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Weekly Update from the Open Journal of Astrophysics – 02/08/2025

Posted in OJAp Papers, Open Access, The Universe and Stuff with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , on August 2, 2025 by telescoper

It’s Saturday morning again, and it’s the start of a new month, so it’s time for an update of papers published at the Open Journal of Astrophysics. Since the last update we have published five new papers, which brings the number in Volume 8 (2025) up to 110, and the total so far published by OJAp up to 345. I expect we’ll the total number we published last year (120) sometime this month. I predict that by the end of this year we will have published around 180 papers in Volume 8 and around 400 altogether.

The papers published this week, with their overlays, are as follows. You can click on the images of the overlays to make them larger should you wish to do so.

The first paper to report is “The matter with(in) CPL” by Leonardo Giani (U. Queensland, Australia), Rodrigo Von Marttens (Universidade Federal da Bahia, Brazil) and Oliver Fabio Piattella (Universita degli Studi dell’Insubria, Italy). This was published on Monday 29th July 2025 in the folder Cosmology and Nongalactic Astrophysics. This article presents a new parameterization of the standard model and its implications for the interpretation of cosmological observations.

The overlay is here:

 

The officially-accepted version can be found on arXiv here.

The second paper of the week, published on Tuesday 30th July in the folder Instrumentation and Methods for Astrophysics, is “An automated method for finding the most distant quasars” by Lena Lenz, Daniel Mortlock, Boris Leistedt & Rhys Barnett (Imperial College London, UK) and Paul C. Hewett (U. Cambridge, UK)”.  This paper presents an automated, reproduceable and objective high-redshift quasar selection pipeline, tested on simulations and real data from the Sloan Digital Sky Survey (SDSS) and UKIRT Infrared Deep Sky Survey (UKIDSS). The overlay is here:

 

You can find the officially accepted version of the paper on arXiv here.

The third paper of the week is “Early Post Asymptotic Giant Branch Instability: Does it Affect White Dwarf Hydrogen Envelope Mass?” by James MacDonald (University of Delaware, USA). This one was published on Friday 1st Auguest (i.e. yesterday) in the folder Solar and Stellar Astrophysics. It is an investigation into whether Early Post AGB Instability (EPAGBI) can affect determinations of the total abundance of hydrogen in white dwarf stars.

The overlay is here:

The final version is on arXiv here.

 

The fourth paper of the week, also published on Friday 1st August, is “Light Echoes of Time-resolved Flares and Application to Kepler Data” by Austin King and Benjamin C. Bromley (University of Utah, USA).  This describes a new model for circumstellar disks that incorporates echoes produced by extended, time-resolved flares. It is published in the folder Solar and Stellar Astrophysics. Here is the overlay:

You can find the officially-accepted version on arXiv here.

 

 

The fifth and final article published this week, also published on Friday 1st August,  is “Wide Binaries from Gaia DR3 : testing GR vs MOND with realistic triple modelling” by Charalambos Pittordis, Will Sutherland and Paul Shepherd (Queen Mary, University of London, UK). This presents a test for modified gravity from a sample of wide-binary stars from Gaia DR3, finding that (unmodified) Newtonian gravity provides a better fit to the data. It is in the folder Astrophysics of Galaxies.

The overlay is here:

 

You can find the officially-accepted version on arXiv here.

And that’s all the papers for this week. I’ll do another update next Saturday.

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Cosmology Results from DESI

Posted in Astrohype, The Universe and Stuff with tags , , , , , , on March 20, 2025 by telescoper

Yesterday evening (10pm Irish Time) saw the release of new results from the Dark Energy Spectroscopic Instrument (DESI), completing a trio of major announcements of cosmological results in the space of two days (the Atacama Cosmology Telescope and the Euclid Q1 release being the others). I didn’t see the DESI press conference but you can read the press release here.

There were no fewer than eight DESI papers on the astro-ph section of the arXiv this morning. Here are the titles with links:

You can see from the titles that the first seven of these relate to the second data release (DR2; three years of data) from DESI; the last one listed here is a description of the first data release (DR1), which is now publicly available.

