First Detection of the 21cm Cosmic Dawn Signal | Hacker News Reader

First Detection of the 21cm Cosmic Dawn Signal

astrobites.org

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

> 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...

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?

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

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 [2] https://www.nrao.edu/pr/1999/74mhz/

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.

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

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."

Breakthrough Message project would probably consider this.

I remember in Contact the signal came through on hydrogen times pi.

Yeah I think that's more for dramatic purposes rather than anything practical

Of those many possibilities, Carl Sagan chose the product of pi times the hydrogen spin-flip transition frequency, for use in his novel Contact. In fact, several SETI scientists in Australia had independently arrived at that same target frequency when Carl was writing his novel in 1985, and were already monitoring there. So, great minds think alike (let us hope on other planets, as well as our own!)

http://www.setileague.org/askdr/HtimesPi.htm

As an arbitrary number it is fine, but to me it sounds like "Mysterium Cosmographicum"

In essence, while it could be, I think it's a plausible explanation given our understanding at the time but it might 'get old' soon.

For all we know maybe they might be trying to communicate in an ultra-wide BW that sounds like noise to us. Or using some other method we know nothing about or have just heard about it, like Higgs field perturbations, or ultra-high frequencies.

Which number is it that you’re taking to be arbitrary? I would expect that the product of two non-arbitrary numbers to be non-arbitrary. (Although I think tau might be a small improvement upon pi.)

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

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.

> 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, linked, describes the 21cm emission as occurring once every 10 million years for a single atom.

It is the 21cm hydrogen line from the early universe. It is redshifted from 1420 MHz down to 200 MHz to 9 MHz; they were looking at 78 MHz.

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