Archive for Cosmology

Negative Mass, Phlogiston and the State of Modern Cosmology

Posted in Astrohype, The Universe and Stuff with tags , , on December 7, 2018 by telescoper

A graphical representation of something or other.

I’ve noticed a modest amount of hype – much of it gibberish – going around about a paper published in Astronomy & Astrophysics but available on the arXiv here which entails a suggestion that material with negative mass might account for dark energy and/or dark matter. Here is the abstract of the paper:

Dark energy and dark matter constitute 95% of the observable Universe. Yet the physical nature of these two phenomena remains a mystery. Einstein suggested a long-forgotten solution: gravitationally repulsive negative masses, which drive cosmic expansion and cannot coalesce into light-emitting structures. However, contemporary cosmological results are derived upon the reasonable assumption that the Universe only contains positive masses. By reconsidering this assumption, I have constructed a toy model which suggests that both dark phenomena can be unified into a single negative mass fluid. The model is a modified ΛCDM cosmology, and indicates that continuously-created negative masses can resemble the cosmological constant and can flatten the rotation curves of galaxies. The model leads to a cyclic universe with a time-variable Hubble parameter, potentially providing compatibility with the current tension that is emerging in cosmological measurements. In the first three-dimensional N-body simulations of negative mass matter in the scientific literature, this exotic material naturally forms haloes around galaxies that extend to several galactic radii. These haloes are not cuspy. The proposed cosmological model is therefore able to predict the observed distribution of dark matter in galaxies from first principles. The model makes several testable predictions and seems to have the potential to be consistent with observational evidence from distant supernovae, the cosmic microwave background, and galaxy clusters. These findings may imply that negative masses are a real and physical aspect of our Universe, or alternatively may imply the existence of a superseding theory that in some limit can be modelled by effective negative masses. Both cases lead to the surprising conclusion that the compelling puzzle of the dark Universe may have been due to a simple sign error.

For a skeptical commentary on this work, see here.

The idea of negative mass is no by no means new, of course. If you had askedk a seventeenth century scientist that question and the chances are the answer would  have involved the word phlogiston, a name derived from the Greek  φλογιστόν, meaning “burning up”. This “fiery principle” or “element” was supposed to be present in all combustible materials and the idea was that it was released into air whenever any such stuff was ignited. The act of burning separated the phlogiston from the dephlogisticated “true” form of the material, also known as calx.

The phlogiston theory held sway until  the late 18th Century, when Antoine Lavoisier demonstrated that combustion results in an increase in weight implying an increase in mass of the material being burned. This poses a serious problem if burning also involves the loss of phlogiston unless phlogiston has negative mass. However, many serious scientists of the 18th Century, such as Georg Ernst Stahl, had already suggested that phlogiston might have negative weight or, as he put it, `levity’. Nowadays we would probably say `anti-gravity.

Eventually, Joseph Priestley discovered what actually combines with materials during combustion:  oxygen. Instead of becoming dephlogisticated, things become oxidised by fixing oxygen from air, which is why their weight increases. It’s worth mentioning, though, the name that Priestley used for oxygen was in fact “dephlogisticated air” (because it was capable of combining more extensively with phlogiston than ordinary air). He  remained a phlogistonian longer after making the discovery that should have killed the theory.

The standard cosmological model involves the hypothesis that about 75% of the energy budget of the Universe is in the form of “dark energy”. We don’t know much about what this is, except that in order to make our current understanding work out it has to act like a source of anti-gravity. It does this by violating the strong energy condition of general relativity.

Dark energy is needed to reconcile three basic measurements: (i) the brightness distant supernovae that seem to indicate the Universe is accelerating (which is where the anti-gravity comes in); (ii) the cosmic microwave background that suggests the Universe has flat spatial sections; and (iii) the direct estimates of the mass associated with galaxy clusters that accounts for about 25% of the mass needed to close the Universe.

A universe without dark energy appears not to be able to account for these three observations simultaneously within our current understanding of gravity as obtained from Einstein’s theory of general relativity.

I’ve blogged before, with some levity of my own, about how uncomfortable this dark energy makes me feel. It makes me even more uncomfortable that such an enormous  industry has grown up around it and that its existence is accepted unquestioningly by so many modern cosmologists.

Isn’t there a chance that, with the benefit of hindsight, future generations will look back on dark energy in the same way that we now see the phlogiston theory?

Or maybe, as the paper that prompted this piece might be taken to suggest, the dark energy really is something like phlogiston. At least I prefer the name to quintessence. However, I think the author has missed a trick. I think to create a properly trendy cosmological theory he should include the concept of supersymmetry, according to which there should be a Fermionic counterpart of phlogiston called the phlogistino..

