BOSS: A Ruler to Measure Them All

Amidst the release of a treasure trove of astronomical data, scientists announce the most precise “standard ruler” yet for cosmological distances.

At the winter meeting of the American Astronomical Society, astronomers released more than 100 terabytes of data as part of one of the richest databases in astronomical history. Scouring the sky since 2000, the Sloan Digital Sky Survey (SDSS) now contains 470 million stars and galaxies, a number that boggles comprehension.

artist's conception of BAO

An artist's conception of the BOSS measurement scale of the universe. Baryon acoustic oscillations are the tendency of galaxies and other matter to cluster in spheres, which originated as density waves traveling through the plasma of the early universe. The clustering is greatly exaggerated in this illustration. The radius of the spheres (white line) is the scale of a “standard ruler” that astronomers can use to determine, within one percent accuracy, the large-scale structure of the universe and how it has evolved.
Zosia Rostomian / Lawrence Berkeley NationalLaboratory)

A significant chunk of that data set comes from the Baryon Oscillation Spectroscopic Survey (BOSS), an SDSS survey that covered 25% of the sky over the past seven years. The goal: detect the imprint of primordial sound waves, called baryon acoustic oscillations, which directly link the universe’s infancy to its adulthood. (These are totally different from the primordial polarization imprint that’s been in the news this last year.)

BOSS adds the third dimension to 2D sky pictures by measuring spectra of 1.4 million galaxies (relatively nearby) and 300,000 quasars (relatively far away), thereby revealing the objects’ distances. That’s a monumental effort best appreciated by realizing that for every one of SDSS’s 5 million spectra, including the 1.7 million in BOSS, a man or woman hand-placed an optical fiber into a metal plate drilled with holes at that object’s location in the sky. Two people can place 1,000 fibers in 40 minutes, so placing all the fibers took 417 workdays.

In the case of BOSS, this massive effort resulted in a 3D map of the universe that covers huge volumes of space. And that’s exactly what’s required to peer into the universe’s past.

The Universe: From Infancy to Adulthood

The newborn universe was a vastly different place than what we see today. Photons and ionized matter mingled together in a hot, clumpy primordial soup. The photons were trapped in the clumps — they couldn’t get far without encountering more of the dense plasma — and they exerted pressure from within. The pressure waves that rippled through the universe were akin to sound waves in Earth’s atmosphere.

BOSS plate

Astronomer Michael Wood-Vasey (University of Pittsburgh) holds up one of the 2,000 plates used in the BOSS project. Each of the 1,000 holes drilled in a single plug plate captures the light from a specific galaxy, quasar, or other target, and conveys its light to a sensitive spectrograph through an optical fiber. The plates are marked to indicate which holes belong to which bundles of the thousand optical fibers that carry the object’s light.
Monica Young / Sky & Telescope

The waves sloshed around for a long time, roughly 380,000. But as soon as the soup cooled enough for electrons and protons to combine, photons made their escape. With no pressure to push matter apart, gravity took over. The remnants of the primordial ripples imprinted themselves on the collapsing clumps of gas and dark matter, which would eventually become galaxies and galaxy clusters.

These remnant ripples aren’t self-evident when you look at a slice of sky. They only make themselves known in huge statistical samples by galaxies’ slight preference to lie 500 million light-years apart, instead of, say, 400 million or 600 million light-years. Yet that tiny statistical effect provides a direct link between fluctuations in the primordial soup and the cosmic web of galaxies that we see in the universe today.

BOSS surveyed huge volumes of space to see this effect, and although the final data analysis isn’t expected until later this spring, 85% of the data has already been analyzed. The primordial sound waves are detected to an extremely high precision: in technical terms, the total detection is 10 sigma. That translates as, “There’s no real question anymore about whether [these waves] exist,” says SDSS-III director Daniel Eisenstein (Harvard University).

BOSS detected ripples by looking at populations of relatively nearby galaxies, divided into two groups whose light has traveled for 3.5 billion and 5.7 billion years, respectively, and by looking at more distant quasars, whose light has traveled 11 billion years to Earth. These two data sets sandwich the era when the universe's expansion began accelerating.

Normally, astronomers need to calculate distances to these objects using their redshifts, measuring how far spectral lines shift due to the expansion of the universe. But that requires models of how fast the universe is expanding.

