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The Universe: the 2014 Model © 2014, Astronomical Society of the Pacific No. 87 • Spring 2014 www.astrosociety.org/uitc 390 Ashton Avenue, San Francisco, CA 94112 The Universe: the 2014 Model. Part II. Cosmological Puzzles (and Ways to Resolve Them) by Bruce Partridge (Haverford College) n Part I, we presented evidence that (a) the it difficult to detect: we can’t “see” it. All cosmic Universe is expanding, isotropic and homo- Dark Matter does is to exert gravity. The notion geneous; (b) it has nearly flat, Euclidean there is a different (and ”Dark”) form of matter has Igeometry; and (c) it is filled with the cooled-down a long history. Fritz Zwicky first posited it some 80 remnants of a Hot Big Bang. years ago. His studies of the motions of galaxies in We also raised a puzzling issue centering on the clusters of galaxies, like the one shown in Figure 1, average density of the Universe. If the curvature of indicated they would fly away unless there was suf- space is small, the total density ought to be near the ficient mass to hold them together gravitationally. special value of 9 x 10-30 gm/cm3. If we count up The mass of all the galaxies alone was not enough all the ordinary matter in atoms, planets, stars and — more was needed. But it doesn’t show up in pho- galaxies, however, this observed density is much tos like Figure 1 — hence it must be “Dark.” smaller. Are we missing something? Using a similar argument, Vera Rubin made the argument for Dark Matter both more quantitative Dark Matter and more convincing. She showed that the rota- Yes. One contributor to the density is Dark Matter, tion of spiral galaxies (Figure 2) would rip them Figure 1. A cluster of galaxies (Abell S1077) held together gravitationally. matter that, unlike ordinary matter, does not apart unless more matter was present in them than (Image: ESA/Hubble & NASA. Acknowledgement: N. Rose) interact with light in any way. Here it is important just the mass of their stars alone. Dark Matter was to point out that Dark Matter is not the same as needed to hold in the rapidly moving stars in the clusters) contain Dark Matter, and that the mass black matter (say a charcoal briquette). Charcoal outer reaches of the galaxies. of all the galaxies is too small to bring the average does interact with light — by absorbing it. Dark You may have noticed by now what seems like a density up to 9 x 10-30 gm/cm3. Surely, you may Matter does nothing to light. That, of course, makes contradiction. I’ve written both that galaxies (and be thinking, the Dark Matter must count when we Universe in the Classroom No. 87 • Spring 2014 Page 1 by yet another puzzle — just why 1025 in size. This huge change of scale stretched out is the geometry of the Universe any curvature to make it essentially flat. A soccer so flat? In principle, the Universe ball with a radius of 12 cm has very evident curva- could have any old curvature, ture; but a soccer ball of radius 12 cm x 1025 would positive or negative, big or small. be so huge as to be imperceptibly curved. This huge It is perfectly possible to imagine expansion flattens the Universe. In addition, the a Universe so strongly curved it expansion would stretch tiny regions that did have never gets any bigger than a soccer time to settle into homogeneity up to astronomical ball, with a radius of curvature of scales (as explored in the box below). Adding an roughly 12 cm. Indeed, “Why is early phase of explosive expansion thus solves both the Universe so flat?” is equiva- puzzles. This suggestion also solved some other lent to the question “Why is the pesky problems and, more importantly, it is consis- Universe so big?” tent with all current cosmological observations. It Figure 2. If only the mass of visible stars contributed to the gravitational force holding a galaxy together, we would expect a sharply decreasing rotational speed as the distance from Related to that question is one has come to be called “Inflation.” the galactic center increases beyond a few thousands of light years (as shown in red). Instead, more: Why is the Universe so Because it so successfully solves these puzzles, the measured velocities change little with distance from the center. Additional mass — Dark Matter — is required to hold the stars in. (Image from SuperCDMS at Queen’s University) homogeneous (so closely the same Inflation is now accepted as part of the standard everywhere)? How could two picture of the history of the Universe. There are dif- chunks of the Universe, located a ferences of opinion on when it started (but certainly add up the mass of galaxies. Even if we include it, long way apart, “know” to have the same density? very early) and how long it lasted. There are also however, the average density in any chunk of the One answer to these questions — but not a very many suggestions for what drives the rapid expansion Universe remains far too low to ensure a flat geom- satisfactory one — is to say, “That’s just the way it (about the same number of ideas — if not more — etry. Something is still missing. Some of this miss- is.” But scientists like to probe deeper and to mini- than the number of theorists working on Inflation!). ing mass, of course, could lie outside the galaxies: mize the number of explanations that come down It is slightly unsettling that we seem to have there is no reason there shouldn’t be Dark Matter to unspecified causes. replaced a couple of large questions about the outside as well as inside the galaxies. If we can’t see curvature and homogeneity of the Universe with it, how can we know how much Dark Matter there Inflation another one about the physics behind Inflation. is, and where it is? In the early 1980s, two young theorists in the US and There is hope in the fact different theories of It turns out there is a way to detect Dark Matter, a the (then) Soviet Union came up with a neat way of Inflation make different predictions about observ- method that depends on its gravitational action alone; ensuring both homogeneity and flat geometry. able phenomena. In other words, these predictions we will come back to that towards the end of this Alan Guth and Andrei Linde independently sug- are testable. They thus fit in the framework of nor- installment. For the moment we will stay with what gested the Universe went through a very early phase mal science. Can we devise experiments or observa- we have learned so far: there is Dark Matter present of very rapid expansion. That solves the problems, tions that can discriminate among the various ideas (exactly how much there is, we will leave to Part III in but the numbers involved are staggering. We now about Inflation? We will answer that question in the this series) and the Universe has close to flat geometry. think this phase kicked in something like 10-32 sec last installment of this three-part series. after the Big Bang, and lasted something like 10-30 A second reason to take seriously the idea the But Why Is the Geometry Flat? sec. In that sliver of time, the Universe expanded Universe once expanded explosively is that the Meanwhile, some quizzical theorists were troubled by a huge amount; again in rough figures, by about Universe appears to have entered a second phase of Universe in the Classroom No. 87 • Spring 2014 Page 2 quick moment in the distant past; it is a property of Once the expansion of the Universe was discov- A requirement for any chunk of the Universe our present Universe. ered, however, the cosmological constant was no to be homogeneous is that all parts of it have had time to interact with each other. longer thought to be static. Thus Einstein aban- Dark Energy Otherwise, there is no way to ensure all parts doned the cosmological constant, and it faded from of the chunk have the same density. Since So what drives this later phase of expansion? It discussion. The statement that the cosmological physical interactions can’t propagate faster appears to be a mysterious substance called Dark constant was “the biggest blunder of my life” is than the speed of light, c, we need to start Energy. Like Dark Matter, Dark Energy does exert attributed — probably erroneously — to Einstein. with chunks smaller than ct. Here, t is the a gravitational pull, so it can be thought of as In any case, it may be that declaring the cosmologi- time available for physics to act to make the contributing to the overall density of “stuff” in the cal constant a blunder was in fact a bigger blunder. chunk homogeneous. This is at most the Universe. Indeed, Dark Energy contributes even Since it acts to counter gravity, the cosmological time since the Universe began. So let’s run more to the total density than Dark Matter, solving constant can produce the run-away expansion we the sums for a chunk that starts out a mere the puzzle we started with. It is not just Dark Matter see in the Universe today. 10-23 cm in size at the beginning of Inflation. that brings the density of the Universe up to of Introducing the cosmological constant, how- If Inflation begins at 10-32 sec, ct is equal to 9 x 10-30 gm/cm3, but Dark Energy as well.
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