Archive for Cosmology

Status of Dark Matter in the Universe [CEA]

Posted in The Universe and Stuff with tags , on January 11, 2017 by telescoper

Courtesy of arXiver, here’s a nice review article if you want to get up to date with the latest ideas and evidence about Dark Matter…

arXiver

http://arxiv.org/abs/1701.01840

Over the past few decades, a consensus picture has emerged in which roughly a quarter of the universe consists of dark matter. I begin with a review of the observational evidence for the existence of dark matter: rotation curves of galaxies, gravitational lensing measurements, hot gas in clusters, galaxy formation, primordial nucleosynthesis and cosmic microwave background observations. Then I discuss a number of anomalous signals in a variety of data sets that may point to discovery, though all of them are controversial. The annual modulation in the DAMA detector and/or the gamma-ray excess seen in the Fermi Gamma Ray Space Telescope from the Galactic Center could be due to WIMPs; a 3.5 keV X-ray line from multiple sources could be due to sterile neutrinos; or the 511 keV line in INTEGRAL data could be due to MeV dark matter. All of these would require further confirmation in other experiments…

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Galaxy Formation in the EAGLE Project

Posted in The Universe and Stuff with tags , , , on December 8, 2016 by telescoper

Yesterday I went to a nice Colloquium by Rob Crain of Liverpool John Moores University (which is in the Midlands). Here’s the abstract of his talk which was entitled
Cosmological hydrodynamical simulations of the galaxy population:

I will briefly recap the motivation for, and progress towards, numerical modelling of the formation and evolution of the galaxy population – from cosmological initial conditions at early epochs through to the present day. I will introduce the EAGLE project, a flagship program of such simulations conducted by the Virgo Consortium. These simulations represent a major development in the discipline, since they are the first to broadly reproduce the key properties of the evolving galaxy population, and do so using energetically-feasible feedback mechanisms. I shall present a broad range of results from analyses of the EAGLE simulation, concerning the evolution of galaxy masses, their luminosities and colours, and their atomic and molecular gas content, to convey some of the strengths and limitations of the current generation of numerical models.

I added the link to the EAGLE project so you can find more information. As one of the oldies in the audience I can’t help remembering the old days of the galaxy formation simulation game. When I started my PhD back in 1985 the state of the art was a gravity-only simulation of 323 particles in a box. Nowadays one can manage about 20003 particles at the same time aas having a good go at dealing not only with gravity but also the complex hydrodynamical processes involved in assembling a galaxy of stars, gas, dust and dark matter from a set of primordial fluctuations present in the early Universe. In these modern simulations one does not just track the mass distribution but also various themrmodynamic properties such as temperature, pressure, internal energy and entropy, which means that they require large supercomputers. This certainly isn’t a solved problem – different groups get results that differ by an order of magnitude in some key predictions – but the game has certainly moved on dramatically in the past thirty years or so.

Another thing that has certainly improved a lot is data visualization: here is a video of one of the EAGLE simulations, showing a region of the Universe about 25 MegaParsecs across. The gas is colour-coded for temperature. As the simulation evolves you can see the gas first condense into the filaments of the Cosmic Web, thereafter forming denser knots in which stars form and become galaxies, experiencing in some cases explosive events which expel the gas. It’s quite a messy business, which is why one has to do these things numerically rather than analytically, but it’s certainly fun to watch!

Does the fine structure constant vary?

Posted in The Universe and Stuff with tags , , on November 16, 2016 by telescoper

No.

A Non-accelerating Universe?

Posted in Astrohype, The Universe and Stuff with tags , , , , , on October 26, 2016 by telescoper

There’s been quite a lot of reaction on the interwebs over the last few days much of it very misleading; here’s a sensible account) to a paper by Nielsen, Guffanti and Sarkar which has just been published online in Scientific Reports, an offshoot of Nature. I think the above link should take you an “open access” version of the paper but if it doesn’t you can find the arXiv version here. I haven’t cross-checked the two versions so the arXiv one may differ slightly.

