
First Detection of the 21cm Cosmic Dawn Signal - radicaldreamer
https://astrobites.org/2018/03/14/first-detection-of-the-21cm-cosmic-dawn-signal/
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amluto
This is incredibly cool. The signal is hard to detect because it's quite faint
and there are tons of other signals superimposed on it: the galactic
synchrotron, other astronomical sources, and terrestrial radio (FM, for
example). And the neutral hydrogen involved can both emit and absorb 21cm
light. Getting it right is _hard_ , but various groups are working on it.

Once we can not just detect the signal but measure its spatial and frequency
variation, we'll get a very nice map of the earlier universe. In particular,
the 21cm line is very narrow, so its power spectrum as observed from Earth
gives the strength of the signal as a function of redshift, which is strongly
related to distance. If we can get some decent spatial resolution, this will
give us a 3D map of the density of neutral hydrogen on the early universe.

This will give great data for cosmological calculations. Most useful
cosmological observations right now are either 2-dimensional (e.g. the cosmic
microwave background, which we can resolve angularly with great detail but
represents a very narrow slice of time) or barely 3-dimensional (supernova
observations, for example, which let us map a much smaller range of times than
we can potentially map with 21cm observation).

(Part of my thesis was on an approach to detecting the 21cm signal and its
spatial variations.)

Followup: A friend who works on HERA, which is also looking for the 21cm
signal, forwarded this discussion:
[https://twitter.com/UCBProf/status/969071237405097985](https://twitter.com/UCBProf/status/969071237405097985)

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JumpCrisscross
> _the galactic synchrotron_

"The plane of the Milky Way is dominated by diffuse radio emission with a
brightness temperature of thousands of Kelvin at low radio frequencies (Zheng
et al. 2016). This emission originates from relativistic electrons interacting
with the Galactic magnetic field."

[https://uhra.herts.ac.uk/bitstream/handle/2299/17665/stw2959...](https://uhra.herts.ac.uk/bitstream/handle/2299/17665/stw2959.pdf?sequence=2)

~~~
Sniffnoy
Question: Why the galactic synchrotron and not, say, the solar synchrotron?
I'm assuming the latter exists but is negligible for some reason? Why is that?

~~~
eosophos
Apparently the particles need to be relativistic, maybe the Sun doesn't
generate enough acceleration compared to the central galactic supermassive
black hole?

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kawfey
21cm was initially misleading; that's the origination of the hydrogen line at
1420MHz; by the time it reaches earth it is doppler shifted down (redshift) to
78MHz!

I worked as an antenna engineer on the VLA's low-band Epoch of Reionization
expansion project in 2014[1], which focused on 74MHz signals. They've been
looking at this frequency since 1999 [2] but who knew they were 4MHz low! (I
doubt that they just didn't look there, likely some other effects or local RFI
problems, as Australia is a lot more remote than New Mexico.)

[1]
[http://lwa.phys.unm.edu/users14/Ellingson_MJP.pdf](http://lwa.phys.unm.edu/users14/Ellingson_MJP.pdf)
[2] [https://www.nrao.edu/pr/1999/74mhz/](https://www.nrao.edu/pr/1999/74mhz/)

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nategri
Quasi-related fact:

Radio frequencies with a wavelength of 21cm are one of SETI's favorite places
to look for artificial signals. Since it originates from atomic hydrogen (of
which there is a _lot_ ), it's one of the notes that the universe likes to hum
to itself---and radio-using species everywhere would probably figure that out.

~~~
duncan_bayne
So, do _we_ deliberately transmit at that wavelength, with the intention of
attracting the attention of ETIs?

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pharrington
It's certainly one of the better ideas for that purpose. I like to imagine,
though, that the sufficiently advanced aliens, upon receiving the signal,
would say "huh, there's this weird low energy 1420Mhz emission coming from
Sol. Guess we'll file it next to the other million aberrant emissions in the
Local Group we can't explain."

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Sniffnoy
OK, I don't really understand this. What exactly is this cosmic dawn signal
that's distinct from all the other things on 21cm?

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manicdee
Apologies if this is oversimplified.

In the (time shortly after the) beginning the universe was formless and void,
and all that existed was hydrogen and hydrogen was all that existed. Over time
the hydrogen clumped together through various forces such as gravity.
Eventually enough hydrogen clumped together that the force of gravity was
sufficient to trigger nuclear fusion. And then there was light.

Hydrogen can absorb or (suitably excited) emit light at 21cm wavelength
specifically, better than all other wavelengths. So from the background
smorgasbord of frequencies emitted by the extremely hot hydrogen of the first
stars (“First Light”) the cold hydrogen would have absorbed some as heat, and
absorbed more at 21cm as excitation energy.

So assuming a relatively linear distribution of wavelengths in First Light
(you could call it “white” light) there will be a portion where photons at
21cm were absorbed by hydrogen.

So the Cosmic Dawn signal is distinct because of what it is missing.

~~~
ballenarosada
> Hydrogen can absorb or (suitably excited) emit light at 21cm wavelength
> specifically, better than all other wavelengths.

Based on the article, I don't think this is right.
[http://astronomy.swin.edu.au/cosmos/S/Spin-
flip+Transition](http://astronomy.swin.edu.au/cosmos/S/Spin-flip+Transition),
linked, describes the 21cm emission as occurring once every 10 million years
for a single atom.

