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Dark Matter from the Dark Energy Survey

Posted in The Universe and Stuff with tags , , , on April 14, 2015 by telescoper

I’m a bit late onto this story which has already been quite active in the media today, and has generated an associated flurry of activity on social media, but I thought it was still worth passing it on via the medium of this blog. The Dark Energy Survey has just released a number of papers onto the arXiv, the most interesting of which (to me) is entitled Wide-Field Lensing Mass Maps from DES Science Verification Data. The abstract reads as follows (the link was added by me):

Weak gravitational lensing allows one to reconstruct the spatial distribution of the projected mass density across the sky. These “mass maps” provide a powerful tool for studying cosmology as they probe both luminous and dark matter. In this paper, we present a weak lensing mass map reconstructed from shear measurements in a 139 deg^2 area from the Dark Energy Survey (DES) Science Verification (SV) data overlapping with the South Pole Telescope survey. We compare the distribution of mass with that of the foreground distribution of galaxies and clusters. The overdensities in the reconstructed map correlate well with the distribution of optically detected clusters. Cross-correlating the mass map with the foreground galaxies from the same DES SV data gives results consistent with mock catalogs that include the primary sources of statistical uncertainties in the galaxy, lensing, and photo-z catalogs. The statistical significance of the cross-correlation is at the 6.8 sigma level with 20 arcminute smoothing. A major goal of this study is to investigate systematic effects arising from a variety of sources, including PSF and photo-z uncertainties. We make maps derived from twenty variables that may characterize systematics and find the principal components. We find that the contribution of systematics to the lensing mass maps is generally within measurement uncertainties. We test and validate our results with mock catalogs from N-body simulations. In this work, we analyze less than 3% of the final area that will be mapped by the DES; the tools and analysis techniques developed in this paper can be applied to forthcoming larger datasets from the survey.

This is by no means a final result from the Dark Energy Survey, as it was basically put together in order to test the telescope, but it is interesting from the point of view that it represents a kind of proof of concept. Here is one of the key figures from the paper which shows a reconstruction of the mass distribution of the Universe (dominated by dark matter) obtained indirectly by the Dark Energy Survey using distortions of galaxy images produced by gravitational lensing by foreground objects, onto which the positions of large galaxy clusters seen in direct observations have been plotted. Although this is just a small part of the planned DES study (it covers only 0.4% of the sky) it does seem to indicate that the strong concentrations of dark matter (red) do corrrelate with the positions of concentrations of galaxy clusters.

DES_MAP

It all seems to work, so hopefully we can look forward to lots of interesting science results in future!

P.S. When I first saw the map it looked like a map of the North of England Midlands and I was surprised to see that the survey showed such strong support for the Greens…

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Last Call for Cosmology Talks at NAM 2015!

Posted in The Universe and Stuff with tags , on April 13, 2015 by telescoper

Just a quick post about this year’s forthcoming Royal Astronomical Society National Astronomy Meeting, which will be taking place at the splendid Venue Cymru conference centre, Llandudno, North Wales, from Sunday 5th July to Thursday 9th July 2015. I’m on the Scientific Organizing Committee for NAM 2015 and as such I’ll be organizing a part of this meeting, namely a couple of sessions on Cosmology under the title Cosmology Beyond the Standard Model, with the following description.

Recent observations, particularly those from the Planck satellite, have provided strong empirical foundations for a standard cosmological model that is based on Einstein’s general theory of relativity and which describes a universe which is homogeneous and isotropic on large scales and which is dominated by dark energy and matter components. This session will explore theoretical and observational challenges to this standard picture, including modified gravity theories, models with large-scale inhomogeneity and/or anisotropy, and alternative forms of matter-energy. The aim will be to both take stock of the evidence for, and stimulate further investigation of, physics beyond the standard model.

The deadline for submitting abstracts for this and other sessions was originally 1st April, but this has been extended until 14th April (i.e. tomorrow). The cosmology sessions are shaping up to be very interesting indeed, but I might be able to squeeze in one or two more talks. If you’ve been prevaricating about submitting a proposal, then please get your finger out and visit the NAM2015 website right now. This is your last chance!

NAM is a particularly good opportunity for younger researchers – PhD students and postdocs – to present their work to a big audience so I particularly encourage such persons to submit abstracts. Would more senior readers please pass this message on to anyone they think might want to give a talk?

If you have any questions please feel free to use the comments box (or contact me privately).