Anyway, here is the abstract:

The ‘standard’ model of cosmology is founded on the basis that the expansion rate of the universe is accelerating at present — as was inferred originally from the Hubble diagram of Type Ia supernovae. There exists now a much bigger database of supernovae so we can perform rigorous statistical tests to check whether these ‘standardisable candles’ indeed indicate cosmic acceleration. Taking account of the empirical procedure by which corrections are made to their absolute magnitudes to allow for the varying shape of the light curve and extinction by dust, we find, rather surprisingly, that the data are still quite consistent with a constant rate of expansion.

Obviously I haven’t been able to repeat the statistical analysis but I’ve skimmed over what they’ve done and as far as I can tell it looks a fairly sensible piece of work (although it is a frequentist analysis). Here is the telling plot (from the Nature version)  in terms of the dark energy (y-axis) and matter (x-axis) density parameters:

lambda

Models shown in this plane by a line have the correct balance between Ωm, and ΩΛ to cancel out the decelerating effect of the former against the accelerating effect of the latter (a special case is the origin on the plot, which is called the Milne model and represents an entirely empty universe). The contours show “1, 2 and 3σ” contours, regarding all other parameters as nuisance parameters. It is true that the line of no acceleration does go inside the 3σcontour so in that sense is not entirely inconsistent with the data. On the other hand, the “best fit” (which is at the point Ωm=0.341, ΩΛ=0.569) does represent an accelerating universe.

I am not all that surprised by this result, actually. I’ve always felt that taken on its own the evidence for cosmic acceleration from supernovae alone was not compelling. However, when it is combined with other measurements (particularly of the cosmic microwave background and large-scale structure) which are sensitive to other aspects of the cosmological space-time geometry, the agreement is extremely convincing and has established a standard “concordance” cosmology. The CMB, for example, is particularly sensitive to spatial curvature which, measurements tells us, must be close to zero. The Milne model, on the other hand, has a large (negative) spatial curvature entirely excluded by CMB observations. Curvature is regarded as a “nuisance parameter” in the above diagram.

I think this paper is a worthwhile exercise. Subir Sarkar (one of the authors) in particular has devoted a lot of energy to questioning the standard ΛCDM model which far too many others accept unquestioningly. That’s a noble thing to do, and it is an essential part of the scientific method, but this paper only looks at one part of an interlocking picture. The strongest evidence comes from the cosmic microwave background and despite this reanalysis I feel the supernovae measurements still provide a powerful corroboration of the standard cosmology.

Let me add, however, that the supernovae measurements do not directly measure cosmic acceleration. If one tries to account for them with a model based on Einstein’s general relativity and the assumption that the Universe is on large-scales is homogeneous and isotropic and with certain kinds of matter and energy then the observations do imply a universe that accelerates. Any or all of those assumptions may be violated (though some possibilities are quite heavily constrained). In short we could, at least in principle, simply be interpreting these measurements within the wrong framework, and statistics can’t help us with that!

KiDS-450: Testing extensions to the standard cosmological model [CEA]

Posted in The Universe and Stuff with tags , , , on October 19, 2016 by telescoper

Since I’ve just attended a seminar in Cardiff by Catherine Heymans on exactly this work, I couldn’t resist reblogging the arXiver entry for this paper which appeared on arXiv a couple of days ago.

The key finding is that the weak lensing analysis of KIDS data (which is mainly to the distribution of matter at low redshift) does seem to be discrepant with the predictions of the standard cosmological model established by Planck (which is sensitive mainly to high-redshift fluctuations).

