Archive for Cosmology

Cosmological Non-Linearities as an Effective Fluid

Posted in The Universe and Stuff with tags , , , , , , , on November 27, 2020 by telescoper

We know our Universe is inhomogeneous, comprising regions of high density (galaxies and clusters of galaxies) as well as regions of much lower density (e.g. cosmic voids). Our standard cosmological models are based on exact solutions of Einstein’s equations of general relativity that assume homogeneity and isotropy. The general assumption is that if we confine ourselves to large enough scales the effect of the clumpiness of matter can either be disregarded or treated using perturbation theory. As far as we can tell, that approach works reasonably well but we know it must fail on smaller scales where the structure is in the non-linear regiome where it can’t be described accurately using perturbation theory because the fluctuations are so large.

From time to time I’ve idly wondered whether it might be possible to understand the effect of these non-linearities in general relativity by treating them as a kind of fluid with an energy-momentum tensor that acts as a correction to that of the perfect fluid form of the background cosmological model. This would have to be done via some sort of averaging so it would be an effective, coarse-grained description rather than an exact treatment. It is clear though that non-linearities would generate departures from the perfect fluid form, particularly resulting in off-diagonal terms in the energy-momentum tensor corresponding to anisotropic stresses (e.g. viscosity terms).

Anyway, a recent exchange on Twitter relating to a new paper that has just appeared revealed that far cleverer people than me had looked at this in quite a lot of detail a decade ago:

You can find the full paper here.

There are quite a lot of subtleties in this – how to do the spatial averaging, how to do the time-slicing, etc – which I don’t fully understand but at least I’m reassured that it isn’t a daft idea to try thinking of things this way!

P.S. The relativistic simulations reported in this paper could in principle be used to estimate the parameters mentioned in the abstract above, if that hasn’t been done before!

 

Astrophysics & Cosmology Masterclass at Maynooth

Posted in Education, Maynooth, The Universe and Stuff with tags , , on November 24, 2020 by telescoper

Stealing the idea from our long-running Particle Physics Master Class (which sadly had to be cancelled this year for Covid-19 related reasons, but will resume in 2021), the Department of Theoretical Physics at Maynooth University has decided to launch a Masterclass in Astrophysics & Cosmology. The first one will be on January 14th 2021.

This will be a half-day virtual event via Zoom. It’s meant for school students in their 5th or 6th year of the Irish system, but there might be a few of them or their teachers who see this blog so I thought I’d share the news here. You can find more information, including instructions on how to book a place, here.

Here is the official poster and the programme:

I’ll be talking about cosmology early on, while John Regan will talk about black holes. After the coffee break one of our PhD students will talk about why they wanted to study astrophysics. Then I’ll say something about our degree programmes for those students who might be interested in studying astrophysics and/or cosmology as part of a science course. We’ll finish with questions either about the science or the study!

Cosmology Talks: Mateja Gosença & Bodo Schwabe on Simulating Mixed Fuzzy and Cold Dark Matter

Posted in The Universe and Stuff with tags , , , , , , on November 11, 2020 by telescoper

It’s been too long since I shared one of those interesting cosmology talks on the Youtube channel curated by Shaun Hotchkiss. This channel features technical talks rather than popular expositions so it won’t be everyone’s cup of tea but for those seriously interested in cosmology at a research level they should prove interesting.

Anyway, although I’ve been too busy to check out the talks much recently I couldn’t resist sharing this one not only because it’s on a topic I find interesting (and have worked on) but also because one of the presenters (Mateja Gosença) is a former PhD student of mine from Sussex! So before I go fully into proud supervisor mode, I’ll just say that the talk is about AxioNyx, which is a new public code for simulating both ultralight (or “Fuzzy”, so called because its Compton de Broglie wavelength is large enough to be astrophysically relevant) dark matter (FDM) and Cold dark matter (CDM) simultaneously. The code simulates the FDM using adaptive mesh refinement and the CDM using N-body particles.

P. S. The paper that accompanies this talk can be found on the arXiv here.

New Publication at the Open Journal of Astrophysics!

Posted in Open Access, The Universe and Stuff with tags , , , , , , on October 2, 2020 by telescoper

Time to announce another new paper in the Open Journal of Astrophysics. The latest publication is by Amy Louca and Elena Sellentin, both of the Sterrewacht Leiden in the The Netherlands, and is entitled The impact of signal-to-noise, redshift, and angular range on the bias of weak lensing 2-point functions. This is another one for the Cosmology and Nongalactic Astrophysics folder.

