Archive for dark matter

Cosmological Constraints on Alternative Gravity Theories

Posted in The Universe and Stuff with tags , , , , , , , on July 11, 2022 by telescoper

The standard model of cosmology is based on Einstein’s theory of general relativity. In order to account for cosmological observations this has required the introduction of dark matter – which also helps explain the properties of individual galaxies – and dark energy. The result model, which I would describe as a working hypothesis, is rather successful but it is reasonable to question whether either or both of the dark components can be avoided by adopting an alternative theory of gravity instead of Einstein’s.

There is an interesting paper by Kris Pardo and David Spergel on arXiv that argues that none of the modifications of Einstein’s theory currently on the market is able to eliminate the need for dark matter. Here is the abstract of this paper:

It’s a more sophisticated version of an argument that has been going around at least in qualitative form for some time. The gist of it is that the distinctive pattern of fluctuations in the cosmic microwave background, observed by e.g. the Planck experiment, arise from coupling between baryons and photons in the early Universe. Similar features can be observed in the distribution of galaxies – where they are called Baryon Acoustic Oscilations (BAO) at a more recent cosmic epoch, but they are are much weaker. This is easily explicable if there is a dark matter component that dominates gravitational instability at late times but does not couple to photons via electromagnetic interactions. This is summed up in the following graphic (which I think I stole from a talk by John Peacock) based on data from about 20 years ago:

If there were no dark matter the coherent features seen in the power spectrum of the galaxy distribution would be much stronger; with dark matter dominating they are masked by the general growth of the collisionless component so their relative amplitude decreases.

The graphic shows how increasing the dark matter component from 0.1 to 0.3, while keeping the baryon component fixed, suppresses the wiggles corresponding to BAOs. The data suggest a dark matter contribution at the upper end of that range, consistent with the standard cosmology.

Of course if there are were no baryons at all there wouldn’t be fluctuations in either the CMB polarization or the galaxy distribution so both spectra would be smooth as shown in the graphic, but in that case there wouldn’t be anyone around to write about them as people are made of baryons.

This general conclusion is confirmed by the Pardo & Spergel paper, though it must be said that the argument doesn’t mean that modified gravity is impossible. It’s just that it seems nobody has yet thought of a specific model that satisfies all the constraints. That may change.

Fuzzy Cosmology at ITP2022

Posted in Biographical, Talks and Reviews, The Universe and Stuff with tags , , , on May 26, 2022 by telescoper

As I usually do when I give a talk (which hasn’t been for a while) I’ve uploaded the slides for the presentation I gave at the Irish Theoretical Physics meeting at DIAS this morning. The title of the talk was Fuzzy Cosmology and the abstract reads:

I discuss some applications of the Schrodinger-Poisson wave-mechanical approach to
cosmological structure formation. The most obvious use of this formalism is to “fuzzy” dark matter,
i.e. dark matter consisting of extremely light particles whose effective de Broglie wavelength is
sufficiently large to be astrophysically relevant, but it can be used to model more general scenarios
and has a number of advantages over standard methods based on Eulerian perturbation theory. I
illustrate the formalism with some calculations for cosmic voids and discuss its application to the
cosmological reconstruction problem(s).

I think it went reasonably well despite there being a hitch at the start because the touchpad on my laptop stopped working. Fortunately I was able to produce an emergency mouse. Anyway, here is a picture of me taken during the talk to prove I was there..

New Publication at the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags , , , on February 21, 2022 by telescoper

It’s time yet again to announce a new publication in the Open Journal of Astrophysics! This one is the 3rd paper in Volume 5 (2022) and the 51st in all. We actually published this on Friday, byt I’ve only just got around to announcing it here now.

The latest publication is entitled Differentiable Predictions for Large Scale Structure with SHAMNet and is written by Andrew Hearin, Nesar Ramachandra and Matthew R. Becker of the Argonne National Laboratory and Joseph DeRose of the Lawrence Berkeley Laboratory (both institutions being in the USA).

Here is a screen grab of the overlay which includes the abstract:

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. This paper is in our popular Cosmology and Non-galactic Astrophysics section.

P. S. Here’s a bit of feedback from the author of this paper about the referees:

They reviewed the paper in conscientious detail, and every comment was thoughtful. We feel that our paper has materially improved in clarity as a result of their critique.”

