Archive for Cosmology

The Hubble Constant: A Historical Review

Posted in The Universe and Stuff with tags , , , , , on May 29, 2023 by telescoper

I bookmarked this paper on arXiv a week or so ago with the intention of sharing it here, but evidently forgot about it. Anyway, as its name suggests, it’s a review by Brent Tully from a historical perspective of measurements of the Hubble Constant. I’m not sure whether it is intended for publication in a book – as it opens with the heading “Chapter 1” – but it’s well worth reading whatever its purpose. Here is the abstract:

For 100 years since galaxies were found to be flying apart from each other, astronomers have been trying to determine how fast. The expansion, characterized by the Hubble constant, H0, is confused locally by peculiar velocities caused by gravitational interactions, so observers must obtain accurate distances at significant redshifts. Very nearby in our Galaxy, accurate distances can be determined through stellar parallaxes. There is no good method for obtaining galaxy distances that is applicable from the near domain of stellar parallaxes to the far domain free from velocity anomalies. The recourse is the distance ladder involving multiple methods with overlapping domains. Good progress is being made on this project, with satisfactory procedures and linkages identified and tested across the necessary distance range. Best values of H0 from the distance ladder lie in the range 73 – 75 km/s/Mpc. On the other hand, from detailed information available from the power spectrum of fluctuations in the cosmic microwave background, coupled with constraints favoring the existence of dark energy from distant supernova measurements, there is the precise prediction that H0 = 67.4 to 1%. If it is conclusively determined that the Hubble constant is well above 70 km/s/Mpc as indicated by distance ladder results then the current preferred LambdaCDM cosmological model based on the Standard Model of particle physics may be incomplete. There is reason for optimism that the value of the Hubble constant from distance ladder observations will be rigorously defined with 1% accuracy in the near future.

Brent Tully, arXiv:2305.11950

Here is the concluding paragraph:

As the 20th century came to an end, ladder measurements of the Hubble constant were at odds with the favored cosmological model of the time of cold dark matter with Λ =0. The new favorite became the ΛCDM model with dark energy giving rise
to acceleration of space in a topologically flat universe. Yet ladder measurements, continuously improving, create doubts that this currently favorite model is complete. Yes, there is a Hubble tension.

R.I.P. Jim Hartle (1939-2023)

Posted in The Universe and Stuff with tags , , , , on May 27, 2023 by telescoper

It’s another one of the occasions on which I have to use this blog to pass on some sad news. Renowned physicist James B. Hartle has passed away.

Jim Hartle’s scientific work was concerned with the application of Einstein’s theory of general relativity to astrophysics, especially gravitational waves, relativistic stars, black holes, and cosmology, specifically the theory of the wave function of the universe. For much of his career he was interested in the earliest moments of the big bang where the subjects of quantum mechanics, gravity theory and cosmology overlap, leading among other things to the Hartle-Hawking conjecture.

Jim Hartle was one of the speakers at the very first scientific conference I attended in Cargèse, Corsica way back in 1986. I remember his lectures very well after all these years, not least because he was so witty. I remember his response when someone asked him about the existence of large dimensionless numbers in cosmology: “…it’s a property that numbers have that some of them are larger than others.”

Condolences to his family, friends and colleagues. Rest in peace, Jim Hartle (1939-2023).

The Euclid Launch Kit!

Posted in The Universe and Stuff with tags , , , , , on May 17, 2023 by telescoper

As the launch of the European Space Agency’s Euclid mission approaches, though we don’t know official launch date yet, the associated publicity machines are ramping up for the big occasion. The latest bit of merch is the Euclid Launch Kit.

Sadly, this does not allow you to build your own Falcon 9 launcher which is what I inferred from the name. What it is is an interactive PDF file that allows you to navigate around and learn things about the satellite, its orbit, its instruments and the science case. I think it’s pretty good. You can download it here. It’s over 100 MB though, so beware if you have a very slow connection.

To whet your appetite, here some graphics extracted from the launch kit. You can click on the tiles to make them bigger.

The two science images relate to weak gravitational lensing and baryon acoustic oscillations; for more details you can click on the relevant links.

New Publication at the Open Journal of Astrophysics

Posted in OJAp Papers, The Universe and Stuff with tags , , , , , , , on April 25, 2023 by telescoper

It’s time once more to announce a new paper at the Open Journal of Astrophysics. The latest paper is the 13th paper so far in Volume 6 (2023) and the 78th in all. This one is another for the folder marked Cosmology and NonGalactic Astrophysics and its title is “The catalog-to-cosmology framework for weak lensing and galaxy clustering for LSST”.

