Archive for Cosmology

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Hot News! Supernova in M82

Posted in The Universe and Stuff with tags , , , , , on January 22, 2014 by telescoper

Very exciting news today – a supernova has gone off in Messier 82. In fact, according to this sequence of images from Japan it actually started to brighten about a week ago:

psn-m82

Being arranged in Japanese fashion, you have to read these from top to bottom but starting at the right, so the supernova can be seen to be steadily brightening, i.e. decreasing in magnitude from 17.0 to 11.9. That means it’s now visible with binoculars and will have been seen already by many amateur astronomers. The exciting question this time is whether we’ll get any neutrinos from it!

UPDATE: I’m told that, close as it is, M82 is probably too far to detect neutrinos. Boo.

This is the nearest supernova since 1987a which was observed in, er, 1987. This is the nearest Type Ia supernova for a very long time (possibly 1937), so it’s of considerable interest for the use of such objects in cosmology. There have been other close ones since the nearest one I can remember, 1987a, which was observed in, er, 1987 but all have been Type II.

UPDATE: Thanks for the people who pointed out my error which I’ve left in to show that I don’t know much about supernovae so you shouldn’t phone me up to ask.

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Cosmology and the Constants of Nature

Posted in The Universe and Stuff with tags , , , , on January 20, 2014 by telescoper

Just a brief post to advertise a very interesting meeting coming up in Cambridge:

–o–

Cosmology and the Constants of Nature

DAMTP, University of Cambridge

Monday, 17 March 2014 at 09:00 – Wednesday, 19 March 2014 at 15:00 (GMT)

Cambridge, United Kingdom

The Constants of Nature are quantities, whose numerical values we know with the greatest experimental accuracy – but about the rationale for those values, we have the greatest ignorance. We might also ask if they are indeed constant in space and time, and investigate whether their values arise at random or are uniquely determined by some deep theory.

This mini-series of talks is part of the joint Oxford-Cambridge programme on the Philosophy of Cosmology which aims to introduce philosophers of physics to fundamental problems in cosmology and associated areas of high-energy physics.

The talks are aimed at philosophers of physics but should also be of interest to a wide range of cosmologists.  Speakers will introduce the physical constants that define the standard model of particle physics and cosmology together with the data that determine them, describe observational programmes that test the constancy of traditional ʽconstantsʼ, including the cosmological constant, and discuss how self-consistent theories of varying constants can be formulated.

Speakers:

John Barrow, University of Cambridge

John Ellis, King’s College London

Pedro Ferreira, University of Oxford

Joao Magueijo, Imperial College, London

Thanu Padmanabhan, IUCAA, Pune

Martin Rees, University of Cambridge

John Webb, University of New South Wales, Sydney

Registration is free and includes morning coffee and lunch. Participants are requested to register at the conference website where the detailed programme of talks can be found:

http://www.eventbrite.co.uk/e/cosmology-and-the-constants-of-nature-registration-9356261831

For enquiries about this event please contact Margaret Bull at mmp@maths.cam.ac.uk

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Cosmological Tanka

Posted in Poetry, The Universe and Stuff with tags , , , on January 14, 2014 by telescoper

Most readers of this blog will be familiar with the form of Japanese poetry known as Haiku. I’ve even had a go at producing some cosmological Haiku myself. I suspect rather fewer will have come across another form known as Tanka. Being 31 syllables long rather than the 17 of Haiku, these are not quite as short but still quite a challenge to write.  They comprise 5 lines with a 5-7-5-7-7 pattern of syllables. I’m told by Japanese friends that Tanka are specifically written to celebrate a special event or to capture the mood of a particular moment. Here is an exquisite example by a famous poet called Otomo No Yakamochi:

From outside my house,
only the faint distant sound
of gentle breezes
wandering through bamboo leaves
in the long evening silence.

I’ve had a go at composing a couple of Tanka to do with specific moments in cosmology. Here’s one about the epoch of recombination:

An electron finds
a proton and marries it;
they make hydrogen.
Simultaneous weddings
free light across the cosmos.

I was talking to some students about the spherical collapse model so here’s a Tanka for that:

I was more dense than
my surroundings, expanded
more slowly, then stopped.
Now I must start to collapse;
soon I shall virialize.

Further attempts welcome through the comments box!

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Physics World Plug

Posted in Books, Talks and Reviews, The Universe and Stuff with tags , , , on January 7, 2014 by telescoper

Just time for a quick bit of shameless self-promotion. This month’s Edition of Physics World has an article by me as cover feature. Here’s a sneak preview, but to read the whole thing you’ll have to rush out and buy a copy! Alternatively, you can find it online here.

