Alternative to Dark Energy

Transcription

1 National National Aeronautics Aeronautics and and Space Space Administration Administration Alternative to Dark Energy Space Weather by John T. Clarke Taken from: Hubble 2010: Science Year in Review TakenProduced from: by NASA Goddard Space Flight Center and the Space Telescope Science Institute. Hubble 2011: Science Year in Review The full contents of this book include Hubble science articles, an overview of Producedthe bytelescope, NASA Goddard and more. SpaceThe Flight complete Centervolume and its component sections are and the Space available Telescope for download Science online Institute. at: The full contents of this book include Hubble science articles, an overview of the telescope, and more. The complete volume and its component sections are available for download online at:

2 Ruling Out One Alternative to Dark Energy In the early 20th century, Dr. Edwin Hubble s observations of Cepheid variable stars in what was then known as the Great Andromeda Nebula ultimately led to the realization that the universe is not static, but expanding in size. (See article on page 77). Now, nearly a century later, astronomers have discovered that the universe is not only expanding, but its rate is accelerating. Scientists reached this conclusion by measuring the distance to remote supernovas and comparing their results to the expected distances based upon a constant expansion rate. Finding them in disagreement, cosmologists quickly focused on theoretical models that could account for this unexpected observation. Their leading explanation is that the universe is filled with dark energy, believed to be a repulsive form of gravity. According to this theory, dark energy is evenly distributed in space, maintains a constant density as space expands, and works to counteract the gravitational force that galaxies exert upon each other. Gravitational force, if acting alone, would cause the expansion of the universe to decelerate or possibly stop. However, in order to explain certain cosmological observations made to date, dark energy would have to comprise approximately 73 percent of the total mass-energy budget of the entire universe. The very idea that a pervasive dark energy drives the universe is so radical that some scientists have offered other interpretations of the data. One alternative postulates that the solar system is located within a giant cosmic void an enormous bubble of relatively empty space, eight billion light-years across. In this scenario, the lower-density bubble would expand faster than the rest of the more massive universe around it. To an observer inside the bubble, it would appear as though a dark-energy-like force was pushing the entire universe apart, but this would be incorrect. To assess which models best described cosmic reality, astronomers needed a more precise determination of the universe s rate of expansion, a value hitherto thought to be fixed and thus named the Hubble Constant (abbreviated as H0). A group Spiral galaxy NGC 5584 lies 72 million light-years away in the constellation Virgo, the Virgin. It was one of eight galaxies astronomers recently studied with Hubble to measure the expansion rate of the universe and refine key properties of the cosmos. 131

3 called The Supernova H0 for the Equation of State (SHOES) team, led by Maryland-based astronomer Adam Riess, recently used Hubble observations to recalculate this value in the present time. A key part of their effort was remeasuring the distances to various galaxies. Quantifying these distances required the researchers to find stars or other objects that could serve as reliable distance indicators, just as Edwin Hubble once did. These objects had to have a known intrinsic brightness, that is, a characteristic luminosity that has not been dimmed by distance, an atmosphere, or stellar dust. The scientists could then infer the object s true distance by comparing its intrinsic brightness with its measured brightness. Cepheid variables remain one of the most reliable tools for measuring short distances. These pulsating stars brighten and dim at rates (or periods) that correspond quite predictably to their intrinsic brightness. However, Cepheids are too dim to be seen in very distant galaxies. To determine these longer distances, Riess team chose a special class of exploding stars called Type Ia supernovas (see sidebar on page 134). These stellar explosions are brilliant enough to be seen far across the universe but are less precise as measuring tools than Cepheids. The SHOES team wanted to achieve better accuracy from these Type Ia supernovas. To do this, they focused their efforts on nearby galaxies with newly discovered supernovas of this type. Then, they set out to locate as many Cepheid variables as possible within the same galaxy. Knowing the galaxies distances based upon the combined Cepheid data taken for each of them, they used this information to calibrate the distance measurement derived from the Type Ia supernovas. The astronomers could then more confidently determine distances to the farthest galaxies using supernova-based data to better understand the history of the universe s expansion rate. Riess and his team combined new Hubble observations with previous ones from a related investigation in 2009 and found more than 600 Cepheids in eight galaxies. Each galaxy also contained a recent Type Ia supernova. Galaxy NGC 5584 in the constellation Virgo was one of those observed. Among the thousands of stars resolved in this spiral galaxy by Hubble s Wide Field Camera 3 (WFC3), 250 were identified as Cepheid variables. Using them, NGC 5584 s distance was determined to be 72 million light-years. Another key galaxy in the study was NGC Its distance of approximately 27 million light-years was also known quite accurately through a separate technique using radio observations. 132

4 Location of supernova Cepheid variable periods in NGC 5584 >60 days days <30 days Adam Riess and his team identified many Cepheid variable stars and a recent Type Ia supernova in spiral galaxy NGC In this image, the locations of the Cepheids are circled and the lengths of their periods color coded. 133

