Archive for 21cm absorption

EDGES and Foregrounds

Posted in Astrohype, The Universe and Stuff with tags , , , on September 3, 2018 by telescoper

Earlier this year I wrote a brief post about paper by Bowman et al. from the EDGES experiment that had just come out in Nature reportining the detection of a flattened absorption profile in the sky-averaged radio spectrum, centred at a frequency of 78 megahertz, largely consistent with expectations for the 21-centimetre signal induced by early stars. It caused a lot of excitement at the time; see, e.g., here.
The key plot from the paper is this:

At the time I said that I wasn’t entirely convinced. Although the paper is very good at describing the EDGES experiment, it is far less convincing that all necessary foregrounds and systematics have been properly accounted for. There are many artefacts that could mimic the signal shown in the diagram.

I went on to say

If true, the signal is quite a lot larger than amplitude than standard models predict. That doesn’t mean that it must be wrong – I’ve never gone along with the saying `never trust an experimental result until it is confirmed by theory’ – but it’s way too early to claim that it proves that some new exotic physics is involved. The real explanation may be far more mundane.

There’s been a lot of media hype about this result – reminiscent of the BICEP bubble – and, while I agree that if it is true it is an extremely exciting result – I think it’s far too early to be certain of what it really represents. To my mind there’s a significant chance this could be a false cosmic dawn.

I gather the EDGES team is going to release its data publicly. That will be good, as independent checks of the data analysis would be very valuable.

Well, there’s a follow-up paper that I missed when it appeared on the arXiv in May the abstract of which reads:

We have re-analyzed the data in which Bowman et al. (2018) identified a feature that could be due to cosmological 21-cm line absorption in the intergalactic medium at redshift z~17. If we use exactly their procedures then we find almost identical results, but the fits imply either non-physical properties for the ionosphere or unexpected structure in the spectrum of foreground emission (or both). Furthermore we find that making reasonable changes to the analysis process, e.g., altering the description of the foregrounds or changing the range of frequencies included in the analysis, gives markedly different results for the properties of the absorption profile. We can in fact get what appears to be a satisfactory fit to the data without any absorption feature if there is a periodic feature with an amplitude of ~0.05 K present in the data. We believe that this calls into question the interpretation of these data as an unambiguous detection of the cosmological 21-cm absorption signature.

You can read the full paper here (PDF). I haven’t kept up with this particular story, so further comments/updates/references are welcome through the box below!