Sins of Omission

Posted in The Universe and Stuff with tags , , , , , on February 20, 2022 by telescoper

There’s a paper recently published in Nature Astronomy by Moreno et al, which you can find on the arXiv here. The title is Galaxies lacking dark matter produced by close encounters in a cosmological simulation and the abstract is here:

The standard cold dark matter plus cosmological constant model predicts that galaxies form within dark-matter haloes, and that low-mass galaxies are more dark-matter dominated than massive ones. The unexpected discovery of two low-mass galaxies lacking dark matter immediately provoked concerns about the standard cosmology and ignited explorations of alternatives, including self-interacting dark matter and modified gravity. Apprehension grew after several cosmological simulations using the conventional model failed to form adequate numerical analogues with comparable internal characteristics (stellar masses, sizes, velocity dispersions and morphologies). Here we show that the standard paradigm naturally produces galaxies lacking dark matter with internal characteristics in agreement with observations. Using a state-of-the-art cosmological simulation and a meticulous galaxy-identification technique, we find that extreme close encounters with massive neighbours can be responsible for this. We predict that approximately 30 percent of massive central galaxies (with at least 1011 solar masses in stars) harbour at least one dark-matter-deficient satellite (with 108 – 109 solar masses in stars). This distinctive class of galaxies provides an additional layer in our understanding of the role of interactions in shaping galactic properties. Future observations surveying galaxies in the aforementioned regime will provide a crucial test of this scenario.

It’s quite an interesting result.

I’m reminded of this very well known paper from way back in 1998, available on arXiv here, by Priya Natarajan, Steinn Sigurdsson and Joe Silk, with the abstract:

We propose a scenario for the formation of a population of baryon-rich, dark matter-deficient dwarf galaxies at high redshift that form from the mass swept out in the Intergalactic Medium (IGM) by energetic outflows from luminous quasars. We predict the intrinsic properties of these galaxies, and examine the prospects for their observational detection in the optical, X-ray and radio wavebands. Detectable thermal Sunyaev-Zeldovich decrements (cold spots) on arc-minute scales in the cosmic microwave background radiation maps are expected during the shock-heated expanding phase from these hot bubbles. We conclude that the optimal detection strategy for these dwarfs is via narrow-band Lyman-α imaging of regions around high redshift quasars. An energetically scaled-down version of the same model is speculated upon as a possible mechanism for forming pre-galactic globular clusters.

It’s true that the detailed mechanism for forming dwarf galaxies with low dark matter densities is different in the two papers, but it does show that the issue being addressed by Moreno et al. had been addressed before. It seems to me therefore that the Natarajan et al. paper is clearly relevant background to the Moreno et al. one. I always tell junior colleagues to cite all relevant literature. I wonder why Moreno et al. decided not to do that with this paper?

Had Moreno et al. preprinted their paper before acceptance by Nature Astronomy I’m sure someone would have told them of this omission. This is yet another reason for submitting your papers to arXiv at the same time as you submit them to a journal rather than waiting for them to be published.

Cosmology Talks: Alvaro Pozo on Potential Evidence for Wave Dark Matter

Posted in The Universe and Stuff with tags , , , , , , , , , on February 19, 2021 by telescoper

It’s time I shared another 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. This is quite a recent one, from about a week ago.

In the talk, Alvaro Pozo tells us about a recent paper where he an collaborators detect the transition between a core (flat density profile) and halo (power law density profile) in dwarf galaxies. The full core + halo profile matches very closely what is expected in simulations of wave dark matter (sometimes called “fuzzy” dark matter), by which is meant dark matter consisting of a particle so light that its de Broglie wavelength is long enough to be astrophysically relevant. That is, there is a very flat core, which then drops off suddenly and then flattens off to a decaying power-law profile. The core matches the soliton expected in wave dark matter and the halo matches an outer NFW profile expected outside the soliton. They also detect evidence for tidal stripping of the matter in the galaxies. The galaxies closer to the centre of the Milky Way have their transition point between core and halo happen at smaller densities (despite the core density itself not being systematically smaller). The transition also appears to happen closer to the centre of the galaxy, which matches simulations. Of course the core+halo pattern they have clearly observed might be due to something else, but the match between wave dark matter simulations and observations is impressive. An important  caveat is that the mass for the dark matter that they use is very small and in significant tension with Lyman Alpha constraints for wave-like dark matter. This might indicate that the source of this universal core+halo pattern they’re observing comes from something else, or it might indicate that the wave dark matter is more complicated than represented in the simplest models.

P. S. The papers that accompany this talk can be found here.

P.P.S. If you’re interested in wave dark matter there is a nice recent review article by Lam Hui here.

Cosmology Examination Results

Posted in Education, Maynooth, The Universe and Stuff with tags , , , , on July 14, 2020 by telescoper

The examination season in Maynooth being now over, and the results having been issued, I thought I’d pass on the results not for individual students but for the Universe as a whole.

