Radio Astronomers listen to the Early Universe at 78 MHz with a Dipole and Custom SDR

Radio astronomers from Arizona State University and MIT have recently observed a predicted radio phenomenon that originates from the very first stars formed in the Universe.

Hydrogen tends to emit radio signals in the 21cm (1.4 GHz) region of the frequency spectrum. An emission from a single Hydrogen atom is very rare, but since there is so much Hydrogen in space a bump at 1.4 GHz can be observed on the frequency spectrum if a sensitive radio is used with a directional antenna pointing up at the sky. This is a moderate difficulty experiment that can be performed by amateur radio astronomers today with cheap RTL-SDRs or other SDRs together with some LNAs. 

The astronomers in this experiment focus on a distortion in the 21cm line signal that is expected to have been created when the first stars formed. The their paper they write:

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.

The results show a successful detection of the expected phenomena at 78 MHz, confirming the age at when the first stars have been predicted to have begun forming. The phenomena is detected at 78 MHz instead of 1.4 GHz because the wavelength of a Hydrogren line signal gets stretched the further the source is from us, due to the redshift doppler effect from the expansion of the Universe. This detection is from some of the furthest (and thus oldest) stars in the Universe, so a big stretch is expected.

The experiment consisted of a broadband blade dipole which was set up in the Australian outback. Since the cosmic signal is expected to be detected right in the middle of the broadcast FM band, a dedicated radio-quiet location is required to stand any chance of detection. The receiving SDR hardware consists of an LNA, line amp, filtering and a 14-bit ADC that is connected to a PC.

It seems possible that this experiment could be repeated by amateur radio astronomers with commercial SDR hardware, but the biggest challenge would probably be finding a very radio-quiet location without broadcast FM radio signals.

The 78 MHz Cosmic Signal SDR Detection Setup
The 78 MHz Cosmic Signal SDR Detection Setup
Dipole antenna with 30mx30m ground plane
Dipole antenna with 30mx30m ground plane

6 comments

  1. biff

    Who falls for this? Astronomy has turned full circle back to astrology…
    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.
    Exactly the argument the astrologists fought against regarding red shifts, talk about cake and eat it…

  2. Randall Wayth

    It is great that the SDR community is excited about astrophysics, but I’m afraid it is a bridge too far to call their spectrometer a custom SDR. Radio astronomers have been building custom digital acquisition systems and spectrometers in hardware and software for several decades. Interested readers might read about CSIRO’s patent windfall from wifi resulting from innovations originally based in radio astronomy:
    http://blog.patentology.com.au/2012/04/story-behind-csiros-wi-fi-patent.html

    • Mike

      SDR is at it’s core level just an ADC with DSP software. I’d say that anything that receives and processes radio signals with an ADC can be classed under the SDR umbrella.

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