Obviously there is a lot of information to digest in these papers so here are two members of the DESI collaboration talking with Shaun Hotchkiss on Cosmology Talks about the key messages from the analysis of Baryon Acoustic Oscillations (the BAO in the titles of the new papers):

A lot has been made in the press coverage of these results about the evidence that the standard cosmological model is incomplete; see, e.g., here. Here are a few comments.

As I see it, taken on their own, the DESI BAO results are broadly consistent with the ΛCDM model as specified by the parameters determined by the Cosmic Microwave Background (CMB) inferred from Planck. Issues do emerge, however, when these results are combined with other data sets. The most intriguing of these arises with the dark energy contribution. The simplest interpretation of dark energy is that it is a cosmological constant (usually called Λ) which – as explained here – corresponds to a perfect fluid with an equation-of-state p=wρc2 with w=-1. In this case the effective mass density of the dark energy ρ remains constant as the universe expands. To parametrise departures from this constant behaviour, cosmologists have replaced this form with the form w(a)=w0+wa(1-a) where a(t) is the cosmic scale factor. A cosmological constant Λ would correspond to a point (w0=-1, wa=0) in the plane defined by these parameters, but the only requirement for dark energy to result in cosmic acceleration is that w<-1/3, not that w=-1.

The DESI team allow (w0, wa) to act as free parameters and let the DESI data constrain them, either alone or in combinations with other data sets, finding evidence for departures from the “standard values”. Here’s an example plot:

The DESI data don’t include the standard point (at the intersection of the two dashed lines) but the discrepancy gets worse when other data (such as supernovae and CMB) are folded in, as in this picture. The weight of evidence suggests a dark energy contribution which is decreasing with time.

These results are certainly intriguing, and a lot of credit is due to the DESI collaboration for working so hard to identify and remove possible systematics in the analysis (see the papers above) but what do they tell us about ΛCDM?

My view is that we’ve never known what the dark energy actually is or why it is so large that it represents 70% of the overall energy density of the Universe. The Λ in ΛCDM is really just a place-holder, not there for any compelling physical reason but because it is the simplest way of accounting for the observations. In other words, it’s what it is because of Occam’s Razor and nothing more. As with any working hypothesis, the standard cosmological model will get updated whenever new information comes to light (as it is doing now) and/or if we get new physical insights into the origin of dark energy.

Do the latest observations cast doubt on the standard model? I’d say no. We’re seeing an evolutionary change from “We have no idea what the dark energy is but we think it might be a cosmological constant” to “We still have no idea what the dark energy is but we think it might not be a cosmological constant”.

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Timescape versus Dark Energy?

Posted in Astrohype, Open Access, The Universe and Stuff with tags , , , , , , , on January 2, 2025 by telescoper

Just before the Christmas break I noticed a considerable amount of press coverage claiming that Dark Energy doesn’t exist. Much of the media discussion is closely based on a press release produced by the Royal Astronomical Society. Despite the excessive hype, and consequent initial scepticism, I think the paper has some merit and raises some interesting issues.

The main focus of the discussion is a paper (available on arXiv here) by Seifert et al. with the title Supernovae evidence for foundational change to cosmological models. This paper is accompanied by a longer article called Cosmological foundations revisited with Pantheon+ (also available on arXiv) by a permutation of the same authors, which goes into more detail about the analysis of supernova observations. If you want some background, the “standard” Pantheon+ supernova analysis is described in this paper. The reanalysis presented in the recent papers is motivated an idea called the Timescape model, which is not new. It was discussed by David Wiltshire (one of the authors of the recent papers) in 2007 here and in a number of subsequent papers; there’s also a long review article by Wiltshire here (dated 2013).

So what’s all the fuss about?

Simulation of the Cosmic Web

In the standard cosmological model we assume that, when sufficiently coarse-grained, the Universe obeys the Cosmological Principle, i.e. that it is homogeneous and isotropic. This implies that the space-time is described by a Friedmann–Lemaître–Robertson–Walker metric (FLRW) metric. Of course we know that the Universe is not exactly smooth. There is a complex cosmic web of galaxies, filaments, clusters, and giant voids which comprise the large-scale structure of the Universe. In the standard cosmological model these fluctuations are treated as small perturbations on a smooth background which evolve linearly on large scales and don’t have a significant effect on the global evolution of the Universe.

