Like a BOSS: How Astronomers are Getting Precise Measurements of the Universe’s Expansion Rate

Astrophysicists studying the expansion of the Universe with the largest galaxy catalogs ever assembled are ushering in an exciting era of precision cosmology. Last week, the Sloan Digital Sky Survey (SDSS) issued its final public data release, and scientists working in its largest program, the Baryon Oscillation Spectroscopic Survey (BOSS) also presented their final results at the American Astronomical Society meeting in Seattle, Washington.

By mapping over 10,000 square degrees — 25% of the sky — BOSS is “measuring our universe’s accelerated expansion with the world’s largest extragalactic redshift survey,” according to SDSS-III Director Daniel Eisenstein of the Harvard-Smithsonian Center for Astrophysics. The BOSS results include new and precise measurements of the universe’s expansion rate (called the “Hubble constant”) and matter density, which includes dark matter, stars, gas, and dust.

BOSS conducted its observations at 2.5-meter Sloan Foundation Telescope at Apache Point Observatory in New Mexico, producing spectra and spatial positions for 1.5 million galaxies and 300,000 quasars in a volume equivalent to a cube with length 8.5 billion light-years on a side (see image above). Astronomers used this rich dataset to map the objects’ distributions and to detect the characteristic scale imprinted by baryon acoustic oscillations in the early universe. Sound waves propagate outward with time, like ripples spreading in a pond, and are indicated by a large-scale clustering signal in the positions of galaxies relative to each other (see illustration below). By analyzing this signal at different times, it is possible to study the behavior of the mysterious “dark energy” causing the accelerating expansion of the universe.

An illustration of the concept of baryon acoustic oscillations, imprinted in the early universe and seen today in galaxy surveys. (courtesy:  Chris Blake and Sam Moorfield)
An illustration of the concept of baryon acoustic oscillations, imprinted in the early universe and seen today in galaxy surveys. (courtesy: Chris Blake and Sam Moorfield)

In BOSS’s final results, hundreds of scientists in the international collaboration measured this scale with unprecedented precision. In particular, Ashley Ross from Ohio State University presented results that demonstrated the power of combining an analysis of the transverse and line-of-sight distributions of galaxies. In a paper by Eric Aubourg and collaborators, BOSS astronomers measured the cosmic distance scale of galaxies in the “local” universe and of quasars in the distance universe with impressively small systematic errors—at less than the 1% level—when combined with cosmic microwave background constraints. Their cosmological analysis yields a measurement of the Hubble constant and of the matter density of the universe consistent with a “flat” cold dark matter cosmology with a cosmological constant (see below). Cosmological models including curvature, evolving dark energy, or massive neutrinos are not completely ruled out but are less supported by the data than before. Other results from the collaboration will be submitted for publication in the coming months.

Cosmological constraints on the Hubble parameter h, matter density Ωm, and curvature parameter Ωk from BOSS's baryon acoustic oscillations (BAO) combined with supernovae (SN) and Planck results. (Courtesy: Aubourg et al. 2014)
Cosmological constraints on the Hubble parameter h, matter density Ωm, and curvature parameter Ωk from BOSS’s baryon acoustic oscillations (BAO) combined with supernovae (SN) and Planck results. (Courtesy: Aubourg et al. 2014)

The BOSS dataset “represents the gold standard in mapping out the network of galaxies that comprises the large-scale structure of the Universe…The data enables us to trace, with greater precision than ever before, the presence of dark energy, the behaviour of gravity on cosmic scales, and the effect of massive neutrinos,” says Chris Blake of Swinburne University, not affiliated with the collaboration.

Where will the BOSS team go from here? The collaboration has begun work on SDSS-IV, whose six-year mission includes an ambitious extended BOSS (eBOSS) survey. According to eBOSS Targeting Coordinator Jeremy Tinker of New York University, eBOSS observations of over 700,000 quasars will precisely measure the distance scale “at a much higher redshift regime that is not covered by current large-scale surveys.”

You can read more about BOSS and updates about the three other componenets of the SDSS in our previous article here.
SDSS website

(Full disclosure: Ramin Skibba had been a member of the BOSS collaboration during 2010-2012.)

Hearing the Early Universe’s Scream: Sloan Survey Announces New Findings

Imagine a single mission that would allow you to explore the Milky Way and beyond, investigating cosmic chemistry, hunting planets, mapping galactic structure, probing dark energy and analyzing the expansion of the wider Universe. Enter the Sloan Digital Sky Survey, a massive scientific collaboration that enables one thousand astronomers from 51 institutions around the world to do just that.

At Tuesday’s AAS briefing in Seattle, researchers announced the public release of data collected by the project’s latest incarnation, SDSS-III. This data release, termed “DR12,” represents the survey’s largest and most detailed collection of measurements yet: 2,000 nights’ worth of brand-new information about nearly 500 million stars and galaxies.