IMG-20140107-00256

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The Cosmic Web at Sussex

Posted in Books, Talks and Reviews, The Universe and Stuff with tags , on December 10, 2013 by telescoper

Yesterday I had the honour of giving an evening lecture for staff and students at the School of Mathematical and Physical Sciences at the University of Sussex. The event was preceded by a bit of impromptu twilight stargazing with the new telescope our students have just purchased:

IMG-20131209-00241 IMG-20131209-00243

You can just about see Venus in the second picture, just to the left of the street light.

Anyway, after briefly pretending to be a proper astronomer it was down to my regular business as a cosmologist and my talk entitled The Cosmic Web. Here is the abstract:

The lecture will focus on the large-scale structure of the Universe and the ideas that physicists are weaving together to explain how it came to be the way it is. Over the last few decades, astronomers have revealed that our cosmos is not only vast in scale – at least 14 billion light years in radius – but also exceedingly complex, with galaxies and clusters of galaxies linked together in immense chains and sheets, surrounding giant voids of (apparently) empty space. Cosmologists have developed theoretical explanations for its origin that involve such exotic concepts as ‘dark matter’ and ‘cosmic inflation’, producing a cosmic web of ideas that is, in some ways, as rich and fascinating as the Universe itself.

And for those of you interested, here are the slides I used for your perusal:

It was quite a large (and  very mixed) audience; it’s always difficult to pitch a talk at the right level in those circumstances so that it’s not too boring for the people who know something already but not too challenging for those who don’t know anything at all. A couple of people walked out about five minutes into the talk, which doesn’t exactly inspire a speaker with confidence, but overall it seemed to go down quite well.

Most of all, thank you to the organizers for the very nice reward of a bottle of wine!

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Statistical Challenges in 21st Century Cosmology

Posted in The Universe and Stuff with tags , , on December 2, 2013 by telescoper

I received the following email about a forthcoming conference which is probably of interest to a (statistically) significant number of readers of this blog so I thought I’d share it here with an encouragement to attend:

–o–

IAUS306 – Statistical Challenges in 21st Century Cosmology

We are pleased to announce the IAU Symposium 306 on Statistical Challenges in 21st Century Cosmology, which will take place in Lisbon, Portugal from 26-29 May 2014, with a tutorial day on 25 May.  Apologies if you receive this more than once.

Full exploitation of the very large surveys of the Cosmic Microwave Background, Large-Scale Structure, weak gravitational lensing and future 21cm surveys will require use of the best statistical techniques to answer the major cosmological questions of the 21st century, such as the nature of Dark Energy and gravity.

Thus it is timely to emphasise the importance of inference in cosmology, and to promote dialogue between astronomers and statisticians. This has been recognized by the creation of the IAU Working Group in Astrostatistics and Astroinformatics in 2012.

IAU Symposium 306 will be devoted to problems of inference in cosmology, from data processing to methods and model selection, and will have an important element of cross-disciplinary involvement from the statistics communities.

Keynote speakers

• Cosmic Microwave Background :: Graca Rocha (USA / Portugal)

• Weak Gravitational Lensing :: Masahiro Takada (Japan)

• Combining probes :: Anais Rassat (Switzerland)

• Statistics of Fields :: Sabino Matarrese (Italy)

• Large-scale structure :: Licia Verde (Spain)

• Bayesian methods :: David van Dyk (UK)

• 21cm cosmology :: Mario Santos (South Africa / Portugal)

• Massive parameter estimation :: Ben Wandelt (France)

• Overwhelmingly large datasets :: Alex Szalay (USA)

• Errors and nonparametric estimation :: Aurore Delaigle (Australia)

You are invited to submit an abstract for a contributed talk or poster for the meeting, via the meeting website. The deadline for abstract submission is 21st March 2014. Full information on the scientific rationale, programme, proceedings, critical dates, and local arrangements will be on the symposium web site here.

Deadlines

13 January 2014 – Grant requests

21 March 2014 – Abstract submission

4 April 2014 – Notification of abstract acceptance

11 April 2014 – Close of registration

30 June 2014 – Manuscript submission

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A Dark Energy Mission

Posted in The Universe and Stuff with tags , , on November 16, 2013 by telescoper

Here’s a challenge for cosmologists and aspiring science communicators out there. Most of you will know the standard cosmological model involves a thing, called Dark Energy, whose existence is inferred from observations that suggest that the expansion of the Universe appears to be accelerating.

That these observations require something a bit weird can be quickly seen by looking at the equation that governs the dynamics of the cosmic scale factor R for a simple model involving matter in the form of a perfect fluid:

\ddot{R}=-\frac{4\pi G}{3} \left( \rho + \frac{3p}{c^2}\right) R

The terms in brackets relate to the density and pressure of the fluid, respectively. If the pressure is negligible (as is the case for “dust”), then the expansion is always decelerating because the density of matter is always positive quantity; we don’t know of anything that has a negative mass.

