The Mystery Of Our (Mostly) Missing Universe
By Judith E Braffman-Miller
Almost 14 billion years ago, our Universe burst into existence in the form of an unimaginably tiny soup of densely packed, searing-hot particles, commonly referred to as “the fireball”. Spacetime has been expanding–and cooling off–from this original brilliant, fiery, glaring state ever since. But what is our Universe made of, and has its composition evolved over time? It is often said that most of our Universe is “missing”, composed as it mostly is of a mysterious substance that we call dark energy. The elusive dark energy is causing our Universe to accelerate in its relentless expansion, and it is generally believed to be a property of Space itself. In August 2017, scientists announced that they now have a new window from which they can study our Universe’s mysterious properties, thanks to an international collaboration of more than 400 scientists called the Dark Energy Survey (DES), that is helping to shed new light on the secretive structure of our mostly missing Cosmos.
On large scales, the entire Universe appears the same wherever we look–displaying a foamy, bubbly appearance, with extremely heavy filaments that braid themselves around each other, weaving a web-like structure that is appropriately called the Cosmic Web. The filaments of the Cosmic Web shine with the fierce fires of a myriad of stars that outline enormous sheets and intertwining braids that host the starlit galaxies of the visible Universe. Immense dark, empty–or almost empty–Voids interrupt this weird, twisting, transparent web-like structure. The Voids contain few galaxies, and this makes them appear to be almost entirely empty. In dramatic contrast, the heavy starry filaments, that compose the Cosmic Web, weave themselves around these dark caverns creating what looks like a convoluted, twisted knot.
We live in a mysterious Universe–most of which we are unable to see. The galaxies, galaxy clusters and superclusters are all imprisoned in halos composed of invisible non-atomic dark matter. This unidentified material knits the heavy filaments of the great Cosmic Web into a remarkable tapestry that extends throughout all of Spacetime. Scientists are almost certain that the dark matter really exists because of its observable gravitational influence on those objects and structures that can be seen–such as stars, galaxies, and clusters and superclusters of galaxies.
The most recent measurements suggest that our Universe is composed of approximately 70% dark energy and 25% dark matter. As of today, the origin and nature of the mysterious dark matter and dark energy remain elusive. A much smaller percentage of our Universe is composed of the badly misnamed “ordinary” atomic matter–the familiar material that composes all of the elements listed in the Periodic Table. “Ordinary matter”–which is really extraordinary stuff–is comparatively scarce in the Cosmos. However, this runt of the Cosmic litter of three is what makes up the stars, planets, moons, people, and all of the rest of the Universe that human beings perceive as familiar. It is also the precious material that allowed life to emerge and evolve in our Universe.
However, the Cosmos may be even more bizarre than we are capable of imagining it to be. Modern scientific cosmology began with Albert Einstein who, in the early decades of the 20th century, applied his theories of Relatively—Special (1905) and General (1915)–to our “Cosmic habitat”. At the start of the 20th century, our Milky Way was believed to be the entire Universe, and it was also thought that the Universe was both static and eternal. However, we now know otherwise.
Our Universe does evolve in Time, and there is much, much more of the vast Cosmos than our own home Galaxy. It is generally thought that the Universe was born about 13.8 billion years ago, when Space itself ripped apart, in an event scientists call the Inflationary Big Bang. At the moment of its mysterious birth, in the smallest fraction of a second, the Universe expanded exponentially to balloon to macroscopic size–beginning as an incredibly tiny Patch that was smaller than a proton. Spacetime has been expanding from this initial brilliant state, and cooling off, ever since. All of the galaxies are drifting away from one another, and our Universe has no center. Indeed, everything is floating away from everything else, as a result of the expansion of Spacetime. The expansion of the Universe is frequently likened to a loaf of leavening raisin bread. The dough expands, taking the raisins along for the ride. The raisins become progressively more widely separated from one another because the dough is expanding.
Georges Henri Joseph Edouard Lemaitre (1894-1966) was a Belgian astronomer, priest, and professor of physics at the Catholic University of Louvain. Lemaitre was one of the first to suggest that our Universe is not static–that it is expanding. He also formulated the theory that would eventually be termed the Big Bang Universe. Lemaitre once commented that “The evolution of the world may be compared to a display of fireworks that has just ended: some few wisps, ashes, and smoke. Standing on a cooled cinder, we see the slow fading of the suns, and we try to recall the vanished brilliance of the origins of the worlds.”