Could this discrepancy be interpreted as evidence of something going on beyond the standard cosmology? Read the paper to explore some possibilities!

arXiver

http://arxiv.org/abs/1610.04606

We test extensions to the standard cosmological model with weak gravitational lensing tomography using 450 deg$^2$ of imaging data from the Kilo Degree Survey (KiDS). In these extended cosmologies, which include massive neutrinos, nonzero curvature, evolving dark energy, modified gravity, and running of the scalar spectral index, we also examine the discordance between KiDS and cosmic microwave background measurements from Planck. The discordance between the two datasets is largely unaffected by a more conservative treatment of the lensing systematics and the removal of angular scales most sensitive to nonlinear physics. The only extended cosmology that simultaneously alleviates the discordance with Planck and is at least moderately favored by the data includes evolving dark energy with a time-dependent equation of state (in the form of the $w_0-w_a$ parameterization). In this model, the respective $S_8 = sigma_8 sqrt{Omega_{rm m}/0.3}$ constraints agree at the $1sigma$ level, and there is `substantial concordance’ between…

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A Universe of Two Trillion Galaxies

Posted in The Universe and Stuff with tags , , , , , on October 13, 2016 by telescoper

I just saw a press-release that describes a paper, just out, authored by Chris Conselice et al from the University of Nottingham (in the Midlands), with this here abstract:

conselice

The key conclusion of this paper is that when the universe was only a few billion years old there were about ten times as many galaxies in a given volume of space as there are within a similar volume today, but most of these galaxies were much lower mass systems than, e.g., the Milky Way. In fact their masses are similar to those of the satellite galaxies surrounding the Milky Way. These objects are numerous but so faint that even in very deep surveys with very big telescopes they are very easy to miss.

Here’s an image from a deep survey: this is from the Hubble Space Telescoper Great Observatories Deep Survey (HST-GOODS).

hst_goods-south

You can click on this to make it larger if you wish. This is typical of a “pencil beam” survey. It opens a very small window on the heavens – about a millionth of its total area of the sky – in a direction chosen to avoid having too many bright stars from our own Galaxy getting in the way. When you look at such a patch with a big telescope for a long time, what you see is basically all galaxies. The few stars in the above image can be identified by the diffraction patterns they produce, but almost every fuzzy blob in the picture is a galaxy. It looks like there are a lot of galaxies in this image, but the real number seems to be substantially higher than we thought.

When I’ve given popular talks about this kind of thing I’ve always said something like “There are at least as many galaxies in the observable Universe as there are stars in our own Galaxy”. It turns out that I was wise to include the “at least as”. There are about 100 billion (1011) stars in the Milky Way, but the latest estimate is now that there are two trillion (2 ×1012) galaxies in the observable Universe. I quote Douglas Adams:

“The Universe, as has been observed before, is an unsettlingly big place, a fact which for the sake of a quiet life most people tend to ignore. Many would happily move to somewhere rather smaller of their own devising, and this is what most beings in fact do.

I believe this explains a lot about modern politics.

 

General Relativity and Cosmology: Unsolved Questions and Future Directions [CL]

Posted in The Universe and Stuff with tags , on October 12, 2016 by telescoper

I missed this when it appeared on the arXiv last week, but now that I’ve read it I couldn’t resist reblogging this nice review of the current state of General Relativity and its alternatives, with an emphasis on the cosmological ramifications.

arXiver

http://arxiv.org/abs/1609.09781

For the last 100 years, General Relativity (GR) has taken over the gravitational theory mantle held by Newtonian Gravity for the previous 200 years. This article reviews the status of GR in terms of its self-consistency, completeness, and the evidence provided by observations, which have allowed GR to remain the champion of gravitational theories against several other classes of competing theories. We pay particular attention to the role of GR and gravity in cosmology, one of the areas in which one gravity dominates and new phenomena and effects challenge the orthodoxy. We also review other areas where there are likely conflicts pointing to the need to replace or revise GR to represent correctly observations and consistent theoretical framework. Observations have long been key both to the theoretical liveliness and viability of GR. We conclude with a discussion of the likely developments over the next 100 years.

Read this paper…

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