Archive for Cosmology

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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)

 

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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:

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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).

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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.

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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!

 

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The Big Bang Exploded?

Posted in Biographical, The Universe and Stuff with tags , , , on October 15, 2018 by telescoper

I suspect that I’m not the only physicist who receives unsolicited correspondence from people with wacky views on Life, the Universe and Everything. Being a cosmologist, I probably get more of this stuff than those working in less speculative branches of physics. Because I’ve written a few things that appeared in the public domain, I probably even get more than most cosmologists (except the really famous ones of course).

Many “alternative” cosmologists have now discovered email, and indeed the comments box on this blog, but there are still a lot who send their ideas through regular post. Whenever I get a envelope with an address on it that has been typed by an old-fashioned typewriter it’s a dead giveaway that it’s going to be one of those. Sometimes they are just letters (typed or handwritten), but sometimes they are complete manuscripts often with wonderfully batty illustrations. I remember one called Dark Matter, The Great Pyramid and the Theory of Crystal Healing. I used to have an entire filing cabinet filled with things like his, but I took the opportunity of moving from Cardiff some time ago to throw most of them out.

One particular correspondent started writing to me after the publication of my little book, Cosmology: A Very Short Introduction. This chap sent a terse letter to me pointing out that the Big Bang theory was obviously completely wrong. The reason was obvious to anyone who understood thermodynamics. He had spent a lifetime designing high-quality refrigeration equipment and therefore knew what he was talking about (or so he said). He even sent me this booklet about his ideas, which for some reason I have neglected to send for recycling:

His point was that, according to the Big Bang theory, the Universe cools as it expands. Its current temperature is about 3 Kelvin (-270 Celsius or thereabouts) but it is now expanding and cooling. Turning the clock back gives a Universe that was hotter when it was younger. He thought this was all wrong.

The argument is false, my correspondent asserted, because the Universe – by definition – hasn’t got any surroundings and therefore isn’t expanding into anything. Since it isn’t pushing against anything it can’t do any work. The internal energy of the gas must therefore remain constant and since the internal energy of an ideal gas is only a function of its temperature, the expansion of the Universe must therefore be at a constant temperature (i.e. isothermal, rather than adiabatic). He backed up his argument with bona fide experimental results on the free expansion of gases.

I didn’t reply and filed the letter away. Another came, and I did likewise. Increasingly overcome by some form of apoplexy his letters got ruder and ruder, eventually blaming me for the decline of the British education system and demanding that I be fired from my job. Finally, he wrote to the President of the Royal Society demanding that I be “struck off” and forbidden (on grounds of incompetence) ever to teach thermodynamics in a University. The copies of the letters he sent me are still will the pamphlet.

I don’t agree with him that the Big Bang is wrong, but I’ve never had the energy to reply to his rather belligerent letters. However, I think it might be fun to turn this into a little competition, so here’s a challenge for you: provide the clearest and most succint explanation of why the temperature of the expanding Universe does fall with time, despite what my correspondent thought.

Answers via the comment box please!

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EDGES and Foregrounds

Posted in Astrohype, The Universe and Stuff with tags , , , on September 3, 2018 by telescoper

Earlier this year I wrote a brief post about paper by Bowman et al. from the EDGES experiment that had just come out in Nature reportining the detection of a flattened absorption profile in the sky-averaged radio spectrum, centred at a frequency of 78 megahertz, largely consistent with expectations for the 21-centimetre signal induced by early stars. It caused a lot of excitement at the time; see, e.g., here.
The key plot from the paper is this:

At the time I said that I wasn’t entirely convinced. Although the paper is very good at describing the EDGES experiment, it is far less convincing that all necessary foregrounds and systematics have been properly accounted for. There are many artefacts that could mimic the signal shown in the diagram.

I went on to say

If true, the signal is quite a lot larger than amplitude than standard models predict. That doesn’t mean that it must be wrong – I’ve never gone along with the saying `never trust an experimental result until it is confirmed by theory’ – but it’s way too early to claim that it proves that some new exotic physics is involved. The real explanation may be far more mundane.

There’s been a lot of media hype about this result – reminiscent of the BICEP bubble – and, while I agree that if it is true it is an extremely exciting result – I think it’s far too early to be certain of what it really represents. To my mind there’s a significant chance this could be a false cosmic dawn.

I gather the EDGES team is going to release its data publicly. That will be good, as independent checks of the data analysis would be very valuable.

Well, there’s a follow-up paper that I missed when it appeared on the arXiv in May the abstract of which reads:

We have re-analyzed the data in which Bowman et al. (2018) identified a feature that could be due to cosmological 21-cm line absorption in the intergalactic medium at redshift z~17. If we use exactly their procedures then we find almost identical results, but the fits imply either non-physical properties for the ionosphere or unexpected structure in the spectrum of foreground emission (or both). Furthermore we find that making reasonable changes to the analysis process, e.g., altering the description of the foregrounds or changing the range of frequencies included in the analysis, gives markedly different results for the properties of the absorption profile. We can in fact get what appears to be a satisfactory fit to the data without any absorption feature if there is a periodic feature with an amplitude of ~0.05 K present in the data. We believe that this calls into question the interpretation of these data as an unambiguous detection of the cosmological 21-cm absorption signature.