When we refer to the observable, or visible, Universe we are referring to the relatively small region of the entire Universe that we can observe. The rest of it–the lion’s share of it–is located far, far beyond what we call the cosmological horizon. The light traveling to us from those unimaginably remote regions of Spacetime, far beyond the horizon of our visibility, has not had sufficient time to reach us since the Big Bang because of the expansion of the Universe. No known signal can travel faster than light in a vacuum, and this sets something of a universal speed limit that has made it impossible for us to directly observe these extremely remote domains of Spacetime.
The temperature throughout that original primordial fireball was almost uniform. This very small deviation from perfect uniformity resulted in the formation of everything that we are, and all that we can ever know. Before the Inflation occurred, that extremely small primordial Patch was completely homogeneous, smooth, and appeared to be the same in every direction. It is generally thought that Inflation explains how that entirely smooth and homogeneous Patch began to ripple.
The extremely tiny fluctuations, the primordial ripples in Spacetime, occurred in the smallest units that we can measure (quantum). The theory of Inflation explains how these quantum fluctuations, in the smooth and isotropic baby Universe, would eventually grow into large-scale structures like galaxies, galaxy clusters, and superclusters. To paraphrase the late Dr. Carl Sagan of Cornell University, we are the eyes of the Universe seeing itself. But, of course, nothing with eyes to see existed as yet in these initial moments of the birth of Spacetime.
The weird quantum world is a foamy, jittery arena, where absolutely nothing can stay perfectly still. The originally smooth and isotropic Universe formed little hills and valleys. The valleys ultimately grew emptier and emptier; the hills higher and heavier. This is because of the force of gravity. Gravity drew the original material of the baby Universe into the heavier hills, that eventually acquired increasingly more and more of the matter making up the primordial soup. The impoverished plains, that were devoid of the same powerful gravitational lure possessed by the hills, became increasingly more barren of this primordial broth. As time passed, larger and larger structures formed within our Universe’s wealthier and more massive hills. This is because the hills exerted an increasingly more powerful pull on the primordial material–and the heavier the hills became, the more powerful their gravitational attraction grew. The large-scale structure of the Universe began as tiny variations in the density of matter in the ancient Cosmos. Some domains of Spacetime received a much higher density of matter than others, simply as a result of mere chance. The rich get richer and the poor get poorer, as a result of jittery quantum fluctuations. The distribution of wealth in the Universe is completely random. Powerful gravitational attraction made more and more matter clump together in the more richly endowed regions of the Cosmos.
Universe Gone “Missing”
Two future space missions depend on data derived from DES: The European Space Agency’s (ESA’s) Euclid mission (which has significant NASA participation) and NASA’s own Wide-Field Infrared Survey Telescope (WFIRST) mission. Both space missions are expected to launch in the 2020s, and they are designed to investigate the myriad mysteries concerning the secretive nature of the Universe.
“With this study, we are showcasing what’s going to be possible with these much more complex observatories,” commented Dr. Andres Plazas Malagon in an August 4, 2017 Jet Propulsion Laboratory (JPL) Press Release. Dr. Malagon is a postdoctoral researcher at JPL, who helped characterize DES’s Dark Energy Camera detectors and who also participated in detector studies for WFIRST. The JPL is in Pasadena, California.
According to Albert Einstein’s Theory of General Relativity, gravity should slow down the rate of the Universe’s expansion. However, in 1998, two teams of astronomers observing distant supermovae made the surprising discovery that the Universe is not slowing down at all–in fact, it is speeding up! In order to explain this puzzling observation, scientific cosmologists were forced to confront two possibilities: either 70% of the Universe is in an exotic form, now termed dark energy, or General Relativity must be replaced by a new theory of how gravity operates on cosmic scales.
DES is designed to search for the origin of the accelerating Universe and help to reveal the true nature of the dark energy by measuring the 14-billion-year-old history of the universal expansion with high precision. More than 400 scientists from over 25 institutions in the United States, the United Kingdom, Brazil, Spain, Germany, Switzerland, and Australia are participating in this project. The collaboration has constructed a very sensitive 570-Megapixel digital camera, dubbed DECam, mounted on the Blanco 4-meter telescope at the Optical Astronomy Observatory’s 4-meter Cerro Tololo Inter-American Observatory, located high in the Chilean Andes. Its derived data are processed at the National Supercomputing Applications at the University of Illinois at Urbana-Champaign
Over five years (2013-2018), the DES collaboration is using 525 nights of observation to carry out a deep, wide-area survey to record new information about 300 million galaxies that are billions of light-years from our planet. The survey is imaging 5000 square degrees of the southern sky in five optical filters in order to obtain detailed information about each galaxy being targeted. A fraction of the survey time is being used to study smaller regions of the sky approximately once a week in order to discover and observe thousands of supernovae and other forms of astrophysical transients.
