Archive for Cosmic Microwave Background

Latest Results from the South Pole Telescope

Posted in The Universe and Stuff with tags , , , , on December 13, 2022 by telescoper

Just time for a quick post to point out the latest results from the South Pole Telescope (SPT) have now hit the arXiv. The measurements were made in 2018 but the outcome of a full analysis of temperature and polarization has only just appeared.

Here’s a grab of the abstract:

The key figures showing the constraints on the Hubble Constant H0 and the parameter S8 are shown here:

As you can see, the results from SPT-3G are consistent with the standard cosmological model and agree on H0 with Planck rather than the higher value obtained from local measurements. If you thought there was Hubble tension before this measurement, then you will still think so now!

Save the Holmdel Antenna!

Posted in History, The Universe and Stuff with tags , , , , on December 11, 2022 by telescoper

I’ve used the above image hundreds of times in popular talks. It shows Robert W. Wilson (left) and Arno A. Penzias (right) standing in front of the famous horn antenna that (accidentally) discovered what we now know to be the cosmic microwave background radiation left over after the Big Bang. Penzias and Wilson made their historic measurements in 1964, published their results in 1965, and received the Nobel Prize for Physics in 1978.

At the time of their historic experiment, the scientists were working at Bell Telephone Laboratories at Holmdel, New Jersey, on Project Echo. The antenna was built to receive radio signals bounced off a passive satellite in a low Earth orbit to check the feasibility of satellite radio communication. They found excess noise in their receiver, which was eventually identified as a relic of a time when the Universe was extremely hot. Coincidentally, the theory of this yet undiscovered radiation was being worked on by Bob Dicke and his group in Princeton at about the same time (and also in New Jersey). Discussions ensued, and the discovery paper by Penzias & Wilson appeared in the Astrophysical Journal in 1965 beside a paper by Dicke et al. giving the theoretical interpretation.

Anyway, in case you were wondering whatever happened to the Holmdel Antenna, it is still there in Holmdel (at the top of Crawford Hill) and in 1988 was declared a National Historic Landmark:

Bell Labs (as it was usually known) was acquired by Nokia in 2016 and subsequently called Nokia Bell Labs. In 2019, however, Nokia put the entire Holmdel site up for sale and redevelopment of the entire site is currently being considered. This would not only bring to an end the connection between Holmdel and the telecommunications industry but also places a big question mark over the famous antenna. A petition has been raised to secure the future of this extremely important piece of scientific history. I encourage you to read more about the situation here and consider signing the petition.

Simons Observatory News

Posted in Cardiff, Science Politics, The Universe and Stuff with tags , , , on October 18, 2022 by telescoper

It seems a lot longer than four years ago that I drew the attention of readers of this blog to the science case for the Simons Observatory, the next big thing in ground-based studies of the cosmic microwave background.

The Simons Observatory Site in Chile, as it appeared four years ago

Obviously a couple of years of pandemic have intervened, amongst other things, but I was delighted to read yesterday that the UK has invested £18M in the Simons Observatory, which will enable further development of the facility at Cerro Toco, high above the Atacama Desert in Chile.

Simons Observatory in May 2022

The project was already a large international collaboration led from the USA, but the new funds from UKRI mean that six UK institutions will now join. These are (in alphabetical order): Cambridge; Cardiff; Imperial College London; Manchester; Oxford; and Sussex. Although I’m not involved in this project myself I know many people at these institutions (two of which I have worked at) and elsewhere who will be absolutely thrilled to be able to participate in this exciting project. Congratulations to them!

It would have been great if Ireland had been able to get involved in the Simons Observatory, but sadly fundamental science of this type is not a priority for the powers that be in Irish science funding. This is unfortunate because I think membership of international consortia like this would enable a small country to punch above its weight in science. Still, at least the UK PI, Prof. Michael Brown (Manchester), is an Irishman…

New Publication at the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags , , , , , , , on December 6, 2021 by telescoper

Time to announce yet another publication in the Open Journal of Astrophysics. This one is the 15th paper in Volume 4 (2021) and the 46th in all.

The latest publication is entitled  Interplanetary Dust as a Foreground for the LiteBIRD CMB Satellite Mission by Ken Ganga (Paris), Michele Maris (Trieste) and Mathieu Remazeilles (Santander) on behalf of the LiteBIRD collaboration. For information about the LiteBIRD mission see here.

Here is a screen grab of the overlay which includes the abstract:

You can find the paper on the Open Journal of Astrophysics site here and can also read it directly on the arXiv here.

