8. Galaxies and Cosmology
8.1 Types of galaxies
There are three major classes of galaxies, with the first two being the most important. Spirals
Our Milky Way is a typical fairly large spiral galaxy. There are classification schemes for spiral galaxies but you don't need to know them Ellipticals
Elliptical galaxies have much less rotation than spiral galaxies. They are a bit like giant globular clusters, with little interstellar matter and few young stars. Elliptical galaxies are rather like just the halo component of spiral galaxies. Some look circular, but others are more elongated. Elliptical galaxies range from even more massive than large spiral galaxies, to much smaller. Elliptical galaxies are in some ways like larger versions of globular clusters IrregularsThe Magellanic Clouds are the nearest galaxies to us and are small irregular galaxies
Irregulars usually have a lot of gas, and are making new stars like spirals Why do these different types of galaxies exist?Maybe how much rotation in the cloud that formed them. Another likely possibility is that ellipticals form from the merger of spirals.
8.2 Distances to galaxiesTypical distances:Distances to galaxies are measured in Megaparsecs (millions of parsecs) or millions of light years
Hubble ultradeep field press release Cepheid variablesDistances to galaxies are very hard to obtain. They are too far away to use parallax. The distances to nearby galaxies are obtained by studying Cepheid variables. These are stars which vary in brightness in a recognizable way. From the speed of the variations we can determine their luminosity, and from the luminosity and their apparent brightness we can determine their distances. |
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Hubble's LawCepheids are only good for glaxies that are near enough to us that we can watch individiual stars with a telescope. For more distant galaxies we use Hubble's Law t was discovered in the 1920s by using the Doppler effect (redshift), that the speed at which a galaxy is moving away from us is proportional to its distance. . (We can't determine any sideways movement). This is called "Hubble's Law" and is one of the most important laws in astronomy. Farther away a galaxy is the faster it is moving away from us We can write an equation V = H x d H is called Hubble's constant. Its value is 72 km per second per Mpc.or 22 km per second per million light years. V is the velocity Originally astronomers used observations of nearby galaxies, whose distance they knew, to establish the correctness of the law and to measure Hubble's constant. More often, nowadays, we use Hubble's law to determine the distance to a galaxy. All we have to do is to measure the velocity by measuring the redshift, then divide by Hubble's constant. This is the only method that there is for most of the galaxies in the Universe. |
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8.3 Clusters of galaxiesGalaxies are found in clusters of galaxies even more than stars are found in clusters of stars. Nearly every galaxy is in a cluster This is the cluster we belong to is called the Local Group. It surrounds us
Major clusters can contain thousands of galaxies. The galaxy clusters themselves are grouped into superclusters, so that there is an enormous amount of "structure" in the universe. On a large scale it is incorrect to think of galaxy clusters as clumps with space in between. The most recent research indicates that galaxies are spread out in filaments with large spaces called "Voids" between them. 8.4 Galaxy CollisionsGalaxies are relatively much closer than stars, so collisions between galaxies occur more often than collisions between stars Movies of collision between galaxies Although individual stars rarely collide during a galaxy collision, the gas clouds do collide, These collisions lead to the collapse of the clouds to form new stars. Gas can also fall onto the centers of the galaxies: if there is already a black hole there the gas falls onto the accretion disk, gets very hot, and radiates a lot of power. Galaxies with very strong emission from their nuclei are called QUASARS
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8.5 Masses of galaxies and clustersGalaxiesThe main way of determining the mass of a galaxy is to do more or less what we did with our own galaxy, namely orbits of stars, as in our galaxy. Galaxies rotate too slowly for us to see any actual motions. What we measure are the Doppler shifts of stars or interstellar gas at different distances from the center of the galaxy. This actually tells us how the mass is distributed in a galaxy. The total masses of galaxies are up to about 1013 times the mass of the Sun, but there are many smaller ones, down to 108 solar masses. When we study the rotation curves of many spiral galaxies we get a strange result. There seems to be extra mass that is not producing as much light as we would expect if the matter were in the form of normal stars. We call this extra mass dark matter. This is a major mystery in astronomy right now. There are signs that there is 3-10 times more dark matter than regular mass in galaxies. The dark matter in galaxies seems to be spread more widely than the visible matter. It is not concentrated into stars, or even into the disks of spiral galaxies. Clusters of galaxiesThe galaxies in a cluster are held to each other by gravity. We can estimate the average gravitational force in a cluster by looking at how the galaxies move with respect to each other. We cannot see them move sideways, but we can can measure their Doppler shifts. When we do this we find that there is much more mass in the clusters of galaxies than we see in the form of starlight from the galaxies themselves. This is the second piece of evidence for dark matter. Dark matterDark matter is deduced to exist from the studies of galaxies and galaxy clusters described above. One of the most powerful ways of measuring the amount of dark matter in the Universe is