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The Hot Big Bang Meet George Gamow "Gamow was fantastic in his ideas. He was right, he was wrong. More often wrong than right. Always interesting;… …and when his idea was not wrong it was not only right, it was new.” Some Serious Science Gamow asked (in the 1930s): Where do the heavy elements come from? You know the answer! They are created in massive stars: elements that are less massive than iron form during the stable lifetimes of such stars elements that are more massive than iron form during supernova events But that understanding came later. So Gamow Speculated (before we understood stellar evolution) Where could it be hot and dense enough to cook up the heavy elements? Answer: in the dense early universe – if it had been hot. The Ylem With that motivation, Gamow suggested a hot origin to the universe, in what he called the ‘ylem’ – a ‘cosmic egg’. (The word comes from ancient Latin and Greek words for matter.) Remember that this would have occupied all space, with no ‘outside.’ Do not visualize a hot blob sitting in a vast emptiness! It’s Time to Reconsider the Grenade Analogy We explored the analogy to a grenade to understand the calculation of the time since an explosion. I said then that the analogy was a very poor one. It’s time now to tell you why. One Reason A grenade explodes into pre-existing space that surrounds it. The fragments move through that space. But the universe has no surrounding space into which material moves. New, expanding space appears everywhere ‘between the galaxies’ and carries the material with it. There is not, never was, and never will be any ‘outside’to the universe. The Second Reason… A grenade expands because it gets very hot at the centre (thanks to the rapid chemical reactions of the explosive material in its core). This creates a huge outward pressure, so the grenade fractures and explodes into the surrounding lowerdensity space. By contrast, the universal expansion requires no heat. The expansion simply ‘is’. The Name ‘Big Bang’ is Utterly Misleading Fred Hoyle coined the term ‘Big Bang’ in a derisive way – he didn’t believe in it! But the term caught on, and we still refer to the “Hot Big Bang.“ Unfortunately, it conjurs up misleading images of a bomb exploding (noisily!). It’s Not a Huge ‘Kablooie’ as it is so often depicted (often with sound effects!) So Please Remember… The present expansion of the universe does not necessarily imply that it started hot! Gamow’s idea was imaginative speculation, intended to solve the problem of heavy element formation. On the other hand, we should not rule out the possibility that it might have been hot. So Gamow’s speculation was certainly worth considering. His Idea Thermonuclear reactions would take place in the early moments, perhaps building up all the heavy elements. It Doesn’t Work! The rapid expansion dilutes the gas (the density drops as the particles move farther apart), and the temperature also drops. Any thermonuclear reactions would quickly die down. In fact, it’s fairly easy to show that the reactions would not last long enough to create much in the way of heavy elements So Gamow’s imaginative idea was untenable. But Not a Complete Failure! We now know that Gamow was partly right! Most of the helium and some light elements were indeed formed in the early universe. (We will return to this in a bit.) Still, Imagine the Circumstances… …if the early universe had been extremely hot. There would have been: rapidly-moving particles, in a fantastically dense plasma; and a ‘sea’ of energetic radiation everywhere. But the material would have been so dense that it would be opaque. (Radiation would not get far!) With the Passage of Time, the Temperature Would Fall Think of the temperature as a measure of the highly energetic motion of the particles, or the nature of light present – not in human terms of ‘how hot it would feel!’ These are unimaginably high temperatures. As the Expansion Continues The density drops, the freely-moving electrons get captured by the protons --- and the radiation is now able to flow unimpeded, like when a fog dissipates. The universe becomes ‘transparent.’ Consequently If we look around, we should find vestiges of that radiation just reaching us now, after flowing through space unimpeded for billions of years. Moreover, the Cosmological Principle means that this radiation should be uniform in all directions, coming in from all sides. So why is the sky not bright everywhere? Remember the Redshift The continual‘stretching’ of space means that the light just reaching us now has been redshifted to very long wavelengths (low energies). So-called ‘relic radiation’ actually is now reaching us from billions of light years away, after flowing through space for billions of years! In that sense, then, the sky around us truly is ‘bright everywhere’ – just not at the wavelengths to which the human eye is sensitive! Gamow Actually Predicted This! He noted that we should be in a ‘bath’ of radiation, as though sitting in an oven at a very low temperature. (His estimate: about 10 degrees above absolute zero.) It would have looked much hotter at earlier stages. This Was Untestable! At the time of Gamow’s speculations, here was no technology to search for the background radiation. Moreover, as we came to understand the details of thermonuclear reactions in stars, Gamow was proven wrong about the origin of the heaviest elements For both these reasons, his work was dismissed and forgotten about. A Couple of Decades Later In the 1960s: Dicke (Princeton) thought again about a hot early universe (but not to explain heavy elements). Amazingly, he did not know about Gamow’s earlier work and predictions! He suggested that Peebles and Wilkinson search for the radiation (now feasible) Meet Jim Peebles On A Recent Visit Scooped! Penzias and Wilson, Nobel Laureates The CMBR (Cosmic Microwave Background Radiation) Proof of a hot early universe! Improved Observations Soon Followed And Better Still From Space The ‘dots’ are observations; the line is from theory. This is a Very Low Temperature It’s like we are in an ‘oven’, immersed in a bath of radiant energy at a temperature of just under 3K (very close to Gamow’s prediction). The universe was clearly hot in its earliest phases. Gamow was right to suggest that possibility, even if (in fact) it did not lead to his proposed result of creating all the heavy elements. It’s Not Absolutely Uniform [it would be astonishing if it were!] We see a ‘dipole’ – that is, the radiation from one side of the sky has a spectrum that is a little ‘hotter’ than that from the other side, owing to the Doppler shift and the fact that the Earth has its own ‘peculiar’ (individual) motion through the large-scale distribution of matter. As We Look Out, We Are Looking Back in Time Further Vindication for Gamow: Cosmic Nucleosynthesis We now know where the H and He came from! Consider The Role of Light At one stage in the early universe, light (that is, the enormous density and energy of the radiation) was more important dynamically than matter. Moreover, at one stage, that’s essentially all there was. In a very real sense, the matter we see now ‘crystallized out’ from the enormous radiation field. The early universe was ’radiation dominated’ How Things Change: A Succinct Representation And again: Cosmic Evolution Creation Stories Light, sometimes in the form of a specific Sun god, plays a significant role in creation stories from many cultures and societies. This is of course understandable, given the importance of light (especially that of the Sun) for our existence on Earth. (You can explore many of these on the web; there are both commonalities and striking differences over many representations.) In the Beginning… Haydn’s “Creation” Oratorio http://www.physics.queensu.ca/~hanes/A102-Movies/CreationMusic.mp4 https://www.youtube.com/watch?v=l07oRR4u-rk - scroll to the 9m 30s mark