This standard model is very successful in accounting for many things but only at the expense of introducing dark energy whose origin is uncertain but which accounts for about 70% of the energy density of the Universe. Among other things, this accounts for the apparent acceleration of the Universe inferred from supernovae measurements.

The standard cosmology’s energy budget

The approach taken in the Timescape model is to dispense with the FLRW metric, and the idea of separating the global evolution from the inhomogeneities. The idea instead is that the cosmic structure is essentially non-linear so there is no “background metric”. In this model, cosmological observations can not be analysed within the standard framework which relies on the FLRW assumption. Hence the need to reanalyse the supernova data. The name Timescape refers to the presence of significant gravitational time-dilation effects in this model as distinct from the standard model.

I wrote before in the context of a different paper:

….the supernovae measurements do not directly measure cosmic acceleration. If one tries to account for them with a model based on Einstein’s general relativity and the assumption that the Universe is on large-scales is homogeneous and isotropic and with certain kinds of matter and energy then the observations do imply a universe that accelerates. Any or all of those assumptions may be violated (though some possibilities are quite heavily constrained). In short we could, at least in principle, simply be interpreting these measurements within the wrong framework…

So what to make of the latest papers? I have to admit that I didn’t follow all the steps of the supernova reanalysis. I hope an expert can comment on this! I will therefore restrict myself to some general comments.

  • My attitude to the standard cosmological model is that it is simply a working hypothesis and we should not elevate it to a status any higher than that. It is based not only on the Cosmological Principle (which could be false), but on the universal applicability of general relativity (which might not be true), and on a number of other assumptions that might not be true either.
  • It is important to recognize that one of the reasons that the standard cosmology is the front-runner is that it provides a framework that enables relatively straightforward prediction and interpretation of cosmological measurements. That goes not only for supernova measurements but also for the cosmic microwave background, galaxy clustering, gravitational lensing, and so on. This is much harder to do accurately in the Timescape model simply because the equations involved are much more complex; there are few exact solutions of Einstein’s equations that can help. It is important that people work on alternatives such as this.
  • Second, the idea that inhomogeneities might be much more important than assumed in the standard model has been discussed extensively in the literature over the last twenty years or so under the heading “backreaction”. My interpretation of the current state of play is that there are many unresolved questions, largely because of technical difficulties. See, for example, work by Thomas Buchert (here and, with many other collaborators here) and papers by Green & Wald (here and here). Nick Kasiser also wrote about it here.
  • The new papers under discussion focus entirely on supernovae measurements. It must be recognized that these provide just one of the pillars supporting the standard cosmology. Over the years, many alternative models have been suggested that claim to “fix” some alleged problem with cosmology only to find that it makes other issues worse. That’s not a reason to ignore departures from the standard framework, but it is an indication that we have a huge amount of data and we’re not allowed to cherry-pick what we want. We have to fit it all. The strongest evidence in favour of the FLRW framework actually comes from the cosmic microwave background (CMB) with the supernovae provide corroboration. I would need to see a detailed prediction of the anisotropy of the CMB before being convinced.
  • The Timescape model is largely based on the non-linear expansion of cosmic voids. These are undoubtedly important, and there has been considerable observational and theoretical activity in understanding them and their evolution in the standard model. It is not at all obvious to me that the voids invoked to explain the apparent acceleration of the Universe are consistent with what we actually see in our surveys. That is something else to test.
  • Finally, the standard cosmology includes a prescription for the initial conditions from which the present inhomogeneities grew. Where does the cosmic web come from in the Timescape model?

Anyway, I’m sure there’ll be a lot of discussion of this in the next few weeks as cosmologists return to the Universe from their Christmas holidays!

Comments are welcome through the box below, especially from people who have managed to understand the cos.

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Cosmology Talks: Recent DESI Power Spectrum Results

Posted in The Universe and Stuff with tags , , , , , on December 12, 2024 by telescoper

Some weeks ago I posted an item about recent results that have emerged from the DESI (Dark Energy Spectroscopic Instrument) Collaboration. I have been a bit busy since then but I just saw that there is one of those Cosmology Talks about these results which I thought I would pass on. The contributors are Arnaud de Mattia, Hector Gil-Marín and Pauline Zarrouk and they are talking about the analsysis they have done using the “full shape” of the galaxy power spectrum. It’s quite a long video, but very illuminating.