to use gravitational lensing. Light from a distant galaxy gets bent by passing near the enormous mass that is in the galaxy cluster. As a result, one may see more than one image of a distant galaxy. By studying the locations and shapes of these images one can work out how much dark matter is in a cluster, and even map it. We now believe that there is at least 10 times more dark matter than matter in the Universe. In other words we know just about nothing about 90% of the Universe. What is dark matter?Ordinary matter is atoms, and the more fundamental particles that make up atoms, like protons. Ordinary matter is "visible" in the sense that it can absorb light, scatter light and emit light. Dark matter seems to be different: it appears to respond to gravity, but to nothing else. Let us examine the possibilities for what it is: Hydrogen and helium. If dark matter were hydrogen or helium gas we would see absorption lines. Some dark matter is hydrogen gas, but certainly not all of it. Hydrogen and helium cannot be solid in space. Rocks or terrestrial-type planets. The problem here is that these are made almost entirely out of "heavy elements". If dark matter were rocks then the ratio of heavy element to hydrogen would have to be more than 10, instead of only 1%. All our theories of the origin of the elements (which are theories that are solidly backed up by observational data) would have to be discarded. Black holes. The only way we know of making black holes is when big stars die. If there had been thousands of times more big stars in the past than we now estimate, we would see plenty of evidence for this in the Galaxy, such as a higher abundance of heavy elements. Machos Acronym for Massive Compact Halo Objects. The idea here is that there are zillions of small stars or large planets. They either do not have nuclear reactions (like Jupiter) or have so few that the stars hardly shine at all. They are so faint that they hide from our telescopes. They are called Halo, because there is evidence that the dark matter in our Galaxy is spread out in the galactic halo, where the globular clusters are, rather than just in the disk of the Galaxy. The problem is that there needs to be an awful lot of them; more than you get by extrapolating the numbers of normal stars. One way of looking for them is to watch for them to move in front of a distant star and amplify the light by gravitational lensing. This has been tried, and the answer seems to be that there are not enough Machos to explain dark matter Wimps Acronym for Weakly Interacting Massive Particle. The idea is that there is a new kind of particle, separate from protons, electrons, quarks etc. To verify this idea scientists want to find actual traces of these particles, or create examples of them in a lab. This is an enormous challenge to physicists. Does Dark Matter matter? ..Although there is a lot of dark matter in the Universe, it is very spread out, and not concentrated at the Earth or even in the Solar System, so there is a negligible amount of it around here. The equivalent of a few atoms per cubic meter! You don't have to worry about walking into it. But the future of the Universe depends overwhelmingly on the amount of dark matter in the Universe.
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8.6 The expansion of the UniverseDoes Hubble's law imply that we are the center of the Universe? It does not. We can understand this by considering the expansion of a loaf of raisin bread Each raisin will see each other raisin moving away from it with a speed that depends on its distance. Thus Hubble's law implies only that the whole universe is expanding, not that we are the center of it. Nowadays astronomers think of space itself expanding, rather than the galaxies moving through space. |
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8.7. The Big BangIf you make a simple extrapolation of Hubble's law backwards, assuming that the galaxies have always moved at the same speed, we find that all the galaxies in the Universe lay on top of each other about 13 billion years ago. This leads to the idea of the Big Bang, namely that the Universe suddenly started expanding from an enormous explosion 13 billion years ago. To some people asking how the Universe started is a religious question. The scientific approach is to go out and make observations that can tell us something about what happened. What we would like to do is look back in time Looking back in timeWe can do this because of light travel time. Light from distant galaxies has been traveling for billions of years. What we see is galaxies as they were billions of years ago. In this way we look at the past. The steady state alternativeWe can get an idea of how cosmologists can use the idea of looking back in time by considering the controversy that raged in the 1960s between two rival cosmology theories. The rival to the Big Bang was the steady state which states that there was no beginning to the Universe. As the galaxies move apart new matter is formed to take its place, so that the average density of the Universe never changes. The steady state theory has the advantage of simplicity. You can test these theories by seeing how far apart (on average) galaxies were in the past. We find that they used to be close together Nowadays there are better tests than this between the two theories, but this is still the simplest to explain. The early universeThe Big Bang occurred about 13 billion years ago You should think of it as an explosion of matter and space, rather than matter into space. We have previously discussed how things tend to get hotter as they are squeezed together. Conversely, things cool as they expand. The Universe has been expanding from a very hot dense beginning. The farther back we go the hotter and denser the universe was. Amazingly, we extrapolate back to when the Universe was only a fraction of a second old, applying the laws of physics that we have discovered in the last 3 centuries, and