You can read the full paper here (PDF). I haven’t kept up with this particular story, so further comments/updates/references are welcome through the box below!

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The Simons Observatory: Science Goals and Forecasts

Posted in The Universe and Stuff with tags , , on August 27, 2018 by telescoper

I haven’t been involved in this project, but several of my former colleagues at Cardiff have beenm and still are, so I know how much work has gone into this (especially by the amazing Erminia Calabrese), so I am happy to share this impressive work here. This long (54 pages) paper, which appeared on the arXiv last week, describes the latest step forward in ground-based cosmology using the cosmic microwave background. It shows just how rapid the onward march of instrumental technology continues to be.

The Simons Observatory Site, in Chile

It is likely that the Simons Observatory (based on a single 6m dish) will form part of the next generation CMB experiment known currently as CMB-S4.

You can download the paper in full from the arXiv here.

The Simons Observatory (SO) is a new cosmic microwave background experiment being built on Cerro Toco in Chile, due to begin observations in the early 2020s. We describe the scientific goals of the experiment, motivate the design, and forecast its performance. SO will measure the temperature and polarization anisotropy of the cosmic microwave background in six frequency bands: 27, 39, 93, 145, 225 and 280 GHz. The initial configuration of SO will have three small-aperture 0.5-m telescopes (SATs) and one large-aperture 6-m telescope (LAT), with a total of 60,000 cryogenic bolometers. Our key science goals are to characterize the primordial perturbations, measure the number of relativistic species and the mass of neutrinos, test for deviations from a cosmological constant, improve our understanding of galaxy evolution, and constrain the duration of reionization. The SATs will target the largest angular scales observable from Chile, mapping ~10% of the sky to a white noise level of 2 μK-arcmin in combined 93 and 145 GHz bands, to measure the primordial tensor-to-scalar ratio, r, at a target level of σ(r)=0.003. The LAT will map ~40% of the sky at arcminute angular resolution to an expected white noise level of 6 μK-arcmin in combined 93 and 145 GHz bands, overlapping with the majority of the LSST sky region and partially with DESI. With up to an order of magnitude lower polarization noise than maps from the Planck satellite, the high-resolution sky maps will constrain cosmological parameters derived from the damping tail, gravitational lensing of the microwave background, the primordial bispectrum, and the thermal and kinematic Sunyaev-Zel’dovich effects, and will aid in delensing the large-angle polarization signal to measure the tensor-to-scalar ratio. The survey will also provide a legacy catalog of 16,000 galaxy clusters and more than 20,000 extragalactic sources.

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Cosmology Big Brother

Posted in Television, The Universe and Stuff with tags , , on August 17, 2018 by telescoper

I saw on Twitter today that the new series of Celebrity Big Brother has just started, though looking at the list of inmates housemates, I’m not sure whether the producers of this show understand the meaning of the word `celebrity’. At any rate, I’ve never heard of most of them.

I get the feeling that the Big Brother franchise may be getting a little tired, so I thought I’d pitch a new variant in order to boost the flagging ratings.

In Cosmology Big Brother a group of wannabe cosmologists live together in a specially-constructed house (with lots of whiteboards) isolated from the outside world (i.e. the arXiv). As the series progresses the furniture and rooms are gradually moved further apart, the temperature of the central heating is turned down, and the contents of the house become progressively more disordered.

Housemates are regularly voted out, at which point they have to enter the `real world’ (i.e. get a job in data science). Eventually only one person remains and whoever that is is awarded a research grant. They can then spend the rest of their life combining their study of cosmology with the usual activities of a Big Brother winner, e.g. opening supermarkets.

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Nature After Planck…

Posted in Maynooth, The Universe and Stuff with tags , , , , , , on July 24, 2018 by telescoper

After last week’s short update about the last tranche of papers from the European Space Agency’s Planck Mission it’s time for another short update about a piece in Nature (by David Castelvecchi) that explains how researchers are moving to smaller projects studying different aspects of the cosmic microwave background.

In the spirit of gratuitous self-promotion I should also mention that there’s a little quote from me in that piece. My comment was hardly profound, but at least it gets Maynooth University a name check…

Much of Davide’s piece echoes discussions that were going on at the meeting I attended in India  last October, but things have moved on quite a bit since then at least as far as space experiments are concerned. In particular, the proposed Japanese mission Litebird has been shortlisted for consideration, though we will have to wait until next year (2019) at the earliest to see if it will be selected. An Indian mission, CMB-Bharat, has also emerged as a contender.

While the end of Planck closes one chapter on CMB research, several others will open. These are likely to focus on polarization, gravitational lensing and on cosmic reionization rather than refining the basic cosmological parameters still further.

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