One component of SDSS is exploring dark energy by “listening” for acoustic oscillation signals from the the acceleration of the early Universe, and the team also shared a new animated “fly-through” of the Universe that was created using SDSS data.

The SDSS-III collaboration is based at the powerful 2.5-meter Sloan Foundation Telescope at the Apache Point Observatory in New Mexico. The project itself consists of four component surveys: BOSS, APOGEE, MARVELS, and SEGUE. Each of these surveys applies different trappings to the parent telescope in order to accomplish its own, unique goal.

BOSS (the Baryon Oscillation Spectroscopic Survey) visualizes the way that sound waves produced by interacting matter in the early Universe are reflected in the large-scale structure of our cosmos. These ancient imprints, which date back to the first 500,000 years after the Big Bang, are especially evident in high-redshift objects like luminous-red galaxies and quasars. Three-dimensional models created from BOSS observations will allow astronomers to track the expansion of the Universe over a span of 9 billion years, a feat that, later this year, will pave the way for rigorous assessment of current theories regarding dark energy.

At the press briefing, Daniel Eistenstein from the Harvard-Smithsonian Center for Astrophysics explained how BOSS requires huge volumes of data and that so far 1.4 million galaxies have been mapped. He indicated the data analyzed so far strongly confirm dark energy’s existence.

This tweet from the SDSS twitter account uses a bit of humor to explain how BOSS works:

APOGEE (the Apache Point Observatory Galactic Evolution Experiment) employs a sophisticated, near-infrared spectrograph to pierce through thick dust and gather light from 100,000 distant red giants. By analyzing the spectral lines that appear in this light, scientists can identify the signatures of 15 different chemical elements that make up the faraway stars – observations that will help researchers piece together the stellar history of our galaxy.

MARVELS (the Multi-Object APO Radial Velocity Exoplanet Large-Area Survey) identifies minuscule wobbles in the orbits of stars, movements that betray the gravitational influence of orbiting planets. The technology itself is unprecedented. “MARVELS is the first large-scale survey to measure these tiny motions for dozens of stars simultaneously,” explained the project’s principal investigator Jian Ge, “which means we can probe and characterize the full population of giant planets in ways that weren’t possible before.”

At the press briefing, Ge said that MARVELS observed 5,500 stars repeatedly, looking for giant exoplanets around these stars. So far, the data has revealed 51 giant planet candidates as well as 38 brown dwarf candidates. Ge added that more will be found with better data processing.

A still photo from an animated flythrough of the universe using SDSS data. This image shows a small part of the large-scale structure of the universe as seen by the SDSS -- just a few of many millions of galaxies. The galaxies are shown in their proper positions from SDSS data. Image credit: Dana Berry / SkyWorks Digital, Inc.
A still photo from an animated flythrough of the universe using SDSS data. This image shows a small part of the large-scale structure of the universe as seen by the SDSS — just a few of many millions of galaxies. The galaxies are shown in their proper positions from SDSS data. Image credit: Dana Berry / SkyWorks Digital, Inc.

SEGUE (the Sloan Extension for Galactic Understanding and Exploration) rounds out the quartet by analyzing visible light from 250,000 stars in the outer reaches of our galaxy. Coincidentally, this survey’s observations “segue” nicely into work being done by other projects within SDSS-III. Constance Rockosi, leader of the SDSS-III domain of SEGUE, recaps the importance of her project’s observations of our outer galaxy: “In combination with the much more detailed view of the inner galaxy from APOGEE, we’re getting a truly holistic picture of the Milky Way.”

One of the most exceptional attributes of SDSS-III is its universality; that is, every byte of juicy information contained in DR12 will be made freely available to professionals, amateurs, and lay public alike. This philosophy enables interested parties from all walks of life to contribute to the advancement of astronomy in whatever capacity they are able.

As momentous as the release of DR12 is for today’s astronomers, however, there is still much more work to be done. “Crossing the DR12 finish line is a huge accomplishment by hundreds of people,” said Daniel Eisenstein, director of the SDSS-III collaboration, “But it’s a big universe out there, so there is plenty more to observe.”

DR12 includes observations made by SDSS-III between July 2008 and June 2014. The project’s successor, SDSS-IV, began its run in July 2014 and will continue observing for six more years.

Here is the video animation of the fly-through of the Universe:

One Percent Measure of the Universe

When it comes to accuracy, everyone strives for a hundred percent, but measuring cosmic distances leaves a bit more to chance. Just days ago, researchers from the Baryon Oscillation Spectroscopic Survey (BOSS) announced to the world that they have been able to measure the distance to galaxies located more than six billion light-years away to a confidence level of just one percent. If this announcement doesn’t seem exciting, then think on what it means to other studies. These new measurements give a parameter to the properties of the ubiquitous “dark energy” – the source of universal expansion.