The most current leading models of the Universe indicate that it is composed primarily of the dark energy and dark matter. The dark matter plays the role of an “invisible glue” that holds galaxies and galaxy clusters together with its powerful gravitational grip, while the dark energy is believed to be responsible for the accelerated expansion of the Universe. Some of the best scientific predictions for the amount of dark matter and dark energy in the Cosmos come from the ESA’s Planck satellite, which observes the light emitted approximately 400,000 years after the Big Bang.
The Mystery Of The (Mostly) Missing Universe
The DES has studied the composition of the more mature Universe. The new results show that there is an agreement with predictions made using Planck measurements of the Universe’s babyhood. This finding helps cosmologists reach a new understanding about how the Universe has evolved since the Big Bang. The DES findings were presented at the American Physical Society’s (APS) Division of Particles and Fields meeting held at the U.S. Department of Energy’s Fermi National Accelerator Laboratory in Batavia, Illinois.
“The Planck results have been the landmark constraints in cosmology. It is truly amazing that you have a model that describes the Universe at 400,000 years old, and now we have a similarly precise measurement of the Universe at 13 billion years [old] that agrees with the model,” commented JPL’s Dr. Tim Eifler in the August 4, 2017 JPL Press Release. Dr. Eifler led the DES analysis team that developed the science software for the interpretation of the results.
The measurements show that approximately 70% of the Universe is contained in the dark energy, about 25% is contained in the dark matter, and that the rest is composed of “ordinary” atomic matter–the “runt” of the cosmic litter. All three measurements agree with other precise measurements made to date. At this point, DES has found no evidence that the quantity of dark energy has changed over time. This finding is consistent with Albert Einstein’s idea of a cosmological constant. Einstein first proposed the concept of a cosmological constant, usually symbolized by the Greek letter lambda (^), as a mathematical fix to General Relativity.
The results are of great importance to scientific cosmologists because they show, for the first time, that observations of the more recent Universe, using gravitational lensing and galaxy clustering, can yield results just as precise as those obtained from the cosmic microwave background (CMB) radiation. The CMB is the primordial light that lingers from the “infant” Universe.
Gravitational lensing is a distribution of matter (such as galaxy clusters) that are situated between a distant source of light and an observer. The foreground object (the lens) bends the light from the background source, as the traveling light wanders in the direction of the observer. Gravitational lensing can reveal the presence of the invisible, ghostly dark matter, because its gravity bends, distorts, and magnifies the path of the light wandering its way through Space from a background object.
“This is the crossover point where gravitational lensing and galaxy clustering measurements and surveys will be the primary driver of what we know about dark energy in the Universe,” noted Dr. Eric Huff in the August 4, 2017 JPL Press Release. Dr. Huff is a JPL researcher who invented a new method of extracting the weak lensing signal that enhances the precision of the DES galaxy shape catalogs. The findings come from the first-year data set collected by the DES, using the Blanco telescope.
In order to measure the dark matter, the researchers first created maps of galaxy positions. Then they measured the shapes of 26 million galaxies to directly map patterns of dark matter over billions of light-years, using gravitational lensing and galaxy clustering.
The DES scientists then went on to develop new methods to detect the very small lensing distortions appearing on the galaxy images. In the process, they created the largest guide ever drawn to help scientists detect the Universe’s mysterious dark matter. The new dark matter map is 10 times the size of the one DES had already released in 2015–and it continues to grow. The DES plans to publish a data set that is even five times larger over the next two years.
Dr. Eifler commented in the August 4, 2017 JPL Press Release: “There is a feeling of true discovery in the collaboration. For the first time, we have the data and tools in hand to see whether Einstein’s cosmological constant prevails. We are all excited to explore the physical nature of dark energy. In particular we want to see if there are hints in the data that suggest modifying the laws of gravity on the largest scales in the Universe.”
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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