b'12 Carnegie Science|Fall 2020How the Universe Got its Structure . . . demonstrated for the first time that the growth of individual protostructures can be calculated and then averaged over all of space.The universe has billions of galaxies0but their distribution is far fromAge of Universe (Bilions of Years) 4 4 2 2 0uniform. Why is there so much6 Age of Universe (Bilions of Years)8 6structure in the universe today, and810 0how did it come about? X (Megaparsecs) 10 2 0-30 -20 -10 0 10 20 304 2A 10-year survey of tens ofY (Megaparsecs) 30 20 100 -10 -20-30 4thousands of galaxies made using theAge of Universe (Bilions of Years) 668 Age of Universe (Bilions of Years)Magellan Baade telescope at Carnegies810Las Campanas Observatory in Chile10Carnegies Daniel Kelson led theprovided a new approach to answering research. Image courtesy Carnegie Observatories this mystery. The work, led byWhy is the distribution of structure in the cosmos not uniform? The universes first Carnegies Daniel Kelson, was publishedstructure originated when some of the material flung outward by the Big Bang in Monthly Notices of the Royal Astronomical Society. overcame its trajectory and collapsed on itself, forming clumps. A team of Carnegie researchers showed that denser clumps of matter grew faster, and less-dense clumps How do you describe the indescribable? asked Kelson. Bygrew more slowly. The groups data revealed the distribution of density in the taking an entirely new approach to the problem. universe over the last 9 billion years. (On the illustration, violet represents low-density Coauthor Andrew Benson explained, Our tactic providesregions and red represents high-density regions.) Working backward in time, the teams findings reveal the density fluctuations (far right, in purple and blue) that newand intuitiveinsights into how gravity drove the growthcreated the universes earliest structure. This aligns with what we know about the of structure from the universes earliest times. This is a direct,ancient universe from the afterglow of the Big Bang, called the cosmic microwave background (CMB) (far right in yellow and green). The researchers achieved their observation-based test of one of the pillars of cosmology. results by surveying the distances and masses of nearly 100,000 galaxies, going back The Carnegie-Spitzer-IMACS Survey was designed to studyto when the universe was only 4.5 billion years old. About 35,000 of the galaxies the relationship between galaxy growth and the surroundingstudied by the Carnegie-Spitzer-IMACS Survey are represented here as small spheres. Image courtesy Daniel Kelson. CMB data is based on observations obtained with Planck, an ESA science environment over the last 9 billion years, when modern galaxiesmission with instruments and contributions directly funded by ESA member states, NASA, and Canada.were defined.The first galaxies formed a few hundred million years after the Big Bang, which started the universe as a hot, murky soup ofPatrick McCarthy, and John S. Mulchaey, as well as Rik extremely energetic particles. As this material expanded, itWilliams, now of Uber Technologiesdemonstrated for the first cooled, and the particles coalesced into neutral hydrogen gas.time that the growth of individual protostructures can be Some patches were denser than others and, eventually, theircalculated and then averaged over all of space. The team gravity overcame the universes outward trajectory and therevealed that denser clumps grew faster, and less-dense clumps material collapsed inward, forming the first structural clumps. grew more slowly.The density differences that allowed for different structuresThe team then worked backward to determine the original to form in some places and not in others have been a long- distributions and growth rates of the fluctuations in density, standing topic of fascination. But until now, astronomerswhich would eventually become the large-scale structures that abilities to model how structure grew in the universe faceddetermined galaxy distributions we see today.mathematical limitations.The work provided a simple, yet accurate, description of The gravitational interactions occurring between all thewhy and how density fluctuations grow the way they do in the particles in the universe are too complex to explain with simplereal universe, as well as in the computational-based work that mathematics, Benson said. underpins our understanding of the infant universe.So, astronomers either used mathematical approximations The findings would not have been possible without the which compromised the models accuracyor large computerallocation of an extraordinary number of observing nights at Las simulations that numerically model all the interactions betweenCampanas.galaxies, but not all the interactions occurring between all of theMany institutions wouldnt have had the capacity to take particles, which was considered too complicated. on a project of this scope on their own, said Observatories A key goal of our survey was to count up the mass presentdirector John Mulchaey. But thanks to our Magellan telescopes, in stars found in an enormous selection of distant galaxies andwe were able to execute this survey and create this novel then use this information to formulate a new approach toapproach to answering a classic question.understanding how structure formed in the universe, KelsonWhile theres no doubt that this project required the explained. resources of an institution like Carnegie, our work also could not The research teamwhich included Carnegies Louishave happened without the tremendous number of additional Abramson, Shannon Patel, Stephen Shectman, Alan Dressler,infrared images that we were able to obtain at Kitt Peak and Cerro Tololo, which are both part of the National Science Background Image: The Magellan telescopes at Carnegies Las CampanasFoundations National Optical-Infrared Astronomy Research Observatory in Chile were crucial to this survey. Image courtesy Yuri Beletsky, Carnegie Institution for Science Laboratory, Kelson added.'