<p>I could not find a freely accessible version of this article, so I am mostly guessing here.
From this article <a href="https://arxiv.org/pdf/1206.0267.pdf" rel="noreferrer">https://arxiv.org/pdf/1206.0267.pdf</a> (especially sections 4 and 5), the main challenge seems to be distinguishing between the different components of the signal:</p>

<blockquote>
  <p>(1) the 21 cm signal coming from the high redshift Universe,
  (2) galactic and extra-galactic foregrounds,
  (3) ionospheric influences,
  (4) telescope response effects,
  (5) Radio frequency interference and
  (6) thermal noise
  Distinguishing between all these components means understanding each of them equally well and probably requires recent knowledge (in cosmology, radio receptors, etc)</p>
</blockquote>

<p>In any case, the idea of using the 21-cm line to probe the early universe seems quite old:
<a href="https://academic.oup.com/mnras/article/188/4/791/999622" rel="noreferrer">https://academic.oup.com/mnras/article/188/4/791/999622</a></p>

CC BY-SA 3.0
2018-03-02T01:36:07.577Z
Thanks, this is a thoughtful answer, and I'll give those a read. You are of course right, the Nature paper is not open access, I'll edit the question accordingly.
uhoh
CC BY-SA 3.0
2018-03-02T06:00:05.233Z
haven't read through it yet, but I think this is it (at least the DM explanation)
https://arxiv.org/abs/1802.10577
Bob
CC BY-SA 3.0
2018-03-03T07:05:22.697Z
See also Pritchard&Loeb "21 cm cosmology in the 21st century" https://arxiv.org/abs/1109.6012.
Urs Schreiber
CC BY-SA 3.0
2018-03-03T08:15:19.270Z
@Bob I wonder if that would shed some light on [this question](https://astronomy.stackexchange.com/q/25339/7982) as well?
uhoh
CC BY-SA 3.0
2018-03-03T08:15:29.190Z
@UrsSchreiber I wonder if that would shed some light on [this question](https://astronomy.stackexchange.com/q/25339/7982) as well? Oh, I'd already linked to it in that question!
uhoh
CC BY-SA 3.0
2018-03-01T22:14:05.613Z
2018-03-01T22:14:05.613Z
user21988
<p>The recent  paper in Nature <a href="https://www.nature.com/articles/nature25792" rel="nofollow noreferrer">An absorption profile centred at 78 megahertz in the sky-averaged spectrum</a> seems (to me at least) to be an incredibly simple and elegant experiment with far-reaching implications.</p>
<p>While the system has been implemented very carefully, it is essentially a spectrally resolved radiometer at radio frequencies below 200 MHz, measuring the sky-averaged power spectrum. And while many of <a href="https://en.wikipedia.org/wiki/List_of_cosmic_microwave_background_experiments" rel="nofollow noreferrer">radiometers in space and on Earth</a> measuring the <a href="https://en.wikipedia.org/wiki/Cosmic_microwave_background" rel="nofollow noreferrer">Cosmic Microwave Background</a> are fairly broad band, measure in the 10's to 100's of GHz range, and intended for very precise absolute intensity measurements of a smooth <a href="https://en.wikipedia.org/wiki/Planck%27s_law" rel="nofollow noreferrer">Plank distribution</a>, the system described here measures between only 50 and 200 <em>MHz</em>, and with a spectral resolution of only 6.1 kHz!</p>
<p>The small set of carefully designed and optimized, antenna(s) and receiver(s) are simple and straightforward; a broadband dipole-type antenna above a large ground plane in a radio-quiet zone, some low noise electronics and what non-radio astronomers might call a very large bandwidth <a href="https://en.wikipedia.org/wiki/Software-defined_radio" rel="nofollow noreferrer">Software-defined Radio</a> or SDRs.</p>
<p>And yet this small, simple receiver, sitting quietly on the ground in Australia's <a href="https://en.wikipedia.org/wiki/Outback" rel="nofollow noreferrer">Outback</a> has produced observations are amazing! They've detected direct evidence of the first stars producing the first starlight in our early universe only about 200 million years after the Big Bang!</p>
<blockquote>
<p><strong>Abstract:</strong></p>
<p>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 megahertz1. 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 predictions2. 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 amplitude3. 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.</p>
</blockquote>
<hr />
<p>I don't see anything about this data that strikes me as unmeasurable with even say 1970's technology if done carefully. The signal is huge, a distinct, broad (about 19 MHz centered at about 87 MHz), several percent (almost 0.6 K) deep <em>dip</em> in an otherwise Rayleigh-Jeans regime blackbody distribution. In <em>20:20 hindsight</em>, it seems to me this could have been detected a long time ago.</p>
<p><strong>Question:</strong> Am I missing something, or is this discovery remarkable not only for it's far reaching implication, but for its simplicity, elegance, and the fact that it could have been discovered quite a long time ago with '60's or '70's technology?</p>
<p><a href="https://i.sstatic.net/GAxE6.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/GAxE6.jpg" alt="enter image description here" /></a></p>
<p><strong>above:</strong> Figure 1 from the <a href="https://www.nature.com/articles/nature25792" rel="nofollow noreferrer">paper</a>.</p>
<p><strong>below:</strong> Figures 2 and then 1 from the paper's <a href="https://www.nature.com/articles/nature25792#extended-data" rel="nofollow noreferrer">Extended data</a>.</p>
<p><a href="https://i.sstatic.net/KQQZC.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/KQQZC.jpg" alt="enter image description here" /></a></p>
<p><a href="https://i.sstatic.net/e1HqW.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/e1HqW.jpg" alt="enter image description here" /></a></p>

<cosmology><radio-astronomy>
Early universe's first starlight (indirectly) detected for the first time, could it have been detectable with 1970's technology?
uhoh
# Early universe's first starlight (indirectly) detected for the first time, could it have been detectable with 1970's technology?