see if it makes sense. We find that it does. We can trace back well before there were any atoms, say about 10-10 seconds. There were only photons. There was so much energy that some of it got converted into matter Initially protons were formed, so that the universe was full of hydrogen nuclei. Nucleosynthesis of helium was possible only during the first 3 minutes, when the density was high enough. When we apply the laws of physics we predict the right amount of helium at the present day. Most helium dates from the big bang, not from the stars. All other elements were formed in the stars, or in stellar explosions Neutral hydrogen and helium atoms formed about 1 million years later as electrons and protons became stabe atoms.. When these atoms were formed photons were produced Radiation from this period can still be detected. It is called the primordial background radiation. We can detect this with special telescopes as a faint microwave hiss. By studying the fluctuations in this background we learn something about what the universe was like before there were any galaxies. Small fluctuations in the density of the gas in the young universe grow to form galaxies and clusters of galaxies, somewhat in the way that stars form, but on a bigger scale.. Understanding exactly how this works is a vibrant area of current cosmology. |
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8.8. The edge of the UniverseEveryone worries about where the edge of the Universe is. It is historically one of the great paradoxes. In the traditional view of the Universe there is no simple answer. But there are a couple of ways of realizing that there are more subtle ways of looking at the question Light HorizonSince the Universe is only about 13 billion years old we cannot detect anything from farther away then 13 billion light years. Curved SpaceWe can only imagine a world of three dimensions. We can get some idea of how limited this view is by imagining a world of only two dimensions, such as the surface of a piece of paper. This gives the idea of "Flatland" Imagine some dumb creatures who can only conceive of two dimentions, forward/back and left/right. They have no notions of up and down. If they inhabit a truly flat plane then they are faced with the problem that their universe either has an edge or goes to infinity. But if they were living on a vary large sphere without knowing realising it. their "universe" would be almost flat but would actually have no edge and would not be infinite. Perhaps our three dimensional universe is actually four dimensional without our realising it.
Nowadays cosmologists can calculate the properties of a Universe with different numbers of dimensions, even though they can't visualize them. |
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8.9. The future of the UniverseThe crucial question about the future of the Universe is whether or not it will continue to expand forever. The gravitational forces between the galaxies tends to brake the expansion. The more mass in the universe, the stronger the gravity slowing down the expansion. The answer depends on average density, which is 4 x 10-30 g/cc. If the average density is higher than this the Universe will collapse on itself. If it is smaller than this it will expand forever. This is about 2 atoms per cubic meter. The problem is actually another example of "escape velocity". Remember the escape velocity is larger the higher the density. The visible matter not enough to stop the expansion, but when you include dark matter it adds up to nearly, but not quite enough to stop the expansion. Dark EnergyIf there is a large amount of dark matter, the expansion of the universe should be slowing down. Astronomers looked at distant galaxies to see if the expansion long ago was different to what it is now. To their surprise they found that the Universe's expansion is accelerating, There seems to be some extra pressure that is forcing things outward. It acts in the opposite way to gravity, but is only important at very large distances. We can't measure this pressure directly on Earth. This pressure is called "Dark Energy". We have very little idea what causes it, and it is very hard to study. This idea is right on the frontiers of science, but almost all the experts I know are enthusiastic about the idea. If these ideas are correct, the Universe will expand for ever. 8.10. Final remarksThinking about how the Universe came into being and how it will end is the ultimate challenge to physics. Solving the mystery will require us to bring together the two main theories of physics; quantum theory, which deals with the very small and relativity which deals with large masses. Normally we only need one or the other, and both give the same answers for the kinds of problems that we use Newton's laws for. But they both are needed when we get large masses in small volumes, as at the start of the Universe. "String Theory" is an attempt to come up with a unified theory, but it has not yet been verified, and many physicists don't like it. Main thing I want you to understand is that it is now possible to use science to address questions that were once only philosophy and religion. The main difference between the scientific approach and the religious one is the approach taken to finding the answer. One depends on observing, experimenting and modifying one's views in the light of experience, the other depends on handed-down information and divine guidance. Reconciling the scientific approach with the religious one is a personal matter which is more important for some people than for others. Some religions are much more flexible than others. |
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The same idea can be used to help understand the concept of space expanding rather than galaxies moving apart. Imagine we had two dimensional universe on the surface of a balloon. Inflating the balloon makes the space of the universe expand, carrying the galaxies with them.