When Dark Energy Turned On

The record of baryon acoustic oscillations (white circles) in galaxy maps helps astronomers retrace the history of the expanding universe. These schematic images show the universe at three different times. The representative-color image on the right shows the "cosmic microwave background," a record of what the very young universe looked like 13.7 billion years ago. The small density variations present then have grown into the clusters, walls, and filaments of galaxies that we see today. These variations included the signal of the original baryon acoustic oscillations (white circle, right). As the universe has expanded (middle and left), evidence of the baryon oscillations has remained, visible in a "peak separation" between galaxies (the larger white circles). The SDSS-III results announced today (middle) are for galaxies 5.5 billion light-years distant, at the time when dark energy turned on. Comparing them with previous results from galaxies 3.8 billion light-years away (left) measures how the universe has expanded with time. Credit: E.M. Huff, the SDSS-III team, and the South Pole Telescope team. Graphic by Zosia Rostomian. High Resolution Image (jpg) - Low Resolution Image (jpg)

Cambridge, MA - Astronomers announced today that they have made the most accurate measurement yet of galaxy distances in the faraway universe, giving an unprecedented look at the time when dark energy turned on. Some five to seven billion years ago, the expansion of the universe stopped slowing due to gravity and started to accelerate due to dark energy. Yet the nature of dark energy remains a puzzle that astronomers are seeking to solve.

The new measurement came from the Baryon Oscillation Spectroscopic Survey (BOSS), which is part of the third Sloan Digital Sky Survey (SDSS-III).

"We see the influence of dark energy on cosmic structure, but we have no idea what it is. The data gathered by this survey will help answer that question," said Daniel Eisenstein (Harvard-Smithsonian Center for Astrophysics), the director of SDSS-III.

"There's been a lot of talk about using galaxy maps to find out what's causing accelerating expansion," said David Schlegel of the U.S. Department of Energy's Lawrence Berkeley National Laboratory, BOSS's principal investigator. "We've been making a map and now we're using it - starting to push our knowledge out to the distances when dark energy turned on."

Investigating dark energy

One of the most amazing discoveries of the last two decades in astronomy, recognized with the 2011 Nobel Prize in Physics, was that not only is our universe expanding, but it is accelerating. Galaxies are becoming farther apart from each other faster and faster with time.

The leading contender for the cause of the accelerating expansion is a postulated new property of space dubbed "dark energy." Alternatively, the universe may be accelerating because gravity deviates from Einstein's General Theory of Relativity and becomes repulsive at very large distances.

Whether the answer to the puzzle of the accelerating universe is dark energy or modified gravity, the first step to finding that answer is to measure accurate distances to as many galaxies as possible. From those measurements, astronomers can trace out the history of the universe's expansion.

BOSS is producing the most detailed map of the universe ever made by using a new custom-designed spectrograph of the SDSS 2.5-meter telescope at Apache Point Observatory in New Mexico to observe more than a million galaxies over six years.

Today's announcement is based on a map of more than 250,000 galaxies created from the first year and a half of BOSS observations. Some of these galaxies are so distant that their light has traveled more than six billion years to reach Earth - nearly half the age of the universe.

Surveying the cosmos

Maps of the universe like BOSS's show that galaxies and clusters of galaxies are clumped together into walls and filaments, with giant voids between. These structures grew out of subtle variations in density in the early universe, which bore the imprint of "baryon acoustic oscillations" - pressure-driven acoustic (sound) waves that passed through the early universe.

Billions of years later, the record of these sound waves can still be read in our universe. "Because of the regularity of the ancient sound waves, there's a slightly increased probability that any two galaxies today will be separated by about 500 million light-years, rather than 400 million or 600 million," said Eisenstein.

In a graph of the number of galaxy pairs by separation distance, that magic number of 500 million light-years shows up as a peak, so astronomers often speak of the "peak separation." The position of this peak depends on the amount of dark energy in the Universe. But measuring the distance between galaxies depends critically on having the right distances to the galaxies in the first place.

That's where BOSS comes in. "We've detected the peak separation more clearly than ever before," said Nikhil Padmanabhan of Yale University. "These measurements allow us to determine the contents of the Universe with unprecedented accuracy."

This release is being issued jointly with the Sloan Digital Sky Survey.

Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science. The SDSS-III web site is http://www.sdss3.org/.

SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, University of Cambridge, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu

Source: HARVARD-SMITHSONIAN CENTER FOR ASTROPHYSICS

More about → When Dark Energy Turned On

Gamma-ray bursts shed light on the nature of dark energy

An artistic image of the explosion of a star leading to a gamma-ray burst
Source: FUW/Tentaris/Maciej Frołow

Dark energy is the basic constituent of the Universe today, one that is responsible for its accelerated expansion. Although astronomers observe the cosmological effects of the impact of dark energy, they still do not know exactly what it is. A new method for measuring the largest distances in the Universe developed by scientists from the Faculty of Physics, University of Warsaw and the University of Naples Federico II helps solve the mystery. A key role is played by the most powerful cosmic explosions – gamma-ray bursts.

