In the study of the universe, size is a relative term. But last year’s scientific discovery of the largest known super-structure in the universe by College of Charleston Astronomy Professor Jon Hakkila and his collaborators has left the researchers and the scientific community dumbfounded.
Hakkila explains how the team identified the Hercules-Corona Borealis Great Wall, why it matters and what its implications could be for our understanding of the universe.
Q: What are the implications of this discovery?
A: The discovery has two major implications. The first is that gamma-ray bursts can be used to study the large-scale structure of the Universe. Gamma-ray bursts are the most luminous sources known, and as such they can be seen over tremendous distances. Thus, gamma-ray bursts can be detected even if the galaxies in which they reside are too faint to be observed. Most gamma-ray bursts are thought to originate in hypernovae, which are beamed supernovae occurring when massive stars die. If you take the immense energy of an exploding star and focus it into a narrow beam, then the light from that beam will be significantly brighter than that of a normal supernova. Gamma-ray bursts should be luminous enough to be seen back to the time that stars first formed, making them ideal tracers of the distribution of matter in the Universe. The second implication of the discovery is that our understanding of universal large-scale structure might need to be modified.
Q: You’ve said that this discovery presents a conundrum because it’s not supposed to be possible. Can you explain this conundrum?
A: One of the standard hypotheses of cosmology (the science describing the structure, origin, and evolution of the Universe) is that the Universe is isotropic (there is no preferred direction) and homogeneous (matter is distributed uniformly everywhere on a large scale). The scale on which the Universe is homogeneous has been thought to be several hundred million light years; things should be smooth on scales larger than this. This observation appears to violate the concept of Universal homogeneity. The nature of science is to propose and test hypotheses based on observational and/or experimental evidence. There is always uncertainty because measurements are not exact. Here we are using a sample of several hundred well-measured gamma-ray burst locations to infer the properties of much larger underlying structures. Our uncertainty would be much smaller if our sample were itself larger, such as several thousand or several million gamma-ray bursts. However, with a detection rate of about one gamma-ray burst per day, and a smaller rate of measuring distances to them, it will take a very long time for us to compile a sample that large.
Q: Can you put the structure’s size and distance into perspective?
A: The structure is about 10 billion light years across (and probably only about a billion light years thick), and is at a distance of about 10 billion light years. That means that it takes 10 billion years for light to travel from one end of the structure to the other, and it has taken about 10 billion years for the light emitted by the structure to reach us. To put this in perspective, it takes light from the Sun about eight minutes to reach us, light from the nearest star over four years to reach us, and light from the nearest large galaxy (Andromeda) only about 2.5 million years to reach us. This structure is very far away, and thus we are seeing a structure that existed long ago, when the Universe was much younger (only about 4 billion years after the Big Bang).
Q: How does this discovery square with the Big Bang theory?
A: This observation does not contradict most evidence for the Big Bang (the idea of an expanding, evolving Universe where every location was once the center). The evidence for the Big Bang is overwhelming. Rather, this observation is confusing because it is inconsistent with models of how quickly the Universe has expanded, and with how much early Universe information has been carried as the Universe evolves.
Q: When did this research take place and with whom did you collaborate?
A: This work was done last year during my sabbatical leave. My collaborators on this project are Dr. Istvan Horvath and Dr. Zsolt Bagoly of the National University of Public Service in Budapest, Hungary. The three of us have known each other and have worked on related gamma-ray burst projects for 20 years now, and this seemed like a great opportunity for collaboration. We are now working on several follow-up papers, with some additional coauthors (mostly Hungarian).
Q: What kind of equipment did you use?
A: This is a statistical data analysis study, so we primarily used computers. However, the data collection process that produced our database involved hundreds of scientists, several orbital satellites, and decades of work. The gamma-ray bursts were primarily observed by NASA’s Swift orbital satellite, although many observations were made by other instruments and orbital experiments including NASA’s Compton Gamma-Ray Observatory, the joint Italian-Dutch satellite BeppoSAX, the European Space Agency’s INTEGRAL satellite, and NASA’s Fermi satellite. The ground-based observations leading to redshift measurements occurred at orbital- and ground-based observatories all over the globe, supported by many different countries and international consortia.
Q: Do you believe that there are even larger structures out there?
A: I would have thought this structure was too big to exist. Even as a coauthor, I still have my doubts. However, we think that there is only a very small chance (significantly less than one in a hundred) that we would randomly have detected so many gamma-ray bursts in one sky location. Thus we believe that the structure exists. There are other structures that appear to violate Universal homogeneity: the Sloan Great Wall and the Huge Large Quasar Group (discovered last year) are two. Thus, there may very well be others, and some could indeed be bigger. Only time will tell.