Here is a screen grab of the overlay:

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

We actually published this one a few days ago but there was a slight delay registering the metadata and also I was very busy, so this post is a little late. With this paper, we have published as many papers so far in 2020 as we did in 2019 so with several more in the pipeline this looks like being our busiest year

Memories of My First Paper

Posted in Biographical, The Universe and Stuff with tags , on September 28, 2020 by telescoper

The death of John Barrow reminded me of a post I did some years ago about my first ever publication, which was published on 15th September 1986 while I was doing my DPhil at Sussex under John’s supervision. I’m mentioning
it hereby way of a postscript to yesterday’s piece.

Here is the front page:

mnras_paper

This was before the days of arXiv so there isn’t a copy on the preprint server, but you can access the whole article here on NASA/ADS.

All right. I know it’s a shitty little paper. But you have to start somewhere!

I’m particularly sad that, looking back, it reads as if I meant to be very critical of the Kaiser (1984) paper that inspired it. I still think that was a brilliant paper because it was based on a very original idea that proved to be enormously influential. The only point I was really making was that a full calculation of the size of the effect Nick Kaiser had correctly identified was actually quite hard, and his simple approximation was of limited quantitative usefulness. The idea was most definitely right, however.

I was just a year into my PhD  DPhil when this paper came out, and it wasn’t actually on what was meant to be the subject of my thesis work (which was the cosmic microwave background), although the material was related.

This paper provides two excellent illustrations of what a good supervisor John was. I was a bit stuck with the project that John had assigned me and eventually admitted to him that I was having problems getting anywhere. I thought he’d assume I was useless and suggest that someone else should supervise me. But no. He said he realised it was a hard problem and sometimes it’s good to think about something else when you’re stuck. So he asked me to look at cluster clustering for a bit. I told him what I found and he said I should write this up as a paper, which I did. Most importantly however the trick I used in simplifying the calculations in this paper turned out to be applicable to the first problem, hotspots in the cosmic microwave background, which led a success in the project and to my second paper. We were both delighted that everything turned out well with that original project.

My original draft of this first paper had John Barrow’s name on it, but he removed his name from the draft (as well as making a huge number of improvements to the text). At the time I assumed that he took his name off because he didn’t want to be associated with such an insignificant paper, but I later realized he was just being generous. It was very good for me to have a sole-author paper very early on. I’ve taken that lesson to heart and have never insisted – unlike some supervisors – in putting my name on my students’ work.

Watch “Why the Universe is quite disappointing really – Episode 7” on YouTube

Posted in The Universe and Stuff, YouTube with tags , , , , on September 3, 2020 by telescoper

Back for Episode 7 of this series in which I explain how we can measure the strength of acoustic waves in early Universe using measurements of the cosmic microwave background, and how that leads to the conclusion that the Big Bang wasn’t as loud as you probably thought. You can read more about this here.

Primordial Figures

Posted in Biographical, The Universe and Stuff with tags , , , , on August 28, 2020 by telescoper

I was rummaging around looking for some things related to a paper I’m struggling to finish before term starts and I found some vintage diagrams. They brought back a lot of memories of working on the textbook I wrote with Francesco Lucchin way back in the 1990s. In particular I remember how long it took to make these figures, when nowadays it would take a few minutes. In fact I’m thinking of setting this as a Computational Physics project for next year. These are not full computations either, just a simple fluid-based approach.

The curves show the evolution of fluctuations in both matter δm and radiation δr on a particular scale (i.e. a Fourier mode of given wavelength) defined as δm=δρmm, etc.  The x-axis shows the cosmic scale factor, which represents the expansion of the Universe and in both cases the universe is flat, i.e. it has a critical density. The first graph shows a universe with only baryonic matter:

Notice the strongly coupled oscillations in matter and radiation until a scale factor of around 10-3, corresponding to a redshift of a thousand or so, which is when matter and radiation decouple. The y-axis is logarithmic so the downward spikes represent zero points.

It is these oscillations which are responsible for the bumps and wiggles in the spectrum of the cosmic microwave background spectrum, as different Fourier modes arrive at the last scattering surface at a different phase of its oscillation. Of course going from the Figure above to the CMB fluctuation spectrum (see below) involves more calculations, and there is now a well-established machinery for doing these with full physical descriptions, but I think the above diagram makes the physical origin of these features clear.

