It wasn’t long ago that we thought our own galaxy was the entire universe. We know now, of course, that we live in an ocean of countless galaxies. A recent estimate of the number of galaxies in the known universe, based on deep sky Hubble imagery, increased the count from about 200 billion to more than 2 trillion!
Two hundred billion is an already unimaginable number, and increasing that to 2 trillion just makes it unimaginable squared. But as our understanding of the largest scale of the universe is improving, many mysteries remain, some of which are touched on in the interview below with professor Brent Tully from the University of Hawaii’s Institute of Astronomy.
The standard model of cosmology is based on Albert Einstein’s general relativity, the Big Bang and subsequent inflation, dark matter and dark energy, among other concepts. As successful as the standard model has been, there are still serious questions and anomalies to be resolved.
Tully has specialized in studying the local cosmic neighborhood, which encompasses many thousands of galaxies — his last major catalog, CosmicFlows3, included about 18,000 galaxies and related objects — and the ways in which our neighborhood is changing over time.
Tully and his team made global news in 2014 with new data and modeling on the Laniakea Supercluster of galaxies, which is centered on what was formerly described as the Great Attractor and consists of about 100,000 galaxies. We live in Laniakea, so it’s exciting to see this new data come to light and inspire not only scientists but also the general public. It’s also been gratifying to see Hawaiian culture come to the fore with the name Laniakea (“immense heaven”), among many recent astronomical names borrowed from the Hawaiian tongue.
Tully has been a professor at the Institute of Astronomy since the 1970s. I met Tully, as I have many other astronomers and cosmologists, at the Imiloa Astronomy Center in Hilo, a true community resource for interesting talks and planetarium shows all year. We conducted this interview via email in mid-2017.
Tam Hunt: What inspired you to study galaxies for a living?
Brent Tully: When I was looking for a Ph.D. thesis topic around 1967, extragalactic astronomy was wide open. Little was known. I felt that I didn’t have to be so smart to make a contribution.
TH: You’ve made the news in recent years with your work identifying our local supercluster of galaxies that you’ve called Laniakea. What is Laniakea, and how did you find out about this new large-scale structure of our universe?
BT: While it was known that galaxies collect in large structures, the meaning of “supercluster” was not well-defined. We were mapping the flow patterns of galaxies and saw that flows are bounded within separated basins of gravitational attraction, analogous to river watersheds on Earth. We called the one that we live in the “Laniakea Supercluster.”
TH: Another important news item this year was the announcement about the first neutron star collision observed with gravitational waves. What does this new finding tell us about our universe and our current physical theories?
BT: This is a broad question and not one that I have any expertise in. It was a big surprise to observe two black holes collide; the occurrences are much more frequent than we could have imagined. Now we have seen neutron stars collide. This was much more expected. It has been fantastic in the case, though, because we have simultaneously observed phenomena of the collision over a wide range of wavelengths. The event was rather close, in an environment that we can study. The event has already given us a lot of insight into a dramatic physical process.
TH: Is there a single book or books that you would recommend for someone not willing to pursue a Ph.D. in cosmology that would provide a good understanding of current science?
BT: I don’t give attention to books for the general reader. I can recommend Scientific American.
TH: One of your main contributions to physics is the Tully-Fisher relation that suggests a regular relationship between intrinsic luminosity of a spiral galaxy and its mass, allowing distances from us to be calculated based on apparent luminosity. How does this work, and how confident are you that this method yields a generally accurate conclusion in most cases? Does this relationship mesh well with the new LIGO/VIRGO-based Hubble Constant calculations (expansion of around 70 kilometers per megaparsec per second)?
BT: The basic idea is simple: The equilibrium rotation speed of massive galaxies is higher than in less massive galaxies, and massive galaxies have more stars so are intrinsically brighter. Hence, there naturally is a relation between the brightness of a galaxy and its rate of rotation. The calibration of the relation is done by considering nearby cases with distances established by other methods. The agreement with the new LIGO/VIRGO estimate is excellent.
