## Zel’dovich Pancake Day!

Posted in The Universe and Stuff with tags , , , on February 16, 2021 by telescoper

Today it’s Shrove Tuesday but unfortunately I forgot to buy shroves yesterday so will have to make do with pancakes instead, but not the usual kind. I’ve blogged before about the Zel’dovich Approximation (published in Zeldovich, Ya.B. 1970, A&A, 5, 84) but there’s no harm in describing this classic again. Here’s the first page of the original paper:

In a nutshell, this daringly simple approximation considers the evolution of particles in an expanding Universe from an early near-uniform state into the non-linear regime as a sort of ballistic, or kinematic, process. Imagine the matter particles are initial placed on a uniform grid, where they are labelled by Lagrangian coordinates $vec{q}$. Their (Eulerian) positions at some later time $t$ are taken to be

$vec{r}(vec(q),t) = a(t) vec{x}(vec{q},t) = a(t) left[ vec{q} + b(t) vec{s}(vec{q},t) right].$

Here the $vec{x}$ coordinates are comoving, i.e. scaled with the expansion of the Universe using the scale factor $a(t)$. The displacement $vec{s}(vec{q},t)$ between initial and final positions in comoving coordinates is taken to have the form

$vec{s}(vec{q},t)= vec{nabla} Phi_0 (vec{q})$

where $Phi_0$ is a kind of velocity potential (which is also in linear Newtonian theory proportional to the gravitational potential).If we’ve got the theory right then the gravitational potential field defined over the initial positions is a Gaussian random field. The function $b(t)$ is the growing mode of density perturbations in the linear theory of gravitational instability.

This all means that the particles just get a small initial kick from the uniform Lagrangian grid and their subsequent motion carries on in the same direction. The approximation predicts the formation of caustics in the final density field when particles from two or more different initial locations arrive at the same final location, a condition known as shell-crossing. The caustics are identified with the main elements we find in large-scale structure. Because the initial collapse is usually along one direction the dominant structures are known as pancakes (or, as Zel’dovich himself might have called them, blini…).

Here’s a picture of a simulation showing these structures from the classic paper of Davis, Efstathiou, Frenk & White (1985):

Despite its simplicity this approximation is known to perform extremely well at reproducing the morphology of the cosmic web, although it breaks down after shell-crossing has occurred. In reality, bound structures are formed whereas the Zel’dovich approximation simply predicts that particles sail straight through the caustic which consequently evaporates.

## The Cosmic Web in Maynooth

Posted in Books, Talks and Reviews, Talks and Reviews, The Universe and Stuff with tags , , on November 5, 2019 by telescoper

Next week (10th to 17th November) in Ireland is Science Week and this will be celebrated by a number of events here in Maynooth, among which is a talk by yours truly on 15th November:

Here is a short description:

How can we map the distribution of galaxies over thousands of millions of light years? What does the Universe look like on these scales? How did get to look like that? And how do we know?This talk will explain how astronomers and cosmologists have come together over the past couple of decades to make huge surveys of the Universe, revealing the existence of a complex but beautiful Cosmic Web’ with vast chains of galaxies strung out around immense dark voids. These observational breakthroughs have been mirrored by advances in theory and computer simulation that allow us to understand how this amazing structure was born 14 billion years ago in the Big Bang and has been growing and evolving ever since. Free and open to TY, 5th and 6th year students, this talk will be of particular interest in those interested astronomy, space, physics and the Universe itself!

It is on in the morning to make it possible for school students to attend and the talk is adapted to this audience, so it won’t be the same as the one I gave in Dublin last week. The timing seems to have worked because the lecture theatre has over 200 seats in it but is already almost full. There are still a few places available so if you’re in the area you can book here.