5 Type Ia Supernovas The Type Ia supernova is an invaluable distance-measuring tool for observational cosmologists astronomers who investigate the nature and origin of the cosmos. Appearing with sudden brilliance, these exploding stars emit four billion times as much light as our Sun for a brief period of time, making them visible far across the universe. Scientists believe Type Ia supernovas arise in tight binary star systems where one member is a white dwarf star. If the two stars are sufficiently close to one another, material from the outer regions of the dwarf s companion is gravitationally pulled away and accreted onto the surface of the dwarf. Over time, enough mass deposits on the white dwarf that the weight of the dwarf s outer layers overcomes the internal force holding up these layers, and the white dwarf gravitationally implodes. Within seconds, the tremendous compressive heat generated at the core of the dwarf ignites a thermonuclear reaction that detonates the star. Astronomer Subrahmanyan Chandrasekhar namesake of NASA s Chandra X-ray Observatory mathematically demonstrated that since the internal force that keeps white dwarfs from gravitationally imploding is constant, one could calculate quite accurately the point at which an accreting white dwarf exceeds this limit and implodes. He calculated that a white dwarf reaches this boundary condition when its total mass is greater than 1.4 solar masses. Since the luminosity of the resulting supernova depends on its mass, supernovas generated this way all shine with the same characteristic brightness. 1 Material accretes onto a white dwarf star within a binary star system until a predictable point. 2 Past this point, the star implodes and then detonates as a supernova. 3 A supernova remnant expands outward to a great distance. Knowing this intrinsic brightness, astronomers can This figure illustrates the demise of a white dwarf star into a compute the distance to any Type Ia supernova by Type Ia supernova. applying the mathematical inverse-square law that governs the brightness of light; doubling the distance to a light source causes it to appear one-quarter as bright. In this way, Type Ia supernovas act like candle flames, which also shine with a predictable brightness, hence astronomers designate these supernovas as standard candles. In the decades that have passed since scientists recognized the expansion of the universe, larger and larger telescopes with increasingly sensitive instruments have enabled astronomers to observe Type Ia supernovas at greater distances. Today, the Hubble Space Telescope is one of the most powerful telescopes for making these discoveries, as its nearinfrared sensitivity and sharpness can detect these supernovas at great distances and capture their light curves. 134 Type 1a Supernovas Unlike candle flames, however, Type Ia supernovas stay at their peak brightness for only a short period of time (a few hours to days) and then fade in a distinctive way. By recording their brightness over time in what astronomers call a light curve, scientists can distinguish them from other varieties of supernovas that have different light curves.

6 Supernova 2007af in NGC G5584 was discovered by Japanese supernova hunter Koichi Itagaki. This galaxy in Virgo is slightly smaller than the Milky Way. (Photo credit: ESO VLT) 135 HUBBLE HUBBLE 2011: 2011: SCIENCE SCIENCE YEAR YEAR IN IN REVIEW REVIEW

7 Future Present 5 Dark energy dominates Decelerating expansion Past BILLIONS OF YEARS Accelerating expansion Dark matter constrains Dark matter dominates 13.7 Dark energy repels BIG BANG This illustration portrays the gravitational force from matter, including dark matter, as a stretched rubber band. The force is inward and has the effect of slowing the expansion of the universe. Dark energy is depicted as a compressed spring. It pushes outward and exerts a repulsive force. Whether deceleration or acceleration occurs depends on the difference, or net force attractive (inward) or repulsive (outward). In the distant past, when the universe was smaller, the attractive force appears to have dominated the repulsive force. Now, however, the repulsive force is dominant, resulting in the accelerating expansion of the universe. Using only data from Hubble s Wide Field Camera 3 (WFC3), Riess s team eliminated the systematic errors that are unavoidably introduced by comparing measurements from different telescopes and cameras. They thereby bridged the rungs in the cosmic distance ladder more simply and accurately. This process is akin to using a single, long tape measure to determine a length rather than a smaller ruler laid end-over-end multiple times. 136

8 Cepheid Variables Cepheid variables belong to a class of very luminous stars that have masses between 5 and 20 times the mass of our Sun. These stars show a strongly correlated relationship between their luminosities and pulsation periods, a quality that makes Cepheids important standard candles for establishing the galactic and extragalactic distance scales. Ursa Major (Big Dipper) Polaris Astronomer Henrietta Swan Leavitt, working at Harvard College Observatory in 1908, discovered the relationship between the oscillation period and luminosity for Cepheids while investigating thousands of variable stars in the Magellanic Clouds. Seven years later, astronomer Harlow Shapley at Princeton University used Cepheids to place constraints on the size and shape of the Milky Way and of the placement of the Sun within it. Ursa Minor (Little Dipper) Cepheid Variables Cepheids oscillate between two states. In the first, the star is physically compact, with internal processes that build up high temperatures and outward-directed pressure gradients within the star. Responding to these pressures, the star expands in size and cools. When fully in this second, expanded state, the outward-directed pressure drops. Without this pressure to support the star against its own gravity, the Cepheid contracts and returns to its compressed state. One of the most-studied Cepheid variable stars is the Pole Star, Polaris, located at the end of the Little Dipper s handle. Only 425 light-years distant, Polaris can be seen to vary in brightness over a period of approximately four days. Long-term trending of the star s brightness and period since the year 1900 indicate the star has brightened (on average) about 15 percent and increased in period by about 15 minutes. Such variations highlight the need for ongoing calibration of the Cepheid period-luminosity relationship that makes these stars valuable as distance indicators. (Photo credit: A. Fujii) Edwin Hubble established the distance to Cepheid variables in the Great Andromeda Nebula in 1924, showing that these stars were so far away they could not be a part of the Milky Way. This eliminated all doubt that the Andromeda Nebula was extragalactic and settled the island universe debate, in which scientists pondered whether the terms Milky Way and universe were synonymous or whether the Milky Way was just one of many galaxies that comprise the universe. Further investigation by astronomers of galactic distances and speeds led to their discovery that the universe is expanding. 137