What is the nature of dark energy, a recently discovered dominant constituent of the Universe today? Is expansion-accelerating dark energy an intrinsic property of space-time itself or rather a field unknown to science? A new distance-measuring method developed by scientists from the Faculty of Physics, University of Warsaw (FUW) and the University of Naples Federico II can provide the answer. “We are able to determine the distance of an explosion on the basis of the properties of the radiation emitted during gamma-ray bursts. Given that some of these explosions are related to the most remote objects in space that we know about, we are able, for the first time, to assess the speed of space-time expansion even in the relatively early periods after the Big Bang,” says Prof. Marek Demiański (FUW). The method was used to verify models of the structure of the Universe containing dark energy.

In 1998, during the analysis of the brightness of Type Ia supernovae, it was discovered that the most remote explosions seemed to be too weak. Type Ia supernovae appear in binary systems. One of the stars is a white dwarf, a relic of an evolutionary cycle of stars similar to the Sun. When the second star of the system enters the red giant phase and swells up, its external layers, containing mainly hydrogen, begin to fall onto the white dwarf, which gradually grows in mass. When the white dwarf reaches 1.4 solar masses, it explodes and is completely torn apart. Since the conditions that trigger the explosion are similar every time, Type Ia supernovae always release more or less the same amount of energy. Astronomers rely on this property to measure distances in space.

The fainter brightness of remote Type Ia supernovae was a clear indication that they were even more distant than assumed. Instead of slowing down the expansion, the Universe was accelerating. A new form of mass-energy – dark energy – needed to be introduced into the theory in order to reconcile the previous models of the Universe with the observations. The calculations indicate the existence of a huge amount of dark energy, nearly 20 times greater than the amount of mass-energy related to the world accessible to human senses. “Overnight, dark energy became, quite literally, the greatest mystery of the Universe,” says Prof. Demiański.

To this day no one knows exactly what dark energy is. There are two models explaining its nature. According to the first one, dark energy is a property described by the famous cosmological constant introduced by Albert Einstein. According to the second model, the accelerated expansion is caused by some unknown scalar field. “In other words, it is either-or: either space-time expands by itself or is expanded by a scalar physical field inside it,” says Prof. Demiański.

Examining the density of dark energy in various periods after the Big Bang can help choose the correct model. If the density remained constant, it would mean that dark energy is related to the cosmological constant, that is to say, the property of space-time. But if the acceleration of the Universe is caused by a scalar field, then, given the swelling-up of space-time, the density of dark energy should change. “This used to be a problem. In order to assess the changes in the density of dark energy immediately after the Big Bang, one needs to know how to measure the distance to very remote objects. So remote that even Type Ia supernovae connected to them are too faint to be observed,” says Prof. Demiański.

The group of Polish and Italian astrophysicists suggested using gamma-ray bursts (GRBs), the most powerful explosions observed in the Universe today, to measure the largest distances in the Universe. They analyzed the so-called long bursts that probably arise during the collapse of the core of a large star. The process leads to the formation of a black hole. The gamma radiation emitted at that time is so intense that even objects that exploded 400 million years after the Big Bang can be observed.

The main problem was how to assess the total energy of a burst. To that end, the scientists analyzed databases of previous gamma explosions. It turned out that a part of the explosions occurred in galaxies the distance to which could be measured using other methods, for example, by means of Type Ia supernovae. “We focused on those instances. We knew the distance to the galaxy and we also knew how much energy of the burst reached the Earth. This allowed us to calibrate the burst, that is to say, to calculate the total energy of the explosion,” explains Prof. Demiański.

The next step was to find statistical dependencies between various properties of the radiation emitted during a gamma-ray burst and the total energy of the explosion. Such relations were discovered. “We cannot provide a physical explanation of why certain properties of gamma-ray bursts are correlated,” points out Prof. Demiański. “But we can say that if registered radiation has such and such properties, then the burst had such and such energy. This allows us to use bursts as standard candles, to measure distances.”

The team of scientists from the universities in Warsaw and Naples, headed by Dr Ester Piedipalumbo, analyzed data gathered by astronomers. Extremely remote gamma-ray bursts are quite rare. The Amanti catalogue listed 95 such phenomena and failed to provide enough clues as to the exact nature of dark energy. “It is quite a disappointment. But what is important is the fact that we have in our hands a tool for verifying hypotheses about the structure of the Universe. All we need to do now is wait for the next cosmic fireworks,” concludes Prof. Demiański.

The insufficient amount of observational data remains the main problem in the data analysis of gamma-ray bursts. For this reason, many groups of astronomers and astrophysicists combine their efforts in order to register them in the fastest and most accurate manner possible. One of such projects is “Pi of the Sky”, a system of robotic telescopes for real-time monitoring of large areas of the sky, co-organized by the Faculty of Physics, University of Warsaw.

