Archive for Cosmic Microwave Background

Primordial Figures

Posted in Biographical, The Universe and Stuff with tags , , , , on August 28, 2020 by telescoper

I was rummaging around looking for some things related to a paper I’m struggling to finish before term starts and I found some vintage diagrams. They brought back a lot of memories of working on the textbook I wrote with Francesco Lucchin way back in the 1990s. In particular I remember how long it took to make these figures, when nowadays it would take a few minutes. In fact I’m thinking of setting this as a Computational Physics project for next year. These are not full computations either, just a simple fluid-based approach.

The curves show the evolution of fluctuations in both matter δm and radiation δr on a particular scale (i.e. a Fourier mode of given wavelength) defined as δm=δρmm, etc.  The x-axis shows the cosmic scale factor, which represents the expansion of the Universe and in both cases the universe is flat, i.e. it has a critical density. The first graph shows a universe with only baryonic matter:

Notice the strongly coupled oscillations in matter and radiation until a scale factor of around 10-3, corresponding to a redshift of a thousand or so, which is when matter and radiation decouple. The y-axis is logarithmic so the downward spikes represent zero points.

It is these oscillations which are responsible for the bumps and wiggles in the spectrum of the cosmic microwave background spectrum, as different Fourier modes arrive at the last scattering surface at a different phase of its oscillation. Of course going from the Figure above to the CMB fluctuation spectrum (see below) involves more calculations, and there is now a well-established machinery for doing these with full physical descriptions, but I think the above diagram makes the physical origin of these features clear.

The CMB power spectrum from Planck

The second diagram shows what happens if you add a third component called `X’ in the Figure below which we take to be cold non-baryonic matter. Because  this stuff doesn’t interact directly with radiation (while baryons do) it doesn’t participate in the oscillations but the density perturbations just carry on growing:

Notice too that at late times (i.e. after the baryonic matter and radiation have decoupled) the baryonic component grows much more quickly than in the first Figure. This is because, when released from the effect of the photon background, baryons start to feel the gravitational pull of the dark matter perturbations.

There’s nothing new in this of course – these Figures are thirty years old and similar were produced even earlier than that – but I still think pictures like these are pedagogically useful,


Cosmology Talks – Deanna Hooper on CMB spectral distortions

Posted in The Universe and Stuff with tags , , , , on May 26, 2020 by telescoper

Here is another one of those Cosmology Talks curated on YouTube by Shaun Hotchkiss. This one was published over a month ago, but I missed it at the time.

In the talk, Deanna Hooper tells us about what we could learn from future measurements of the spectral distortions in the CMB, as well as how spectral distortions complement current and future measurements of CMB anisotropies. I’m particularly interested in this as I wrote a paper on it with John Barrow almost thirty years 30 ago and it’s fascinating to see how far the field has moved on from the theoretical point of view. Our paper was motivated by limits on spectral distortions imposed by the FIRAS instrument on COBE, and there hasn’t been anywhere near as much observational progress since then.

The paper that accompanies this talk can be found here.

Cosmology Talks: Omar Darwish on Lensing Maps

Posted in The Universe and Stuff with tags , , , , , on April 17, 2020 by telescoper

If you are missing your regular seminar experience because of the Coronavirus lockdown, Shaun Hotchkiss has set up a YouTube channel just for you!

The channel features technical talks rather than popular expositions so it won’t be everyone’s cup of tea but for those seriously interested in cosmology at a research level they should prove interesting.

Here’s another example from that series in which Omar Darwish talks about CMB Lensing Maps and specifically about an extremely impressive example thereof which he made using data from the Atacama Cosmology Telescope.

More Cosmic Tension?

Posted in The Universe and Stuff with tags , , , , , , , , , on November 12, 2019 by telescoper

Quite a lot of fuss was being made in cosmological circles while I was away last week concerning a paper that had just been published in Nature Astronomy by Eleonora Di Valentino, Alessandro Melchiorri and Joe Silk that claims evidence from the Planck Cosmic Microwave background and other data that the Universe might be closed (or at least have positive spatial curvature) in contrast to the standard cosmological model in which the spatial geometry is Euclidean. Nature Astronomy is behind a paywall but the paper is available for free on the arXiv here. The abstract reads:

The recent Planck Legacy 2018 release has confirmed the presence of an enhanced lensing amplitude in CMB power spectra compared to that predicted in the standard ΛCDM model. A closed universe can provide a physical explanation for this effect, with the Planck CMB spectra now preferring a positive curvature at more than 99% C.L. Here we further investigate the evidence for a closed universe from Planck, showing that positive curvature naturally explains the anomalous lensing amplitude and demonstrating that it also removes a well-known tension within the Planck data set concerning the values of cosmological parameters derived at different angular scales. We show that since the Planck power spectra prefer a closed universe, discordances higher than generally estimated arise for most of the local cosmological observables, including BAO. The assumption of a flat universe could, therefore, mask a cosmological crisis where disparate observed properties of the Universe appear to be mutually inconsistent. Future measurements are needed to clarify whether the observed discordances are due to undetected systematics, or to new physics, or simply are a statistical fluctuation.