## Hallowe’en at Dias!

Posted in Uncategorized with tags , , on October 29, 2019 by telescoper

I’m interrupting my short break to post a quick reminder that I’m giving a public talk at the Dublin Institute for Advanced Studies (DIAS) this coming Thursday, Dark Matter Day, October 31st 2019, coincidentally the same day as Hallowe’en, or in modern parlance Not-Brexit Day. I am particularly grateful to be invited to give a talk that evening because it allows me to avoid getting involved in trick-or-treat or any of that nonsense.

Here is the nice advert the people at DIAS have made for the event:

The talk is free, but you need to sign up here as the venue is not infinitely large and is already almost full. You can also find some more details about the talk there.

## Dark Matter Day at DIAS

Posted in Books, Talks and Reviews, Talks and Reviews, The Universe and Stuff with tags , , on October 7, 2019 by telescoper

Just a quick post to mention that I’m giving a public talk at the Dublin Institute for Advanced Studies (DIAS) on Dark Matter Day, October 31st 2019, coincidentally the same day as Halloween. I am particularly grateful to be invited to give a talk that evening because it allows me to avoid getting involved in trick-or-treat or any of that nonsense.

Here is the nice advert the people at DIAS have made for the event:

The talk is free, but you need to sign up here as the venue is not infinitely large. You can also find some more details about the talk there.

## Gas Filaments in the Cosmic Web

Posted in Astrohype, The Universe and Stuff with tags , , , , , on October 4, 2019 by telescoper

I saw that there’s a new paper that has just been published in the journal Science by Umehata et al with the title Gas filaments of the cosmic web located around active galaxies in a protocluster. In case you run into a paywall at Science, you may of course, find the paper on the arXiv here.

The abstract reads:

Cosmological simulations predict the Universe contains a network of intergalactic gas filaments, within which galaxies form and evolve. However, the faintness of any emission from these filaments has limited tests of this prediction. We report the detection of rest-frame ultraviolet Lyman-alpha radiation from multiple filaments extending more than one megaparsec between galaxies within the SSA 22 proto-cluster at a redshift of 3.1. Intense star formation and supermassive black-hole activity is occurring within the galaxies embedded in these structures, which are the likely sources of the elevated ionizing radiation powering the observed Lyman-alpha emission. Our observations map the gas in filamentary structures of the type thought to fuel the growth of galaxies and black holes in massive proto-clusters.

The existence of a complex cosmic web of filaments and voids has been known about for some time as it is revealed on large scales by the distribution of galaxies through redshift surveys:

You can see all my posts agged with Cosmic Web’ here. There are also good theoretical reasons (besides numerical simulations) for believing this is what the large-scale distribution of matter should look like. Roughly speaking, dense knots of matter lie at the vertices of a three-dimensional pattern traced out by one-dimensional structures.

We have also known for some time, however, that there is more going on in cosmic structure than is revealed by light from stars in galaxies. In particular the way gas flows along the filaments into the knots plays an important role in galaxy and cluster formation. This paper reveals the distribution of gas around a giant cluster that has formed at such a node using observations made using the European Southern Observatory’s MUSE instrument.

Here’s a pretty picture:

I found out about this paper from a news piece in the Guardian with the title Scientists observe mysterious cosmic web directly for first time. That’s sufficiently misleading for me to cross-file the paper under Astrohype’ because, as I explained above, we have been observing the cosmic web for decades. It is however only just becoming possible to observe the diffuse gas rather than having to join the dots between the galaxies so it is an exciting result. My complaint, I suppose, is that the word directly’ is doing a lot of heavy lifting in the title!

## New Publication at the Open Journal of Astrophysics!

Posted in OJAp Papers, Open Access, The Universe and Stuff with tags , , , , , , , on August 7, 2019 by telescoper

Just before I went off on my break I published another new paper at The Open Journal of Astrophysics, but I didn’t get time to write a post about before going on leave. In fact I completed the publication process using a WIFI connection in the departure lounge at Dublin Airport!