New Publication at the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags , , , , , , on September 23, 2021 by telescoper

Time to announce another publication in the Open Journal of Astrophysics. This one is the tenth paper in Volume 4 (2021) and the 41st in all. We actually published this one a couple of days ago but I’ve been so busy with start-of-term shenanigans that I didn’t get time to announce it until this morning.

The latest publication is entitled Consequences of constant elevation scans for instrumental systematics in Cosmic Microwave Background Experiments. The authors are Daniel B. Thomas & Nialh McCallum of Queen Mary, University of London, and Michael Brown of the University of Manchester.

Here is a screen grab of the overlay which includes the abstract:

You can click on the image to make it larger should you wish to do so. You can find the arXiv version of the paper here. This one is also in the folder marked Instrumentation and Methods for Astrophysics, though it is obviously of relevance to Cosmology and Non-Galactic Astrophysics too.

New Publication at the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags , , , , , , on August 19, 2021 by telescoper

Time to announce another publication in the Open Journal of Astrophysics. This one is the eighth paper in Volume 4 (2021) and the 39th in all.

The latest publication is entitled A Detailed Description of the CAMSPEC Likelihood Pipeline and a Reanalysis of the Planck High Frequency Maps. The authors are George Efstathiou and Steven Gratton of the Kavli Institute for Cosmology at the University of Cambridge.

Here is a screen grab of the overlay which includes the abstract:

You can click on the image to make it larger should you wish to do so. You can find the arXiv version of the paper here. This one is also in the folder marked Cosmology and Nongalactic Astrophysics.

This is a long and detailed paper, running to 92 pages in PDF form. Our Editorial process relies on referees being willing to volunteer their time to read and comment on submissions and this one in particular required a great deal of effort. I am always grateful to referees but in this case I am even more grateful than usual the diligence displayed during and the many useful comments received. I know who our reviewers are and they know who they are, but shall remain anonymous!

Searching for the Predicted Peaks in the CMB Power Spectrum

Posted in The Universe and Stuff with tags , , on July 11, 2021 by telescoper

I came across this on social media and thought I’d share it here. It’s a nice graphical demonstration of the interplay between theory and experiment in the field of cosmic microwave background physics. The video was created by Forrest Fankhauser and Lloyd Knox at the University of California, Davis with funding from the National Science Foundation.

As someone famous once said: “we’ve come a long way from pigeon shit…”

New Publication at the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags , , , , , on July 7, 2021 by telescoper

Time to announce another publication in the Open Journal of Astrophysics. This one was actually published last Friday, but I didn’t get time to post about it until just now. It is the fifth paper in Volume 4 (2021) and the 36th paper in all.

The latest publication is entitled Gravitational Wave Direct Detection does not Constrain the Tensor Spectral Index at CMB Scales and the author is Will Kinney of the State University of New York at Buffalo (which is SUNY Buffalo, for short).

Here is a screen grab of the overlay which includes the abstract:

You can click on the image to make it larger should you wish to do so. You can find the arXiv version of the paper here. This one is, fairly obviously, in the Cosmology and Nongalactic Astrophysics folder..

Over the last few months I have noticed that it has taken a bit longer to get referee reports on papers and also for authors to complete their revisions. I think that’s probably a consequence of the pandemic and people being generally overworked. We do have a number of papers at various stages of the pipeline, so although we’re a bit behind where we were last year in terms of papers published I think may well catch up in the next month or two.

I’ll end with a reminder to prospective authors that the OJA  now has the facility to include supplementary files (e.g. code or data sets) along with the papers we publish. If any existing authors (i.e. of papers we have already published) would like us to add supplementary files retrospectively then please contact us with a request!

Cosmology and the Born-Again Bayesians!

Posted in Bad Statistics, Biographical, The Universe and Stuff with tags , , , , on May 10, 2021 by telescoper

The other day, via Twitter, I came across an interesting blog post about the relatively recent resurgence of Bayesian reasoning in science. That piece had triggered a discussion about why cosmologists seem to be largely Bayesian in outlook, so I thought I’d share a few thoughts about that. You can find a lot of posts about various aspects of Bayesian reasoning on this blog, e.g. here.

When I was an undergraduate student I didn’t think very much about statistics at all, so when I started my DPhil studies I realized I had a great deal to learn. However, at least to start with, I mainly used frequentist methods. Looking back I think that’s probably because I was working on cosmic microwave background statistics and we didn’t really have any data back in 1985. Or actually we had data, but no firm detections. I was therefore taking models and calculating things in what I would call the forward direction, indicated by the up arrow. What I was trying to do was find statistical descriptors that looked likely to be able to discriminate between different models but I didn’t have the data.

