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.)

The Beginning of the Universe in 3 Minutes

Illustration of the Big Bang Theory

One of the greatest mysteries is how the Universe began — and also how and why does it appear to be ever-expanding? CERN physicist Tom Whyntie shows how cosmologists and particle physicists are exploring these questions by replicating the heat, energy, and activity of the first few seconds of our Universe, from right after the Big Bang.

Supernova Primo – Out To Far Frontiers

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Its nickname is SN Primo and it’s the farthest Type Ia supernova to have its distance spectroscopically confirmed. When the progenitor star exploded some 9 billion years ago, Primo sent its brilliant beacon of light across time and space to be captured by the Hubble Space Telescope. It’s all part and parcel of a three-year project dealing specifically with Type Ia supernovae. By splitting its light into constituent colors, researchers can verify its distance by redshift and help astronomers better understand not only the expanding Universe, but the constraints of dark energy.

“For decades, astronomers have harnessed the power of Hubble to unravel the mysteries of the Universe,” said John Grunsfeld, associate administrator for NASA’s Science Mission Directorate in Washington. “This new observation builds upon the revolutionary research using Hubble that won astronomers the 2011 Nobel Prize in Physics, while bringing us a step closer to understanding the nature of dark energy which drives the cosmic acceleration.”

Type Ia supernovae are theorized to have originated from white dwarf stars which have collected an excess of material from their companions and exploded. Because of their remote nature, they have been used to measure great distances with acceptable accuracy. Enter the CANDELS+CLASH Supernova Project… a type of census which utilizes the sharpness and versatility of Hubble’s Wide Field Camera 3 (WFC3) to aid astronomers in the search for supernovae in near- infrared light and verify their distance with spectroscopy. CANDELS is the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey and CLASH is the Cluster Lensing and Supernova Survey with Hubble.

“In our search for supernovae, we had gone as far as we could go in optical light,” said Adam Riess, the project’s lead investigator, at the Space Telescope Science Institute and The Johns Hopkins University in Baltimore, Md. “But it’s only the beginning of what we can do in infrared light. This discovery demonstrates that we can use the Wide Field Camera 3 to search for supernovae in the distant Universe.”

However, discovering a supernova like Primo just doesn’t happen overnight. It took the research team several months of work and a huge amount of near-infrared images to locate the faint signature. After capturing the elusive target in October 2010, it was time to employ the WFC3’s spectrometer to validate SN Primo’s distance and analyze the spectra for confirmation of a Type Ia supernova event. Once verified, the team continued to image SN Primo for the next eight months – collecting data as it faded away. By engaging the Hubble in this type of census, astronomers hope to further their understanding of how such events are created. If they should discover that Type Ia supernova don’t always appear the same, it may lead to a way of categorizing those changes and aid in measuring dark energy. Riess and two other astronomers shared the 2011 Nobel Prize in Physics for discovering dark energy 13 years ago, using Type Ia supernova to plot the Universe’s expansion rate.

“If we look into the early Universe and measure a drop in the number of supernovae, then it could be that it takes a long time to make a Type Ia supernova,” said team member Steve Rodney of The Johns Hopkins University. “Like corn kernels in a pan waiting for the oil to heat up, the stars haven’t had enough time at that epoch to evolve to the point of explosion. However, if supernovae form very quickly, like microwave popcorn, then they will be immediately visible, and we’ll find many of them, even when the Universe was very young. Each supernova is unique, so it’s possible that there are multiple ways to make a supernova.”

Original Story Source: Hubble Site News Release.

The Expanding Universe – Credit To Hubble Or Lemaitre?

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Perhaps one of the greatest astronomical discoveries of the 20th century may have gone down in the history books as credited to the wrong person. Now known as the Hubble Constant, the theory of an expanding Universe was first speculated by Belgian priest and cosmologist, Father Georges Lemaitre. How did this oversight occur? It may very well be the hand of the man himself who was unpretentious enough to pass on his findings.

According to the the November 10th issue of the journal Nature, astrophysicist Mario Livio of the Space Telescope Science Institute is calling for closure about a conspiracy theory of who should be properly credited for the discovery of the expansion theory. For almost a hundred years we’ve been led to believe American astronomer Edwin P. Hubble was the man who explained the universal expansion in 1929 – although he never won a Nobel prize for his work. His findings were based on the achievements of Vesto Slipher, who – through the use of redshift – calculated recessional velocities and paired them with distances to the same galaxies as Hubble’s work. This led Hubble to demonstrate that the further away a galaxy was, the faster it would recede… the Hubble Constant.

However, two years before Hubble published his work, a quiet man called Georges Lemaitre published the same conclusions based on Slipher’s same redshift data and Hubble’s calculated distances.

Father Georges Lemaitre and Albert Einstein - Historical Image
How did this happen and why didn’t Father Lemaitre get credit? According to news release, it may have been because the original paper was published in French, in a rather obscure Belgian science journal called the Annales de la Societe Scientifique de Bruxelles (Annals of the Brussels Scientific Society). Chances are, we never would have known except for a later translation which was published in the Monthly Notices of the Royal Astronomical Society in 1931… a paper which just “left out” Lemaitre’s 1927 calculations! Of course, there were people who knew these passages had been omitted since 1984 and the ensuing debate accused not only the editors of the Monthly Notices, but Hubble as well.

However, before any accusations can be made, let it be noted that astrophysicist Mario Livio combed through an exhaustive archive of hundreds of letters to the Royal Astronomical Society and the RAS meeting minutes – as well as Father Lemaitre’s Archive. What he found was the good Father had simply omitted the passages himself when he translated the papers to English. In one of two “smoking-gun letters” uncovered by Livio, Lemaitre wrote to the editors: “I did not find advisable to reprint the provisional discussion of radial velocities which is clearly of no actual interest, and also the geometrical note, which could be replaced by a small bibliography of ancient and new papers on the subject.”

What is left for us to ponder is “why” Georges Lemaitre didn’t want to take credit for this discovery. Can there really be an altruistic scientist? One who puts the simple act of discovery above himself?

Livio concludes, “Lemaitre’s letter also provides an interesting insight into the scientific psychology of some of the scientists of the 1920s. Lemaitre was not at all obsessed with establishing priority for his original discovery. Given that Hubble’s results had already been published in 1929, he saw no point in repeating his more tentative earlier findings again in 1931.”

Excuse me, folks… After having read the original news release, I think we should rename the Hubble Telescope to read the “Humble Telescope”.

Original Story Source: Hubblesite News Release.