The Simons Observatory: Science Goals and Forecasts

I haven’t been involved in this project, but several of my former colleagues at Cardiff have beenm and still are, so I know how much work has gone into this (especially by the amazing Erminia Calabrese), so I am happy to share this impressive work here. This long (54 pages) paper, which appeared on the arXiv last week, describes the latest step forward in ground-based cosmology using the cosmic microwave background. It shows just how rapid the onward march of instrumental technology continues to be.

The Simons Observatory Site, in Chile

It is likely that the Simons Observatory (based on a single 6m dish) will form part of the next generation CMB experiment known currently as CMB-S4.

You can download the paper in full from the arXiv here.

The Simons Observatory (SO) is a new cosmic microwave background experiment being built on Cerro Toco in Chile, due to begin observations in the early 2020s. We describe the scientific goals of the experiment, motivate the design, and forecast its performance. SO will measure the temperature and polarization anisotropy of the cosmic microwave background in six frequency bands: 27, 39, 93, 145, 225 and 280 GHz. The initial configuration of SO will have three small-aperture 0.5-m telescopes (SATs) and one large-aperture 6-m telescope (LAT), with a total of 60,000 cryogenic bolometers. Our key science goals are to characterize the primordial perturbations, measure the number of relativistic species and the mass of neutrinos, test for deviations from a cosmological constant, improve our understanding of galaxy evolution, and constrain the duration of reionization. The SATs will target the largest angular scales observable from Chile, mapping ~10% of the sky to a white noise level of 2 μK-arcmin in combined 93 and 145 GHz bands, to measure the primordial tensor-to-scalar ratio, r, at a target level of σ(r)=0.003. The LAT will map ~40% of the sky at arcminute angular resolution to an expected white noise level of 6 μK-arcmin in combined 93 and 145 GHz bands, overlapping with the majority of the LSST sky region and partially with DESI. With up to an order of magnitude lower polarization noise than maps from the Planck satellite, the high-resolution sky maps will constrain cosmological parameters derived from the damping tail, gravitational lensing of the microwave background, the primordial bispectrum, and the thermal and kinematic Sunyaev-Zel’dovich effects, and will aid in delensing the large-angle polarization signal to measure the tensor-to-scalar ratio. The survey will also provide a legacy catalog of 16,000 galaxy clusters and more than 20,000 extragalactic sources.

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13 Responses to “The Simons Observatory: Science Goals and Forecasts”

  1. Peter any comments on https://arxiv.org/abs/1808.04597?

    • telescoper Says:

      Yes. It’s irrelevant to this blog post.

      • I know. I should have mentioned OT. But was hoping for some expert insight from you on this.

      • Also irrelevant to the blog post, but as I have answered your question elsewhere: if this conclusion is true, then the authors have to explain why the CMB indicates acceleration (derived from the values of lambda and Omega), and indeed essentially all other cosmological observations which are precise enough. That’s why the current standard model is called the cosmological model.

        Which is more likely: this paper got something wrong, or essentially all other cosmological observations are wrong?

        To get back on topic, perhaps the Simons Observatory will confirm the concordance model.

  2. Philip, In any field there is a confirmation bias (cf https://arxiv.org/abs/1112.3108 for an example in Cosmology). Independent of CMB and other probes of DE, there is still an important question of whether the SN data alone provided smoking gun evidence for DE.

    • telescoper Says:

      The SN data on their own do not provide compelling evidence for dark energy. They never have and probably never will.

      • I suppose this depends on one’s demands. Certainly if you just do a chi-square minimization at face value, then non-accelerating models are excluded at more then three sigma.

      • telescoper Says:

        That’s not the point I was making, The SN measurements don’t on their own provide evidence for FRW at all, so one can appeal to, e.g., inhomogeneous cosmologies to fit them. And even if you believe there is direct evidence of acceleration (which there isn’t), it could be modified gravity rather than dark energy.

      • Right. It is well known that a LTB (no, nothing to do with Pride Cymru!) model can fit any mz relation. However, one has to ask whether the cure is worth than the disease (which I don’t even consider a disease). At some level, we know nothing: everything could be a joke played on us by some super-being or whatever. The sensible approach, though, is that there is a huge amount of evidence that FLRW is a good approximation to the universe on large scales and, within that framework, the supernova data do indicate acceleration. There was a time when they were the only cosmological test which, by itself, indicated acceleration. That is no longer the case; the CMB alone, even without additional assumptions (other than FLRW) now indicates this.

        “Direct evidence for acceleration” again depends on the definition. Bob Kirshner mentioned this in his book, but I really don’t see it. At one extreme, of course, all we measure are photon counts as a function of position, whether in photometry or spectroscopy, but that is not my point. “Direct evidence for acceleration” would, in my book, be measuring a velocity then measuring a different velocity sometime later. Anything else is deduction. Basically, all cosmological tests measure some combination of lambda and Omega and from this one can compute the acceleration, so in that sense it is always computed. Differences are quantitative: some chains of induction are longer.

      • telescoper Says:

        I don’t think there’s really ever `direct evidence’ in cosmology (as it is understood in the context of forensic science) . But `circumstantial evidence’ is not the same as `no evidence’.

      • I don’t think there’s really ever `direct evidence’ in cosmology (as it is understood in the context of forensic science) . But `circumstantial evidence’ is not the same as `no evidence’.

        My point exactly.

    • I’ve actually cited this paper. 🙂 However, the general tone of the paper is the opposite: it appears that measurements are converging on the real values; the stuff about confirmation is merely a caveat.

  3. sphaerenklang Says:

    With this level of sensitivity one is in Clover – if not well beyond.

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