#dark-energy-spectroscopic-instrument — Public Fediverse posts

A DESI Milestone

Yesterday the Open Journal of Astrophysics published a paper by Porredon et al which will feature in the usual Saturday round-up. That paper, which is based on the First Data Release from the Dark Energy Spectroscopic Instrument (DESI) reminded me that I should mention that DESI recently reached an amazing milestone – it has now mapped the positions and redshifts of 47 million galaxies and quasars! There is a full press-release about this achievement here.

Here’s a little video showing how the survey works:

https://www.youtube.com/watch?v=6H3diAK_KIc

There are more videos and other graphics in the press release.

Here’s a nice picture showing a thin slice through the full survey that reveals the characteristic “cosmic web” of the large-scale structure of the Universe in all its glory:

This progress is great, but it really makes me feel old. Forty years ago, in 1986, I had just started my PhD. The state-of-the-art galaxy redshift survey slice then is shown in this plot, from de Lapparent et al 1986 (ApJLett 302, L1), one of the first papers I read as a research student (I got it in 1985 as a preprint), which contains just 1,100 galaxies:

It is worth mentioning that although DESI has now covered its original target area, it will continue until 2028. You can never have too many galaxy redshifts!

A DESI Milestone

Yesterday the Open Journal of Astrophysics published a paper by Porredon et al which will feature in the usual Saturday round-up. That paper, which is based on the First Data Release from the Dark Energy Spectroscopic Instrument (DESI) reminded me that I should mention that DESI recently reached an amazing milestone – it has now mapped the positions and redshifts of 47 million galaxies and quasars! There is a full press-release about this achievement here.

Here’s a little video showing how the survey works:

https://www.youtube.com/watch?v=6H3diAK_KIc

There are more videos and other graphics in the press release.

Here’s a nice picture showing a thin slice through the full survey that reveals the characteristic “cosmic web” of the large-scale structure of the Universe in all its glory:

This progress is great, but it really makes me feel old. Forty years ago, in 1986, I had just started my PhD. The state-of-the-art galaxy redshift survey slice then is shown in this plot, from de Lapparent et al 1986 (ApJLett 302, L1), one of the first papers I read as a research student (I got it in 1985 as a preprint), which contains just 1,100 galaxies:

It is worth mentioning that although DESI has now covered its original target area, it will continue until 2028. You can never have too many galaxy redshifts!

Neat!

"The Dark Energy Spectroscopic Instrument has completed its originally planned five-year mission and mapped more than 47 million galaxies and quasars, creating the largest high-resolution 3D map of our universe to date."

https://newscenter.lbl.gov/2026/04/15/desi-completes-planned-3d-map-of-the-universe-and-continues-exploring/

#science #ScienceNews #space #DESI #DarkEnergySpectroscopicInstrument

Neat!

"The Dark Energy Spectroscopic Instrument has completed its originally planned five-year mission and mapped more than 47 million galaxies and quasars, creating the largest high-resolution 3D map of our universe to date."

https://newscenter.lbl.gov/2026/04/15/desi-completes-planned-3d-map-of-the-universe-and-continues-exploring/

#science #ScienceNews #space #DESI #DarkEnergySpectroscopicInstrument

Dark Energy Survey Year Y6 Results Day!

This morning’s arXiv announcement contained a number of papers related to the Dark Energy Survey Y6 analysis. There is also a Zoom webinar later today at 10.30 Central Time (16.30 GMT’; 13.30 in Greeland). Details can be found here.

You can find links to and abstracts of all the papers here, but I thought it would be useful to provide arXiv links to the latest batch here.

• arXiv:2601.14559 Dark Energy Survey Year 6 Results: Cosmological Constraints from Galaxy Clustering and Weak Lensing – this is the key summary paper.
• arXiv:2601.14484 Dark Energy Survey Year 6 Results: MagLim++ Lens Sample Selection and Measurements of Galaxy Clustering
• arXiv:2601.14864 Dark Energy Survey: DESI-Independent Angular BAO Measurement
• arXiv:2601.15175 Dark Energy Survey Year 6 Results: Galaxy-galaxy lensing
• arXiv:2601.14833 Dark Energy Survey Year 6 Results: Magnification modeling and its impact on galaxy clustering and galaxy-galaxy lensing cosmology
• arXiv:2601.14859 Dark Energy Survey Year 6 Results: Weak Lensing and Galaxy Clustering Cosmological Analysis Framework

A number of DES Y6 papers already published – including several in the Open Journal of Astrophysics – are listed here.

