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Thursday, 25 June 2009


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The universe, in the instant after the Big Bang, was an undifferentiated plasma of quarks and gluons. As it expanded, it cooled, to the point where quark triplets could be bound to form nucleons (protons and neutrons, in roughly equal numbers). Free neutrons are unstable, with a half life of the order of ten minutes. In the time it took for the plasma to cool to the point where neutrons and protons could combine to form nuclei, enough of the neutrons had decayed (into a proton, an electron and an electron antineutrino, thus leaving an excess of protons or hydrogen nuclei) that the primordial elemental abundances were 75% hydrogen, 25% helium, traces of deuterium, even tinier traces of lithium, and essentially nothing else. All other elements from beryllium on up were synthesised in stars, with those beyond the atomic mass of iron (56) being created during supernova explosions. The plasma was still very hot, hence ionised, which strongly scatters photons. Hence it was opaque. It cooled further until about 300,000 years after the Big Bang it was cool enough for electrons to be bound to nuclei to form neutral atoms. At this point the matter in the universe decoupled from the electromagnetic radiation and the universe became transparent. The relict radiation became what is now the Cosmic Microwave Background.

Gravity in the primordial universe was initially unified with the other three forces (electromagnetic, strong nuclear, weak nuclear). Almost immediately, this symmetry was broken as the plasma cooled. The next symmetry to break was the grand unification symmetry when the strong force broke off from the electroweak force. Some very weird quantum mechanical effects then appear to have acted as a strongly repulsive force which almost instantaneously inflated the universe by a factor of about 10^26. Anisotropies in the pre-inflation universe were smeared out almost to nothing, which is why the curvature of the universe is so startlingly close to zero. The minute anisotropies that remained were sufficient to drive galaxy and eventually star formation.

No-one knows how gravity works, although the best candidate is that, analogously with the other forces, it is mediated by a vector boson, in this case a neutral, massless spin-2 particle called a graviton. Gravitons are astonishingly weakly coupled to normal matter which is why gravity is such a weak force compared to the others.

Thanks a million, David, my first headache had just subsided and now you've started a second!

More seriously, thank you for that and on behalf of my sad, old memory I apologise for my incorrect remembrance of the hydrogen/helium ratios shortly after the Big Bang. I am about to totter off to bed so I will leave a close scrutiny of your description until tomorrow, however, it would be helpful if you could explain what "anisotropies" are in as near to layman's language as you can get.

Also, what do you think of the 'Monster Question' as indicated by Dyson's quote?

An anisotropy is the opposite of an isotropy. A system is said to be isotropic if, from a given point, it looks the same in all directions. Closely allied is homogeneous, where the system looks the same at all points. A key assumption of the evolution of the early universe is that it was isotropic and homogeneous - and to a degree that remained inexplicable until the idea of inflation. It's worth remarking just how close to uniformity the early universe was. A deviation by parts per trillion would yield a situation that looks very different from what we see today. Over the 13.7 billion years since the Big Bang, the remnant irregularities have been sufficient to build galaxies and clusters of galaxies, but a little too much irregularity would have left us with nothing more than a universe filled with black holes. In a similar vein, the rate of expansion of the universe is astoundingly close to the critical value. Almost imperceptibly slower, and the universe would have collapsed in on itself again; just a fraction faster, and it would have expanded too fast for galaxy formation.

I'm agnostic about all teleological statements such as Dyson's. What exactly he is saying is a bit opaque. If he is saying that the Universe is constructed in such a way that life can eventually arise, then that strikes me as nothing more than the weak form of the Anthropic Principle, which is more than a little tautological. If he is remarking the contingency of having a universe conducive to life, then I think he's on shakier ground. In a one-off event (and the Universe is the ultimate one-off event), there is no meaningful way of ascribing a probability of how 'likely' it was. If you find a single raffle ticket on the ground with the number 613 on it, what is the likelihood it is the winning ticket? There is no way of making this calculation. So I don't think a universe where the laws 'tend' towards the existence of life can be distinguished from one in which the laws are not incompatible with life.

A couple of good books in this area are Steven Weinberg's The First Three Minutes and at a more technical level Igor Novikov's The Evolution of the Universe.

David, thank you very, very much for all of that especially as I suspect you have better things to do than teach me Cosmology 101. Even so, I am going to be tiresome and ask you some more - but if I become too irritating just swat me like a fly!

I understand the effect produced by anisotropes or isotropes but are they actually 'things' (particles, forces?) that produce these effects or are the words simply descriptions of a system. (I ask because you talk of the anisotropies being virtually wiped out.)

I have read elsewhere (and here I paraphrase in my non-scientific language) that the very, very slight imperfections in the initial 'explosion' were critical to the eventual formation of stars and galaxies. Also, that there are several key measurements which, had any of them altered by a tad one way or the other the universal outcome would have been very different from what we have now. I know that has led some people to suspect 'the hidden hand' of a highly 'intelligent designer' but like you, I simply add it to the list of mysteries and enjoy the puzzle of it all.