More about → Gamma-ray bursts shed light on the nature of dark energy

‘Zombie' Stars Key to Measuring Dark Energy

Chandra X-ray image of Tycho's supernova remnant. This Type Ia supernova was observed by Tycho Brahe in 1572, and today is just an expanding ball of gas. Astronomers used to have to wait years for a close, bright supernova to learn about them. Today big surveys are discovering supernovae by the thousands. Credit: NASA/Chandra X-ray Observatory

Supernova 1994D. The supernova is the bright point in the lower-left. It is a type Ia thermonuclear supernova like those described by Howell. The supernova is on the edge of galaxy NGC 4526, depicted in the center of the image.

Credit: NASA/Hubble Space Telescope

D. Andrew Howell
Credit: Katrina Marcinowski

(Santa Barbara, Calif.) –– "Zombie" stars that explode like bombs as they die, only to revive by sucking matter out of other stars. According to an astrophysicist at UC Santa Barbara, this isn't the plot for the latest 3D blockbuster movie. Instead, it's something that happens every day in the universe –– something that can be used to measure dark energy.

This special category of stars, known as Type Ia supernovae, help to probe the mystery of dark energy, which scientists believe is related to the expansion of the universe.

Andy Howell, adjunct professor of physics at UCSB and staff scientist at Las Cumbres Observatory Global Telescope Network (LCOGT), wrote a review article about this topic, published recently in Nature Communications. LCOGT, a privately funded global network of telescopes, works closely with UCSB.

Supernovae are stars that have been observed since 1054 A.D., when an exploding star formed the crab nebula, a supernova remnant.

More recently, the discovery of dark energy is one of the most profound findings of the last half-century, according to Howell. Invisible dark energy makes up about three-fourths of the universe. "We only discovered this about 20 years ago by using Type Ia supernovae, thermonuclear supernovae, as standard or ‘calibrated' candles," said Howell. "These stars are tools for measuring dark energy. They're all about the same brightness, so we can use them to figure out distances in the universe."

These supernovae are so bright that they shine with the approximate power of a billion suns, noted Howell.

He calls Type Ia supernovae "zombie" stars because they're dead, with a core of ash, but they come back to life by sucking matter from a companion star. Over the past 50 years, astrophysicists have discovered that Type Ia supernovae are part of binary systems –– two stars orbiting each other. The one that explodes is a white dwarf star. "That's what our sun will be at the end of its life," he said. "It will have the mass of the sun crammed into the size of the Earth."

The white dwarf stars that tend to explode as Type Ia supernovae have approximately the same mass. This was considered a fundamental limit of physics, according to Howell. However, in an article in Nature about five years ago, Howell reported his discovery of stars that go beyond this limit. These previously unknown Type Ia supernovae have more than typical mass before they explode –– a fact that confounds scientists.

Howell presented a hypothesis to understand this new class of objects. "One idea is that two white dwarfs could have merged together; the binary system could be two white dwarf stars," he said. "Then, over time, they spiral into each other and merge. When they merge, they blow up. This may be one way to explain what is going on."

Astrophysicists are using Type Ia supernovae to build a map of the history of the universe's expansion. "What we've found is that the universe hasn't been expanding at the same rate," said Howell. "And it hasn't been slowing down as everyone thought it would be, due to gravity. Instead, it has been speeding up. There's a force that counteracts gravity and we don't know what it is. We call it dark energy."

The new findings relate to Einstein's concept of the cosmological constant. This is a term he added into his equations to make them valid. However, Einstein did it because he thought the universe was static; he didn't know the universe was expanding. When it was revealed that the universe is expanding, Einstein believed this concept was his biggest blunder. "It turns out that this cosmological constant was actually one of his greatest successes," said Howell. "This is because it's what we need now to explain the data."

He said that dark energy is probably a property of space. "Space itself has some energy associated with it," said Howell. "That's what the results seem to indicate, that dark energy is distributed everywhere in space. It looks like it's a property of the vacuum, but we're not completely sure. We're trying to figure out how sure are we of that –– and if we can improve Type Ia supernovae as standard candles we can make our measurements better."

Throughout history, people have noticed a few supernovae so bright they could be seen with the naked eye. With telescopes, astronomers have discovered supernovae farther away. "Now we have huge digital cameras on our telescopes, and really big telescopes," said Howell, "We've been able to survey large parts of the sky, regularly. We find supernovae daily." Astronomers have discovered thousands of supernovae in recent years.

During his career, Howell has used these powerful telescopes to study supernovae. Currently, besides teaching at UCSB, he is involved in LCOGT's detailed study of supernovae that is aimed at helping to understand dark energy. With this extensive network of observatories, it will be possible to study the night sky continuously.

"The next decade holds real promise of making serious progress in the understanding of nearly every aspect of supernovae Ia, from their explosion physics, to their progenitors, to their use as standard candles," writes Howell in Nature Communications. "And with this knowledge may come the key to unlocking the darkest secrets of dark energy."

CONTACT

Gail Gallessich
805-893-7220

George Foulsham
805-893-3071

More about → ‘Zombie' Stars Key to Measuring Dark Energy