Quasars Help Shed Light on Dark Energy Mystery
There’s exciting news today for those following the quest to comprehend dark energy — the mysterious repulsive force that fills the universe, causing its expansion to accelerate. New results from the Baryon Oscillation Spectroscopic Survey (BOSS) — part of the Sloan Digital Sky Survey (SDSS-III) — relying on data from quasars have enabled physicists to produce a detailed 3D “map” of the early universe a whopping 11.5 billion years ago.
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That’s the period where physicists think dark energy was not yet dominant over the mutual gravitational pull of all the matter in the universe. The new BOSS measurements indicate that before that critical point 11 billion years ago, the expansion of the universe was actually slowing down. These findings should shed light on that critical transition point where dark matter started to dominate.
“If we think of the universe as a roller coaster, then today we are rushing downhill, gaining speed as we go,” said Nicolas Busca (French Centre National de la Recherche), one of the lead authors on the study, said via press release. “Our new measurement tells us about the time when the universe was climbing the hill — still being slowed by gravity.”
Some 80 years after Edwin Hubble and Georges Lemaitre made the first
measurements of how fast our nearby universe was expanding, the BOSS
collaboration has done the same for our universe as it was 11 billion
The revolutionary 1998 discovery that led to the theory of dark energy relied on studying the red shifts of bright light from supernovae. BOSS, in contrast, looks at something called baryonic acoustic oscillation (BAO). This phenomenon is the result of pressure waves (sound, or acoustic waves) propagating through the early universe in its earliest hot phase, when everything was just one big primordial soup.
Those sound waves created pockets where the density differed in regular intervals or periods, a “wiggle” pattern indicative of oscillation, or vibration. Then the universe cooled sufficiently for ordinary matter and light to go their separate ways, the former condensing into hydrogen atoms.
We can still see signs of those variations in temperature in the cosmic microwave background (CMB), thereby giving scientists a basic scale for BAO — a “standard ruler,” if you will, to compare the size of the universe at various points as it evolved through time.
Earlier this year, BOSS announced the first results from their collaboration, revealing the most precise measurements ever made of the large-scale structure of the universe between five to seven billion years ago.
The results were significant because that time frame is the era when dark energy “turned on.”
That earlier survey looked at galaxies, but as useful as the results were, the galaxy-measuring approach was not sufficient to map structures that are as far away as 11.5 billion years, because those galaxies are just too faint.
So BOSS scientists turned to a cunning new technique that relied on the light from quasars to measure the clumping of hydrogen gas between galaxies in the distant universe.
“Quasars” are short for “quasi-stellar radio sources,” regions around the black holes at the center of massive galaxies that give off a great deal of radiation. Black holes provide the power source. As matter — gas and dust, primarily — wanders near a black hole, it doesn’t cross the event horizon and fall into the hole directly. Instead, it forms an accretion disk, and the light is the result of the energy produced as the black hole gobbles up that gas and dust.
Quasars have long been a boon to astronomers: images of a double quasar helped confirm Einstein’s prediction of gravitational lensing in 1979. And in September, quasar data from the Massive Compact Halo Objects (MACHO) project were used as cosmic mileposts to map the structure and expansion history of the universe.
How do quasars help us view the distant universe? They let us “see” all that intergalactic hydrogen gas clustering between galaxies, because that gas absorbs some of the light from the quasars just behind it.
Physicists can look at the spectra of quasar light to figure out how it changes as that light moves through space and time. In particular, the spectrum changes as intervening gas absorbs some of the quasar’s light — a phenomenon known as the “Lyman-alpha forest.”
Brookhaven’s Anze Slosar, another contributor to this research, described the technique as “measuring the shadows cast by gas along a single line billions of light years long.” But it’s difficult to see the Lyman-alpha forest for the trees. The tricky part, Slosar added, was “combining all those one-dimensional maps into a three-dimensional map. It’s like trying to see a picture that’s been painted on the quills of porcupines.”
Physicists weren’t sure at first if this unusual approach would work, but over the last year, as the data was analyzed, it became clear that the measurements matched perfectly with theoretical predictions of where the BOA “peak” should be.
So the picture of our universe, a mere three billion years after the big bang, shows that dark energy worked much the same way it does today — a constant part of space throughout the cosmos that gradually became more dominant as matter in the universe moved further and further apart.
“It looks like the roller coaster crested the hill just about seven billion years ago, and we’re still going,” said Busca.
Images: Top: Artist’s rendering of ULAS J1120+0641, a very distant quasar powered by a black hole with a mass two billion times that of the Sun. Credit: M. Kornmesser, European Southern Observatory. Center: An illustration of the concept of baryon acoustic oscillations, which are imprinted in the early universe. Credit: Chris Blake and Sam Moorfield. Bottom: Light from distant quasars gets absorbed by hydrogen gas, leaving behind a “forest” of absorption lines in the spectrum. Credit: Zosia Rostomian, LBNL; Nic Ross, BOSS/LBNL; Springel et al, Virgo Consortium and the Max Planck Institute for Astrophysics.