, however, that GEODEs could contribute in this way. With the corrected formalism, Croker and Weiner showed that if a fraction of the oldest stars collapsed into GEODEs, instead of black holes, their averaged contribution today would naturally produce the required uniform Dark Energy.
In an email to The Daily Galaxy, Croker wrote, “GEODEs are an example of cosmologically coupled compact objects, solutions to General Relativity that respond to the universe’s expansion. What makes GEODEs useful cosmologically is that they are maximally coupled. They gain in mass exactly the same amount as they dilute in volume as the universe expands, giving a density that mimics a cosmological constant.”
The results of this study also apply to the colliding double star systems observable through gravitational waves by the LIGO-Virgo collaboration. In 2016, LIGO announced the first observation of what appeared to be a colliding double black hole system. Such systems were expected to exist, but the pair of objects was unexpectedly heavy–roughly 5 times larger than the black hole masses predicted in computer simulations.
A LIGO-Virgo Illusion?
Using the corrected formalism, Croker and Weiner considered whether LIGO-Virgo is observing double GEODE collisions, instead of double black hole collisions. They found that GEODEs grow together with the universe during the time leading up to such collisions. When the collisions occur, the resulting GEODE masses become 4 to 8 times larger, in rough agreement with the LIGO-Virgo observations.
Croker and Weiner were careful to separate their theoretical result from observational support of a GEODE scenario, emphasizing that “black holes certainly aren’t dead. What we have shown is that if GEODEs do exist, then they can easily give rise to observed phenomena that presently lack convincing explanations. We anticipate numerous other observational consequences of a GEODE scenario, including many ways to exclude it. We’ve barely begun to scratch the surface.”
The Last Word
“Recently, our collaboration has considered the possibility of cosmological coupling in the black hole populations observable to LIGO-Virgo,” Croker added in his email to The Daily Galaxy. “With no modifications to typical assumptions about isolated massive binary star evolution, we found that a modest coupling gave good qualitative agreement with the observed populations. Going forward, we anticipate that gravitational wave observatories will allow constraint of coupling strength versus composition through the study of merger rate evolution and differences between binary black hole and black hole-neutron star populations.”
“The James Webb Space Telescope, currently on its month-long journey to its L2 parking slot, will provide unprecedented measurement of the high redshift UV and optical flux, which are proxies for early star formation rate”, Croker told The Daily Galaxy. “These data can be used to determine whether GEODE production at high redshift is sufficient to lay the foundation for the observed dark energy density at the present day.”