The lead author is Judit Prat of the University of Chicago (Illinois, USA) and there are 21 co-authors from elsewhere in the USA and in the UK. The paper is written on behalf of the LSST Dark Energy Science Collaboration (LSST DESC), which is the international science collaboration that will make high accuracy measurements of fundamental cosmological parameters using data from the Rubin Observatory Legacy Survey of Space and Time (LSST). The OJAp has published a number of papers involving LSST DESC, and I’m very happy that such an important consortium has chosen to publish with us.

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

You can click on the image of the overlay to make it larger should you wish to do so. You can find the officially accepted version of the paper on the arXiv here.

New Results from the Atacama Cosmology Telescope

Posted in The Universe and Stuff with tags , , , , , , on April 12, 2023 by telescoper

I wish to draw your attention to a clutch of new papers out on the arXiv today (here, here and here) which describe latest results from the Atacama Cosmology Telescope (ACT for short). There was a webinar about this yesterday, which I failed to attend because I forgot about it.

The first of the papers listed above summarizes the key science results, which include a mass map obtained from gravitational lensing of the cosmic microwave background and its implications for cosmology.

As cosmic background photons propagate freely through space, i.e. without scattering, from the time of recombination to the observer, they are deflected by the gravitational effect of the large-scale distribution of matter in the Universe. This lensing effect leaves imprints in the temperature and polarization anisotropies, which can be used to reconstruct a map of the lensing potential, the gradient of which determines the lensing deflections. Structures in the CMB temperature pattern look bigger on the sky if we view them through an overdense clump of dark matter. By looking how the typical size of hot and cold spots in the CMB temperature map vary across the sky, it is possible to reconstruct the lensing deflections and hence the distribution of dark matter integrated along the line of sight. Since the structure through which the radiation passes is changing with time, this sort of map can provide constraints on models for the evolution of structure.

To cut a long story short, here is the map obtained using Data Release 6 of the ACT data over about 25% of the sky:

There’s a lot of information in the three papers but the key conclusion can be found in the last sentence of the abstract of the first paper:

Our results provide independent confirmation that the universe is spatially flat, conforms with general relativity, and is described remarkably well by the ΛCDM model, while paving a promising path for neutrino physics with gravitational lensing from upcoming ground-based CMB surveys.

Nothing revolutionary, then, but interesting nevertheless. There is an article on the BBC website about these results.

What’s a good Cosmological Model?

Posted in Books, The Universe and Stuff with tags , , , on April 2, 2023 by telescoper

Some years ago – actually about 30! – I wrote a book with George Ellis about the density of matter in the Universe. Many of the details in that book are of course out of date now but the main conclusions still stand. We started the book with a general discussion of cosmological models which I think also remains relevant today so I thought I’d do a quick recap here.

Anyone who takes even a passing interest in cosmology will know that it’s a field that’s not short of controversy, sometimes reinforced by a considerable level of dogmatism in opposing camps. In understanding why this is the case, it is perhaps helpful to note that much of the problem stems from philosophical disagreements about which are the appropriate criteria for choosing a “good” (or at least acceptable) theory of cosmology. Different approaches to cosmology develop theories aimed at satisfying different criteria, and preferences for the different approaches to a large extent reflect these different initial goals. It would help to clarify this situation if one could make explicit the issues relating to choices of this kind, and separate them from the more `physical’ issues that concern the interpretation of data.

The following philosophical diversion was intended to initiate a debate within the cosmological community. Some cosmologists in effect claim that there is no philosophical content in their work and that philosophy is an irrelevant and unnecessary distraction from their work as scientists. I would contend that they are, whether they like it or not, making philosophical (and, in many cases, metaphysical) assumptions, and it is better to have these out in the open than hidden.

To provide a starting point for, consider the following criteria, which might be applied in the wider context for scientific theories in general, encapsulating the essentials of this issue:

One can imagine a kind of rating system which judges cosmological models against each of these criteria. The point is that cosmologists from different backgrounds implicitly assign a different weighting to each of them, and therefore end up trying to achieve different goals to others. There is a possibility of both positive and negative ratings in each of these areas.

Note that such categories as “importance”, “intrinsic interest” and “plausibility” are not included. Insofar as they have any meaning apart from personal prejudice, they should be reflected in the categories above, and could perhaps be defined as aggregate estimates following on from the proposed categories.

Category 1(c) (“beauty”) is difficult to define objectively but nevertheless is quite widely used, and seems independent of the others; it is the one that is most problematic . Compare, for example, the apparently “beautiful” circular orbit model of the Solar System with the apparently ugly elliptic orbits found by Kepler. Only after Newton introduced his theory of gravitation did it become clear that beauty in this situation resided in the inverse-square law itself, rather than in the outcomes of that law. Some might therefore wish to omit this category.

One might think that category 1(a) (“logical consistency'”) would be mandatory, but this is not so, basically because we do not yet have a consistent Theory of Everything.

