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:
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!
In the Dark 
April 30, 2026 at 8:16 am
Your message reminded me of the days of preprints! Writing to authors to ask them to send you copies of their published papers (offprints as they were called) and to be added to their circulation list for preprints of their work. Some organisations had preprint series (such as RGO I think) and would send these out to people on their circulation lists.
I remember when interviewed for my lectureship in 1989, the external assessor (Mike Seaton) asked me what I expected the big discoveries to be in the next 30 years. Cannot recall what I said but I am sure it was completely wrong.
April 30, 2026 at 8:32 am
Yes, this was long before arXiv so we existed on printed preprints – they used to come in the mail on an almost daily basis.
April 30, 2026 at 2:34 pm
Good old preprints! I remember the many times I’d visit our department’s little astronomy library to see if any new items had appeared on the preprint display shelf. It was the only way to keep up with the latest news, and since there were only a handful of new deliveries each week, it was actually possible to do so.
Not so easy these days, with tens of papers a day appearing on arXiv …
May 2, 2026 at 11:14 am
[…] parameters. The analysis also introduces a new blinding procedure to prevent confirmation bias. See this post for news of an important DESI […]