The only way to make the expansion of such a universe actually accelerate is to fill it with some sort of stuff that has

\left( \rho + \frac{3p}{c^2} \right) < 0.

In the lingo this means that the strong energy condition must be violated; this is what the hypothetical dark energy component is introduced to do. Note that this requires the dark energy to exert negative pressure, ie it has to be, in some sense, in tension.

However, there’s something about this that seems very paradoxical. Pressure generates a force that pushes, tension corresponds to a force that pulls. In the cosmological setting, though, increasing positive pressure causes a greater deceleration while to make the universe accelerate requires tension. Why should a bigger pushing force cause the universe to slow down, while a pull causes it to speed up?

The lazy answer is to point at the equation and say “that’s what the mathematics says”, but that’s no use at all when you want to explain this to Joe Public.

Your mission, should you choose to accept it, is to explain in language appropriate to a non-expert, why a pull seems to cause a push…

Your attempts through the comments box please!

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Tension in Cosmology?

Posted in Astrohype, Bad Statistics, The Universe and Stuff with tags , , , on October 24, 2013 by telescoper

I noticed this abstract (of a paper by Rest et al.) on the arXiv the other day:

We present griz light curves of 146 spectroscopically confirmed Type Ia Supernovae (0.03<z<0.65) discovered during the first 1.5 years of the Pan-STARRS1 Medium Deep Survey. The Pan-STARRS1 natural photometric system is determined by a combination of on-site measurements of the instrument response function and observations of spectrophotometric standard stars. We have investigated spatial and time variations in the photometry, and we find that the systematic uncertainties in the photometric system are currently 1.2% without accounting for the uncertainty in the HST Calspec definition of the AB system. We discuss our efforts to minimize the systematic uncertainties in the photometry. A Hubble diagram is constructed with a subset of 112 SNe Ia (out of the 146) that pass our light curve quality cuts. The cosmological fit to 313 SNe Ia (112 PS1 SNe Ia + 201 low-z SNe Ia), using only SNe and assuming a constant dark energy equation of state and flatness, yields w = -1.015^{+0.319}_{-0.201}(Stat)+{0.164}_{-0.122}(Sys). When combined with BAO+CMB(Planck)+H0, the analysis yields \Omega_M = 0.277^{+0.010}_{-0.012} and w = -1.186^{+0.076}_{-0.065} including all identified systematics, as spelled out in the companion paper by Scolnic et al. (2013a). The value of w is inconsistent with the cosmological constant value of -1 at the 2.4 sigma level. This tension has been seen in other high-z SN surveys and endures after removing either the BAO or the H0 constraint. If we include WMAP9 CMB constraints instead of those from Planck, we find w = -1.142^{+0.076}_{-0.087}, which diminishes the discord to <2 sigma. We cannot conclude whether the tension with flat CDM is a feature of dark energy, new physics, or a combination of chance and systematic errors. The full Pan-STARRS1 supernova sample will be 3 times as large as this initial sample, which should provide more conclusive results.

The mysterious Pan-STARRS stands for the Panoramic Survey Telescope and Rapid Response System, a set of telescopes cameras and related computing hardware that monitors the sky from its base in Hawaii. One of the many things this system can do is detect and measure distant supernovae, hence the particular application to cosmology described in the paper. The abstract mentions a preliminary measurement of the parameter w, which for those of you who are not experts in cosmology is usually called the “equation of state” parameter for the dark energy component involved in the standard model. What it describes is the relationship between the pressure P and the energy density ρc2 of this mysterious stuff, via the relation P=wρc2. The particularly interesting case is w=-1 which corresponds to a cosmological constant term; see here for a technical discussion. However, we don’t know how to explain this dark energy from first principles so really w is a parameter that describes our ignorance of what is actually going on. In other words, the cosmological constant provides the simplest model of dark energy but even in that case we don’t know where it comes from so it might well be something different; estimating w from surveys can therefore tell us whether we’re on the right track or not.

The abstract explains that, within the errors, the Pan-STARRS data on their own are consistent with w=-1. More interestingly, though, combining the supernovae observations with others, the best-fit value of w shifts towards a value a bit less than -1 (although still with quite a large uncertainty). Incidentally  value of w less than -1 is generally described as a “phantom” dark energy component. I’ve never really understood why…

So far estimates of cosmological parameters from different data sets have broadly agreed with each other, hence the application of the word “concordance” to the standard cosmological model.  However, it does seem to be the case that supernova measurements do generally seem to push cosmological parameter estimates away from the comfort zone established by other types of observation. Could this apparent discordance be signalling that our ideas are wrong?

