This episode is supported by The Great Courses Plus. Did all of dark energy just vanish? A team of scientists have analyzed new data, and claim that we need to completely rethink its existence. Back in 1998 two independent teams of astronomers made an extremely controversial announcement: that the universe is not only expanding, but that expansion is accelerating. They discovered this by watching for the explosions of Type 1-A supernovae. These exploding white dwarf stars have predictable brightnesses that allow astronomers to figure out how far away they are. With a set of reliable distances extending back several billion years, the teams were able to map the expansion history of the universe. They expected to see that this expansion rate was slowing down due to the gravitational effect of all of the matter in the universe. Instead, they found the expansion rate has been accelerating for half of the age of the universe. It appeared that something was acting to counter gravity. An outward pressure that has come to be known as "dark energy." The finding led to a shared Nobel Prize for the leaders of the supernova hunter teams, Adam Riess, Brian Schmidt, and Saul Perlmutter. For a deep dive into the mysteries of dark energy, we actually already made this entire playlist on the topic. It's pretty hardcore, so definitely check it out, but maybe after this video. So why are we talking about dark energy again? Because another team has just announced a new analysis of updated supernova data. They claim the data are consistent with there being no dark energy; no accelerating expansion. They suggest the universe may be just expanding at a constant rate: never speeding up, but also not slowing down. This claim is just as controversial as the original discovery of dark energy because dark energy is now the "industry standard." Of course, the media jumped all over this and provided almost no useful info. We thought it would be a good idea to figure out whether we should throw away nearly 20 years of work on dark energy, and delete our playlist based on this result. In October 2016, the team of Nielsen, Guffanti, and Sarkar published a paper titled "Marginal Evidence for Cosmic Acceleration from Type IA Supernovae." It appeared in the prestigious Nature journal, and that helped it get a lot of attention. As with the initial discovery of dark energy, these scientists used Type 1-A supernovae to track the expansion history of the universe. However, in the 18 years since the first studies, we've observed a lot more of these exploding white dwarfs. 740 of them, compared to the ten used by Riess and Schmidt, and the 49 by Perlmutter. The new guys claim that the much larger sample is consistent with there being no acceleration, and more data equals more confidence, right? So, did dark energy just go away? Actually not at all, and here's why: the new study actually agrees with the old dark energy result... mostly. It finds that an accelerating universe containing dark energy still fits the data best. The difference is that it finds that the data is also consistent with a wider range of possible expansion histories, and that range now includes a history in which there never was any acceleration. An accelerating expansion is still preferred, it's just that a non-accelerating history is not excluded with quite the same confidence. I want to talk about how scientists analyze astrophysical data like this, so let's talk numbers. The new study claims a 3-sigma confidence that there is a positive cosmological constant. So, quick aside: the cosmological constant, written as lambda, is the thing you add to Einstein's equations of general relativity to give the anti-gravitational effect of dark energy. If the cosmological constant exists, and is larger than zero, then dark energy is a real thing. Okay, so 3-sigma confidence in a positive cosmological constant basically means this: if you repeated this experiment many, many times, about 0.27% of the time the uncertainties, so the messiness in the data, would cause a universe with no dark energy to just happen to look like one with dark energy. So on average, about 1 in 300 experiments gives you a false 3-sigma result. But given that many thousands of different experiments are being run by professional scientists at any one time, false 3-sigma results do happen. So for a really big scientific claim like the existence of dark energy, 3-sigma just doesn't cut it. Scientists like to get at least 5-sigma significance. False positive 5-sigma results only happen once per 3.5 million experiments. So the new result doesn't give a high enough significance from the supernova data alone. No one could claim a proof based on that; it's at best a very strong hint. But here's something the press skipped: the original papers by our Nobel Laureates also claimed a significance of 3-sigma or lower for a positive cosmological constant based on that early supernova data alone. So why did anyone pay any attention? Because they didn't consider the supernova data alone, and nor should we. If we include other stuff about the universe, our confidence in the existence of dark energy rockets well above 5-sigma for all of these studies. We can't yet observe dark energy directly; we can only infer its existence based on how it affects the expansion of the universe. But that means we have to consider all off the things affecting that expansion when we decide whether dark energy is part of that equation. In fact, there are only two factors that can change the way our universe expands: there are things that tend to accelerate expansion, which we call dark energy, and there are things that slow expansion, which is just the gravitational effect of regular energy, and that's mostly dark matter, but also stars, planets, gas, radiation, et cetera. This graph is how we like to show the balance of these energy types: dark energy on the Y-axis, and normal energy (so, matter) on the X. To be a bit more precise, these numbers, Omega Lambda and Omega m, represent the fraction of the total energy in the universe that these two types would comprise, assuming that the universe is flat, which it probably is, and I'll get back to that. Those blue ovals represent the ranges of combinations of matter and dark energy that are consistent with the new supernova measurements. Now, statistical hypothesis testing is a whole big topic, and I'll put some links in the description, but very crudely, the inner circle is the most likely region, that's the 95% confidence, 1-sigma region. But really, Omega-Lambda and Omega-m could be anywhere in here, although the further from the center, the less likely. The big controversy here is that the 3-sigma contour touches the zero Omega-lambda line. So if we consider the supernova data by itself, represented by these contours there appears to be a small chance that we lie on this part of the graph: little or no dark energy. Except that bottom-left corner of the graph also