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New Results from DESI

Posted in Barcelona, The Universe and Stuff with tags , , , , , , , on November 20, 2024 by telescoper
The Mayall Telescope at Kitt Peak, in which DESI is housed. This PR image was taken during a meteor shower, which is not ideal observing conditions. Picture Credit: KPNO/NOIRLab/NSF/AURA/R. Sparks

I’ve just got time between meetings to mention that a clutch of brand new papers has emerged from the DESI (Dark Energy Spectroscopic Instrument) Collaboration. There is a press release discussing the results from the Lawrence Berkeley Laboratory here and one from the ICCUB in Barcelona here; several members of the group I visited there during sabbatical are working on DESI. Congratulations to them.

I haven’t had time to read them yet, but a quick skim suggests that the results are consistent with the standard cosmological model.

The latest batch contains three Key Publications:

together with the companion supporting papers:

The links lead to the arXiv version of these papers. These articles can also be found, along with previously released publications by the DESI Collaboration, here.

Anyone who has read the latest papers is welcome to comment through the box below!

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Space Week 2024: The Universe according to Euclid

Posted in Biographical, Books, Talks and Reviews, Euclid, The Universe and Stuff with tags , , , , , on October 10, 2024 by telescoper

I had a very busy day yesterday culminating in the Space Week event I blogged about a few weeks ago. There was a good attendance – lots of young kids as well as adults – and the lecture room was very full. We could probably have filled a much bigger room, actually, but had been moved to a smaller venue and had to close registrations very early to avoid having too many people. I’d guess we had about 350. My talk was the last one, and didn’t finish until 8.30 by which time I was definitely ready for a pint.

You can find the slides I used for my presentation, The Universe according to Euclid, here.

There was an official photographer there who took quite a few pictures but I haven’t seen any of them yet. I’ll post a selection if and when I get them.

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Coming of Age in a Low-Density Universe

Posted in Biographical, Open Access, The Universe and Stuff with tags , , , , on August 25, 2024 by telescoper

I was reminded just now that 30 years ago today, on 25th August 1994, this review article by myself and George Ellis was published in Nature (volume 370, pp. 609–615).

Sorry for the somewhat scrappy scanned copy. The article is still behind a paywall. No open access for the open Universe!

Can this really have been 30 years ago?

Anyway, that was the day I officially became labelled a “crank”, by some, although others thought we were pushing at an open door. We were arguing against the then-standard cosmological model (based on the Einstein – de Sitter model), but the weight of evidence was already starting to shift. Although we didn’t predict the arrival of dark energy, the arguments we presented about the density of matter did turn out to be correct. A lot has changed since 1994, but we continue to live in a Universe with a density of matter much lower than the critical density and our best estimate of what that density is was spot on.

Looking back on this, I think valuable lessons would be learned if someone had the time and energy to go through precisely why so many papers at that time were consistent with a higher-density Universe that we have now settled on. Confirmation bias undoubtedly played a role, and who is to say that it isn’t relevant to this day?

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Evolving Dark Energy or Supernovae Systematics?

Posted in The Universe and Stuff with tags , , , , , , , on August 15, 2024 by telescoper

A few months ago I posted an item about the release new results from the Dark Energy Spectroscopic Instrument (DESI). That was then followed by a presentation explaining the details which you can find here to find out more about the techniques involved. At the time the new DESI results garnered a lot of media attention much of it about claims that the measurements provided evidence for “New Physics”, such as evolving dark energy. Note that the DESI results themselves did not imply this. Only when combined with supernova measurements did this suggestion arise.

Now there’s a new preprint out by George Efstathiou of Cambridge. The abstract is here:

Recent results from the Dark Energy Spectroscopic Instrument (DESI) collaboration have been interpreted as evidence for evolving dark energy. However, this interpretation is strongly dependent on which Type Ia supernova (SN) sample is combined with DESI measurements of baryon acoustic oscillations (BAO) and observations of the cosmic microwave background (CMB) radiation. The strength of the evidence for evolving dark energy ranges from ~3.9 sigma for the Dark Energy 5 year (DES5Y) SN sample to ~ 2.5 sigma for the Pantheon+ sample. Here I compare SN common to both the DES5Y and Pantheon+ compilations finding evidence for an offset of ~0.04 mag. between low and high redshifts. Correcting for this offset brings the DES5Y sample into very good agreement with the Planck LCDM cosmology. Given that most of the parameter range favoured by the uncorrected DES5Y sample is discrepant with many other cosmological datasets, I conclude that the evidence for evolving dark energy is most likely a result of systematics in the DES5Y sample.