As you can see Dark Energy is top of the class, with a good II.1 (Upper Second Class). A few years ago this candidate looked likely to get a mark over 70% and thus get First Class Honours, but in the end fell just short. Given the steady performance and possible improvement in future I think this candidate will probably be one to reckon with in a future research career.

In second place, a long way behind on about 27%, is Dark Matter. This candidate only answered some of the questions asked, and those not very convincingly. Although reasonably strong on theory, the candidate didn’t show up at all in the laboratory. The result is a fail but there is an opportunity for a repeat at a future date, though there is some doubt as to whether the candidate would appear.

At the bottom of the class on a meagre 5% we find Ordinary Matter. It seems this candidate must have left the examination early and did not even give the correct name (baryons) on the script. Technically this one could repeat but even doing so is unlikely even to get an Ordinary Degree. I would suggest that baryons aren’t really cut out for cosmology and should make alternative plans for the future.


P.S. Photons and neutrinos ceased interacting with the course some time ago. Owing to this lack of engagement they are assumed to have dropped out, and their marks are not shown.





Varun Sahni on Dark Matter & Dark Energy

Posted in The Universe and Stuff with tags , , , , on June 17, 2020 by telescoper

I’m very happy to be able to share a couple of lectures by esteemed cosmologist and erstwhile co-author Varun Sahni of the Inter University Centre for Astronomy & Astrophysics (IUCAA) in Pune, India. They’re at an introductory level appropriate for a summer school so I think quite a lot of students will find them interesting and informative!

The Largest Known Spiral Galaxy – UGC 2885

Posted in The Universe and Stuff with tags , , , , , on January 6, 2020 by telescoper

So here I am, Christmas break over, waiting in Cardiff Airport for my flight back to civilization. I thought I’d take the opportunity to share this wonderful picture, courtesy of the Hubble Space Telescope, of the galaxy UGC 2885 – the largest known spiral galaxy. You can click on the picture to make it bigger or if you really are a size queen you can download an ultra-high=resolution version here.

UGC 2885 is located about 71 Mpc (232 million light-years) from us, in the direction of the constellation Perseus. The galaxy is 2.5 times wider than our own Milky Way and contains approximately 10 times as many stars. A number of foreground stars (in our Galaxy), identified by the diffraction crosses produced by unresolved point sources, can be seen in the image, including one superimposed on the disk of the galaxy, to the left of its centre. The galaxy UGC 2885 has been nicknamed “Rubin’s galaxy” after Vera Rubin, the astronomer who studied the rotation of the galaxy and found evidence for dark matter therein.

There is a very interesting and informative thread on Twitter by Benne Holwerda covering the background to this latest image of the galaxy:

If you click on the above it will take you to Twitter where you can read the series of linked tweets on this subject by clicking on `show this thread’.

New Publication at the Open Journal of Astrophysics!

Posted in Open Access, The Universe and Stuff with tags , , , , , , on December 11, 2019 by telescoper

We seem to be having an end-of-year rush and have published yet another new paper at The Open Journal of Astrophysics!

Here is a grab of the overlay:

The authors are Aditi Krishak (IISER Bhopal) and Shantanu Desai (IIT Hyderabad), both in India.

You can find the accepted version on the arXiv here. This is another one for the `Cosmology and Nongalactic Astrophysics’ section with a cross-listing in `Instrumentation and Methods for Astrophysics’, both of which are proving rather popular.

We would be very 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 so wish..

With the Cosmic Web in Mind..

Posted in Astronomy Lookalikes, The Universe and Stuff with tags , , , , , on November 23, 2019 by telescoper

Some time ago I posted one of my Astronomy Look-alikes about the remarkable similarity between the structure of the human brain and that revealed by computer simulations of the large-scale structure of the Universe:

I wonder whether this means that the Cosmic Web is really just all in the mind?

Anyway I just came across an article by Franco Vazza and Alberto Fenetti that takes the comparison between brain cells (among other things) and the Cosmic Web a bit further, including a look at the corresponding power spectra:

The main point to take from this picture is that many naturally occurring patterns have approximately power-law power spectra, at least over a limited range of scales. However, as I have pointed out before on this blog, the power spectrum on its own does not really quantify pattern in any meaningful way. Here for example are two patterns with exactly the same power spectrum:

The point is that the power spectrum does not contain any information about the phase correlations of the Fourier modes, which are important in quantifying localised features. For further discussion of this issue, see here.