TH: Your initial Tully-Fisher relation 1977 paper estimated the Hubble Constant at 80 kilometers per megaparsec per second (km/MPar/s), substantially higher than the 70.0+12.0-8.0 calculated by the recent LIGO results, or the 73.00±1.75 calculated in Riess et al. 2016, and also your 2016 CosmicFlows3 estimate of 75±2. Planck and SDSS have calculated values around 67. Some estimates before 1950 were over 200. So it seems that estimates of the constant have consistently been falling in the past few decades. If so, do you see it falling further as new data comes in? Is there a reason for this apparent steady fall in our best estimates of the local value of the Hubble constant?
BT: There were problems that are indisputable with the early estimates of the Hubble Constant so they should not be part of the discussion. Fisher and I offered a promising new way to measure distances in 1977, but the reference zero-point (the distances of our calibrators) was very poorly known. Our initial estimate of the constant was very tentative. The current estimates are much more strongly anchored. There is a correct value for the Hubble Constant that the universe knows and it is for us to find out. It is idle talk to speculate on the value based on past trends.
TH: How does redshift allow us to judge cosmological distances? Does this technique mesh well with the Tully-Fisher relation?
BT: The relationship between redshift and distance (the Hubble Law) needs a “zero-point” calibration. If everything is done properly, then Tully-Fisher (TF) distances and redshift distances mesh on average. In detail, there may be local departures from the Hubble Law that TF distances pick up but redshift distances miss.
TH: As marvelous as our growing understanding of the universe is, it’s no secret that there are a number of serious problems with today’s standard model of cosmology (known technically as the lambda Cold Dark Matter or LCDM model), such as the dark matter problem (identifying what exactly dark matter is), the flatness problem (why is our universe apparently almost perfectly flat despite this being a highly unusual configuration?), the “small-scale crisis,” the horizon problem, problems with inflation theory identified by Paul Steinhardt and others, and many other problems. Given all of these ongoing problems, are you confident that we’re generally on the right track with the standard model?
BT: In the classroom, I speculate to students about the probability that what I am telling them today would be what I would be telling them a hundred years from now. That probability depends on the specific topic. Overall, I consider that we are on the right track. Thirty years ago, the standard model held that the universe had “critical” density of matter sufficient to stop expansion with no dark energy. It was evident to me that this model was critically flawed. The current paradigm is much more robust to observations. Obviously, our lack of understanding of dark matter and dark energy are huge unknowns, and the biggest question of all remains concerning the birth of the universe.
TH: The dark matter problem has remained particularly pesky since there has been no good evidence of what dark matter actually is, decades after the search began in earnest. At what point do you think the failure to find any of the various candidates for dark matter should lead to a serious rethinking of the dark matter hypothesis?
BT: It is always time to consider alternatives to the existing popular hypothesis. People are doing that. Maybe the dark matter hypothesis is wrong, but to me it remains the most plausible idea among those currently on the market.
TH: I read and hear from various astronomers that it seems we may be missing something fundamental in the current standard model. For example, Lee Smolin writes about in both his 2006 book, The Trouble with Physics, and his 2013 book, Time Reborn, that perhaps we are missing something important about the nature of time. Do you have an opinion on whether we are indeed missing something fundamental or not in our current approach to understanding the universe?
BT: Obviously, we don’t have a complete understanding of the universe, given that we postulate the existence of dark matter and dark energy without knowledge of what is giving rise to either. And whatever was going on at the initiation of our universe is complete speculation. That said, I don’t understand Smolin’s concerns.
TH: In discussing the distances of galaxies and our cosmos more generally, it’s hard not to implicate Halton Arp and his work on anomalous redshifts. Arp is well-known for his 1966 Atlas of Peculiar Galaxies, a respected early text in the field, but today he is often dismissed as a crank because of his insistence that redshift anomalies represented serious issues with the notion of redshift being caused only through cosmological expansion (Hubble’s Law). It’s not well-known that he compiled literally hundreds of anomalous redshifts and published in respected journals like The Astrophysical Journal on many of his findings up until his death in 2013. What is your view of Arp’s work on anomalous redshift objects? Was he chasing phantoms, or is it possible that there are factors behind redshift, as Arp suggested throughout his career, other than cosmological expansion?