The CMB power spectrum from Planck

The second diagram shows what happens if you add a third component called `X’ in the Figure below which we take to be cold non-baryonic matter. Because  this stuff doesn’t interact directly with radiation (while baryons do) it doesn’t participate in the oscillations but the density perturbations just carry on growing:

Notice too that at late times (i.e. after the baryonic matter and radiation have decoupled) the baryonic component grows much more quickly than in the first Figure. This is because, when released from the effect of the photon background, baryons start to feel the gravitational pull of the dark matter perturbations.

There’s nothing new in this of course – these Figures are thirty years old and similar were produced even earlier than that – but I still think pictures like these are pedagogically useful,

 

New KiDS on the Blog!

Posted in The Universe and Stuff with tags , , , , , , , , , , on August 5, 2020 by telescoper

The above image is from the Kilo Degree Survey, performed using the OmegaCAM instrument on the European Southern Observatory’s VST Survey Telescope at Cerro Paranal in Northern Chile. I got it by googling `Pictures of KiDS’, which was probably unwise.

Here’s another picture, of part of the survey region.

A few people have asked me why I didn’t post about the new results from KiDs which came out last week. The answer is simply that I’ve been a bit busy, but here we go now with a post on the blog about the new KiDs papers. These appear as a bunch of five on the arXiv:

KiDS-1000 Methodology: Modelling and inference for joint weak gravitational lensing and spectroscopic galaxy clustering analysis

KiDS-1000 catalogue: weak gravitational lensing shear measurements

KiDS-1000 catalogue: Redshift distributions and their calibration

KiDS-1000 Cosmology: Cosmic shear constraints and comparison between two point statistics

KiDS-1000 Cosmology: Multi-probe weak gravitational lensing and spectroscopic galaxy clustering constraints

The result that stands out from the latest release is the suggestion that the Universe is about 8% less clumpy than the standard cosmological model suggests. The level of clumpiness is quantified by the parameter S8 which, according to Planck, has a value 0.832 ± 0.013 whereas KiDS gives 0.776 (+0.020/-0.014), a discrepancy of about 3σ. It’s not only the Hubble constant that is causing a bit of tension in cosmological circles!

 

 

More Lockdown Perspectives on the Hubble Tension and thoughts on the future of scientific publications

Posted in Open Access, The Universe and Stuff with tags , , , , on July 28, 2020 by telescoper

This is interesting. Remember last week when I posted about a paper by George Efstathiou on the Hubble Constant controversy. This is the abstract.

Well, a new version of the paper has just appeared on the arXiv that includes some comments in response from the SH0ES team.

It is of course interesting in itself to see the cut and thrust of scientific debate on a live topic such as this, but in my mind at least it raises interesting questions about the nature of scientific publication. To repeat something I wrote a a while ago, it seems  to me that the scientific paper published in an academic journal is an anachronism. Digital technology enables us to communicate ideas far more rapidly than in the past and allows much greater levels of interaction between researchers. I agree with Daniel Shanahan that the future for many fields will be defined not in terms of “papers” which purport to represent “final” research outcomes, but by living documents continuously updated in response to open scrutiny by the research community.

The Open Journal of Astrophysics is innovative in some ways but remains wedded to the paper as its fundamental object, and the platform is not able to facilitate interaction with readers. Of course one of the worries is that the comment facilities on many websites tend to get clogged up with mindless abuse, but I think that is manageable. I have some ideas on this, but for the time being I’m afraid all my energies are taken up with other things so this is for the future.

I’ve long argued that the modern academic publishing industry is not facilitating but hindering the communication of research. The arXiv has already made academic journals virtually redundant in many of branches of  physics and astronomy; other disciplines will inevitably follow. The age of the academic journal is drawing to a close, and it is consequently time to rethink the concept of a paper.

A Lockdown Perspective on the Hubble Tension (plus Poll)

Posted in The Universe and Stuff with tags , , on July 22, 2020 by telescoper

These are busy days in cosmological circles, especially regarding the Hubble Constant controversy. The latest contribution to appear on the arXiv is by George Efstathiou of Cambridge. Here is the abstract:

I don’t know if George has voted in my ongoing poll relating to this issue, but I bet that if he did he would vote low – along with the majority (so far):

Incidentally, I have seen no evidence of Russian interference in the voting.