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Circular Polarization in the Cosmic Microwave Background?

Posted in The Universe and Stuff with tags , , , , on November 23, 2018 by telescoper

Some years ago I went to a seminar on the design of an experiment to measure the polarization of the cosmic microwave background. At the end of the talk I asked what seemed to me to be an innocent question. The point of my question was the speaker had focussed entirely on measuring the intensity of the radiation (I) and the two Stokes Parameters that measure linear polarization of the radiation (usually called Q and U). How difficult, I asked, would it be to measure the remaining Stokes parameter V (which quantifies circular polarization)?

There was a sharp intake of breath among the audience as if I had uttered an obscenity, and the speaker responded with a glare and a curt `the cosmic microwave background is not circularly polarized’. It is true that in the standard cosmological theory the microwave background is produced by Thomson scattering in the early Universe which produces partial linear polarization, so that Q and U are non-zero, but not circular polarization, so V=0. However, I had really asked my question because I had an idea that it might be worth measuring V (or at least putting an upper limit on it) in order to assess the level of instrumental systematics (which are a serious issue with polarization measurements).

I was reminded of this episode when I saw a paper on the arXiv by Keisuke Inomata and Marc Kamionkowski which points out that the CMB may well have some level of circular polarization. Here is the abstract of the paper:

(You can click on the image to make it more readable.) It’s an interesting calculation, but it’s hard to see how we will ever be able to measure a value of Stokes V as low as 10-14.

A few years ago there was a paper on the arXiv by Asantha Cooray, Alessandro Melchiorri and Joe Silk which pointed out that the CMB may well have some level of circular polarization. When light travels through a region containing plasma and a magnetic field, circular polarization can be generated from linear polarization via a process called Faraday conversion. For this to happen, the polarization vector of the incident radiation (defined by the direction of its E-field) must have non-zero component along the local magnetic field, i.e. the B-field. Charged particles are free to move only along B, so the component of E parallel to B is absorbed and re-emitted by these charges, thus leading to phase difference between it and the component of E orthogonal to B and hence to the circular polarization. This is related to the perhaps more familiar process of which causes the plane of linear polarization to rotate when polarized radiation travels through a region containing a magnetic field.

Here is the abstract of that paper:

(Also clickable.) This is a somewhat larger effect but differs from the first paper in that it is produced by foreground processes rather than primordial physics. In any case a Stokes V of 10-9 is also unlikely to be measurable at any time in the foreseeable future.

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:

534515-112918 (2)

 

Eight Papers from the Dark Energy Survey

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

Just a quick post to point out the exciting news that this week a clutch of papers on cosmology using Type 1a Supernovae have been released by the Dark Energy Survey team. Naturally, all of them are on the arXiv. You can also read them here. For convenience I’ve provided links below to arXiv versions through their titles:

  1. Steve: A hierarchical Bayesian model for Supernova Cosmology
  2. First Cosmology Results Using Type Ia Supernovae from the Dark Energy Survey: Effects of Chromatic Corrections to Supernova Photometry on Measurements of Cosmological Parameters
  3. First Cosmology Results using Type Ia Supernova from the Dark Energy Survey: Simulations to Correct Supernova Distance Biases
  4. First Cosmology Results Using Type Ia Supernovae From the Dark Energy Survey: Photometric Pipeline and Light Curve Data Release
  5. First Cosmology Results Using Type Ia Supernovae From the Dark Energy Survey: Analysis, Systematic Uncertainties, and Validation
  6. First Cosmological Results using Type Ia Supernovae from the Dark Energy Survey: Measurement of the Hubble Constant
  7. Cosmological Constraints from Multiple Probes in the Dark Energy Survey
  8. First Cosmology Results using Type Ia Supernovae from the Dark Energy Survey: Constraints on Cosmological Parameters

Here’s a plot showing some of the cosmological constraints:

The parameter plotted on the vertical axis is the dark energy equation of state parameter, w, and w=-1 corresponds to a cosmological constant.

For those of youparticularly interested in the Hubble constant, the headline value from Paper 6 is H0 = 67.77 +/- 1.30 km s-1 Mpc-1. This closer to the value obtained from Planck and in tension with other values as I’ve blogged about before, and gives me an excuse to continue my online poll:

Stokes, Lonsdale and DCU

Posted in Cosmic Anomalies, Maynooth, Talks and Reviews, The Universe and Stuff with tags , , , on November 2, 2018 by telescoper

On Wednesday I took a trip from Maynooth into Dublin to give a talk at the Centre for Astrophysics and Relativity at Dublin City University (DCU). I’ve stolen the above picture, which someone took near the start of the talk, from Twitter.