Primordial sound waves provide a ruler independent of cosmological models. If you know how big the ripples should be (information that can be found in cosmic microwave background fluctuations), and you measure how big they appear on the sky, you get a measure of distance. With the results announced at AAS, these distances are now known to an accuracy of 1%.

Alternatively, you can forgo the cosmic microwave background measurements and just calculate relative ripple sizes at different distances to see how quickly the ruler expands over cosmic time. Either way, both measurements provide excellent (and unsurprising) agreement with the leading cosmological model, including a mysterious dark energy whose nature has stayed constant since the Big Bang.

But perhaps the most interesting results from BOSS are yet to come, as astronomers continue to apply the enormous data set to quasar physics, galaxy evolution, and other science.

6 thoughts on “BOSS: A Ruler to Measure Them All

  1. Peter WilsonPeter Wilson

    Don’t forget the Lorentz transformation.
    A galaxy 3.5 bly away is receding at 0.23c, and is therefore shortened in our direction by a factor of 0.94. In other words, it is 6% farther away than it looks. With these distances now known to an accuracy of 1%, is that before or after the Lorentz transformation?

  2. Howard RitterHoward Ritter

    The Lorentz contraction shortens the dimension of a moving object along the line of motion. It does not affect the distance from the object to an external observer moving at a different velocity.

    Can someone comment on whether the cosmological expansion of the universe, which is not a proper motion of galaxies THROUGH space but an increase in distance due to the expansion OF space, even produces a Lorentz contraction? It’s my impression that it does not. As a case in point, the cosmological red shift is due to a stretching of the wavelength of light in transit across cosmological distances as the universe expands, not due to an intrinsic motion of the distant object. There are, of course, red shifts (and blue shifts) that ARE due to intrinsic motions, but these are separate from the cosmological red shift.

    1. BRUCEEDWARDARNOLD

      There is both proper motion of the expansion of the Universe and an expansion of space. The rediscovery of the cosmic ether means that it may have been expanding before the expansion of matter. That ether is limited to expanding at the speed of light in the initial Big Bang is of no doubt, but once matter began to condense and it too was expanding at the speed of light the ether then could expand at the speed of light relative to the matter, this is the period we call “inflation” and the expansion proceeded at the speed of light squared. Much or most of the Universe expanded beyond what amounts to an event horizon where the speed of light was exceeded relative to the inner Universe. No matter where we are relative to the origin point of the Big Bang we can not see beyond this event horizon and whatever our motion, the edge of the Universe that we see will be redshifted uniformly. Again referring to my other post, the Lorentz contraction does not exist as it was based on an errant concept and Michelson’s failure to consider the Doppler Effect in the design of his interferometer. As to the stretching of space, this would not also stretch the wavelengths of light already emitted, thus this light would appear to be blue shifted, shorter relative to the space it is traversing.

    2. Peter WilsonPeter Wilson

      That’s a good question, Howard.
      It’s my impression that relative velocity of any sort produces a Lorentz contraction: remote galaxies should appear foreshortened; galaxy clusters should appear squished; the gaps between them should appear compressed; etc. If the objects and distance between them are all compressed along our line-of-sight, then the total distance to them will appear compressed. If a galaxy is compressed 6% in our direction, the total distance to it will be reduced by only 3%, because the length-reduction is cumulative. But if they claim a measurement accuracy of 1%, it matters whether the Lorentz contraction applies or not.

  3. BRUCEEDWARDARNOLD

    There is no Lorentz Transformation. The Lorentz Transformation was an ad hock fix by Lorentz, and Fitzgerald, to explain the null result of A. A. Michelson’s 1881 interferometry experiment looking for the ether. Michelson figured that he was looking for about a wavelength of change in the two million wavelengths per meter of his instrument. Michelson however failed to consider the Doppler Effect, which at the Earth’s revolving around the Sun at 30 km/sec should have shown up as 200+/- wavelengths of change in his interferometer. This famous “failed” experiment, failed in a way that scientists have missed until now. Michelson found the ether, but what he proved is that the ether is fully bound to both the motion and the rotation of the Earth. This is even more so when one fully considers the full motion of the Earth as it moves through the Universe. The speed of light is only constant to the ether in which it travels, and the Doppler Effect occurs as as light transitions from one ether to another.

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