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Darkness


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As a particle physicist, I study the elementary particles and how they interact on the most fundamental level. For most of my research career, Ive been using accelerators, such as the electron accelerator at Stanford University, just up the road, to study things on the smallest scale. But more recently, Ive been turning my attention to the universe on the largest scale. Because, as Ill explain to you, the questions on the smallest and the largest scale are actually very connected. So Im going to tell you about our twenty-first-century view of the universe, what its made of and what the big questions in the physical sciences are -- at least some of the big questions.

So, recently, we have realized that the ordinary matter in the universe -- and by ordinary matter, I mean you, me, the planets, the stars, the galaxies -- the ordinary matter makes up only a few percent of the content of the universe. Almost a quarter, or approximately a quarter of the matter in the universe, is stuff thats invisible. By invisible, I mean it doesnt absorb in the electromagnetic spectrum. It doesnt emit in the electromagnetic spectrum. It doesnt reflect. It doesnt interact with the electromagnetic spectrum, which is what we use to detect things. It doesnt interact at all. So how do we know its there? We know its there by its gravitational effects. In fact, this dark matter dominates the gravitational effects in the universe on a large scale, and Ill be telling you about the evidence for that.

What about the rest of the pie? The rest of the pie is a very mysterious substance called dark energy. More about that later, OK. So for now, lets turn to the evidence for dark matter. In these galaxies, especially in a spiral galaxy like this, most of the mass of the stars is concentrated in the middle of the galaxy. This huge mass of all these stars keeps stars in circular orbits in the galaxy. So we have these stars going around in circles like this. As you can imagine, even if you know physics, this should be intuitive, OK -- that stars that are closer to the mass in the middle will be rotating at a higher speed than those that are further out here, OK.

So what you would expect is that if you measured the orbital speed of the stars, that they should be slower on the edges than on the inside. In other words, if we measured speed as a function of distance -- this is the only time Im going to show a graph, OK -- we would expect that it goes down as the distance increases from the center of the galaxy. When those measurements are made, instead what we find is that the speed is basically constant, as a function of distance. If its constant, that means that the stars out here are feeling the gravitational effects of matter that we do not see. In fact, this galaxy and every other galaxy appears to be embedded in a cloud of this invisible dark matter. And this cloud of matter is much more spherical than the galaxy themselves, and it extends over a much wider range than the galaxy. So we see the galaxy and fixate on that, but its actually a cloud of dark matter thats dominating the structure and the dynamics of this galaxy.

And again, what we find is that there is much more mass there than can be accounted for by the galaxies that we see. Or if we look in other parts of the electromagnetic spectrum, we see that theres a lot of gas in this cluster, as well. But that cannot account for the mass either. In fact, there appears to be about ten times as much mass here in the form of this invisible or dark matter as there is in the ordinary matter, OK. It would be nice if we could see this dark matter a little bit more directly. Im just putting this big, blue blob on there, OK, to try to remind you that its there. Can we see it more visually? Yes, we can.

So this light ray will not continue in a straight line, but would rather bend and could end up going into our eye. Where will this observer see the galaxy? You can respond. Up, right? We extrapolate backwards and say the galaxy is up here. Is there any other ray of light that could make into the observers eye from that galaxy? Yes, great. I see people going down like this.