I think the important point to take from this study is that estimates of cosmological parameters obtained from Planck are relatively indirect, in that they involve the simultaneous determination of several parameters some of which are almost degenerate. For example, the `anomalous’ lensing amplitude discussed in this paper is degenerate with the curvature so that changing one could mimic the effect on observables of changing the other; see Figure 2 in the paper.

It’s worth mentioning another (and, in my opinion, better argued) paper on a similar topic by Will Handley of Cambridge which is on the arXiv here. The abstract of this one reads:

The curvature parameter tension between Planck 2018, cosmic microwave background lensing, and baryon acoustic oscillation data is measured using the suspiciousness statistic to be 2.5 to 3σ. Conclusions regarding the spatial curvature of the universe which stem from the combination of these data should therefore be viewed with suspicion. Without CMB lensing or BAO, Planck 2018 has a moderate preference for closed universes, with Bayesian betting odds of over 50:1 against a flat universe, and over 2000:1 against an open universe.

Figure 1 makes a rather neat point that the combination of Planck and Baryon Acoustic Oscillations does not separately give consistent values for the Hubble constant and the curvature and neither does the combination of Planck and direct Hubble constant estimates:

I don’t know what the resolution of these tensions is, but I think it is a bit dangerous to dismiss them simply as statistical flukes. They might be that, of course, but they also might not be. By shrugging one’s shoulders and ignoring such indications one might miss something very fundamental. On the other hand, in my opinion, there is nothing here that definitely points the finger at spatial curvature either: it is possible that there is something else missing from the standard model that, if included, would resolve these tensions. But what is the missing link?

Answers on a postcard, or through the comments box.

A Nobel Prize for Jim Peebles!

Posted in The Universe and Stuff with tags , , , , , , on October 8, 2019 by telescoper

I’ve just dashed back in excitement to the office from two hours of mandatory Financial Report Training to write a quick post before my 12 o’clock lecture on Astrophysics & Cosmology because of the news about the award of the 2019 Nobel Prize for Physics.

My recent post was half right in the sense that half this year’s prize goes to Michel Mayor and Didier Queloz for the discovery of an extrasolar planet. I don’t know either of them personally, but heartiest congratulations to both!

My heart lept with joy, however, to see the other half of the prize go to Jim Peebles (above) for his work on theoretical cosmology. Much of the reason for that is that I’ve had the great honour and pleasure to meet Jim many times over the years. He is not only a truly great scientist but also a extremely nice man whose kindness and generosity is universally recognized. He’s not known as `Gentleman Jim’ for nothing!

The other reason for the excitement is that I was completely taken by surprise by the announcement. I had feared that his chance of winning a Nobel Prize had passed – I argued at the time that Jim should have been awarded a share of the 2006 Nobel Prize because without his amazing pioneering theoretical work the importance of the cosmic microwave background for cosmology and the large-scale structure of the Universe would not have been established so rapidly. As an author of the first paper to provide a theoretical interpretation of the signal detected by Penzias and Wilson, Jim was there right at the start of the modern era of cosmology and his subsequent work constructed the foundations of the theory of structure formation through gravitational instability. I was sad that he didn’t get a share in 2006 for this work, but am absolutely delighted that this has been rectified now!

This was one of the first cosmology books I ever bought. It’s an amazing piece of work that has been essential reading for cosmologists for almost 40 years!

Congratulations to Jim!

Now let me think about what to say to my students about this!

A Bayesian Look at Cosmic Anomalies

Posted in Cosmic Anomalies with tags , , , on March 3, 2019 by telescoper

I’ve posted a few times on this blog about Cosmic Anomalies, by which I mean apparent departures from the predictions of the standard cosmological model. From time to time I also talk about this subject at seminars and conferences.

There’s an interesting new paper on this topic on the arXiv now by Shaikh et al., with the following abstract:

You can click on the image to make it larger. You can also find the PDF version of the full paper here.

I find this Bayesian analysis of two of the apparent anomalies (low amplitude in the power spectrum at large angular scales and hemispherical power asymmetry) may be different manifestations of the same underlying phenomenon, which would make them easier to account for without invoking new physics. Rather than being two independent statistical flukes these measurements might both be the result of one, which would be more likely to occur in the standard model. This analysis however suggests that this might not be the case after all, and these are two different things after all. This presupposes, however, that the model chosen to describe the asymmetries is appropriate. Anyway, this paper is well worth a read if you’re into Bayesian model testing (which you should be)…

This also gives me the excuse to post the following poll, which has been running for several years (even longer than Brexit):

Circular Polarization in the Cosmic Microwave Background?