My talk was very general, as it was not a specialist cosmology audience, and was similar to the talks I was giving a few years ago about the Axle of Elvis Axis of Evil. If anyone is interested in the slides, here they are.

Confusingly, Dublin City University (DCU) consists of the same combination of quarks as University College Dublin (UCD), but I managed to find my way to the correct campus via Drumcondra Railway Station (which is next to historic Croke Park). Anyway, there was quite a big audience and not all of them fell asleep (even though I did go on too long) so by that measure at least the talk was moderately successful. Thanks to everyone there for their hospitality during the afternoon!

Incidentally, my talk was in the Lonsdale Building which is right next to the Stoke Building. Both are named in honour of famous Irish-born scientists. physicist George Stokes (who was born in Skreen, in County Sligo, but spent most of his adult life in Cambridge) and crystallographer Kathleen Lonsdale (who was born in Newbridge, County Kildare, but moved to England when she was only five).

The Signs of Age

Posted in Biographical, The Universe and Stuff with tags , , , , on October 23, 2018 by telescoper

I was feeling very tired yesterday evening and in my vegetative state I suddenly realised that last month I missed a significant personal anniversary. In September 1988, now over thirty years ago I submitted my DPhil thesis at the University of Sussex. Here it is..

It was to be another couple of months until I had my viva (an experience I’d definitely rather forget) so I didn’t get to receive the postgraduate degree formally until the following summer, but at least I finished and submitted within the three years my funding allowed. Incidentally, mine was one of the first generation of theses at the University of Sussex to be typeset in LaTeX. At least I avoided the hassle of having carbon copies made!

The field of cosmology has changed so much in the three intervening decades that I’m sure current graduate students would find my thesis as incredibly simple-minded as I do. There weren’t any measurements of CMB temperature patterns in those days (the COBE results were not announced not until 1992) so I had to generate simulated observations, for example. Still, a few of the things in my thesis have stood the test of time, in the form of papers that still get cited to this day. I was lucky that my research  was in an area that was about to take off, rather than one that was already in decline, and that there will still problems around that were easy enough for me to tackle!

The way of working was very different too: the fact that my generation didn’t have computers on our desks makes younger graduate students wonder how we managed to do anything at all! I still amuse my colleagues with my habit of writing out bits of code in longhand on paper  and `desk-checking’ them before typing them in.

The fact that I now have over 30 years’ postdoctoral experience definitely adds to the feeling of getting very old, along with the all-pervading fatigue, the random aches and pains that afflict me from time to time, failing eyesight, and the tendency of Facebook to send me advertisements about stairlifts, hearing aids, and (worst of all) golf equipment.

The start of University term in late September brings with it a new intake of students that always looks even  younger than the last. That produces a strange alternation of feelings. On the one hand, working in a University means that you’re always surrounded by bright young students which is a good thing when you’re getting on a bit in that it reminds you that you were once like that. On the other, the proliferation of young persons around does force you to face up to how old you actually are.

I remember some years ago I was teaching a module on astrophysics as part of which I did a lecture on supernovae. In the middle of that I said to my class: “of course, you will all remember SN 1987A” (which was detected while I was a PhD student). Blank faces. I then realized that none of them had even been born in 1987. Nowadays it is the case that I was already a Professor when all my undergraduate students were born.

But these signs of age are as nothing compared to the shock I underwent when a few months ago I discovered that I’m older than Nigel Farage.

Counting String Theory Standard Models

Posted in The Universe and Stuff with tags , on October 22, 2018 by telescoper

I saw a paper on the arXiv and couldn’t resist a (snarky) comment. Here is the abstract:

We derive an approximate analytic relation between the number of consistent heterotic Calabi-Yau compactifications of string theory with the exact charged matter content of the standard model of particle physics and the topological data of the internal manifold: the former scaling exponentially with the number of Kahler parameters. This is done by an estimate of the number of solutions to a set of Diophantine equations representing constraints satisfied by any consistent heterotic string vacuum with three chiral massless families, and has been computationally checked to hold for complete intersection Calabi-Yau threefolds (CICYs) with up to seven Kahler parameters. When extrapolated to the entire CICY list, the relation gives about 1023 string theory standard models; for the class of Calabi-Yau hypersurfaces in toric varieties, it gives about 10723 standard models.

Isn’t  10723 also the number of angels that can dance on the head of a pin? That number of models for the price of one theory looks like a bargain to me!

But, seriously, people often complain that string theory isn’t really scientific because it isn’t predictive. That clearly isn’t true. String theory is the most predictive theory ever: it can predict anything you want!