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A Birthday Message to Donald Lynden-Bell

Posted in The Universe and Stuff with tags , on April 12, 2015 by telescoper

On Friday being the second Friday of the month of April I went up to London for the regular Open Meeting of the Royal Astronomical Society and afterwards to dinner with the RAS Club. Unusually for club dinners, we were provided with champagne before the toasts but it was a while before I realized why. A distinguished member and indeed former President of the club, Prof. Donald Lynden-Bell, had recently celebrated his eightieth birthday and we were all invited to drink his health.

Donald is an amazing character, not least because he hasn’t changed a bit since I first met him over thirty years ago when he was lecturer for one of the courses I took in the first year. His research has spanned an enormous breadth of subjects, from theoretical topics in classical and quantum physics to astrophysics and cosmology, including data analysis. Anyway, it was great that he was there to receive the toast in person. I’ll take the opportunity here to say a more public Happy Birthday!

On the way home I posted on Facebook that Donald had just celebrated his eightieth birthday. One of my astronomer friends, Manuela Magliocchetti, posted a charming comment about him that I’m sharing here (below) publicly in a slightly edited form, with her permission. By the way, in the interest of full disclosure, I should point out that I subsequently had the honour to be the External Examiner for Manuela’s PhD…

–o–

I just learnt that today from Peter there were celebrations at the RAS Dining Club for the 80th birthday of Professor Donald Lynden-Bell. Since I basically owe my scientific carrier to him, I thought I’d  thank him publicly now.

It was summer 1995 and I had pestered my undergraduate supervisor to send me to Cambridge to attend the conference on Gravitational Dynamics that had been organized for the 60th birthday of Donald (gee, already 20 years ago!), since all my undergrad thesis was on some evidence of a phenomenon (gravothermal catastrophe) that he first theorized in a breakthrough paper published in 1968 that by then I knew by heart. So he definitely was my scientific hero.

At the end of the conference I knocked at his office door to ask him whether it was possible for me to apply for a PhD position at Cambridge. He let me in, but did not even allow me to start talking. Instead he started asking me about the thesis work I had done, since in Italy everyone in Physics has to produce some original work in order to be awarded the undegraduate degree. He had me writing on his blackboard for about an hour (which felt like centuries to me) about all my results, asking genuinely interested questions, discussing, and in some bits  criticizing my work. He was very pushy (as I learned later, this  was his style) and was talking oh-so-very fast.

I was soo unsettled and scared and not even sure I was understanding all his points correctly: my English was so basic… After all this torture, he suddently stopped and, with his slightly squeaky voice, went:” So, why are you here?” I very humbly answered that it was to have information on how to apply to Cambridge for a PhD position. He then looked at me, then at the blackboard, then at me again and told me what I wrote on the blackboard indeed was PhD work. I answered that no, it was just undergraduate work. At that point he jumped off his chair, grabbed my arm and dragged me to the secretary of the Isaac Newton Scholarship, introducing me to her and telling her that I would be applying for both a PhD position at the Institute of Astronomy at University of Cambridge and for the scholarship. So I did apply, and in the end got both and found myself thrown in that fantastically stimulating environment which is Cambridge and the IoA.

Thank you so much Donald! Forever grateful. Without you all this and what happened next, including my present job and career and even my kids, since I met their (astronomer) dad over there, would not have been possible!

 

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Why the Big Bang wasn’t as loud as you think…

Posted in The Universe and Stuff with tags , , , , , on March 31, 2015 by telescoper

So how loud was the Big Bang?

I’ve posted on this before but a comment posted today reminded me that perhaps I should recycle it and update it as it relates to the cosmic microwave background, which is what I work on on the rare occasions on which I get to do anything interesting.

As you probably know the Big Bang theory involves the assumption that the entire Universe – not only the matter and energy but also space-time itself – had its origins in a single event a finite time in the past and it has been expanding ever since. The earliest mathematical models of what we now call the  Big Bang were derived independently by Alexander Friedman and George Lemaître in the 1920s. The term “Big Bang” was later coined by Fred Hoyle as a derogatory description of an idea he couldn’t stomach, but the phrase caught on. Strictly speaking, though, the Big Bang was a misnomer.

Friedman and Lemaître had made mathematical models of universes that obeyed the Cosmological Principle, i.e. in which the matter was distributed in a completely uniform manner throughout space. Sound consists of oscillating fluctuations in the pressure and density of the medium through which it travels. These are longitudinal “acoustic” waves that involve successive compressions and rarefactions of matter, in other words departures from the purely homogeneous state required by the Cosmological Principle. The Friedman-Lemaitre models contained no sound waves so they did not really describe a Big Bang at all, let alone how loud it was.

However, as I have blogged about before, newer versions of the Big Bang theory do contain a mechanism for generating sound waves in the early Universe and, even more importantly, these waves have now been detected and their properties measured.

Planck_CMB

The above image shows the variations in temperature of the cosmic microwave background as charted by the Planck Satellite. The average temperature of the sky is about 2.73 K but there are variations across the sky that have an rms value of about 0.08 milliKelvin. This corresponds to a fractional variation of a few parts in a hundred thousand relative to the mean temperature. It doesn’t sound like much, but this is evidence for the existence of primordial acoustic waves and therefore of a Big Bang with a genuine “Bang” to it.

A full description of what causes these temperature fluctuations would be very complicated but, roughly speaking, the variation in temperature you corresponds directly to variations in density and pressure arising from sound waves.

So how loud was it?

The waves we are dealing with have wavelengths up to about 200,000 light years and the human ear can only actually hear sound waves with wavelengths up to about 17 metres. In any case the Universe was far too hot and dense for there to have been anyone around listening to the cacophony at the time. In some sense, therefore, it wouldn’t have been loud at all because our ears can’t have heard anything.

Setting aside these rather pedantic objections – I’m never one to allow dull realism to get in the way of a good story- we can get a reasonable value for the loudness in terms of the familiar language of decibels. This defines the level of sound (L) logarithmically in terms of the rms pressure level of the sound wave Prms relative to some reference pressure level Pref

L=20 log10[Prms/Pref].

(the 20 appears because of the fact that the energy carried goes as the square of the amplitude of the wave; in terms of energy there would be a factor 10).

There is no absolute scale for loudness because this expression involves the specification of the reference pressure. We have to set this level by analogy with everyday experience. For sound waves in air this is taken to be about 20 microPascals, or about 2×10-10 times the ambient atmospheric air pressure which is about 100,000 Pa.  This reference is chosen because the limit of audibility for most people corresponds to pressure variations of this order and these consequently have L=0 dB. It seems reasonable to set the reference pressure of the early Universe to be about the same fraction of the ambient pressure then, i.e.

Pref~2×10-10 Pamb.

The physics of how primordial variations in pressure translate into observed fluctuations in the CMB temperature is quite complicated, because the primordial universe consists of a plasma rather than air. Moreover, the actual sound of the Big Bang contains a mixture of wavelengths with slightly different amplitudes. In fact here is the spectrum, showing a distinctive signature that looks, at least in this representation, like a fundamental tone and a series of harmonics…

Planck_power_spectrum_orig

 

If you take into account all this structure it all gets a bit messy, but it’s quite easy to get a rough but reasonable estimate by ignoring all these complications. We simply take the rms pressure variation to be the same fraction of ambient pressure as the averaged temperature variation are compared to the average CMB temperature,  i.e.

Prms~ a few ×10-5Pamb.

If we do this, scaling both pressures in logarithm in the equation in proportion to the ambient pressure, the ambient pressure cancels out in the ratio, which turns out to be a few times 10-5. With our definition of the decibel level we find that waves of this amplitude, i.e. corresponding to variations of one part in a hundred thousand of the reference level, give roughly L=100dB while part in ten thousand gives about L=120dB. The sound of the Big Bang therefore peaks at levels just a bit less than 120 dB.

cooler_decibel_chart

As you can see in the Figure above, this is close to the threshold of pain,  but it’s perhaps not as loud as you might have guessed in response to the initial question. Modern popular beat combos often play their dreadful rock music much louder than the Big Bang….

A useful yardstick is the amplitude  at which the fluctuations in pressure are comparable to the mean pressure. This would give a factor of about 1010 in the logarithm and is pretty much the limit that sound waves can propagate without distortion. These would have L≈190 dB. It is estimated that the 1883 Krakatoa eruption produced a sound level of about 180 dB at a range of 100 miles. By comparison the Big Bang was little more than a whimper.

PS. If you would like to read more about the actual sound of the Big Bang, have a look at John Cramer’s webpages. You can also download simulations of the actual sound. If you listen to them you will hear that it’s more of  a “Roar” than a “Bang” because the sound waves don’t actually originate at a single well-defined event but are excited incoherently all over the Universe.

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Praise, by R.S. Thomas

Posted in Poetry, The Universe and Stuff with tags , , , on March 29, 2015 by telescoper

Today is Palm Sunday, the start of what Christians call “Holy Week”, which culiminates in Easter. It’s also the birthday of the great Welsh poet R.S. Thomas, who was born on this day in 1913. Thomas spent much of his life as an Anglican priest. I’m not a Christian but I am drawn to the religious verse of R.S. Thomas not only for its directness and lack of artifice but also the honesty with which he addresses the problems his faith sets him. There are many atheists who think religion is some kind of soft option for those who can’t cope with life in an unfriendly universe, but reading R.S. Thomas, whose faith was neither cosy nor comfortable, led me to realise that is very far from the case. I recommend him as an antidote to the simple-minded antagonism of people like Richard Dawkins. There are questions that science alone will never answer, so we should respect people who search for a truth we ourselves cannot understand.

And whether or not it is clear to you, no doubt the universe is unfolding as it should. Therefore be at peace with God, whatever you conceive Him to be.

I will be offline for the Easter holiday so I thought I’d post a poem that I find appropriate to the time of year. You can read it as Praise for God, or for Nature, or for both. I don’t think it matters.

I praise you because
you are artist and scientist
in one. When I am somewhat
fearful of your power,
your ability to work miracles
with a set-square, I hear
you murmuring to yourself
in a notation Beethoven
dreamed of but never achieved.
You run off your scales of
rain water and sea water, play
the chords of the morning
and evening light, sculpture
with shadow, join together leaf
by leaf, when spring
comes, the stanzas of
an immense poem. You speak
all languages and none,
answering our most complex
prayers with the simplicity
of a flower, confronting
us, when we would domesticate you
to our uses, with the rioting
viruses under our lens.

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Quantum Technology and the Frontier of Computing

Posted in The Universe and Stuff with tags , , , on March 26, 2015 by telescoper

Here’s a short video I just found featuring our own Winfried Hensinger, Professor of Quantum Technologies at the University of Sussex.

It’s part of a pilot documentary that explores the connection between science fiction and science reality. Here is the official blurb:

“The science fiction genre has a history of playing with our imagination; inventing “impossible” technologies and concepts such as time travel and teleportation. The “spooky” discoveries that quantum physicists have recently made are challenging the very “impossibility” of sci-fi. This documentary will explore the ways in which sci-fi has catalysed the imagination of scientists who are pioneering these discoveries.

The theme will explore the causal link between science and science fiction, using the inner workings of the quantum computer that Dr Winfried Hensinger is currently developing as a case study. Dr Hensinger, the head of the Sussex Ion Quantum Technology research group, was inspired early on by the well known 60s science fiction television show Star Trek. Having led multiple breakthroughs in the field of Quantum Computing research, he speaks to the importance of not losing our imagination, citing his childhood desire to be the science officer on Star Trek’s Enterprise as the prime motivator of going into the scientific field. Exploring the relationship between the human beings developing this technology and the non-human genre of science fiction, we will demonstrate that the boundaries between imagination and reality are blurrier than conventionally thought.”

I’ll take the opportunity presented by this video to remind you that the University of Sussex is the only university in the UK to offer an MSc course in Quantum Technologies, and this year there are special bursaries that make this an extremely attractive  option for students seeking to extend their studies into this burgeoning new area. We’ve already seen a big surge in applications for this course this course so if you’re thinking of applying don’t wait too long or it might fill up!

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NAM 2015: Abstract Submission Reminder!

Posted in The Universe and Stuff with tags , on March 24, 2015 by telescoper

Time for another quick plug of this year’s  Royal Astronomical Society National Astronomy Meeting, which will be taking place at the splendid Venue Cymru conference centre, Llandudno, North Wales, from Sunday 5th July to Thursday 9th July 2015. I’ve posted about  this before, but I thought I’d post it again because there is just a week left to submit an abstract for a contributed talk or poster; the deadline for doing that is April 1st. I’m actually on the Scientific Organizing Committee for NAM 2015 and as such I’ll be organizing part of this meeting, namely a couple of sessions on Cosmology under the title Cosmology Beyond the Standard Model, with the following description.

Recent observations, particularly those from the Planck satellite, have provided strong empirical foundations for a standard cosmological model that is based on Einstein’s general theory of relativity and which describes a universe which is homogeneous and isotropic on large scales and which is dominated by dark energy and matter components. This session will explore theoretical and observational challenges to this standard picture, including modified gravity theories, models with large-scale inhomogeneity and/or anisotropy, and alternative forms of matter-energy. The aim will be to both take stock of the evidence for, and stimulate further investigation of, physics beyond the standard model.

It’s obviously quite a broad remit so I hope that there will be plenty of contributed talks and posters. NAM is a particularly good opportunity for younger researchers – PhD students and postdocs – to present their work to a big audience so I particularly encourage such persons to submit abstracts. Would more senior readers please pass this message on to anyone they think might want to give a talk?   To further whet your appetite, here are some pictures of lovely Llandudno  I took at the last National Astronomy Meeting there, back in 2011.     If you have any questions please feel free to use the comments box (or contact me privately).

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The Eclipse Coincidence Question

Posted in The Universe and Stuff with tags , , , , on March 22, 2015 by telescoper

The day before last week’s (partial) solar eclipse I posted an item in which I mentioned the apparent coincidence that makes total eclipses possible, namely that the Moon and Sun have very similar angular sizes when seen from Earth.

In the interest of balance I thought I would direct you to a paper by Steve Balbus that develops a detailed argument to the contrary along the lines I described briefly in my earlier post. I am not entirely convinced but do read it and make up your own mind:

Here is the abstract:

The nearly equal lunar and solar angular sizes as subtended at the Earth is generally regarded as a coincidence. This is, however, an incidental consequence of the tidal forces from these bodies being comparable. Comparable magnitudes implies strong temporal modulation, as the forcing frequencies are nearly but not precisely equal. We suggest that on the basis of paleogeographic reconstructions, in the Devonian period, when the first tetrapods appeared on land, a large tidal range would accompany these modulated tides. This would have been conducive to the formation of a network of isolated tidal pools, lending support to A.S. Romer’s classic idea that the evaporation of shallow pools was an evolutionary impetus for the development of chiridian limbs in aquatic tetrapodomorphs. Romer saw this as the reason for the existence of limbs, but strong selection pressure for terrestrial navigation would have been present even if the limbs were aquatic in origin. Since even a modest difference in the Moon’s angular size relative to the Sun’s would lead to a qualitatively different tidal modulation, the fact that we live on a planet with a Sun and Moon of close apparent size is not entirely coincidental: it may have an anthropic basis.

I don’t know if it’s a coincidence or not but I always follow the advice given by my role model, Agatha Christie’s Miss Marple in Nemesis: “Any coincidence is worth noticing. You can throw it away later if it is only a coincidence.”..

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That Was The Eclipse That Was..

Posted in The Universe and Stuff with tags , , , on March 20, 2015 by telescoper

It’s been an astronomical and meteorological rollercoaster of a morning. When I woke up at 6am this morning, the Sun was just rising. There was hazy cloud and there seemed to be every chance that it would break up to allowing viewing of today’s partial solar eclipse. Unfortunately, however, the cloud thickened rather than breaking up so I abandoned my plan to watch from the seafront and headed up to the University of Sussex campus at Falmer. Being higher up and a few miles inland, the weather is often clearer in Falmer than in Brighton. Unfortunately it was even murkier when I got here, so I assumed I was in for a disappointing morning.

Nevertheeclipse_cloudsless I did join the large gathering in Library Square to experience the event. First contact between the Moon and the Sun happened at about 8.30, but the cloud cover was total at that point so nothing was visible. It did get gradually darker, but this happened slowly enough for eyes to adapt to the dark so it wasn’t all that noticeable to us humans. The birds on campus certainly noticed, however, and began to perform display roosting behaviour thinking it was evening. It also got really very cold.

Around 9.30 the coverage of the Sun by the Moon was about its maximum – 85% or so – but everything was still enshrouded in cloud. The crowd waited patiently in the gloom. It was a very British experience, a large group of people sharing their sense of collective disappointment in appropriately stoical fashion.

eclipse_crowd

Gradually the wind seemed to increase, pushing the clouds over more quickly and causing them to break up. Then, suddenly, a small gap in the cloud opened up and there was the eclipse. For about a second. It may have been only a moment, but it generated a huge cheer which, I should add, wasn’t entirely ironic. The breaking up of the clouds continued and we were treated to several good views of the main event. It was definitely worth it.

Most of the pictures I took didn’t come out at all well, but here is one with the famous Meeting House in the foreground:

eclipse_meetingAnd here’s a rather nice picture from John Sander of the International Office at Sussex University, showing yours truly during the final stages of the eclipse..

Eclipse_me

I’ll add more pictures as I find them. Please feel free to share your comments and observations via the appropriate box!

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Life, the Universe and the Partial Eclipse..

Posted in Astrohype, The Universe and Stuff with tags , , on March 19, 2015 by telescoper

As you will no doubt be aware, tomorrow there will be a Partial Eclipse of the Sun visible from the United Kingdom. Here’s a handy guide, courtesy of the Met Office, to the time and maximum fraction of the Sun’s disk that will be obscured.

Eclipse

Unfortunately the weather forecast for Brighton isn’t marvellous so it’s possible that the main event will be obscured by cloud and all we experience is that an already dark and gloomy morning gets even darker and gloomier.

However, in the event that the weather forecast turns out to be inaccurate, which is far from unheard of, please make sure you follow the official Royal Astronomical Society guidelines to make sure you observe it safely.

And while I’m at it, here is a video of a nice lecture by Ian Ridpath explaining all about Solar and Lunar Eclipses.

As a spectacle a partial solar eclipse is pretty exciting – as long as it’s not cloudy – but even a full view of one can’t really be compared with the awesome event that is a total eclipse. I’m lucky enough to have observed one and I can tell you it was truly awe-inspiring.

If you think about it, though, it’s rather odd that such a thing is possible at all. In a total eclipse, the Moon passes between the Earth and the Sun in such a way that it exactly covers the Solar disk. In order for this to happen the apparent angular size of the Moon (as seen from Earth) has to be almost exactly the same as that of the Sun (as seen from Earth). This involves a strange coincidence: the Moon is small (about 1740 km in radius) but very close to the Earth in astronomical terms (about 400,000 km away). The Sun, on the other hand, is both enormously large (radius 700,000 km) and enormously distant (approx. 150,000,000 km). The ratio of radius to distance from Earth of these objects is almost identical at the point of a a total eclipse, so the apparent disk of the Moon almost exactly fits over that of the Sun. Why is this so?

The simple answer is that it is just a coincidence. There seems no particular physical reason why the geometry of the Earth-Moon-Sun system should have turned out this way. Moreover, the system is not static. The tides raised by the Moon on the Earth lead to frictional heating and a loss of orbital energy. The Moon’s orbit is therefore moving slowly outwards from the Earth. I’m not going to tell you exactly how quickly this happens, as it is one of the questions I set my students in the module Astrophysical Concepts I’ll be starting in a few weeks, but eventually the Earth-Moon distance will be too large for total eclipses of the Sun by the Moon to be possible on Earth, although partial and annular eclipses may still be possible.

It seems therefore that we just happen to be living at the right place at the right time to see total eclipses. Perhaps there are other inhabited moonless planets whose inhabitants will never see one. Future inhabitants of Earth will have to content themselves with watching eclipse clips on Youtube.

Things may be more complicated than this though. I’ve heard it argued that the existence of a moon reasonably close to the Earth may have helped the evolution of terrestrial life. The argument – as far as I understand it – is that life presumably began in the oceans, then amphibious forms evolved in tidal margins of some sort wherein conditions favoured both aquatic and land-dwelling creatures. Only then did life fully emerge from the seas and begin to live on land. If it is the case that the existence of significant tides is necessary for life to complete the transition from oceans to solid ground, then maybe the Moon played a key role in the evolution of dinosaurs, mammals, and even ourselves.

I’m not sure I’m convinced of this argument because, although the Moon is the dominant source of the Earth’s tides, it is not overwhelmingly so. The effect of the Sun is also considerable, only a factor of three smaller than the Moon. So maybe the Sun could have done the job on its own. I don’t know.

That’s not really the point of this post, however. What I wanted to comment on is that astronomers generally don’t question the interpretation of the occurence of total eclipses as simply a coincidence. Eclipses just are. There are no doubt many other planets where they aren’t. We’re special in that we live somewhere where something apparently unlikely happens. But this isn’t important because eclipses aren’t really all that significant in cosmic terms, other than that the law of physics allow them.

On the other hand astronomers (and many other people) do make a big deal of the fact that life exists in the Universe. Given what we know about fundamental physics and biology – which admittedly isn’t very much – this also seems unlikely. Perhaps there are many other worlds without life, so the Earth is special once again. Others argue that the existence of life is so unlikely that special provision must have been made to make it possible.

Before I find myself falling into the black hole marked “Anthropic Principle” let me just say that I don’t see the existence of life (including human life) as being of any greater significance than that of a total eclipse. Both phenomena are (subjectively) interesting to humans, both are contingent on particular circumstances, and both will no doubt cease to occur at some point in perhaps not-too-distant the future. Neither tells us much about the true nature of the Universe.

Perhaps we should just face up to the fact that we’re just not very significant….

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