“There are not many things in our daily lives that we know to one-percent accuracy,” said David Schlegel, a physicist at Lawrence Berkeley National Laboratory (LBNL) and the principal investigator of BOSS. “I now know the size of the universe better than I know the size of my house.”

The research team’s findings were presented at the meeting of the American Astronomical Society by Harvard University astronomer Daniel Eisenstein, the director of the Sloan Digital Sky Survey III (SDSS-III), the worldwide organization which includes BOSS. They are detailed in a series of articles submitted to journals by the BOSS collaboration last month, all of which are now available as online preprints.

“Determining distance is a fundamental challenge of astronomy,” said Eisenstein. “You see something in the sky — how far away is it? Once you know how far away it is, learning everything else about it is suddenly much easier.”

When it comes to measuring distances in space, astronomers have employed many methods. To measure distances to planets has been accomplished using radar, but it has its constraints and going further into space means a less direct method. Even though they have been proved to be amazingly accurate, there is still an uncertainty factor involved – one that is expressed as a percentage. For example, if you were to measure the distance from an object 200 miles away to within a true value of 2 miles, then you have measured with an accuracy of 1%. Cosmically speaking, just a few hundred stars and a handful of star clusters are actually close enough to have their distances so accurately predicted. They reside within the Milky Way and are just a few thousand light-years away. BOSS takes it to the extreme… its measurements go well beyond our galactic boundaries, more than a million times further, and maps the Universe with unparalleled accuracy.

Thanks to these new, highly-accurate distance measurements, BOSS astronomers are making headway in the field of dark energy. “We don’t yet understand what dark energy is,” explained Eisenstein, “but we can measure its properties. Then, we compare those values to what we expect them to be, given our current understanding of the universe. The better our measurements, the more we can learn.”

Just how is it done? To achieve a one-percent measurement at six billion light years isn’t as easy as measuring a solar system object, or even one contained within our galaxy. That’s where the BOSS comes into play. It’s the largest of the four projects that make up the Sloan Digital Sky Survey III (SDSS-III), and was built to take advantage of this technique: measuring the so-called “baryon acoustic oscillations” (BAOs), subtle periodic ripples in the distribution of galaxies in the cosmos. These ripples are the signature of pressure waves which once cruised the early Universe at a time when things were so hot and dense that photons marched along with baryons – the stuff which creates the nuclei of atoms. Since the size of the ripple is known, that size can now be measured by mapping galaxies.

“With these galaxy measurements, nature has given us a beautiful ruler,” said Ashley Ross, an astronomer from the University of Portsmouth. “The ruler happens to be half a billion light-years long, so we can use it to measure distances precisely, even from very far away.

Using its specialized instrumentation which can make detailed measurements of a thousand galaxies at a time, BOSS took on a huge challenge – mapping the location of more than a million galaxies. “On a clear night when everything goes perfectly, we can add more than 8000 galaxies and quasars to the map,” said Kaike Pan, who leads the team of observers at the SDSS-III’s Sloan Foundation 2.5-meter Telescope at Apache Point Observatory in New Mexico.

Although the BOSS research team presented its early galaxy maps and beginning BAO measurements a year ago, this new data covers twice as much territory and gives an even more accurate measurement – including those to nearby galaxies. “Making these measurements at two different distances allows us to see how the expansion of the universe has changed over time, which will help us understand why it is accelerating,” explained University of Portsmouth astronomer Rita Tojeiro, who co-chairs the BOSS galaxy clustering working group along with Jeremy Tinker of New York University.

Also doing a similar study is Mariana Vargas-Magana, a postdoctoral researcher at Carnegie Mellon University. To enable even more accuracy, she’s looking into any subtle effects which could influence the BOSS measurements. “When you’re trying to reach one percent, you have to be paranoid about everything that could go even slightly wrong,” said Vargas-Magana — for example, slight differences in how galaxies were identified could have thrown off the entire measurement of their distribution, so different parts of the sky had to be checked carefully. “Fortunately,” Vargas-Magana said, “there are plenty of careful people on our team to check our assumptions. By the time all of them are satisfied, we are sure we didn’t miss anything.”

As of the present, these new BOSS findings would seem to be consistent with what we consider to be form of dark energy – a constant found throughout the history of the Universe. According to the news release, this “cosmological constant” is one of just six numbers required to create a model which coincides with the scale and structure of the Universe. Schlegel compares this six-number model to a pane of glass, which is pinned in place by bolts that represent different measurements of the history of the Universe. “BOSS now has one of the tightest of those bolts, and we just gave it another half-turn,” said Schlegel. “Each time you ratchet up the tension and the glass doesn’t break, that’s a success of the model.”

Original Story Source: Sloan Digital Sky Survey III News Release. For further reading: Max Planck Institute News Release.