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Star bright, first light — fingerprint hunt

Following up on the “Ultimate how” question in the context of the Big Bang theory, how far back in time can we actually detect evidence, follow a breadcrumbs trail? To a cosmic dawn?, among others, today posted articles about research at the Murchison Radio-Astronomy Observatory (MRO), in particular the MRO’s Experiment to Detect the Global EoR Signature (EDGES) ground-based radio spectrometer.

Astronomers have picked up a long-sought signal from some of the universe’s first stars, determining that these pioneers were burning bright by just 180 million years after the Big Bang.

… the signal that Bowman and his team found was surprisingly strong. It was so strong, in fact, that it hints at a possible interaction between mysterious dark matter and the “normal” stuff that makes up the stars and you and me and everything else we can see in the universe.

The Big Bang theory characterizes the early universe as “suffused with neutral hydrogen atoms, which are good at blocking light.” So, early stars’ “ultraviolet radiation … would excite hydrogen atoms into a different state, causing them to absorb CMB photons” [rather than actually split these atoms].

Detection by the EDGES team required isolating a tiny variation (in this case a dip) within a broader radio spectrum, in this case the cosmic background radiation (CMB). The redshifted 21-cm line.1

“There is a great technical challenge to making this detection,” Peter Kurczynski, the NSF program director who oversaw funding for EDGES, said in a statement. “Sources of noise can be 10,000 times brighter than the signal. It’s like being in the middle of a hurricane and trying to hear the flap of a hummingbird’s wing.

Confidence in identifying the correct variation then permitted a redshift calculation to estimate when those CMB photons were absorbed. Confirming the detection is the next step. Then once again the CMB as a “baby picture” of the early universe [an accidental discovery in 1964] can be “mined” for more information.

Astronomers with the Experiment to Detect the Global EoR (Epoch of Reionization) Signature (EDGES) project reported today (Feb. 28) that they’ve spotted the apparent fingerprints of the universe’s first stars. The signal they detected was twice as intense as they had predicted, suggesting that either the hydrogen gas pervading the early universe was significantly colder than expected, or background radiation levels were significantly hotter than light from the cosmic microwave background, the radiation left over from the Big Bang that formed the universe.2


[1] It’s all in the lines, eh. Wiki:

The line [hydrogen line, 21-centimeter line or H I line] is of great interest in big bang cosmology because it is the only known way to probe the “dark ages” from recombination to reionization. Including the redshift, this line will be observed at frequencies from 200 MHz to about 9 MHz on Earth. It potentially has two applications. First, by mapping the intensity of redshifted 21 centimeter radiation it can, in principle, provide a very precise picture of the matter power spectrum in the period after recombination. Second, it can provide a picture of how the universe was reionized, as neutral hydrogen which has been ionized by radiation from stars or quasars will appear as holes in the 21 cm background.

[2] It’s all about temperature, detecting changes or patterns in the temperature of radiation — cooling and heating.

Gas in the early universe was already extremely cold — the coldest it would ever be, Barkana said, because it had cooled down from the universe’s hot early days but had not yet been heated by starlight.

The tiny amount of cooling is key to why this effect can be observed only in the early universe. With nothing else to heat the gas, it would quickly cool as it interacted with the dark matter, creating the unusual signal observed by the EDGES team. [Gallery: Dark Matter Throughout the Universe]

The signal the EDGES team spotted came from the absorption of cosmic microwave background photons by hydrogen gas. Barkana said it’s possible that something could be strange about the radio background rather than the gas.

“The measurement is solid, and it passed many checks,” he said. “But still, it’s important to get independent confirmation by a different instrument.”


2 thoughts on “Star bright, first light — fingerprint hunt

  1. Here’s the link to the original Nature journal article, “An absorption profile centred at 78 megahertz in the sky-averaged spectrum.”


    After stars formed in the early Universe, their ultraviolet light is expected, eventually, to have penetrated the primordial hydrogen gas and altered the excitation state of its 21-centimetre hyperfine line. This alteration would cause the gas to absorb photons from the cosmic microwave background, producing a spectral distortion that should be observable today at radio frequencies of less than 200 megahertz. Here we report the detection of a flattened absorption profile in the sky-averaged radio spectrum, which is centred at a frequency of 78 megahertz and has a best-fitting full-width at half-maximum of 19 megahertz and an amplitude of 0.5 kelvin. The profile is largely consistent with expectations for the 21-centimetre signal induced by early stars; however, the best-fitting amplitude of the profile is more than a factor of two greater than the largest predictions. This discrepancy suggests that either the primordial gas was much colder than expected or the background radiation temperature was hotter than expected. Astrophysical phenomena (such as radiation from stars and stellar remnants) are unlikely to account for this discrepancy; of the proposed extensions to the standard model of cosmology and particle physics, only cooling of the gas as a result of interactions between dark matter and baryons seems to explain the observed amplitude. The low-frequency edge of the observed profile indicates that stars existed and had produced a background of Lyman-α photons by 180 million years after the Big Bang. The high-frequency edge indicates that the gas was heated to above the radiation temperature less than 100 million years later.

  2. After writing my blog post yesterday, I received a copy of Sean Carroll’s post on the topic, “Dark Matter and the Earliest Stars.” His post summarizes the detection well. It’s good to see commentary by a theoretical physicist who has written about physical cosmology.

    … any morsel of information we can scrounge up is very helpful in putting together a picture of how the universe evolved from a relatively smooth plasma to the lumpy riot of stars and galaxies we see today.

    … when stars first start shining, they can very gently excite the gas around them (the 21cm hyperfine transition, for you experts), which in turn can affect the wavelength of radiation that gets absorbed. This shows up as a tiny distortion in the spectrum of the CMB itself. It’s that distortion which has now been observed …

    … it’s a tour de force bit of observational cosmology …

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