represents a universe that has almost no matter in it either. Zero Omega-Lambda, but also zero Omega-m. But our universe definitely has matter in it. We even have a pretty good idea how much. Counting galaxies and weighing dark matter tells us that Omega-m is probably around 0.3, but it's at least around 0.2. That means we can rule out this entire section of the graph. That alone rules out the region of the supernova results that suggests there's no dark energy. Now this was known in the late 90's, which is why the first supernova results were taken seriously. Another really powerful piece of evidence is that the balance of Omega-Lambda and Omega-m define the geometry of the universe. If these add together to equal one, then the universe is flat, and by that I mean that parallel lines stay parallel, and the angles of triangles add up to 180 degrees, and all the regular rules of geometry work. In a relatively empty, expanding universe as is represented by this part of the graph, space would not be flat. It would have a weird, hyperbolic curvature. I'm going to have to refer you to another vid for details on universe geometries, but the main point is that we can also figure out where we should be on this graph by measuring the geometry of this universe. And as I also talk about in that episode, we can do this using the patterns in the cosmic microwave background to measure the angles of universe-sized triangles. Their geometry appears to be very flat, so our universe should lie on this line here. In fact, I oversimplified slightly, but the CMB results place our universe somewhere in these orange regions. The 1, 2, and 3-sigma contours based on the CMB measurements. That little region where the supernova and CMB results overlap represents the most likely combination of dark energy and matter. When you mathematically combine the certainty contours of two completely independent measurements they give you a much tighter range of possibilities. And the "no dark energy" region down there is so far from the combined likely region that we can rule it out with much more confidence than even 5-sigma. And there's other evidence leading us away from that little corner also: like baryon acoustic oscillations, but I'm gonna have to leave that one to our previous episodes also. But to wrap up: everything we know about the way gravity works, combined with the expansion history that we measure tells us that there has to be something out there countering the gravitational effect of matter and flattening the geometry of space. It's not even like accelerating expansion is that weird anymore. We're pretty sure it happened even more quickly in a separate episode soon after the big bang in the event we call "cosmic inflation." So this is a thing that seems to happen in our universe. Dark energy, whatever it is, is still a thing. But this new study is still very important. It demonstrates one of the greatest qualities of the scientific process and culture. No matter how well-accepted a result is, everything we think we know is always subject to being questioned and retested. We now need to understand why the new result edged the confidence down. There were differences between the experiments, so what effect did these differences have? You can be sure that many scientists will be taking a long and careful look at the evidence for dark energy, and its effect on the expansion of spacetime. Thanks to The Great Courses Plus for sponsoring this episode. The Great Courses Plus is a digital learning service that allows you to learn about a range of topics from ivy league professors and other educators from around the world. Go to thegreatcoursesplus.com/spacetime and get access to a library of different video lectures about science, math, history, literature, or even how to cook, play chess, or become a photographer. New subjects, lectures, and professors are added every month. So Keivan Stassun's course, "The Life and Death of Stars," gave me some great insights into the nature of the weird corpses left after stars die. With The Great Courses Plus, you can watch as many different lectures as you want, any time, anywhere without tests or exams. Help support the series and start your one month trial by clicking the link in the description or going to thegreatcoursesplus.com/spacetime. It's been a few weeks since we did comment responses, so I had some catching up to do. I want to respond to some of your thoughts on both colonizing Mars, as well as on the Many Worlds interpretation of quantum mechanics. Mars first: uh, some of you criticized my centrifuge city idea as being a bit overkill. That maybe 0.4g would be just fine for bone health, or at any rate we could just wear heavier clothes. Sure, maybe a centrifuge city is extravagant. EXTRAVAGANTLY AWESOME!! Anyway, walking around in moon boots and lead bodysuits for your entire life is both unstylish, and it only helps bones and muscles. It's not clear how organs respond to low g, especially the heart, and especially over many decades. My point is that there are plausible options, and a giant rotating maglev ring city in a sealed taurus is actually plausible. Besides, when the rebellion starts, the centrifuge cities will be an important touchstone in the conflict between Earthers and the .4g For True Martians crowds. Okay, on to Many Worlds. A few of you asked how the double slit experiment is actually performed. Do the slits need to be really tiny? How far apart are they? Et cetera. Well, the cool thing about the double slit experiment is that it's really easy to do with light. You get the best separation between interference bands when the distance between the slits is similar to the wavelength of the light, and with slit widths significantly narrower than that separation. So that's 500 nanometers' separation for visible light. This still works, however, for much larger separations and wider slits. Remember, this was first performed by Thomas Young in 1803 by punching a pair of holes in a screen. In fact, you can see diffraction patterns with your own eyes; you don't need two slits. A single slit also produces dark bands of destructive interference. Make a narrow gap with your fingers and put them really close to your eye and look at a light. You'll see dark vertical lines. Those are fringes of destructive interference just like the double slit experiment. It's an example of Fraunhofer diffraction. In fact, it even works if you place a single finger close to your eye.
Unfortunately, there's so much encouragement to misrepresent science in the media. Mainly that the media makes some money, and the scientists have a better chance of getting grants.
I'm not implying that these folks misrepresented their findings, just that it has been known to happen.
It's a serious problem, and it impedes real science.
What an amazing video! You guys are awesome for putting this together.
Though that would mean that light energy is the same true for dark matter in its model.