In the 1920s, astronomers learned that the universe was expanding, as predicted by Einstein’s general theory of relativity. This led to a debate among astrophysicists between those who believed the universe began with a big bang and those who believed the universe existed in a steady state. In the 1960s, the first measurements of the cosmic microwave background (CMB) indicated that the former was the most likely scenario. And in the 1990s, the Hubble Deep Fields provided the deepest images of the Universe ever recorded, revealing galaxies as they appeared just a few hundred million years after the Big Bang.
Over time, these discoveries led to an amazing realization: the rate at which the universe is expanding (aka the Hubble constant) was not constant over time! This led to the theory of dark energy, an invisible force that counteracts gravity and accelerates this expansion. In a series of articles, an international team of researchers led by the University of Hawaii reported that black holes in old and dormant galaxies were growing faster than expected. This represents (they claim) the first evidence that black holes could be the source of dark energy.
The research was conducted by astronomers and astrophysicists from the University of Hawaii, the Kavli Institute for Cosmological Physics, the Enrico Fermi Institute, the European Southern Observatory (ESO), the Netherlands Institute for Space Research (SRON), and the National Radio Astronomy Observatory ( NRAO), the Instituto de Astrofísica e Ciências do Espaço (IA), the Mitchell Institute for Fundamental Physics and Astronomy and several universities. Their findings appeared in two articles published in The Astronomical Journal and The Astronomical Journal Letters.
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According to the most widely accepted model of the universe, dark energy accounts for 68% of the mass-energy content in the universe. This theory revived an idea Einstein had proposed but later discarded – that there is a “cosmological constant” (represented by the scientific symbol delta) that “holds back” gravity and prevents the universe from collapsing in on itself. The force and dark matter (which accounts for 26.8% of the mass-energy content) are integral parts of the most widely used cosmological model today, known as the Lambda-Cold Dark Matter (LCDM) model.
The main argument behind dark energy is that there is a special type of energy in spacetime (called vacuum energy) that is pushing the universe apart. There are a few problems with this theory, however, not the least of which is that there is no direct evidence of this mysterious energy. Furthermore, while this vacuum energy is consistent with quantum mechanics, all attempts to calculate it using quantum field theory have dried up. There is also the question of how this energy coincides with supermassive black holes (SMBHs) in our Universe.
By the 1970s, astronomers had determined that the persistent radio source at the center of our galaxy (Sagittarius A*) was a black hole with a mass of 40 million suns. Further observations showed that most massive galaxies had SMBHs in their core region, giving rise to active galactic nuclei (AGNs) or quasars. The extremely strong gravity of SMBHs causes surrounding matter to fall in around them, forming accretion disks and powerful relativistic jets in which matter is accelerated to near the speed of light (releasing enormous amounts of radiation in the process).
The presence of these mammoths at the centers of the most massive galaxies would require an extremely powerful force to counter them. This is especially true when dealing with singularities, which are thought to exist at their cores, where the very laws of physics break down and become indistinguishable. This led to an exotic theory known as “cosmological coupling,” which states that SMBHs could have tremendous vacuum energy and that they are the reason the universe is expanding.
In their paper, Duncan Farrah’s team (an astronomer at the University of Hawai’i at Manoa and a former PhD at Imperial College) reports the first observational evidence that black holes are gaining mass in a manner consistent with this That They Contain Vacuum While astrophysicists have searched for a theoretical solution to the problem of dark energy and black holes, the team’s findings reportedly represent the first observational evidence that black holes are the source of dark energy.
If true, the finding renders the formation of singularities at the core of black holes obsolete and resolves a long-standing debate. It also means that nothing else is needed (no new forces or modified theories of gravity) for our cosmological models to make sense. dr Chris Pearson, a researcher from RAL Space, a research council overseen by the UK’s Science and Technology Facilities Council (STFC), and Dr. Dave Clements from the Department of Physics at Imperial College co-authored the studies.
“If the theory is correct, then it will revolutionize the field of cosmology because we finally have a solution to the origin of the dark energy that has puzzled cosmologists and theoretical physicists for more than 20 years,” Pearson said in an RAL Space press release. “This is a really surprising result. We started by studying how black holes grow over time and may have found the answer to one of cosmology’s biggest problems,” added Clements.
… call for extraordinary evidence
The team reached this conclusion by studying the evolutionary history of SMBHs at the centers of giant elliptical galaxies. This refers to a type of “early galaxy” that formed early in the Universe and has since stopped forming stars (also known as “dormant galaxies”). Decades of observations have shown that black holes can increase their mass in two ways: by accreting matter or by merging with black holes. As they indicated in their first work, the team studied giant elliptical galaxies at redshifts less than z<2 (as they looked nine billion years ago).
Artist’s impression of merging black holes in the early Universe. Photo credit: LIGO/Caltech/MIT/R. injured (IPAC)
These dormant galaxies have little material left for their SMBHs to accrete, meaning further growth cannot be explained by the two mechanisms above. The team then compared observations of these elliptical galaxies – which still appear young – to local galaxies dating back to around 6.6 billion years ago, which have since been inactive. These observations showed that the SMBHs were 7 to 20 times larger than nine billion years ago, much larger than predicted by accretion or mergers.
In their second paper, they also note how measurements of related galaxy populations at different points in their evolution (about 7.2 billion years ago) showed a similar correlation between the mass of SMBHs and the size of the Universe. This represents the first evidence of “cosmological coupling”, showing that the expansion of the universe and the growth of SMBHs are related. If confirmed by further observations, it could effectively redefine our understanding of the universe and the nature of black holes. As Farrah concluded:
“We’re actually saying two things at once: that there’s evidence that the typical black hole solutions don’t work for you on a long, long timescale, and that we have the first proposed astrophysical source of dark energy. That doesn’t mean that other people haven’t suggested sources of dark energy, but this is the first observational paper in which we don’t add anything new to the universe as a source of dark energy: black holes in Einstein’s theory of gravity are the dark energy.”
Correlation, not coupling?
Of course, these claims were met with some skepticism by the astronomical/astrophysical community. In particular, the authors’ claim that their observations provide evidence of linkage has been questioned for confusing correlation with causation. Astrophysicist, author, science communicator, and Forbes Senior Contributor Ethan Siegel addressed this in a recent issue of Ask Ethan — a special series in his Starts with a Bang! column, in which he answers questions from the audience. In examining their research, Siegel notes that the authors’ conclusions rest on an important assumption.
Expansion of the universe from the big bang to today. Image Credit: NASA/WMAP Science Team
This assumption is that there is “a universal relationship between the mass of the central black hole and the mass of the stars within a galaxy that may evolve over cosmic time, but should be universal at any given time.” From this, they compared the SMBHs they selected for their sample data to see if there is a “coupling parameter” (represented as k) that has the same value over cosmic time. In the end, the team found that k has a non-zero value with 99.8% confidence. Although seemingly compelling, this conclusion boils down to an assumed relationship. As Ethan concluded:
“The authors assume the existence of a coupling that does not exist and attribute the perceived evolution of black hole-star mass ratios to coupling as these galaxies and their black holes evolve. Because we’re only measuring each galaxy in a “snapshot” in time, we have no way of knowing how an individual object is evolving, and with this particular method, the authors of the article fool themselves and, by extension, anyone who believes them.”
At the risk of repeating the overused adage, “Extraordinary claims require extraordinary evidence.” The ability to repeatedly verify results is one of the most important criteria for evidence to be considered sound. This means that results must be verifiable again and again and (preferably) using different methods. The authors recognize this and hope that repeated observations will confirm it. But for now, the claim they made remains exceptional and (given the implications) warrants further investigation.