The big picture Sound waves at the beginning of time may help scientists How astronomers find the universe’s expansion rate today. by C. Renée James measure the cosmos ost astronomical distance determinations rely on cosmic standard candles — objects researchers have established maintain a known intrinsic brightness. But if you want a shot at understand- ing the nature of dark energy and how this mys- terious force is accelerating the expansion of our universe, you Mmust measure distances like a BOSS. To carry out the Baryon Oscillation Spectroscopic Survey (BOSS), you need dozens of observational and theoretical astrono- mers, programmers, and engineers. You need a dedicated telescope that allows you to obtain vast amounts of data spanning at least a quarter of the sky — and more is always better. You also need a large scope, one with the ability to peer deep into the universe to get the picture in three dimensions, pulling information about both distance and motion from the data. Only then will a standard ruler come into focus. It’s a ruler that formed with the universe more than 13 billion years ago and has been expanding with it ever since. This ruler can tell astronomers precisely what the cosmos was doing at every stage in its life since then. If you can figure that out, you might finally understand the mysterious forces driving the behavior of everything we see. And that understanding could very well hold the key to the next life- changing breakthrough in physics. Catching waves Drop a single pebble into a pond, and watch the ripples radiate out- ward. Then simultaneously drop a handful of pebbles into the pond, and take a snapshot of the ripples after a few seconds. Imag- ine them radiating outward not just on the pond’s surface, but as the surfaces of spheres moving in every direction. Further imagine a series of ever-growing enlargements of this extra-dimensional snapshot, erasing the ripples and replacing them with dots that congregate on their edges. Finally, try imagining giving that image to a complete stranger, asking only that they divine the conditions of the original pond and a precise history of the subsequent enlargements. Welcome to the study of baryon acoustic oscillations (BAOs). “Baryon” is a catchall term for the majority of normal, everyday Waves — similar to those made by rocks falling into a pond — undulated through the early universe at nearly 60 percent the speed of light. To find C. Renée James is a professor of astronomy at Sam Houston State evidence for those waves today, astronomers search for telltale features 490 million light-years in diameter that formed in the cosmos’ first 100 seconds, University in Huntsville, Texas, and author of Science Unshackled (Johns expanded, and interacted with other such ripples. ASTRONOMY: ROEN KELLY Hopkins University Press, 2014). © 2015 Kalmbach Publishing Co. This material may not be reproduced in any WWW.ASTRONOMY.COM 45 form without permission from the publisher. www.Astronomy.com This pushback caused an acoustic (compression) wave that traveled What’s more, the scale imprinted on the CMB provides astron- EVOLUTION OF A BAO through the photon-baryon fluid at the speed of sound — but not omers with a long-awaited standard ruler. In today’s universe, that How a BAO moves through time the speed of sound as we measure it today on Earth. Unlike every- last point of photon-baryon contact translates to a huge sphere of 3.8 billion years ago 1 2 day sound waves that crawl through comparatively cold, empty air, material with a radius of some 150 megaparsecs (Mpc), or about early acoustic waves raced through the cosmos at nearly 60 percent 490 million light-years. the speed of light. 5.5 billion years ago Complicating the situation was the presence of dark matter — Setting the standard 13.8 billion the universe’s hypothetical invisible mass that, according to theory, “The early universe is a remote place to calibrate one’s ruler,” says years ago accounts for some phenomena astronomers observe. Immune to Eisenstein. But having such a standard can circumvent some of light’s push, dark matter was largely a spectator to the fight the problems that plague modern distance determinations. between the baryons and photons. Slight variations in the universe Astronomers realized more than a century ago that some directed the dark matter into clumps. objects act as standard candles — phenomena of known Outweighing normal matter by a factor of six, dark matter’s intrinsic brightnesses like certain variable stars or particular gravity tried to coax the massive baryons toward the denser regions stellar explosions called type Ia supernovae. If scientists 3 4 of the cosmos, but the radiation pressure fought back. Light pushed observe the apparent brightness of a