SCIENCE AT THE SHINE DOME canberra 6 - 8 may 2009
Symposium: Evolution of the universe, the planets, life and thought
Friday, 8 May 2009
Professor Matthew Colless FAA
Anglo Australian Observatory, Sydney
Matthew Colless is an astronomer who works on observational cosmology, studying the large-scale structure of the universe in order to understand its formation and evolution. He is director of the Anglo-Australian Observatory and adjunct professor of physics at the University of Sydney. After completing undergraduate studies at Sydney, he obtained a PhD at the University of Cambridge and held research positions at the US National Optical Astronomy Observatories, the Universities of Durham and Cambridge and the Australian National University, before taking up his current role. He is a Fellow of the Australian Academy of Science and chair of the National Committee for Astronomy. He is involved in organising the International Year of Astronomy in Australia.
Evolution of structure in the universe
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Thank you. We have had a wonderful introduction to cosmology from Mike Turner and a fascinating and speculative discussion of the laws of nature from Tamara [Davis]. Before Tamara, as a young woman, takes over from us older chaps, I will sneak in before the end of the session to give a few words on the evolution of structure in the universe.
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Whenever astronomers look, they find structure at all scales throughout the universe.
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Wherever we look for matter, there we find structure.
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We have structure everywhere in astronomy. We have structure on the scale of planets –
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Solar systems –
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Star clouds –
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Galaxies –
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And ultimately here is the Hubble Ultra Deep Field, which is the even deeper version of the field that Mike [Turner] showed you in his talk. For the largest scale structures, this evolution is sufficiently simple that we can actually understand it in great detail, but fortunately it is also sufficiently complex that it encodes a wealth of information about the universe.
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Mike has already shown you a version of this picture, which is the Wilkinson Microwave Anisotropy Probe image of the fluctuations in the microwave background. The time at which this is taking a slice through the universe is 380,000 years after the big bang and, as Mike mentioned, the density contrast is one part in 100,000. These are tiny little fluctuations in the universe.
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We can now make incredibly precise measurements of those fluctuations. Mike has shown you this lovely plot. The dots there are, indeed, measurements of the microwave anisotropy – variations in the size of the fluctuations as a function of scale. This is scale along the x-axis: large wave numbers correspond to small scales, small wave numbers correspond to large scales. And the y-axis gives the size of the fluctuation, effectively. We have this very complex curve, which in fact is a superb fit to the data that we have. It is a wonderful piece of physics and it means that we can do very precise theoretical fitting of our models to the data. So we get many, many fundamental cosmological parameters out of fitting this data.
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Here is a long list of all the parameters that one can get simply by looking at the microwave background and fitting our detailed physical model to that information.
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In fact, we can take a simulated universe and evolve it in our computer to find out how this large-scale structure, seeded by the big bang, evident to us in the microwave background, can be amplified by gravity to form all the structures that we see from the cosmic microwave background (CMB) all the way down to the present day. So we have here a ‘universe in a box’.
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The result that we get from this ‘universe in a box’ is a cosmic web with a complex structure, a very rich structure, that is forming a bottomup hierarchy. That simply means that you form small dense things first and then, gradually, you build those up into larger and larger structures; at the same time, you are collapsing the dimensionality of those structures. You start with large volumes, which collapse down to sheets, which then thin down to filaments, and the filaments feed matter along their lengths towards the nodes. So gradually one is decreasing the dimensionality of the structure with time. So by the present day, about 13.7 billion years after the big bang, the density contrast ranges all the way down to zero but typically it is of order unity, and ranges up to factors of 1,000. So one starts in the big bang with something that has very little density contrast and, as time evolves, you can see sheets, filaments, nodes and all structures forming into this enormously complex cosmic web.
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This structure is different in different universes. If I feed in hypothetically different physical constants – not the genuinely different physical constants that Tamara was talking about, but just imagine different universes – I get different types of large-scale structure. (I am talking about structure on very large scales – scales of millions to billions of light years across.) The sort of structure depends, in detail, on the various constituents of the universe and their relative amounts: the amount of ordinary matter, the amount of neutrinos, the amount of dark matter, the amount of radiation and, of course, the amount of dark energy. It also depends – because we are looking not at matter directly but at galaxies – on how those galaxies are formed. So the recipe for how galaxies form and how the ordinary matter in galaxies is related to the dark matter is also an important point. If we take these different models and we mix different amounts of dark matter in different recipes for galaxy formation, our universe in a box has a very different structural appearance when we look at the density of galaxies on the sky.