I suspect Dyson, who is a very reputable scientist, was probably wondering aloud when he made that remark rather than making a definite pronouncement. Even so, I am taken by your summary:

"I don't think a universe where the laws 'tend' towards the existence of life can be distinguished from one in which the laws are not incompatible with life.

This goes to the heart of the matter because if life has only occurred in our galaxy then that is the equivalent of finding the ticket 613 which turns out indeed to be the winner. One can put no significance to it because it is simply the dead opposite of that piece of American fatalism "Shit happens! (Of course, I assume that life is "A Good Thing" whilst recognising that certain latter-day devils of the Green Movement think it is an appalling thing.) If, however, life has appeared all over the universe then that presents a rather different view. It certainly does not prove 'intent' but it does disprove fluke. Like all the good detectives in the disgraceful 'pulp fiction' I devour by the ton, I distrust co-incidence. One rank outsider winning at fantastic odds I can accept, several and I begin to suspect the game is fixed!

Finally, can I say how very much I envy you! To be able to understand and grapple with these colossal ideas must be exhilerating. I am just so grateful to authors like Davies who can explain some of these concepts to people like me. I have heard of Weinberg's book and only the fear of the little 'Memsahib's' wrath has stopped me from buying it but now - what the hell, I'll just blame you!

An anisotropy is simply a deviation from uniformity in a given direction (and an inhomogeneity is likewise a deviation from uniformity in a given region). The deviation is all that is needed to seed the coalescence of proto-galaxies etc.. If a region of space has a locally-enhanced density of matter compared to its surroundings (for that is what an anisotropy or inhomogeneity means in practice), then provided the enhancement extends over a sufficiently large radius, the excess gravity produced by the extra mass will overcome the repulsive effects of the pressure inside the region and will lead to a contraction that will proceed until equilibrium is re-established. From a somewhat technical standpoint, the scale of a local density enhancement sufficient to cause a gravitational collapse into a galaxy or star-forming region is closely related to the speed of sound in the medium. It was only when the universe cooled sufficiently to allow electrons to bind to nuclei (the recombination epoch) that the radius of perturbations capable of undergoing gravitational collapse became small enough to be contained in the observable universe. The radius is known as the Jeans length, after Sir James Jeans who first derived it. This is all beautifully and lucidly explained in Novikov's book. A naive treatment of density anomalies in the proto-universe points to their being much greater than observed, hence driving the explanation of their having been smeared out to a tremendous degree during the brief inflationary era. If you imagine blowing up a balloon from the size of a grapefruit to several billion light years across, you will see that any surface imperfections will become imperceptibly tiny. It's a testament to our engineers and scientists that we can detect the echoes of these tiny imperfections in the cosmic microwave background, a feat which got George Smoot a well-deserved Nobel Prize.

From the standpoint of one-off events, we at present have exactly one datum concerning the existence of life in the universe: us. We have no means (at present) of determining whether a universe such as the one we see (with, as you note, a myriad of physical parameters, such as the fine structure constant or Planck's constant, that have to be very close to their measured values for us to exist) is the only one compatible with physics, or whether it was a matter of luck. But our existence is a second-order contingency. It is perfectly possible to conceive of a universe in which life could exist, but does not. Further, the discovery of life outside our solar system would change things utterly. If you find two raffle tickets on the ground, then you are able to make a (weak) determination of the probability of having a winner (technically, the probability space becomes sigma-additive at this point). If we are alone in the universe, then it is impossible a priori to assign a probability to our existence. If we are not, then it is possible. Even if the universe is teeming with life, though, it does not prove any teleological effect. It simply lowers the threshold of likelihood for life emerging given the universe we observe.

To summarise: at the present time, we know that if the universe were constituted slightly differently, then our existence would be impossible, but we do not have enough knowledge to determine if such a universe is possible. We know that life can exist, but in the absence of the discovery of life elsewhere in the cosmos, we have no means, even in principle, of deciding how likely it is.

It's kind of fun re-visiting these ideas. I've been out of academia for a long time, but studying astrophysics and cosmology definitely equips one with a powerful lens through which to view the world.

"the scale of a local density enhancement sufficient to cause a gravitational collapse into a galaxy or star-forming region is closely related to the speed of sound in the medium. [My emphasis]

That's extraordinary! Of all the possible critical factors that is not one I would have thought of. (Mischievously, I am tempted to recall the old philosophical question concerning the tree that crashes to the ground in an empty forest and whether or not it makes any noise?!)

"It is perfectly possible to conceive of a universe in which life could exist, but does not.

Well, here I think we return to Dyson's rumination to the effect (and I paraphrase him disgracefully) that somehow, the more we discover concerning the opening moves of the universal gambit, the more it 'seems' to be awaiting life.

I find the whole subject intensely exciting, and again, I am so grateful for people like you and the writers we have discussed, who make it almost(!) possible for people like me to grasp some of it.

In a future post I intend to express my incredulity over the 'universe' that exists at the other end of the scale from the cosmos, the microscopically tiny living cell.

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