standard candle and the baryons outward from the central matter concentration like a know its absolute brightness, then it’s fairly straightfor- multidimensional snowplow. Gravity continued to pull the mate- ward to figure out how far away the object is. rial back to the central dark matter, and the process repeated itself, The problem with standard candles is that the far- wave after wave. ther away they are, the more likely it is that the inter- Then, around 380,000 years after the Big Bang, something new vening universe has tainted their light. Maybe a type happened. The conditions in the rapidly expanding universe Ia supernova appears fainter because it’s more dis- allowed the first atoms to form as electrons and nuclei (composed tant. On the other hand, perhaps there’s simply more dust of protons and neutrons) came together in an event known as obscuring it, or perhaps there was something different about type The record of baryon acoustic oscillations in galaxy maps helps astrono- recombination, although most astronomers agree that it should Ia supernovae in the past. Uncertainties can add up. mers retrace the universe’s history. This illustration presents the cosmos simply be “combination.” A standard ruler, on the other hand, provides a robust way of at three different times. The false-color image on the right shows the This series of four illustrations shows the progression of a single Because neutral atoms are largely immune to light’s push and measuring things because it has a known length. If, for instance, cosmic microwave background, a record of what the universe looked baryon acoustic oscillation by studying just one such event over time. like just 380,000 years after the Big Bang, including an original baryon because there were fewer things to slam into, the photons were we knew that every galaxy had a diameter of 100,000 light-years, acoustic oscillation (white ring). As the universe expanded, evidence of 1) In all images, the blue center represents dark matter while the dark finally free, and for the most part have never been bothered by all we would have to do is look at the angular size of any given gal- outer ring (it’s really a sphere, but we show it in two dimensions) repre- such an oscillation has remained visible as the larger white rings. E.M. HUFF/ THE SDSS-III TEAM/THE SOUTH POLE TELESCOPE TEAM/GRAPHIC BY ZOSIA ROSTOMIAN sents the last point of contact between light and baryons. The red cir- matter since. In fact, this radiation — stretched to microwaves by axy, and we could figure out its distance. cle is a reference to show a standard distance from the center. 2) After the expansion of the universe — now fills the entire sky as the cos- BAOs are not that well behaved, though. When astronomers some time (millions of years), the outer shell, where matter (galaxies) mic microwave background (CMB). Without the snowplows, how- look at the distribution of galaxies in the universe, it’s not clear has accumulated, has expanded much more than the inner portion. ever, a pileup of snow was left at a specific distance away from that which galaxies belong on which ripple. Furthermore, in the 13.8 generation. This reliable ruler helps astronomers precisely measure Different shells have different shapes. Add to that the fact that shells initial central dark matter concentration — the sound horizon. billion years of cosmic evolution, galaxies and dark matter have the expansion rate of the universe — the Hubble constant. interact with other shells over time, and you get an idea how difficult Since the late 1960s, cosmologists have known there should be a been jostling each other around gravitationally. The average sepa- Since the discovery in 1998 of the accelerating expansion of the they are for astronomers to detect. 3) This illustration, the same as #2, greater concentration of matter at the last point of contact between ration is still 150 Mpc, but individual distances might be anywhere universe, the Hubble constant has emerged as the key to under- adds the gravitational influence of material in the space around the sphere and why it’s no longer uniform. Longer arrows represent photons and baryons, a spherical shell at the sound horizon from a from 140 to 160 Mpc, smearing out the signature. standing the nature of dark energy, a component that scientists regions of greater gravity. 4) Several billion years have passed. The central condensation where dark matter tried to gather everything One of the great strengths of BAOs, however, is that the cosmic know little about despite it making up nearly three-quarters of the outer shell now has an even different shape because it has continued to itself. In some respects, it would look much like a snapshot of a sound waves propagated in every direction.