As we continue to navigate the cosmic web of our existence by way of each new discovery and subsequent understanding, we find ourselves a perennial victim of the old adage which dictates that the more we learn, the less we know.
And while we continue to see farther and clearer out into the black waters of space, we become increasingly capable of filling in the more trivial and palpable of the blanks, but it only seems to uncover further mystery, drawing our journey out and harping at our inherent curiosity in mind-bending new ways.
It should perhaps be asked whether the refinement of our technological instrumentation and the cultivation of greater understanding equates to a fundamental shifting of our perspective, though we can’t really tell until we’re steeped in enough retrospection.
Then, every once in a long while, we’re swept off our feet with the kind of existential discovery that results in the most guttural of paradigm shifts, prompting a frenzy of repercussive sequelae that change the fundamental dynamics by which we view our place in the this strange world.
This is where Margaret Geller comes in, along with the monumental discovery made by her and her teammates during a tenure as a Harvard astrophysicist.
What Margaret and her team came to find, as they set out to map the observable universe, would forever change the way we perceived our own universe.
Q: How would you describe the realization you experienced when you had first noted the galactic patterning of the universe - what it had meant to you and what you believe it meant to the astrophysics community as a whole?
In the mid 1980s when my colleagues and I thought about mapping a slice of the universe, the ideas about patterns in the distribution of galaxies were very different from what they are today. The idea of mapping the universe was in its infancy. Mapping the universe is a big idea, but like many big ideas it started in a small way.
In the mid 1980s, people knew there were clusters of galaxies. The general picture was that these clusters were more or less randomly distributed in a sea of randomly distributed individual galaxies and small groups of galaxies.
I wondered whether there were larger patterns, continents and oceans of the universe. By thinking about the continents and oceans on earth, I realized that a thin strip around the earth crosses both continents and oceans…and reveals that both kinds of features are big. The 3D analog of a strip is a slice. Mapping a slice of the nearby universe was a manageable project with a 1.5 meter telescope.
I suggested to my colleague, John Huchra, and a graduate student, Valerie de Lapparent, that we measure redshifts for a thousand galaxies in a strip across the sky. We all agreed that this project would be interesting.
At that time, acquisition of redshifts was a slow process. We observed one galaxy at a time and each observation took half an hour or more. To measure a redshift, we use a diffraction grating to disperse the light from a distant galaxy into its colors. The result is a spectrum, a rainbow of galaxy light. This spectrum contains the telltale signature of the elements in the galaxy. The expansion of the universe shifts these features to redder, longer wavelengths relative to their positions in a laboratory on earth. By measuring the redshift we can derive the distance to each galaxy. In the nearby universe we explored, the distance is simply proportional to the redshift. Once we have the redshift we have a three-dimensional position for each galaxy: we have the latitude and longitude on the sky and the distance from us. With these coordinates we can construct our 3D map.
With about 1000 redshifts in hand, Valerie de Lapparent plotted the first ever map of a slice of the universe. We were all in speechless awe when we saw it. All of the galaxies are in thin structures surrounding vast nearly empty regions. The empty regions are more than two hundred million light years across. The overall pattern in the now iconic map looks like a dancing stick figure. We called the overall structure “bubble-like” to give a visual picture of the broader implications.
It was thrilling to be one of the first three people ever to see this amazing and beautiful pattern. I will never forget that feeling of wonder and awe.
Q: And what has it meant to our collective understanding regarding the structure of our universe?
Soon scientists and members of the public all over the world were sharing the amazement. The presence of a clear pattern inspired both observational and theoretical investigations of the nature of the patterns and their history. John Huchra and I extended our survey to include 15,000 galaxies. The resulting map revealed the then largest known structure in the universe, the Great Wall. The Great Wall stretches across the universe for about seven hundred fifty million light years; its record size was later outdone by the Sloan Great Wall.
The accessibility of our pioneering results attracted media attention all over the world. The maps appeared in every astronomy textbook. The first slice became an icon of twentieth century physics. Posters for scientific meetings included the slice along with the COBE map of the microwave background that reveals the initial seeds of the structure we see. The slice and the COBE map are the foundation for our understanding of what we now call the cosmic web, the pattern of thin filaments and walls of galaxies that defines our universe.
The arresting pattern in the slice has inspired artists. Most recently Jasper Johns included the map in his painting called Slice. This painting was displayed at the major Johns retrospective at the Whitney Museum in 2021–22. The story of patterns in the universe is part of our story. We cannot exist without structure in the universe and our curiosity and technology enable us to uncover the pattern and to understand it.
Q: So how does dark matter fit into the equation?