Again one might think that negative scores in 4(b) (`confirmation’) would disqualify a theory but, again, that is not necessarily so, because measurement processes, may involve systematic errors and observational results are all to some extent uncertain due to statistical limitations. Confirmation can therefore be queried. A theory might also be testable [4(a)] in principle, but perhaps not in practice at a given time because the technology may not exist to perform the necessary experiment or observation.

The idea is that even when there is disagreement about the relative merits of different models or theories, there is a possibility of
agreement on the degree to which the different approaches could and do meet these various criteria. Thus one can explore the degree to which each of these criteria is met by a particular cosmological model or approach to cosmology. We suggest that one can distinguish five broadly different approaches to cosmology, roughly corresponding to major developments at different historical epochs:

These approaches are not completely independent of each other, but any particular model will tend to focus more on one or other aspect and may even completely leave out others. Comparing them with the criteria above, one ends up with a star rating system something like that shown in the Table, in which George and I applied a fairly arbitrary scale to the assignment of the ratings!

To a large extent you can take your pick as to the weights you assign to each of these criteria, but my underlying views is that without a solid basis of experimental support [4(b)], or at least the possibility of confirmation [4(a)], a proposed theory is not a ‘good’ one from a scientific point of view. If one can say what one likes and cannot be proved wrong, one is free from the normal constraints of scientific discipline. This contrasts with a major thrust in modern cosmological thinking which emphasizes criteria [2] and [3] at the expense of [4].

Spectroscopy of High Redshift Galaxies

Posted in The Universe and Stuff with tags , , , , , on March 28, 2023 by telescoper

The tentative identifications of a number of galaxies at high redshift using JWST on the basis of photometric measurements (see, e.g., here and here) have initiated a huge amount of activity in the extragalactic community trying to establish spectroscopic redshifts for these galaxies. Results of this endeavour have started to appear on the arXiv here with this abstract:

During the first 500 million years of cosmic history, the first stars and galaxies formed and seeded the cosmos with heavy elements. These early galaxies illuminated the transition from the cosmic “dark ages” to the reionization of the intergalactic medium. This transitional period has been largely inaccessible to direct observation until the recent commissioning of JWST, which has extended our observational reach into that epoch. Excitingly, the first JWST science observations uncovered a surprisingly high abundance of early star-forming galaxies. However, the distances (redshifts) of these galaxies were, by necessity, estimated from multi-band photometry. Photometric redshifts, while generally robust, can suffer from uncertainties and/or degeneracies. Spectroscopic measurements of the precise redshifts are required to validate these sources and to reliably quantify their space densities, stellar masses, and star formation rates, which provide powerful constraints on galaxy formation models and cosmology. Here we present the results of JWST follow-up spectroscopy of a small sample of galaxies suspected to be amongst the most distant yet observed. We confirm redshifts z > 10 for two galaxies, including one of the first bright JWST-discovered candidates with z = 11.4, and show that another galaxy with suggested z ~ 16 instead has z = 4.9, with strong emission lines that mimic the expected colors of more distant objects. These results reinforce the evidence for the rapid production of luminous galaxies in the very young Universe, while also highlighting the necessity of spectroscopic verification for remarkable candidates.


As the abstract explains, the spectroscopic measurements confirm some – but not all – of the galaxies studied to be at high redshift. One galaxy – the one discussed here (known to its friends as 93316) which appeared to have a redshift of 16.6 ± 0.1 now seems to have a much lower redshift of 4.91. Here’s an image of this object:

The redshift 16.6 object was of some interest to cosmologists because an object of large stellar mass at such a large distance is difficult to reconcile with the standard theory of galaxy formation. That is now apparently out of the way, and the remaining high-z galaxies are not as extreme as this one and pose less of a problem.

While this result may disappoint some, and indeed delight others, it is also interesting to note that there are three similar objects at much the same redshift, which may indicate the presence of some sort of group or cluster:


P.S. It struck me, after writing this, that waiting for spectroscopic confirmation of photometric redshifts is a lot like waiting for VAR to check whether or not to rule out a goal for offside…

Two New Publications at the Open Journal of Astrophysics

Posted in OJAp Papers, The Universe and Stuff with tags , , , , , , , , on March 26, 2023 by telescoper

I just realized that I forgot to advertise on here a couple of recent publications at the Open Journal of Astrophysics – the papers are coming in at quite a rate now – so I’ll catch up with them both in one post.

The first paper of the two is the 10th paper in Volume 6 (2023) and the 75th in all; it was published on 16th March 2023. This one is  in the folder marked Instrumentation and Methods for Astrophysics. The title is “From BeyondPlanck to Cosmoglobe: Open Science, Reproducibility, and Data Longevity” and it is a discussion of the importance of reproducibility and Open Science in CMB science including measures toward facilitating easy code and data distribution, community-based code documentation, user-friendly compilation procedures, etc.  You can find out more about the BeyondPlanck collaboration here and about Cosmoglobe here.