BT: He was chasing phantoms.
TH: Could you explain a little why Arp’s numerous examples of discordant redshifts (collected in one place in his last book, published in 2003, Catalogue of Discordant Redshift Associations) should all be considered phantoms? Would you agree that even one reliable case of discordant redshifts falsifies Hubble’s Law or at least renders it only one factor in determining redshift?
BT: I agree that one reliable example of an object with a very high redshift shown to be nearby would challenge the standard picture that redshifts are correlated with distance. I am not aware of any such example. Any of Arp’s examples that I have looked at are unpersuasive and usually ridiculous.
TH: The general view of science is that it proceeds through observation, hypothesis, then testing that leads to theory creation, and then ongoing testing and elaboration. Hypotheses and theories that lead to incorrect predictions are thought to be falsified. There has been a lot of debate in the philosophy of science community, however, about how accurate this version of events is, with Popperians defending this general view of orderly data gathering and theory testing, and Kuhnians (The Structure of Scientific Revolutions and other works) and others arguing that how science really operates is far less logical, incremental and evidence-based. Many have also suggested that funders (public and private), and the need for obtaining funding with a limited funding pie available, have far too much sway in how science is done. Do your decades as a scientist lead you to believe that the development of cosmological science has been generally fairly orderly and evidence-based, or do you think the Kuhnians have a good point about the more messy version of how science is actually done?
BT: I’ll opt for something in between. Surely we have advanced a long way through a process that is logical, incremental and evidence-based. But the process is messy, with many trips with dead ends (like Arp’s), frequent rediscoveries of unappreciated ideas, and heavy weighting of the voices of pundits with access to the press at the expense of workers in isolated environments.
TH: What is the role of amateur astronomers today in terms of making real scientific breakthroughs? Is this still within reach even though amateurs don’t have access to the massive telescopes that professional astronomers can use (albeit with a lot of competition for time)?
BT: Amateurs are extreme examples of workers in isolated environments.
TH: Looking at your latest work, your papers mapping the dynamics of local galaxies and clusters of galaxies, you use a “gravity shed” approach that mirrors watershed models of terrestrial topography. Could you explain briefly how you calculate your gravity sheds?
BT: We measure the motions of galaxies, in particular, the departures in motions from the overall expansion. We assume, following the standard model of the growth of structure, that these deviant motions arise from gravitational attractors. The various flow patterns are leading to the bottoms of gravitational wells, just as water at different locations flows downhill in whatever different course.
TH: If we come to realize over time that redshift is not caused only by distance (and the expansion of space itself), do you expect that your conclusions regarding the nature of our local gravity sheds will change very much?
BT: Of course, you can hypothesize that our basic understanding of redshifts is flawed and, if so, our observations have to be re-evaluated. That’s the way that science works. Parenthetically, we know that there are other factors affecting redshifts, like local deviant motions and gravitational effects. But the amplitudes of these effects are not large on a cosmological scale. No, I don’t expect these factors to strongly alter the descriptions of gravity sheds.
TH: How would you gauge the chances of large-scale galaxy dynamics being influenced by factors other than gravity, such as electromagnetic forces, in a significant manner? Does the filamentary nature of the large-scale structure of our universe lend any support to the notion that electromagnetic forces play a role at this scale?
BT: Electromagnetic forces can be important on the small spatial and mass scales of jets, but there is no credible evidence that such forces play a role on scales of megaparsecs and trillions of solar masses.
TH: Last, what are you most excited to discover about the big cosmological questions in the coming decade or two?
BT: I would be happy to convincingly identify the full extent of the patch of the universe that we live in that constitutes a “fair sample,” a volume that on average is representative of the universe today. This fair sample patch would have average density and an average representation of galaxies and would be at rest in the reference frame of the cosmic microwave background. Evidently, though, this local fair sample volume is surprisingly large. Our observations are still too limited. It remains to be seen if what we might eventually find challenges the standard Lambda CDM cosmology.
— Tam Hunt is a lawyer and writer.