That’s the line pursued by a Scientific American article on this paper entitled “Leading Dark Energy Theory Incompatible with New Measurement”. This could be true, but I think it’s a bit early to be taking this line when there are still questions to be answered about the photometric accuracy of the Pan-Starrs survey. The headline I would have picked would be more like “New Measurement (Possibly) Incompatible With Other Measurements of Dark Energy”.

But that would have been boring…

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Planck and Being Human

Posted in The Universe and Stuff with tags , , , on October 23, 2013 by telescoper

On Saturday 19th October the instruments and cooling systems on the European Space Agency’s Planck spacecraft were switched off, marking the end of the scientific part of the Planck mission, after about four years of mapping the cosmic microwave background.  Later, a piece of software was uploaded that would prevent  the spacecraft systems being  accidentally switched on again after being switched off and the transmitter from causing interference with any future probes.  Planck is already “parked” indefinitely in a what is called a “disposal” orbit, far from the Earth-Moon system, having been nudged off its perch at the 2nd Lagrangian Point (L2) in August by a complicated series of manoeuvres. On October 21st the spacecraft’s thrusters were fired to burn up the last of its fuel, an important aspect of rendering the spacecraft inert, as required by ESA’s space debris mitigation guidelines.

Planck

These preliminaries having been completed, today, at 12.00 GMT,  a final instruction will be transmitted to the spacecraft  to close it down permanently; thereafter Planck will circle the Sun as a silent memorial to the stunning success it achieved when active. I’m sure all those who worked on the Planck mission will pause as the final shutdown command is given and ponder the lonely future  of the spacecraft that had supplied so much interesting data.

But although this will be the end of the Planck mission, it is by no means the end of the Planck Era. Vast amounts of data still need to be fully analysed and key science results are still in the pipeline,  relating in particular to the polarization of the microwave background radiation. Moreover, the numerous maps, catalogues and other data products will be a priceless legacy to this generation, and no doubt many future generations, of scientists.

The fate of Planck illustrates two contrasting aspects of the human experience. On the one hand, there’s the fragility of our existence in a cosmos too vast for us to comprehend. Like the defunct spacecraft, our Earth too circles this little Sun of ours in a precarious orbit while the rest of the Universe – with its countless billion upon billion of other suns – carries on, oblivious to our very existence. Planck makes us painfully aware of our own insignificance.

But on the other hand there’s the sense of fulfillment, and even of joy, at finding things out. We may have puny monkey brains and many things are likely to remain forever beyond our mental grasp, but trying to figure things out is one of the things that defines us as human.  Experiments like Planck (and, for that matter, the Large Hadron Collider) are not the wasteful extravagance some people claim them to be. We need them not just for the sake of science, but to remind us of our common humanity.

UPDATE: And now, from ESA, confirmation that Planck has received its last command. Goodbye, and enjoy your retirement!

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Updates for Cosmology: A Very Short Introduction?

Posted in Books, Talks and Reviews, The Universe and Stuff with tags , , , , , on October 21, 2013 by telescoper

Yet another very busy day, travelling in the morning and then in meetings all afternoon, so just time for another brief post. I thought I’d take the opportunity to do a little bit of crowdsourcing…

A few days ago I was contacted by Oxford University Press who are apparently considering the possibility of a second edition of my little book Cosmology: A Very Short Introduction, which is part of an extensive series of intensive books on all kinds of subjects.

I really enjoyed writing this book, despite the tough challenge of trying to cover the whole of cosmology in less than 35,000 words and was very pleased with the way it turned out. It has sold over 25000 copies in English and has been published in several other languages.

It is meant to be accessible to the interested layperson but the constraints imposed by the format mean it goes fairly quickly through some quite difficult concepts. Judging by the reviews, though, most people seem to think it gives a useful introduction to the subject, although you can’t please all of the people all of the time!

However, the book was published way back in 2001 and, well, one or two things have happened in the field of cosmology since then.  I have in fact had a number of emails from people asking whether there was going to be a new edition to include the latest developments, but the book is part of a very large series and it was basically up to the publisher to decide whether it wanted to update some, all or none of the series.

Now it seems the powers that be at OUP have decided to explore the possibility further and have asked me to make a pitch for a new edition.  I have some ideas of things that would have to be revised – the section on Dark Energy definitely needs to be updated, and of course first WMAP and then Planck have refined our view of the cosmic microwave background pretty comprehensively?

Anyway, I thought it would be fun to ask people out there who have read it, or even those who haven’t, what they feel I should change for a new edition if there is to be one. That might include new topics or revisions of things that could be improved. Your comments are therefore invited via the famous Comments Box. Please bear in mind that any new edition will be also constrained to be no more than 35,000 words.

Oh, and if you haven’t seen the First Edition at all, why not rush out and buy a copy before it’s too late? I understand you can snap up a copy for just £3 while stocks last. I can assure you all the royalties will go to an excellent cause. Me.

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