Dark matter in the universe is an enduring mystery. The puzzle has been with us since 1937 when Fritz Zwicky first applied Newton’s laws to a few galaxies in the Coma cluster of galaxies. The total mass he derived substantially exceeded the sum of the masses of the stars in the cluster galaxies.
We now know a lot about where the dark matter is but we still don’t know what it is. My colleague and friend, Vera Rubin, played a central role in discovering where the dark matter is. She measured the rotation velocity of material at large radius within spiral galaxies. Surprisingly she found that the velocity remains constant at these large radii. This behavior implies that the mass of the object increases in proportion to the radius. In contrast, the galaxy light decreases steeply as the radius increases. The natural conclusion that has stood the test of time is that the galaxies we observe nestle within extended haloes of dark matter.
The galaxies we observe in our maps of the universe trace the distribution of dark matter. There are a number of ways to measure the amount of dark matter and to discover how well the galaxies trace it. The first is to extend Zwicky’s use of dynamical measurements in systems of galaxies. Where he had measurements for a handful of galaxies, we now have hundreds or even thousands.
A powerful modern approach to measuring the distribution of matter in the universe is gravitational lensing. Galaxies and systems of galaxies act as lenses that magnify and distort the images of more distant objects. Images with Hubble Space Telescope and with ground-based large telescopes like Subaru on Mauna Kea enable a host of detailed measurements.
We now know that about eighty-five percent of the matter in the universe is dark. We also know that on large scales the galaxies we see trace the distribution of the matter we can’t see.
Q: Can you detail a bit about your work with the SHELS and HectoMAP projects?
Among its many goals the HectoMAP survey uses gravitational lensing to compare the distribution of matter in the universe traced by galaxies with the distribution of dark matter. HectoMAP, carried out on the 6.5 meter MMT is a 100,000 galaxy survey of the middle aged universe.
When we look out in space we look back in time. Thus we can observe nearly the entire history of the universe from the age of about 400,000 years when the tiny ripples in the cosmic microwave background provide a picture of the initial irregularities that grow under the influence of gravity to become the patterns we observe al later times.
The HectoMAP survey shows us the universe from an age of about 8 billion years to its current age of nearly fourteen gigayears. We see a similar intricate pattern of dark empty regions or voids surrounded by galaxies in thin walls and filaments. As the universe ages clusters of galaxies and voids grow. With HectoMAP we measure the way clusters accrete mass and compare that growth with simulations to test our understanding of the formation of structure in the universe. The agreement between the data and the models is excellent.
The HectoMAP survey covers a region where there are also deep images taken with HyperSuprimeCam, currently the largest camera in the world. The camera is a prime focus instrument on the Japanese Subaru telescope on Mauna Kea, Hawaii.
HectoMAP reaches just far enough into the universe to optimize the comparison between the patterns traced by the galaxies and the map of the entire matter distribution traced by weak lensing. We have just begun to make that comparison. We have used the redshifts and weak lensing to measure the masses of clusters of galaxies and to derive their dark matter content. As others have found, both methods agree. HectoMAP enables comparison of the galaxy and weak lensing maps. Our adventure in making that comparison is now under way.
During my career the advances in our ability to map the universe have been astounding. For our first slice of the universe, we observed galaxies one at a time. It took twenty-five minutes to measure a redshift for these nearby galaxies. With the MMT we measured two hundred fifty redshifts in an hour and a half. The galaxies are more than ten times as far away.
The change between the first slice and HectoMAP pales in comparison with the DESI Project now underway on the 4-meter telescope on Kitt Peak, Arizona. The DESI instrument enables measurement of five thousand redshifts in a single observation.
The DESI map will be more sparse than HectoMAP but it will reach to earlier epochs in the history of the universe.
More than four hundred people work on DESI, an amazing contrast with the three of us who had the first glimpse of the largest patterns in nature.
Of the numerous questions that bubble up regarding Margaret’s findings, the most pertinent for me considers how her discovery functions to change our fundamental perspective regarding the ways by which reality itself is structured, and the underlying patterns to the fabric of our observable universe that we’re slowly beginning to uncover.
We’re equipped with minds that incessantly pattern our observable world; we have an innate need to map anything and everything, from cartographical endeavors to neurological functions, and we continue to connect the kind of dots that leave us bewildered all the more.
And as we begin to decode the numerous patterns around us — from the Fibonacci sequences of small plants to the gravitational assemblies of distant galaxies, we’re left not so much discouraged as much as we are motivated for more answers.
Thanks to Aldebaran S. for the photo.
To stay up to date with the work Margaret or her team, or to learn more about her work: