Archive for the ‘Astronomy’ Category

Front Cover of Australian PhysicsMy article “Cosmology Q & A” has been published! It appeared in the magazine Australian Physics, 51 (2014) 42-6 and is reproduced here with permission. After a brief overview of modern cosmology, it (tries to) answer the following questions:

  1. Is space expanding, or are galaxies just moving away from us?
  2. Is everything getting bigger?
  3. Ordinary matter and radiation cause the expansion of the universe to decelerate. But our universe is accelerating! How? What is the universe made of?
  4. Dark Energy? Is that like Dark Matter?
  5. How big is the universe?
  6. How big is the universe really?
  7. If the universe were finite, could I see the back of my own head?
  8. Is space expanding faster than the speed of light?
  9. Are there galaxies moving away from us at more than the speed of light?
  10. Light from distant galaxies is observed to be redshifted. Is this because the expansion of space stretches the wavelength, or because is it a Doppler shift due to the recession of the galaxy?
  11. Does the universe have zero total energy?
  12. Energy is not conserved!? Shouldn’t that send shivers up the spine of any physicist?
  13. The very universe, we are told, began in thermal equilibrium. How did equilibrium establish itself so quickly?
  14. How does the initially smooth universe we see in the CMB become today’s universe of stars and galaxies?

As before, further questions in the comments are always welcome.

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I don’t know who Rob Sheldon is, but he doesn’t know much about cosmology. He recently was quoted in this post at uncommondescent.com regarding the geometry of the universe. If I lecture cosmology this year, I’ll set this passage as an assignment: find all the mistakes. It gets more wrong than right. I have an article for “Australian Physics” on common questions about cosmology that I’ll post here once it’s out (a fortnight, maybe). In the meantime, I’ll try to clear up a few things.

The discussion of the mathematics of curvature (flat, positive, negative) is about right. It’s when he discusses the universe that things go wrong.

It takes a lot of effort to find any curvature at all, and certainly it is difficult to get good agreement between different types of measurement.

Nope. That’s why it’s called the “concordance model of cosmology” – because the different measurements converge on the same set of cosmological parameters. For example, this plot.

… a “closed” universe that collapses back down to itself …

A common error. In a matter and radiation-only universe, closed implies collapsing. A cosmological constant and/or dark energy changes this: closed vs. open no longer divides collapse vs. expand forever. Here is the plot you’ll need, from John Peacock’s marvellous Cosmological Physics.

… one would like it to have positive curvature to avoid infinities …

Flat and negatively curved universes can be finite. A flat 3-torus, for example, is finite, unbounded and has a flat geometry. Einstein’s general relativity constrains the geometry of the universe but not its topology. (more…)

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It’s always a nervous moment when, as a scientist, you discover that a documentary has been made on one of your favourite topics. Science journalism is rather hit and miss. So it was when the Australian Broadcasting Corporation (ABC), our public TV network, aired a documentary about the fine-tuning of the universe for intelligent life as part of their Catalyst science series. (I’ve mentioned my fine-tuning review paper enough, haven’t I?).

The program can be watched on ABC iView. (International readers – does this work for you?). It was hosted by Dr Graham Phillips, who has a PhD in Astrophysics. The preview I saw last week was promising. All the right people’s heads were appearing – Sean Carroll, Brian Greene, Paul Davies, Leonard Susskind, Lawrence Krauss, Charley Lineweaver. John Wheeler even got a mention.

Overall – surprisingly OK. They got the basic science of fine-tuning correct. Phillips summarises fine-tuning as:

When scientists look far into the heavens or deeply down into the forces of nature, they see something deeply mysterious. If some of the laws that govern our cosmos were only slightly different, intelligent life simply couldn’t exist. It appears that the universe has been fine-tuned so that intelligent beings like you and me could be here.

Not bad, though I’m not sure why it needed to be accompanied by such ominous music. There is a possibility for misunderstanding, however. Fine-tuning is a technical term in physics that roughly means extreme sensitivity of some “output” to the “input”. For example, if some theory requires an unexplained coincidence between two free parameters, then the “fine-tuning” of the theory required to explain the data counts against that theory. “Fine-tuned” does not mean “chosen by an intelligent being” or “designed”. It’s a metaphor.

Ten minutes in, the only actual case of fine-tuning that had been mentioned was the existence of inhomogeneities in the early universe. Sean Carroll:

If the big bang had been completely smooth, it would just stay completely smooth and the history of the universe would be very, very boring. It would just get more and more dilute but you would never make stars, you would never make galaxies or clusters of galaxies. So the potential for interesting complex creatures like you and me would be there, but it would never actually come to pass. So we’re very glad that there was at least some fluctuation in the early universe.

Paul Davies then discussed the fact that there not only need to be such fluctuations, but they need to be not-too-big and not-too-small. Here’s the scientific paper, if you’re interested.

The documentary also has a cogent discussion of the cosmological constant problem – the “mother of all fine-tunings” – and the fine-tuning of the Higgs field, which is related to the hierarchy problem. Unfortunately, Phillips calls it “The God Particle” because “it gives substance to all nature’s other particles”. Groan.

Once we move beyond the science of fine-tuning, however, things get a bit more sketchy.

The Multiverse

Leonard Susskind opens the section on the multiverse by stating that the multiverse is, in his opinion, the only explanation available for the fine-tuning of the universe for intelligent life. At this point, both the defence and the prosecution could have done more.

Possibilities are cheap. Sean Carroll appears on screen to say “Aliens could have created our universe” and then is cut off. We are told that if we just suppose there is a multiverse, the problems of fine-tuning are solved. This isn’t the full story on two counts – the multiverse isn’t a mere possibility, and it doesn’t automatically solve the fine-tuning problem. (more…)

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I’ve given my talk on the Fine-Tuning of the Universe for Intelligent Life at the UCSC Summer School on Philosophy of Cosmology. The talk is already up on YouTube – see below. The quality isn’t great, but put some headphones on, play with the bass and treble and enjoy.

I’ve given versions of that talk plenty of times, but never with so many of the people whose work I’m discussing in the audience. The questions are always the best part, and this talk was no different.

The other talks I’ve seen have been very good. Fred Adams was engaging and wide-ranging, and Sean Carroll was his usual combination of clarity and insight. Check them out here.

Edit [11/7/2013]: The link to my slides is broken, so while I try to get that fixed, I’ve uploaded the slides on SpeakerDeck here. (WordPress doesn’t seem to allow me to embed the slides in this post.)

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It’s always useful to know a statistics junkie or two. Brendon is our resident Bayesian. Another colleague of mine from Zurich, Ewan Cameron, has recently started Another Astrostatistics Blog. It’s well worth a look.

I’m not a statistics expert, but I’ve had this rant in mind for a while. I’m currently at the “Feeding, Feedback, and Fireworks” conference on Hamilton Island (thanks Astropixie!). There has been some discussion of the problem of reification. In particular, Ray Norris warned that, once a phenomenon is named, we have put it in a box and it is difficult to think outside that box. For example, what was discovered in 1998 was the acceleration of the expansion of the universe. We often call it the discovery of dark energy, but this is perhaps a premature leap from observation to explanation – the acceleration could be being caused by something other than some exotic new form of matter.

There is a broader message here, which I’ll motivate with this very interesting passage from Alfred North Whitehead’s book “Science and the Modern World” (1925):

In a sense, Plato and Pythagoras stand nearer to modern physical science than does Aristotle. The former two were mathematicians, whereas Aristotle was the son of a doctor, though of course he was not thereby ignorant of mathematics. The practical counsel to be derived from Pythagoras is to measure, and thus to express quality in terms of numerically determined quantity. But the biological sciences, then and till our own time, has been overwhelmingly classificatory. Accordingly, Aristotle by his Logic throws the emphasis on classification. The popularity of Aristotelian Logic retarded the advance of physical science throughout the Middle Ages. If only the schoolmen had measured instead of classifying, how much they might have learnt!

… Classification is necessary. But unless you can progress from classification to mathematics, your reasoning will not take you very far.

A similar idea is championed by the biologist and palaeontologist Stephen Jay Gould in the essay “Why We Should Not Name Human Races – A Biological View”, which can be found in his book “Ever Since Darwin” (highly recommended). Gould first makes the point that “species” is a good classification in the animal kingdom. It represents a clear division in nature: same species = able to breed fertile offspring. However, the temptation to further divide into subspecies – or races, when the species is humans – should be resisted, since it involves classification where we should be measuring. Species have a (mostly) continuous geographic variability, and so Gould asks:

Shall we artificially partition such a dynamic and continuous pattern into distinct units with formal names? Would it not be better to map this variation objectively without imposing upon it the subjective criteria for formal subdivision that any taxonomist must use in naming subspecies?

Gould gives the example of the English sparrow, introduced to North America in the 1850s. The plot below shows the distribution of the size of male sparrows – dark regions show larger sparrows. Gould notes:

The strong relationship between large size and cold winter climates is obvious. But would we have seen it so clearly if variation had been expressed instead by a set of formal Latin names artificially dividing the continuum?


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There will be a Solar Eclipse early on the morning of Wednesday 14th November  (less than 1 week away). The path of totality will begin at sunrise in the north of Australia in Arnhem Land and cross Cape York to near Cairns before moving out into the Pacific. For the rest of Australia outside the path of totality the eclipse will be seen as a partial eclipse of the Sun in the early morning or at sunrise. In Sydney the eclipse begins at 7.07am and ends at 9.04am. Mid-eclipse is at 8.03am when 67% of the Sun’s disk will be covered. At that time the Sun will be 27 degrees above the eastern horizon. More details here.

(Don’t stare at the sun, kids! Suitable ‘eclipse glasses’ are available locally from reputable astronomy shops and the Sydney Observatory.)

One of my favourite pieces of science writing is called “Total Eclipse” by Annie Dillard, from her book Teaching a Stone to Talk: Expeditions and Encounters. Here are a few highlights.

Now the sky to the west deepened to indigo, a color never seen. A dark sky usually loses color. This was a saturated, deep indigo, up in the air. Stuck up into that unworldly sky was the cone of Mount Adams, and the alpenglow was upon it. The alpenglow is that red light of sunset which holds out on snowy mountain tops long after the valleys and tablelands are dimmed. “Look at Mount Adams,” I said, and that was the last sane moment I remember.

I turned back to the sun. It was going. The sun was going, and the world was wrong. The grasses were wrong; they were platinum. Their every detail of stem, head, and blade shone lightless and artificially distinct as an art photographer’s platinum print. This color has never been seen on earth. The hues were metallic; their finish was matte. The hillside was a nineteenth-century tinted photograph from which the tints had faded. …

I saw, early in the morning, the sun diminish against a backdrop of sky. I saw a circular piece of that sky appear, suddenly detached, blackened, and backlighted; from nowhere it came and overlapped the sun. It did not look like the moon. It was enormous and black If I had not read that it was the moon, I could have seen the sight a hundred times and never thought of the moon once. (If, however, I had not read that it was the moon – if, like most of the world’s people throughout time, I had simply glanced up and seen this thing – then I doubtless would not have speculated much, but would have, like Emperor Louis of Bavaria in 840, simply died of fright on the spot.) It did not look like a dragon, although it looked more like a dragon than the moon. It looked like a lens cover, or the lid of a pot. It materialized out of thin air – black, and flat, and sliding, outlined in flame. …

The second before the sun went out we saw a wall of dark shadow come speeding at us. We no sooner saw it than it was upon us, like thunder. It roared up the valley. It slammed our hill and knocked us out. It was the monstrous swift shadow cone of the moon. I have since read that this wave of shadow moves 1,800 miles an hour. Language can give no sense of this sort of speed – 1,800 miles an hour. It was 195 miles wide. No end was in sight – you saw only the edge. It rolled at you across the land at 1,800 miles an hour, hauling darkness like plague behind it. Seeing it, and knowing it was coming straight for you, was like feeling a slug of anesthetic shoot up your arm. If you think very fast, you may have time to think, “Soon it will hit my brain.” You can feel the deadness race up your arm; you can feel the appalling, inhuman speed of your own blood. We saw the wall of shadow coming, and screamed before it hit.

This was the universe about which we have read so much and never before felt: the universe as a clockwork of loose spheres flung at stupefying, unauthorized speeds. How could anything moving so fast not crash, not veer from its orbit amok like a car out of control on a turn? … (more…)

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Let’s begin by quoting from Radford Neal:

There is a large literature on the Anthropic Principle, much of it too confused to address.

I’ve previously quoted John Leslie:

The ways in which ‘anthropic’ reasoning can be misunderstood form a long and dreary list.

My goal in this post is to go back to the original sources to try to understand the anthropic principle.

Carter’s WAP

Let’s start with the definitions given by Brandon Carter in the original anthropic principle paper:

Weak Anthropic Principle (WAP): We must be prepared to take account of the fact that our location in the universe is necessarily privileged to the extent of being compatible with our existence as observers.

Carter’s illustration of WAP is the key to understanding what he means. Carter considers the following coincidence: (more…)

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The short version: read this book!

I’ve read quite a few astronomy books in my time, and this is one of the best. The problem with a lot of these books is that, once you’ve read one or two, they start covering the same ground. A novel example or illustration is nice, but you can read Fred Hoyle’s The Nature of the Universe from 1960 (review soon!) and get most of what we know about the lives of stars and the layout of the solar system. The most media-friendly breakthroughs have come in cosmology, which has gained more than its fair share of popular level books on dark energy, dark matter, multiverses and the like.

However, many of the major discoveries of the last few decades have been in fields like high-energy astrophysics, hypervelocity stars, supernovae, black holes, magnetars and the like. Bryan Gaensler gives an outstanding overview of these extreme objects.

A good example is his description of what it would be like to be inside a giant molecular cloud [pg 25]:

“Let’s imagine that one for these [molecular] clouds drifted through our part of the Milky Way, enveloping the Earth, Sun and the rest of the solar system. In the direction from which the cloud approached, there would be a growing inky dark patch, eventually blotting out all the starlight from half the sky. But looking in the other direction, out to free space, we wouldn’t notice any difference at all at first. The stars in that direction would seem just as bright as always.

After about 2000 years (by which point we would have penetrated around 20% of the way into the centre of the cloud), the half of the sky towards the cloud would remain totally black, but now the other half two would have started to fade. Over the centuries, the light from the various stars and constellations would have dimmed by about a factor of six – only about 150 stars would still be bright enough to be visible to the naked eye.

Wait another 2000 years, and the remaining half of the night sky would fade by a factor of 20, leaving only ten stars that we could see unaided. And if 2000 years passed once more (a total of 6000 years since our encounter with the cloud began), there would be no stars left at all visible with the unaided eye.”

This puts me in mind of a quote from Ralph Waldo Emerson: (more…)

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“Leave only three wasps alive in the whole of Europe and the air of Europe will still be more crowded with wasps than space is with stars, at any rate in those parts of the universe with which we are acquainted.”

I love a good illustration.

For whatever reason, I’m drawn to old popular-level science books. I just finished reading “The Stars in Their Courses” by James Jeans, first published in 1931. Jeans is best known in my field for the “Jeans length”. Suppose a cloud of gas is trying to collapse under its own gravity, but is being held back by gas pressure. Jeans showed that there is a critical length scale, such that if the object is smaller than the Jeans length then pressure wins and the cloud is stable, but if it is larger then gravity wins and collapse ensues.

Jeans gives an overview of all of the astronomy of his day. It’s mostly familiar material, of course; the interesting bit is the glimpse inside the mind of the great scientist. Here’s a neat illustration:

“If we could take an ordinary shilling out of our pocket, and heat it up to the temperature of the sun’s centre [40 million kelvin], its heat would shrivel up every living thing within thousands of miles of it.”

Repeating this calculation, I think Jeans is reasoning as follows. A shilling is about 5 grams of copper (specific heat capacity 0.385 J/gram/kelvin), and so at 40,000,000 K we have about 8 \times 10^7 J of energy. This is ‘only’ 20 kg of TNT – most bombs are at least a tonne of TNT equivalent, and they don’t do miles of damage. That much energy could raise the temperature of the surrounding air to boiling point for about a 10 metre radius. Not too promising. However, the coin will be emitting thermal radiation at x-ray wavelengths. A lethal dose of x-rays is about 5 J/kg, so our coin has enough energy to kill about 100,000 people. One must factor in the fraction of energy emitted horizontally, the fraction absorbed by biological material, the cooling of the coin, etc, but certainly it’s a very dangerous coin.

Jeans’ views on cosmology are very revealing. He is writing within 5 years of the discovery of the expansion of the universe by Lemaitre (first!) and Hubble. Jeans says: (more…)

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I’m a great fan a popular science books, particularly when the topic is cosmology or fundamental physics. Susskind’s “The Cosmic Landscape” was particularly enjoyable, though I will take issue with a few things in later posts. For now, here are a few highlights:

I love a good illustration:

A rocket-propelled lemon moving away from you might have the color of an orange or even a tomato if it were going fast enough. While its moving toward you, you might mistake it for a lime.

This is simply the Doppler effect, which we’ve all observed for sound as an ambulance drives past. It works for light as well, but you have to be going close to the speed of light. Using the right formula from Einstein’s special relativity, we find that you must fire a lemon at a tenth of the speed of light to make it look red. About the same speed, but moving toward you, will make it look green.

Susskind gives an excellent account of the fine-tuning of the universe for intelligent life.

[T]he Laws of Physics may not only be variable but are almost always deadly. In a sense the laws of nature are like East Coast weather: tremendously variable, almost always awful, but on rare occasions, perfectly lovely. … One theme has threaded its way through our long and winding tour from Feynman diagrams to bubbling universes: our own universe is an extraordinary place that appears to be fantastically well designed for our own existence. This specialness is not something that we can attribute to lucky accidents, which is far too unlikely. The apparent coincidences cry out for an explanation.

In particular, he takes the discussion to the cutting edge of particle physics, discussing the gauge hierarchy problem:

Physicists puzzled for some time about why the top-quark is so heavy, but recently we have come to understand that it’s not the top-quark that is abnormal: it’s the up- and down-quarks that are absurdly light. The fact that they are roughly twenty thousand times lighter than particles like the Z-boson and the W-boson is what needs an explanation. The Standard Model has not provided one. Thus, we can ask what the world would be like is the up- and down-quarks were much heavier than they are. Once again – disaster!

… the cosmological constant problem:

Throughout the years many people, including some of the illustrious names in physics, have tried to explain why the cosmological constant is small or zero. The overwhelming consensus is that these attempts have not been successful.

… fine-tuning of cosmic inflation needed to give the universe the right amount of lumpiness:

A lumpiness of about 10^-5 is essential for life to get a start. But is it easy to arrange this amount of density contrast? The answer is most decidedly no! The various parameters governing the inflating universe must be chosen with great care in order to get the desired result.

… and even supersymmetry:

The biggest threat to life in an exactly supersymmetric universe [has to do] with chemistry. In a supersymmetric universe every fermion has a boson partner with exactly the same mass, and therein lies the trouble. The culprits are the supersymmetric partners of the electron and the photon. These two particles, called the selectron (ugh!) and the photino, conspire to destroy all ordinary atoms. … in a supersymmetric world, an outer electron can emit a photino and turn into a selectron. … That’s a big problem: the selectron, being a boson, is not blocked (by the Pauli exclusion principle) from dropping down to lower energy orbits near the nucleus. … Goodbye to the chemical properties of carbon – and every other molecule needed by life.

Susskind is also clear to distinguish between the landscape of string theory and a multiverse (or megaverse):

The two concepts – Landscape and megaverse [a.k.a. multiverse] – should not be confused. The Landscape is not a real place. Think of it as a list of all the possible designs of hypothetical universes. Each valley represents one such design. … The megaverse, by contrast, is quite real. The pocket universes that fill it are actual existing places, not hypothetical possibilities.

All in all, the Susskind’s book is highly recommended.

Part 2 of my review is here.

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