Once measurements started to become available the inverse-reasoning part of the diagram indicated by the downward arrow came to the fore. It was only then that it started to become necessary to make firm statements about which models were favoured by the data and which weren’t. That is what Bayesian methods do best, especially when you have to combine different data sets.

By the early 1990s I was pretty much a confirmed Bayesian – as were quite a few fellow theorists -but I noticed that most observational cosmologists I knew were confirmed frequentists. I put that down to the fact that they preferred to think in “measurement space” rather than “theory space”, the latter requiring the inductive step furnished by Bayesian reasoning indicated by the downward arrow. As cosmology has evolved the separation between theorists and observers in some areas – especially CMB studies – has all but vanished and there’s a huge activity at the interface between theory and measurement.

But my first exposure to Bayesian reasoning came long before that change. I wasn’t aware of its usefulness until 1987, when I returned to Cambridge for a conference called The Post-Recombination Universe organized by Nick Kaiser and Anthony Lasenby. There was an interesting discussion in one session about how to properly state the upper limit on CMB fluctuations arising from a particular experiment, which had been given incorrectly in a paper using a frequentist argument. During the discussion, Nick described Anthony as a “Born-again Bayesian”, a phrase that stuck in my memory though I’m still not sure whether or not it was meant as an insult.

It may be the case for many people that a relatively simple example convinces them of the superiority of a particular method or approach. I had previously found statistical methods – especially frequentist hypothesis-testing – muddled and confusing, but once I’d figured out what Bayesian reasoning was I found it logically compelling. It’s not always easy to do a Bayesian analysis for reasons discussed in the paper to which I linked above, but it least you have a clear idea in your mind what question it is that you are trying to answer!

Anyway, it was only later that I became aware that there were many researchers who had been at Cambridge while I was there as a student who knew all about Bayesian methods: people such as Steve Gull, John Skilling, Mike Hobson, Anthony Lasenby and, of course, one Anthony Garrett. It was only later in my career that I actually got to talk to any of them about any of it!

So I think the resurgence of Bayesian ideas in cosmology owes a very great deal to the Cambridge group which had the expertise necessary to exploit the wave of high quality data that started to come in during the 1990s and the availability of the computing resources needed to handle it.

But looking a bit further back I think there’s an important Cambridge (but not cosmological) figure that preceded them, Sir Harold Jeffreys whose book The Theory of Probability was first published in 1939. I think that book began to turn the tide, and it still makes for interesting reading.

P.S. I have to say I’ve come across more than one scientist who has argued that you can’t apply statistical reasoning in cosmology because there is only one Universe and you can’t use probability theory for unique events. That erroneous point of view has led to many otherwise sensible people embracing the idea of a multiverse, but that’s the subject for another rant.

Cosmology Talks: Eiichiro Komatsu & Yuto Minami on Parity Violation in the Cosmic Microwave Background

Posted in Cardiff, Maynooth, The Universe and Stuff with tags , , , , , , , , on December 2, 2020 by telescoper

It’s time I shared another one of those interesting cosmology talks on the Youtube channel curated by Shaun Hotchkiss. This 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.

In this video, Eiichiro Komatsu and Yuto Minami talk about their recent work, first devising a way to extract a parity violating signature in the cosmic microwave background, as manifested by a form of birefringence. If the universe is birefringent then E-mode polarization would change into B-mode as electromagnetic radiation travels through space, so there would be a non-zero correlation between the two measured modes. They  try to measure this correlation using the Planck 2018 data, getting  a 2.4 sigma `hint’ of a result.

A problem with the measurement is that systematic errors, such as imperfectly calibrated detector angles,  could mimic the signal. Yuto and Eiichiro’s  idea was to measure the detector angle by looking at the E-B correlation in the foregrounds, where light hasn’t travelled far enough to be affected by any potential birefringence in the universe. They argue that this allows them to distinguish between the two types of measured E-B correlation. However, this is only the case if there is no intrinsic correlation between the E-mode and B-mode polarization in the foregrounds, which may not be the case, but which they are testing. The method can be applied to any of the plethora of CMB experiments currently underway so there will probably be more results soon that may shed further light on this issue.

Incidentally this reminds me of Cardiff days when work was going on about the same affect using the Quad instrument. I wasn’t involved with Quad but I do remember having interesting chats about the theory behind the measurement or upper limit as it was (which is reported here). Looking at the paper I realize that paper involved researchers from the Department of Experimental Physics at Maynooth University.

P. S. The paper that accompanies this talk can be found here.