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In other words, the structure that we see encodes the physical cosmology behind it. So here are several different simulations of what you might see in a redshift survey of galaxies. If you try to map the positions of galaxies over a large volume of the universe, what do you actually see? If you have different cosmologies and different galaxy formation scenarios, you can end up with what are visibly very different patterns on the sky. In other words, one can simply look at the patterns in cosmology, compare them to the observed universe and decide which of these is, in fact, our universe. Of course, the answer, as we can all see, is the simulation labelled CDM bias #2.
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Rather than do that in a sort of hand-wavy way, it was thought that we now had the technology and the ability to go out and do this for real. So we actually combined two of the telescopes at the Anglo-Australian Observatory here, in north western New South Wales – the UK Schmidt telescope, which took a photographic atlas of the entire southern sky; and the Anglo-Australian four-metre telescope, which is shown in cut-away there, which did the follow-up spectroscopy – to get the redshifts for all our galaxies.
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We used the 2-degree Field (2dF) spectrograph, which is a wonderful Australian invention; it is technology combining robotics and optical fibres, which allows us to obtain spectra for 400 galaxies simultaneously over an area 16 times the size of the full moon.
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With that, we carried out a huge galaxy survey in two slices through the sky. So each of these cones is a slice through the sky. You can think of it as a radar scan through a patch or a strip of sky. In the end, we had 221,000 galaxies mapped by the survey, and we are able to do that comparison directly.
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When we do the comparison in detail, we get the following results. What you get is an accurate measure of the relative amounts of different types of matter in the universe. So we can see that the total amount of matter in stars and ordinary matter is around half a per cent; the amount of neutrinos is perhaps the same or a little less; the amount of free hydrogen and helium is around four per cent; and the cold dark matter – this mysterious unknown particle that Mike was talking about – is around 25 per cent of the total.
One does not get the dark energy directly from this, but one can get it by combining this information either with mapping of the supernovae distances – the work that Brian Schmidt, Saul Perlmutter and others have done – or by looking at the microwave background, which gives you the measures of flatness and geometry in the early universe. In this way, one infers that 70 per cent of the total energy content of the universe is this mysterious dark energy.
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One other thing one can get is a self check. We have assumed that all of this structure has been formed by the gravitational amplification of quantum fluctuations emerging from the big bang and calibrated for us in the CMB. In fact, we can check that that is the case by looking at the sorts of structures we get. We get a particular prediction for these galaxy-galaxy correlation functions – for the sorts of structures that we get, on average, in the universe. It has this rather interesting sort of onion shape, with little pointy bits and rounded bits and so on, and it is quite distinctive; indeed, that is precisely what we see. In other words, we can confirm, in a self consistency check, that the structures that we are looking at are indeed the result of gravitational amplification of those quantum fluctuations.
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So, what is dark energy? Mike has had a go at it; let me have a brief run through it myself. What we have is a diverse set of reasonably robust and relatively precise observations which imply a universe that is spatially flat, has a low density of matter – at least, lower than the critical density of matter – and has an accelerating expansion. These observations are all consistent, in a simple minded way, with an Einstein type universe with a cosmological constant with about 25 per cent of the total energy in (mostly dark) matter and the remainder in dark energy.
The existence of dark energy necessarily implies that there is some new physics. This is one of the key questions for our new century, as Mike’s own US National Academies have reported. Einstein’s cosmological constant works in a phenomenological sense, but it is very difficult to explain theoretically. Mike gave you the kind version, where it is only 55 orders of magnitude out – if you are not so kind, you can in fact argue that it is 122 orders of magnitude out.
There are many alternative theories of dark energy or modified general relativity which might allow you to explain this, but they are extraordinarily hard to distinguish observationally. However, we do have a number of ways that we can do that. I do not have the time to tell you about all of them, but I will tell you about one of them. One of them is to use a feature of the microwave background – indeed, of structure formation in general – as a cosmic standard ruler.
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When you look at this microwave background picture, you notice that there are ‘measles’ of a particular scale. The little dots have a preferred scale. This preferred scale is this sort of size of dot here [pointing to circled dot on slide], which corresponds to the largest peak in this power spectrum of the fluctuations, so you are saying that fluctuations on that scale are stronger; they are stronger because there is a particular effect, an acoustic effect, a balance between the gravity of the matter and the radiation pressure of the photons in the early universe, that gets frozen out when the big bang cools and gives you a preferred scale, which is about 480 million light years across. That is the preferred scale, which we have measured very accurately – to about one per cent precision. In fact, we will measure that more accurately as we get better measurements of the microwave background.
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That scale is absolute, and it can be measured not just from the microwave background but also from the galaxy distribution at later times because it is an enhancement of the number of galaxies that are separated by that preferred scale – a very tiny enhancement, not so strong as in the microwave background. If you look on these scales of about 480 million light years, you find that there is a tiny enhancement in the number of galaxies with that pair separation.