9 With the installation of the Wide Field Camera 3, Hubble gained a sensitive new tool to study the cosmos. Here, astronaut Andrew Feustel is seen maneuvering the camera toward the telescope during the first of five successful spacewalks during Servicing Mission 4 in May WFC3 is the best camera ever flown on Hubble for making critical Cepheid variable star and supernova measurements. Compared to prior instruments, WFC3 provided Reiss with better precision and took a fraction of the time to make observations in both visible and near-infrared light. Observing in near-infrared light reduces the effect of dusty nebulas that often envelope stars, yielding a more accurate measure of a star s true brightness. Riess and his team established a value of 73.8 kilometers per second per megaparsec for H0 and reduced the uncertainty in this measure of the current expansion rate to just 3.3 percent. (A parsec is 3.26 light-years.) This narrows the error margin by 30 percent over Hubble s previous best measurement in Every decrease in uncertainty for this fundamental property of the cosmos helps scientists solidify their understanding of the universe at large, including the relative abundance of ordinary matter, dark matter, and dark energy. Knowing the precise value of the universe s current expansion rate further restricts the range of dark energy s strength and refines a number of other cosmic properties, including the universe s shape. The cosmic void hypothesis, proposed as an alternative to dark energy, requires that H0 be approximately 60 to 65 kilometers per second per megaparsec much slower than what Riess s team measured. By reducing the uncertainty of the Hubble constant s value to 3.3 percent, that is, to the range of 71.4 to 76.2 kilometers per second per megaparsec, the team has eliminated beyond all reasonable doubt the lower number as the true value for H0, thus invalidating the cosmic bubble theory. 138

10 The nature of dark energy remains one of the greatest cosmological mysteries in modern physics. While the existence of a pervasive, repulsive energy may be surprising and counterintuitive, the alternative bubble theory violated a generally held view called the Copernican principle the belief that our place in the universe is altogether ordinary. The bubble theory required that humans live in a very special place, extremely near the center of an enormous and very empty region of space. Scientists estimate that this circumstance had only about a one-in-a-million chance of occurring randomly. By ruling out the bubble hypothesis through a refined assessment of H0, astronomers are closer to the ultimate goal of understanding our remarkable and measurably accelerating universe. Further Reading Andrews, B. Dark Energy Beats Out Rival Bubble Theory. Astronomy 39, no. 7 (July 2011): 17. Carroll, S. Dark Energy & the Preposterous Universe. Sky & Telescope 109, no. 3 (March 2005): Coffey, V. C. Hubble Confirms a Cosmic Jerk. Sky & Telescope 113, no. 3 (March 2007): 20. Glanz, J. Accelerating the Cosmos. Astronomy 27, no. 10 (October 1999). Panek, R. Going Over to the Dark Side. Sky & Telescope 117, no. 2 (February 2009): Riess, A. G., et al. A 3% Solution: Determination of the Hubble Constant with the Hubble Space Telescope and Wide Field Camera 3. The Astrophysical Journal 730, no. 119 (April 1, 2011). Riess, A. G. and M. S. Turner. From Slowdown to Speedup. Scientific American 290, no. 2 (February 2004): Urry, M. Dark Energy, Science s Biggest Mystery, CNN.com, October 9, Dr. Adam Riess, an astronomer at the Space Telescope Science Institute and Krieger-Eisenhower professor in physics and astronomy at The Johns Hopkins University in Baltimore, was awarded the 2011 Nobel Prize in Physics by the Royal Swedish Academy of Sciences. The academy recognized him for leadership in the High-z Team s 1998 discovery that the expansion rate of the universe is accelerating, a phenomenon widely attributed to a mysterious, unexplained dark energy filling the universe. His work culminated in the first detection of an epoch of decelerating expansion preceding the present accelerating period. Dr. Riess has also been awarded the Einstein, Warner, Shaw, Gruber, and Sackler prizes for academic achievement, is the recipient of a MacArthur Fellowship, and was elected to the National Academy of Sciences in Dr. Riess earned his doctorate in astrophysics from Harvard University. 139