The authors are Miguel Aragon Calvo of Instituto de Astronomía at UNAM in Ensenada, Mexico , Mark Neyrinck of the University of the Basque Country and Joseph Silk – yes, that Joseph Silk! – of the Institut dAstrophysique de Paris Université Pierre et Marie Curie. If the Open Journal project is to succeed we need to get some big names submitting papers, and there aren’t many bigger than Joe Silk so I’m very glad to see him among the author list.

You can find the accepted version on the arXiv here. This version was accepted after modifications requested by the referee and editor. Because this is an overlay journal the authors have to submit the accepted version to the arXiv (which we then check against the copy submitted to us) before publishing; version 3 on the arXiv is the accepted version.

You will see that this is  one for the Astrophysics of Galaxies’ folder. We would be happy to get more submissions from other areas, especially Stellar and Planetary astrophysics. Hint! Hint!

P.S. Just a reminder that we now have an Open Journal of Astrophysics Facebook page where you can follow updates from the Journal should you wish..

## 50 Years of the Cosmic Web

Posted in The Universe and Stuff with tags , , , , , , on November 21, 2018 by telescoper

I’ve just given a lecture on cosmology during which I showed a version of this amazing image:

The picture was created in 1977 by Seldner et al. based on the galaxy counts prepared by Charles Donald Shane and Carl Alvar Wirtanen and published in 1967 (Publ. Lick. Observatory 22, Part 1). There are no stars in the picture: it shows the  distribution of galaxies in the Northern Galactic sky. The very dense knot of galaxies seen in the centre of the image is the Coma Cluster, which lies very close to the Galactic North pole.The overall impression  is of a frothy pattern, which we now know as the Cosmic Web. I don’t think it is an unreasonable claim that the Lick galaxy catalogue provided the first convincing evidence of the form of the morphology of the large-scale structure of the Universe.

The original Shane-Wirtanen Lick galaxy catalogue lists counts of galaxies in 1 by 1 deg of arc blocks, but the actual counts were made in 10 by 10 arcmin cells. The later visualization is based on a reduction of the raw counts to obtain a catalogue with the original 10 by 10 arcmin resolution. The map above based on the corrected counts  shows the angular distribution of over 800,000 galaxies brighter than a B magnitude of approximately 19.

The distribution of galaxies is shown only in projection on the sky, and we are now able to probe the distribution in the radial direction with large-scale galaxy redshift surveys in order to obtain three-dimensional maps, but counting so many galaxy images by eye on photographic plates was a Herculean task that took many years to complete. Without such heroic endeavours in the past, our field would not have progressed anything like as quickly as it has.

I’m sorry I missed the 50th anniversary of the publication of the Lick catalogue, and Messrs Shane and Wirtanen both passed away some years ago, but at last I can doff my cap in their direction and acknowledge their immense contribution to cosmological research!

UPDATE: In response to the comments below, I have updated this scan of the original rendition of the Lick counts:

## Celebrating the Sloan Telescope

Posted in The Universe and Stuff with tags , , , , , , , , on May 9, 2018 by telescoper

A little bird tweeted at me this morning that today is the 20th anniversary of first light through the Sloan Telescope (funded by the Alfred P. Sloan Foundation) which has, for the past two decades, been surveying as much of the sky as it can from its location in New Mexico (about 25% altogether): the Sloan Digital Sky Survey is now on its 14th data release.

Here’s a picture of the telescope:

For those of you who want the optical details, the Sloan Telescope is a 2.5-m f/5 modified Ritchey-Chrétien altitude-azimuth telescope located at Apache Point Observatory, in south east New Mexico (Latitude 32° 46′ 49.30″ N, Longitude 105° 49′ 13.50″ W, Elevation 2788m). A 1.08 m secondary mirror and two corrector lenses result in a 3° distortion-free field of view. The telescope is described in detail in a paper by Gunn et al. (2006).

