So here we are today with quite an amazing set of observations and a theory that all seems to more or less hang together. We have a universe that's 13.8 billion years old. We have a universe which is 5% of atoms or barrioms and 70% dark energy, 25% dark matter. And when we use this theory to predict things, we're able to predict in advance every observation we've been able to make so far. So from my perspective, this is truly one of the great triumphs of science in living memory. But from my point of view, whenever I hear something like a great triumph of science and high precision, that sets all the alarm bells ringing in my mind because the human race has been there on cosmology many times dating back to the ancient Egyptians. We've thought we've got it all sussed and we've been wrong over and over and over again. So let's explore a bit how solid this really is. I mean even five years ago wouldn't have such a good consensus. It's very very recent. It hasn't yet had time to survive lots and lots of tests. So how solid is this? I mean if you're having a bad nightmare, what do you think could go wrong with this whole thing? Well I think the big worry is that ultimately scientists are people and people are a herd animal and we tend to follow the leader a little bit too much. And we've had problems in the past where for example if you look at the best value for the speed of light, it converged to the wrong answer and then it switched when people realize there was a mistake. And so there is some worry I guess that maybe we all know what the answer is and we all get the same answer that we're not being quite careful enough as we as we should because we are as I said we are human. So if there's any problem I think that's the one that worries me the most because we do have a consensus of quite disparate methods that are coming to this rather crazy conclusion that 95% of the universe is stuff we don't really see here on earth directly and we have a very good measurement of the age of the universe and all that seems to hang together. So as I said it's more one of whether or not we trust ourselves and whether or not we're being honest with ourselves about the measurements. But each of one of these individual measurements typically doesn't give you an answer to the cosmological parameters gives you some degenerate band and you need multiple measures to cross over and mark the spot. Do we have enough independent measurements because if you get just two things crossing at a spot, any two measurements will do that. It's like we were always told when doing cartography you never just do two lines converging you do three so you get a sense of uncertainty. Do you have enough situations where there are three or four different independent measurements all agree to give us some confidence that really have got a right answer. It's not just a coincidence of two things fitting together. Yeah so if we look at the for example the that whole paradigm having you know dark matter, dark energy and a sort of a 70 30 split with 5% barions to my mind there's really four or five lines of evidence that go together. We have the supernovae that march out acceleration and they give a confidence interval which turns out you can sort of tell that it just doesn't fit a universe without a cosmological constant but maybe they're wrong. Okay? Then you have the large scale structure of galaxies out there and there's two different measurements you can make for those. The old style measurement we just look at how there's spread across the sky and that allows you to very accurately measure the matter density of the universe and that gives you a constraint. The cosmic micro and then there's the barion acoustic oscillations where we see that ruler of splash marks left over from the cosmic microwave background and big bang and that allows you to essentially reproduce the supernova measurements as a ruler moving out to the cosmic microwave background. Now quite remarkably the supernovae and that ruler do the barion acoustic oscillations really agree very accurately I mean as accurately as they should. And it's hard to I mean supernovae could be thrown up if you had some sort of gray dust or something in the universe but that wouldn't affect the ruler measurements. Yeah the ruler length is opposed to brightnesses. That's right. So that concordance is pretty strong and I personally think you almost have to have someone cheating to make that go away. Finally have the cosmic microwave background which sort of works with everything else to do a new set of constraints which is ultimately that the universe is flat and we're beginning to squeeze some more information out of that. All of these different constraints combine into one magic spot which is the model we have. So you really need to have a bunch of them or two of you know more than two of them be wrong at this point and it's possible but they're gonna have to be wrong in a way that conspires and to my mind that would almost have to be human conspiracy a frailty of humans deluding ourselves that we don't really know something as well as we do. And I should say there's other things that fit in we measure the Hubble constant as well and that more or less fits in we go through and we measure things like the age of the oldest stars and that's about the right answer as well. So is this at it all fits together every way we can look at it it it seems too good to go away at this point. Now in some sense this means you've got a lovely mathematical model. We've got a universe flattened out by inflation and dominated by dark matter and dark energy. But isn't this just kicking the can down the road because we don't know what the field was at cause inflation we don't know what dark matter is we don't know what dark energy is. So we've learned as there's something unknown that has a particular equation of state something else unknown that has a different equation of state and something else unknown that must have a Mexican hat with just the right brim shape. So you've just pushed one problem into another one haven't we? Yeah we have and this is a particularly how I say a striking example how we do that but we do that all the time in science. When you talk about how a cell works well cell was made up of atoms and you know we break it down into a cell well what's a cell well we know what a cell looks like but it's not a fundamental description of how human life works nothing is and indeed I would argue there's probably not a fundamental description of anything that we do in science. So yes we have dark matter and we're not quite sure what it is we have dark energy we don't quite know what it is we have something that caused inflation we really don't know what that is but we have those basic ideas in the same way we say that cells are made up of these things that behave in a certain way we have combined things and essentially made a structure which we give an aim to that has a certain level of behavior and so we can test that model which yes has a lot of missing information and we can test it against predictions and the quite remarkable thing is think of what that cosmic microwave background looks like it has 17 little bumps that are exactly where they need to be I mean that's like going through and saying I have a model for Beethoven's brain and you say oh I don't think so I said well my prediction is you know on this day in 1793 he would have done dumb bum bum and if you know that happened you would say oh jeez you're pretty good actually and so that's sort of where we're at I promise you we haven't got everything perfectly right but I think the big concept is right because it's just too good at predicting things and so it may well be a hundred years from now we don't call it dark matter we don't talk about dark energy and it has to behave in this way but it has to behave in that way and so we haven't thrown away Newton's laws of gravity now that we have Einsteinian gravity we know they're not really correct we have a different way of thinking about it but they're still relevant so I think what we've talked about is still going to be relevant now and quite frankly into the distant future