Posted in The Universe and Stuff with tags , , , , on November 23, 2018 by telescoper

Some years ago I went to a seminar on the design of an experiment to measure the polarization of the cosmic microwave background. At the end of the talk I asked what seemed to me to be an innocent question. The point of my question was the speaker had focussed entirely on measuring the intensity of the radiation (I) and the two Stokes Parameters that measure linear polarization of the radiation (usually called Q and U). How difficult, I asked, would it be to measure the remaining Stokes parameter V (which quantifies circular polarization)?

There was a sharp intake of breath among the audience as if I had uttered an obscenity, and the speaker responded with a glare and a curt `the cosmic microwave background is not circularly polarized’. It is true that in the standard cosmological theory the microwave background is produced by Thomson scattering in the early Universe which produces partial linear polarization, so that Q and U are non-zero, but not circular polarization, so V=0. However, I had really asked my question because I had an idea that it might be worth measuring V (or at least putting an upper limit on it) in order to assess the level of instrumental systematics (which are a serious issue with polarization measurements).

I was reminded of this episode when I saw a paper on the arXiv by Keisuke Inomata and Marc Kamionkowski which points out that the CMB may well have some level of circular polarization. Here is the abstract of the paper:

(You can click on the image to make it more readable.) It’s an interesting calculation, but it’s hard to see how we will ever be able to measure a value of Stokes V as low as 10-14.

A few years ago there was a paper on the arXiv by Asantha Cooray, Alessandro Melchiorri and Joe Silk which pointed out that the CMB may well have some level of circular polarization. When light travels through a region containing plasma and a magnetic field, circular polarization can be generated from linear polarization via a process called Faraday conversion. For this to happen, the polarization vector of the incident radiation (defined by the direction of its E-field) must have non-zero component along the local magnetic field, i.e. the B-field. Charged particles are free to move only along B, so the component of E parallel to B is absorbed and re-emitted by these charges, thus leading to phase difference between it and the component of E orthogonal to B and hence to the circular polarization. This is related to the perhaps more familiar process of which causes the plane of linear polarization to rotate when polarized radiation travels through a region containing a magnetic field.

Here is the abstract of that paper:

(Also clickable.) This is a somewhat larger effect but differs from the first paper in that it is produced by foreground processes rather than primordial physics. In any case a Stokes V of 10-9 is also unlikely to be measurable at any time in the foreseeable future.

From Phase Walks to Undergraduate Research

Posted in Education, The Universe and Stuff with tags , , , , , on September 28, 2018 by telescoper

This week I put together a couple of brief descriptions for possible research projects for final-year undergraduate and/or Masters students in the Department of Theoretical Physics at Maynooth University, and I was reminded of the value of projects like this when I found this paper on the arXiv:

In fact the `Phase Walk Analysis’ developed here is based on an original idea I had for an undergraduate summer research project when I was at Nottingham University and have mentioned before on this blog. The student who did the project with me was Andrew Stannard (who is now at King’s College, London) and the work led to a paper that was published in a refereed journal in 2005 and has now been cited 21 times by various authors including the Planck Team.

Although Andrew is now working in a completely different area (Condensed Matter Physics), I like to think this taste of research was of at least some assistance in developing his career. Above all, though, it relates to something I read in the Times Higher by astronomer, Nobel Prize winner, and Vice-Chancellor of the Australian National University, namely that the idea that many politicians seem to have of separating teaching from research in universities is at best misguided and at worst threatens the very idea of a university.

The Simons Observatory: Science Goals and Forecasts

Posted in The Universe and Stuff with tags , , on August 27, 2018 by telescoper

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.

Nature After Planck…

Posted in Maynooth, The Universe and Stuff with tags , , , , , , on July 24, 2018 by telescoper

After last week’s short update about the last tranche of papers from the European Space Agency’s Planck Mission it’s time for another short update about a piece in Nature (by David Castelvecchi) that explains how researchers are moving to smaller projects studying different aspects of the cosmic microwave background.

In the spirit of gratuitous self-promotion I should also mention that there’s a little quote from me in that piece. My comment was hardly profound, but at least it gets Maynooth University a name check…

Much of Davide’s piece echoes discussions that were going on at the meeting I attended in India  last October, but things have moved on quite a bit since then at least as far as space experiments are concerned. In particular, the proposed Japanese mission Litebird has been shortlisted for consideration, though we will have to wait until next year (2019) at the earliest to see if it will be selected. An Indian mission, CMB-Bharat, has also emerged as a contender.

While the end of Planck closes one chapter on CMB research, several others will open. These are likely to focus on polarization, gravitational lensing and on cosmic reionization rather than refining the basic cosmological parameters still further.