Cosmic Dawn?

I’m still in London hoping to get a train back to Cardiff at some point this morning – as I write they are running, but with a reduced service – so I thought I’d make a quick comment on a big piece of astrophysics news. There’s a paper out in this week’s Nature, the abstract of which is

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.

The key plot from the paper is this:

I’ve read the paper and, as was the case with the BICEP2 announcement a few years ago, I’m not entirely convinced. I think the paper is very good at describing the EDGES experiment, but 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.

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.

I’m sorry I haven’t got time for a more detailed post on this, but I have to get my stuff together and head for the train. Comments from experts and non-experts are, as usual, most welcome via the comments box.

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10 Responses to “Cosmic Dawn?”

  1. We were discussing this at coffee time yesterday, and the net conclusion was that we didn’t believe it either. (Well, my colleagues didn’t, and I think I trust their opinion!)

  2. I was very excited when I first saw this result reported, and still am. But a thought occurred to me last night: how often does it happen that an observational signal is larger than expected? Most times this happened in modern physics, it turned out to be a spurious effect, from Weber’s bar to BICEP 2.
    That said, the DM explanation is very exciting. There are some great physics discussions to be had even if the signal turns out to be spurious

  3. The argument that they see it in both of the two low-frequency instruments, when they were in different configurations, and that they don’t see it at a scaled frequency in the high frequency instrument of the same design was interesting – it argues against it being an instrumental effect like a standing wave. But foregrounds/systematics are worrying, and the signal almost looks too clean…

    • What worries me most is that it could be faint RFI signals. In particular at the higher frequency end: FM radio in Australia starts around 87MHz, which is where the high frequency rise is in their data. I’m not sure if there are signals at ~65MHz that might explain the lower-frequency bump…

  4. […] If true, the result would be a major step in our understanding of the formation of stars,  and a major step in the demonstration of the existence of Dark Matter. However, it’s early days – there are many possible sources of a spurious signal and signals that are larger than expected have a poor history in modern physics! There is a nice article on this in The Guardian, and you can see some of the debate on Peter Coles’s blog In the Dark. […]

  5. Eh? I have no idea how a paragraph from my blog wormed its way into these comments, apologies

  6. Michel C. Says:

    Even if the signal is validated, it won’t mean the interpretation is right. Maybe the cosmological model is wrong?

  7. Yes, mebbe. But it could be yet another addition to the collection of indirect evidence for DM.

  8. Michel C. Says:

    All neutrinos we can observe are relativistic. Maybe their probability of interaction (not detection) is proportional to their velocity. Neutrinos could have been produced by matter-antimatter annihilation and most of them have been slowed down by a traffic jam… This is not my favorite DM model, but if it is true then it is a possibility. I am convinced that a solution will throw out the probabilistic model of QM as being fundamental to Physics and it will be relegated to a measurement perspective.

    Cold neutrinos have never been observed, it doesn’t mean there are none… There is a difference between size and cross section. The cross section is associated to the probability of interaction and the size shoud be associated to the particle’s wavelength.

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