I’ll just highlight a couple of points from the first paper listed above, which uses the now standard “3x2pt” analysis, which combines three complementary two-point correlation functions: cosmic shear; galaxy-galaxy lensing and galaxy clustering. The abstract of this paper is as follows:

A notable result is contained in the last sentence. 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  ρ of the dark energy 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. Results last year from DESI suggested a value of w0 different from -1, but DES does not.

I thought I’d add one of the cosmological contraint plots:

The results look qualitatively similar to previous plots but the contours have shifted a bit.

#Cosmology #DarkEnergy #DarkEnergySpectroscopicInstrument #DarkEnergySurvey #DES #DESYear6 #DESI

BBC News - Dark Energy experiment shakes Einstein's theory of Universe - BBC News
https://www.bbc.co.uk/news/articles/c4geldjjge0o
#DarkEnergy #DarkEnergySpectroscopicInstrument #DESI

BBC News - Dark Energy experiment shakes Einstein's theory of Universe - BBC News
https://www.bbc.co.uk/news/articles/c4geldjjge0o
#DarkEnergy #DarkEnergySpectroscopicInstrument #DESI

Cosmology Results from DESI

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:

• DESI DR2 Results I: Baryon Acoustic Oscillations from the Lyman Alpha Forest
• DESI DR2 Results II: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints;
• Construction of the Damped Lyα Absorber Catalog for DESI DR2 Lyα BAO;
• Validation of the DESI DR2 Lyα BAO analysis using synthetic datasets;
• Validation of the DESI DR2 Measurements of Baryon Acoustic Oscillations from Galaxies and Quasars;
• Extended Dark Energy analysis using DESI DR2 BAO measurements;
• Constraints on Neutrino Physics from DESI DR2 BAO and DR1 Full Shape;
• Data Release 1 of the Dark Energy Spectroscopic Instrument.

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):

https://www.youtube.com/watch?v=YiRaDtslycE

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<0 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”.

#baryonAcousticOscillations #cosmologicalConstant #Cosmology #DarkEnergy #DarkEnergySpectroscopicInstrument #OccamSRazor #ShaunHotchkiss

Cosmology Talks: Recent DESI Power Spectrum Results

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.

https://www.youtube.com/watch?v=-2mlU-YzEbw

#Cosmology #CosmologyTalks #DarkEnergy #DarkEnergySpectroscopicInstrument #DESI #Physics

New Results from DESI

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:

• DESI Collaboration et al., DESI 2024 II: Sample Definitions, Characteristics, and Two-point Clustering Statistics
• DESI Collaboration et al., DESI 2024 V: Full-Shape Galaxy Clustering from Galaxies and Quasars
• DESI Collaboration et al., DESI 2024 VII: Cosmological Constraints from the Full-Shape Modeling of Clustering Measurements

together with the companion supporting papers:

• Ishak et al. (2024), Modified Gravity Constraints from the Full Shape Modeling of Clustering Measurements from DESI 2024
• Rosado-Marín et al. (2024), Mitigating Imaging Systematics for DESI 2024 Emission Line Galaxies and Beyond
• Bianchi et al. (2024), Characterization of DESI fiber assignment incompleteness effect on 2-point clustering and mitigation methods for Y1 analysis
• Forero-Sánchez et al. (2024), Analytical and EZmock covariance validation for the DESI 2024 results
• Findlay et al. (2024), Exploring HOD-dependent systematics for the DESI 2024 Full-Shape galaxy clustering analysis

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!

#astronomy #Cosmology #DarkEnergy #DarkEnergySpectroscopicInstrument #darkMatter #DESI #ICCUB #Physics

Dark energy might not be constant after all - Enlarge / The Dark Energy Spectroscopic Instrument (DESI) has made the ... - https://arstechnica.com/?p=2013826 #darkenergyspectroscopicinstrument #baryonicacousticoscillations #astrophysics #darkenergy #astronomy #cosmology #lambdacdm #science #physics #quasars #desi

Dark energy might not be constant after all - Enlarge / The Dark Energy Spectroscopic Instrument (DESI) has made the ... - https://arstechnica.com/?p=2013826 #darkenergyspectroscopicinstrument #baryonicacousticoscillations #astrophysics #darkenergy #astronomy #cosmology #lambdacdm #science #physics #quasars #desi