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That means that we can measure the acoustic scale at various epochs, using the CMB very near to the big bang, and then at subsequent epochs using galaxy surveys at different times. That gives us a way of measuring the geometry of the universe, because we have a standard ruler and we can see how big the physical size appears to us at different times.
So the crucial thing about this is that what sets the scale of that ruler is the dominant dark energy. That, in turn, reveals the equation of state parameter for the dark energy. The equation of state parameter is simply the ratio of the pressure it exerts to its density. That is a vital clue to its nature. If we know what that is, then we have a good idea of where to look for the dark energy. For example, Einstein’s cosmological constant would predict that this equation of state parameter is precisely minus one and, moreover, that it hasn’t changed throughout the entire history of the universe.
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So we are going to go and try and measure this quantity. We are going to use this acoustic ruler to look at different epochs throughout the history of the universe to find out whether the equation of state is a constant and, if so, what its value might be. A group of us – Australian astronomers – are carrying out a new survey, still using the Anglo-Australian telescope and the 2-degree field machine but with a new and more powerful spectrograph, and we are trying to map over 200,000 galaxies again. The reason we are doing it again is that the first survey we did was at very low redshift. We were looking at the nearby (relatively speaking), universe and now we want to go much further back. We are, in fact, going to look about halfway back to the big bang. We are going to look around the time where the dark energy is just beginning to take over from the dark matter as the dominant component in the universe.
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Here are the redshifts that we are accumulating. These are 600,000 redshifts that we have so far measured with the Anglo-Australian Observatory’s telescopes in an array of different surveys. The bright yellow ones are those that we are measuring from the WiggleZ survey, in that intermediate range between the very nearby stuff (in blues and purples) and the very distant universe.
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The WiggleZ survey is named ‘WiggleZ’ because we are looking for little wiggles in the power spectrum. The forecast for what those wiggles should look like is shown in the top left there. The top right shows you the constraints that we actually get from that on two important parameters: the total matter density [horizontal axis], and the equation of state parameter for the dark energy [vertical axis]. The constraints that we will get from the WiggleZ survey are the yellow region and the combined constraints are from folding in what the CMB and the supernovae results give you. We will be able to measure the total matter density to a precision of about two per cent and the dark energy equation of state parameter to a precision of seven per cent. This will be a very significant new piece of evidence regarding the physical nature of this dominant constituent of the universe.
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That is not all – this is just the first step, this is just a tiny baby step, towards what we hope to be able to do in future. There is a vast array of new future dark energy surveys that will be carried out. Some of those, particularly those towards the bottom of the list here, further in the future and more ambitious, will reduce those measurements from several per cent down to a per cent or so precision in measuring the equation of state parameter both now and at a whole series of times into the past, so that we can see how that particular physical constant may evolve with time.
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I am going to leave it there, but will leave you with the standard cosmological joke: no matter what time you start at or what epoch your cosmologist is in, he is still asking the same question. Thank you very much.
Discussion
Brian Schmidt: Thank you, Matthew. We are looking forward to seeing the answer as it comes out. We have time for a couple of questions. I am scared to think what Mike Dopita might ask, but we will let him ask a question nonetheless.
Question: Matthew, is it possible that dark energy does not exist but merely represents a physical phase transition in the matter of the universe?
Matthew Colless: I think dark energy, or at least the phenomenon we call dark energy, certainly exists. The lines of evidence for that are far too strong to ignore. I completely agree with Mike’s interpretation about the alternative idea of simply having a void centred on us as being even less probable than a mysterious new force of nature. But you are right to the extent that we have already seen one previous accelerated expansion of the universe, the inflationary epoch, which of course in many ways looked very similar. It, too, was a runaway expansion of the universe. Indeed, that epoch obviously came to an end – we were no longer inflating in that intermediate epoch, when dark matter ruled. So it is indeed possible, as one of Mike’s various curves for the future history of the universe pointed out, that in fact this current phase of accelerated expansion may also come to an end with another phase change of some sort – and then all bets are off for what happens next.
Question: This is really a question to all three speakers, but Matthew is in the hot seat. When we talk about the way the universe is, I think you might even now say that, if we did not have the dark energy or we did not have the dark matter, we certainly could not have evolved conditions where life could exist and, hence, we would have no observers. In order that we have an intelligent life that can observe the universe, it has to have all or some of these properties. You are just trying, of course, to make a connection with the Sun this afternoon. How far down this path are you willing to go?
Matthew Colless: A good rule for observers in astronomy is that everything is a selection effect until proved innocent – everything you observe should be considered a possible bias of you, as the observer. Indeed, our existence is clearly our strongest single bias. In a multiverse, an anthropic cosmology type view of the universe is simply that we are only observing this particular universe because this is the one we are in. Whether that counts as an answer to your question, I will leave to you. But I am quite happy to let Tamara and Mike give their alternative answers.