A 2.5m telescope of modest size by the standards of modern astronomical research, but the real assets of the Sloan telescope is a giant mosaic camera, highly efficient instruments and a big investment in the software required to generate and curate the huge data sets it creates. A key feature of SDSS is that its data sets are publicly available and, as such, they have been used in countless studies by a huge fraction of the astronomical community.

The Sloan Digital Sky Survey’s original legacy’ survey was basically a huge spectroscopic redshift survey, mapping the positions of galaxies and quasars in three dimensions to reveal the `cosmic web’ in unprecedented detail:

As it has been updated and modernised, the Sloan Telescope has been involved in a range of other surveys aimed at uncovering different aspects of the universe around us, including several programmes still ongoing.

## The Cosmic Web – my Lincoln lecture slides…

Posted in Talks and Reviews, The Universe and Stuff with tags , on February 28, 2017 by telescoper

For those of you who are interested, here are the slides I used for the 1st Annual Robert Grosseteste Lecture on Astrophysics/Cosmology, given at the University of Lincoln on Thursday 23rd February 2017.

## The Zel’dovich Lens

Posted in The Universe and Stuff with tags , , , , on June 30, 2014 by telescoper

Back to the grind after an enjoyable week in Estonia I find myself with little time to blog, so here’s a cute graphic by way of  a postscript to the IAU Symposium on The Zel’dovich Universe. I’ve heard many times about this way of visualizing the Zel’dovich Approximation (published in Zeldovich, Ya.B. 1970, A&A, 5, 84) but this is by far the best graphical realization I have seen. Here’s the first page of the original paper:

In a nutshell, this daringly simple approximation considers the evolution of particles in an expanding Universe from an early near-uniform state into the non-linear regime as a sort of ballistic, or kinematic, process. Imagine the matter particles are initial placed on a uniform grid, where they are labelled by Lagrangian coordinates $\vec{q}$. Their (Eulerian) positions at some later time $t$ are taken to be

$\vec{r}(\vec(q),t) = a(t) \vec{x}(\vec{q},t) = a(t) \left[ \vec{q} + b(t) \vec{s}(\vec{q},t) \right].$

Here the $\vec{x}$ coordinates are comoving, i.e. scaled with the expansion of the Universe using the scale factor $a(t)$. The displacement $\vec{s}(\vec{q},t)$ between initial and final positions in comoving coordinates is taken to have the form

$\vec{s}(\vec{q},t)= \vec{\nabla} \Phi_0 (\vec{q})$

where $\Phi_0$ is a kind of velocity potential (which is also in linear Newtonian theory proportional to the gravitational potential).If we’ve got the theory right then the gravitational potential field defined over the initial positions is a Gaussian random field. The function $b(t)$ is the growing mode of density perturbations in the linear theory of gravitational instability.

This all means that the particles just get a small initial kick from the uniform Lagrangian grid and their subsequent motion carries on in the same direction. The approximation predicts the formation of caustics  in the final density field when particles from two or more different initial locations arrive at the same final location, a condition known as shell-crossing. The caustics are identified with the walls and filaments we find in large-scale structure.

Despite its simplicity this approximation is known to perform extremely well at reproducing the morphology of the cosmic web, although it breaks down after shell-crossing has occurred. In reality, bound structures are formed whereas the Zel’dovich approximation simply predicts that particles sail straight through the caustic which consequently evaporates.

Anyway the mapping described above can also be given an interpretation in terms of optics. Imagine a uniform illumination field (the initial particle distribution) incident upon a non-uniform surface (e.g. the surface of the water in a swimming pool). Time evolution is represented by greater depths within the pool.  The light pattern observed on the bottom of the pool (the final distribution) displays caustics with a very similar morphology to the Cosmic Web, except in two dimensions, obviously.

Here is a very short  but very nice video by Johan Hidding showing how this works:

In this context, the Zel’dovich approximation corresponds to the limit of geometrical optics. More accurate approximations can presumably be developed using analogies with physical optics, but this programme has only just begun.