(Asked by: username_0 on 2018-03-01)

The recent paper in Nature [An absorption profile centred at 78 megahertz in the sky-averaged spectrum](https://www.nature.com/articles/nature25792) seems (to me at least) to be an incredibly simple and elegant experiment with far-reaching implications.

While the system has been implemented very carefully, it is essentially a spectrally resolved radiometer at radio frequencies below 200 MHz, measuring the sky-averaged power spectrum. And while many of [radiometers in space and on Earth](https://en.wikipedia.org/wiki/List_of_cosmic_microwave_background_experiments) measuring the [Cosmic Microwave Background](https://en.wikipedia.org/wiki/Cosmic_microwave_background) are fairly broad band, measure in the 10's to 100's of GHz range, and intended for very precise absolute intensity measurements of a smooth [Plank distribution](https://en.wikipedia.org/wiki/Planck%27s_law), the system described here measures between only 50 and 200 *MHz*, and with a spectral resolution of only 6.1 kHz!

The small set of carefully designed and optimized, antenna(s) and receiver(s) are simple and straightforward; a broadband dipole-type antenna above a large ground plane in a radio-quiet zone, some low noise electronics and what non-radio astronomers might call a very large bandwidth [Software-defined Radio](https://en.wikipedia.org/wiki/Software-defined_radio) or SDRs.

And yet this small, simple receiver, sitting quietly on the ground in Australia's [Outback](https://en.wikipedia.org/wiki/Outback) has produced observations are amazing! They've detected direct evidence of the first stars producing the first starlight in our early universe only about 200 million years after the Big Bang!

> **Abstract:**
>
> 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 megahertz1. 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 predictions2. 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 amplitude3. 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.

---

I don't see anything about this data that strikes me as unmeasurable with even say 1970's technology if done carefully. The signal is huge, a distinct, broad (about 19 MHz centered at about 87 MHz), several percent (almost 0.6 K) deep *dip* in an otherwise Rayleigh-Jeans regime blackbody distribution. In *20:20 hindsight*, it seems to me this could have been detected a long time ago.

**Question:** Am I missing something, or is this discovery remarkable not only for it's far reaching implication, but for its simplicity, elegance, and the fact that it could have been discovered quite a long time ago with '60's or '70's technology?

[![enter image description here](https://i.sstatic.net/GAxE6.jpg)](https://i.sstatic.net/GAxE6.jpg)

**above:** Figure 1 from the [paper](https://www.nature.com/articles/nature25792).

**below:** Figures 2 and then 1 from the paper's [Extended data](https://www.nature.com/articles/nature25792#extended-data).

[![enter image description here](https://i.sstatic.net/KQQZC.jpg)](https://i.sstatic.net/KQQZC.jpg)

[![enter image description here](https://i.sstatic.net/e1HqW.jpg)](https://i.sstatic.net/e1HqW.jpg)

**Tags:** cosmology, radio-astronomy

---

## 1 answers

### Answer by username_1 on 2018-03-01. Score: 5 | Accepted answer

I could not find a freely accessible version of this article, so I am mostly guessing here.
From this article <https://arxiv.org/pdf/1206.0267.pdf> (especially sections 4 and 5), the main challenge seems to be distinguishing between the different components of the signal:

> (1) the 21 cm signal coming from the high redshift Universe,
> (2) galactic and extra-galactic foregrounds,
> (3) ionospheric influences,
> (4) telescope response effects,
> (5) Radio frequency interference and
> (6) thermal noise
> Distinguishing between all these components means understanding each of them equally well and probably requires recent knowledge (in cosmology, radio receptors, etc)

In any case, the idea of using the 21-cm line to probe the early universe seems quite old:
<https://academic.oup.com/mnras/article/188/4/791/999622>

> **Comments:**
> **username_0**: Thanks, this is a thoughtful answer, and I'll give those a read. You are of course right, the Nature paper is not open access, I'll edit the question accordingly.  
> **username_2**: haven't read through it yet, but I think this is it (at least the DM explanation) https://arxiv.org/abs/1802.10577  
> **username_3**: See also Pritchard&Loeb "21 cm cosmology in the 21st century" https://arxiv.org/abs/1109.6012.  
> **username_0**: @Bob I wonder if that would shed some light on [this question](https://astronomy.stackexchange.com/q/25339/7982) as well?  
> **username_0**: @UrsSchreiber I wonder if that would shed some light on [this question](https://astronomy.stackexchange.com/q/25339/7982) as well? Oh, I'd already linked to it in that question!  

---