Here are a couple of figures from the paper illustrating the difference in parameter constraints using the uncorrected (left) and corrected (right) Dark Energy (Survey) 5 year Supernova sample.

The y-axis shows a parameter wa, which is zero in the standard model with non-evolving dark energy; the non-zero value implied by the left hand panel using the uncorrected data.

Just as with the Hubble Tension I blogged about yesterday, the evidence for a fundamental revision of our standard model may be nothing of the sort but some kind of systematic error. I think we can expect a response from the Dark Energy Survey (DES) team. Grab your popcorn.

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Cosmology Talks: Cosmological Constraints from BAO

Posted in The Universe and Stuff, YouTube with tags , , , , , , , , , on April 5, 2024 by telescoper

Here’s another video in the Cosmology Talks series curated by Shaun Hotchkiss. This one very timely after yesterday’s announcement. Here is the description on the YouTube page:

The Dark Energy Spectroscopic Instrument (DESI) has produced cosmological constraints! And it is living up to its name. Two researchers from DESI, Seshadri Nadathur and Andreu Font-Ribera, tell us about DESI’s measurements of the Baryon Acoustic Oscillations (BAO) released today. These results use one full year of DESI data and are the first cosmological constraints from the telescope that have been released. Mostly, it is what you might expect: tighter constraints. However, in the realm of the equation of state of dark energy, they find, even with BAO alone, that there is a hint of evidence for evolving dark energy. When they combine their data with CMB and Supernovae, who both also find small hints of evolving dark energy on their own, the evidence for dark energy not being a cosmological constant jumps as high as 3.9σ with one combination of the datasets. It seems there still is “concordance cosmology”, it’s just not ΛCDM for these datasets. The fact that all three probes are tentatively favouring this is intriguing, as it makes it unlikely to be due to systematic errors in one measurement pipeline.

My own take is that the results are very interesting but I think we need to know a lot more about possible systematics before jumping to conclusions about time-varying dark energy. Am I getting conservative in my old age? These results from DESI do of course further underline the motivation for Euclid (another Stage IV survey), which may have an even better capability to identify departures from the standard model.

P.S. Here’s a nice graphic showing the cosmic web showing revealed by the DESI survey:

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New Publication at the Open Journal of Astrophysics

Posted in OJAp Papers, Open Access, The Universe and Stuff with tags , , , , , on March 12, 2024 by telescoper

It’s my last morning in Phoenix and since I was too busy at the weekend to post the usual update from the Open Journal of Astrophysics I will do so now, before I go to the Airport for my flight home.

Looking at the workflow I see that there is a considerable backlog of papers that have been accepted but are waiting for the authors to put the final version on arXiv.  As a result there is only one paper to report for last week, being the 17th paper in Volume 7 (2024)  and the 132nd altogether; it was published on March 6 2024. I expect more soon!

The title of the latest paper is “Bayesian analysis of a Unified Dark Matter model with transition: can it alleviate the H0tension?” and it  is in the folder marked Cosmology and NonGalactic Astrophysics.  The article presents an investigation using Bayesian techniques of a specific cosmological model, in which dark matter and dark energy are aspects of a single component, with particular emphasis on the Hubble tension.

The authors are seven in number: Emmanuel Frion (University of Helsinki, Finland, and Western University, Canada); David Camarena (University of New Mexico, USA); Leonardo Giani (University of Queensland, Australia); Tays Miranda (University of Helsinki and University of Jyväskylä, both in Finland); Daniele Bertacca (Università degli Studi di Padova, Italy); Valerio Marra (Universidade Federal do Espírito Santo, Brazil and Osservatorio Astronomico di Trieste, Italy);
and Oliver F. Piattella (Università degli Studi dell’Insubria, Como, Italy).

Here is the overlay of the paper containing the abstract:

 

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

 

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