The first author is S. Gerakakis and there are 42 authors in all. This is too many to list individually here but they come from Greece, Norway, Finland, Germany, Italy, and the USA.

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

You can click on the image of the overlay to make it larger should you wish to do so. You can find the officially accepted version of the paper on the arXiv here.

The second paper is the 11th paper in Volume 6 (2023) as well as the 76th in all; this one was published last Thursday (23rd March). This is another for the folder marked Cosmology and Nongalactic Astrophysics. The title is “GLASS: Generator for Large Scale Structure” and the paper is about a new code for the simulation of cosmological observables obtainable from galaxy surveys in a realistic yet computationally inexpensive manner. The code can be downloaded here. This is an interesting approach that contrasts with the “brute force” of full numerical simulations like those I discussed a few days ago.

The authors are Nicolas Tessore (University College London), Arthur Loureiro (UCL, Edinburgh and Imperial College), Benjamin Joachimi (UCL), Maximilian von Wiestersheim-Kramsta (UCL) and Niall Jeffrey (UCL).

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



You can click on the image of the overlay to make it larger should you wish to do so. You can find the officially accepted version of the paper on the arXiv here.

Progress in Computational Cosmology

Posted in Maynooth, The Universe and Stuff with tags , , , , , , on March 23, 2023 by telescoper

We’ve had a visitor in Maynooth for the last couple of days in the form of Mathieu Schaller, who works at Leiden University in the Netherlands. Mathieu was here to work with John Regan’s group on cosmological simulations, but also gave a Theoretical Physics seminar yesterday to a general audience including some of our undergraduate students.

Mathieu’s talk was about a project called FLAMINGO – what is it with cosmologists and acronyms? – which is a suite of simulations designed to be virtual “twins” of the next generation of surveys. This suite includes the largest cosmological simulation ever run to the present time so it can simulate redshift surveys encompassing local volumes near redshift z=0 out to very distant sources at high redshift.

It was a very interesting talk which I thought I would mention here because of one thought that struck me, which is how much the field of computational cosmology has moved on since I started in the field in 1985, almost forty years ago. Not for the first time, it was a seminar that made me feel very old. I’ve been a spectator as far as this is concerned, of course, because I don’t do massive simulation work. Nevertheless these calculations have had a huge impact in the field, and play an important role in, for example, the Euclid mission. They are used both for planning survey strategies and for analyzing the result data.

Take a look at these two pictures, which I’ve chosen to illustrate the progress there has been in the field.

The simulation on the left shows the state-of-the-art when I started my PhD DPhil in 1985 from the classic “DEFW” paper by Davis Efstathiou, Frenk & White; the one on the right I took from Mathieu’s Twitter account. These do no simulate the same volume so the scale looks different, but the morphology of the cosmic web looks similar.

The most obvious change over the years has been the ability to generate colour graphics. The standard cosmological model has also evolved: the one on the right shows a model universe dominated by Cold Dark Matter with no dark energy, while the one on the right is the modern variant known as ΛCDM. The one on the left also is gravitational-only, i.e. no hydrodynamic effects arising from baryonic material., just the effect of the cold dark matter. The simulation on the right includes extensive modelling of baryonic physics. The largest gravity-only simulations that I’m aware of is the Euclid flagship simulation which produces mock galaxy catalogues like this:

The thing that struck me as an oldie, however, is the sheer scale of modern simulations. The DEFW simulations were done by moving N=323 particles around in a box in response to their mutual gravitational interactions. That’s just 32768 particles. The simulations Mathieu talked about involve N=50403 = 125,300,240,064 particles. That’s a factor of almost 4 million bigger. The Flagship simulations are about 16 times bigger than that, with about 2 trillion particles. Impressive! Moore’s Law is a wonderful thing…

New Publication at the Open Journal of Astrophysics

Posted in OJAp Papers, Open Access, The Universe and Stuff with tags , , , , on March 10, 2023 by telescoper

It’s time to announce yet another new paper at the Open Journal of Astrophysics.

The latest paper is the 9th paper in Volume 6 (2023) and the 74th in all. This one is another one for the folder marked Instrumentation and Methods for Astrophysics. The title is “panco2: a Python library to measure intracluster medium pressure profiles from Sunyaev-Zeldovich observations”. The code described in the paper The Python code is available on GitHub and there isextensive technical documentation to complement this paper.

The authors are Florian Kéruzoré (Argonne National Laboratory, USA, and the University of University of Grenoble, France), Frédéric Mayet, Emmanuel Artis, Juan-Francisco Macías-Pérez, Miren Muñoz-Echeverría and Laurence Perotto (all of the University of Grenoble, France) and Florian Ruppin (of the University of Lyon, also in France).

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



You can click on the image of the overlay to make it larger should you wish to do so. You can find the officially accepted version of the paper on the arXiv here.