We now enter the realm of Cosmogony, the study of the origin of the Universe. The general name for the theory of the Universe's origin is the Big Bang theory. While most astronomers accept the basic concepts behind the Big Bang model, there are still some problems with it and some details that need to be worked out. It covers most of the bases, so most astronomers are pretty happy with it.
You'll remember that Einstein didn't propose a model that was anywhere near that of the Big Bang, which includes aspects like the Universe evolving and changing over time. Other astronomers of the time also proposed Universe models that were based on Einstein's General Relativity, but these models did not get much approval since they had bizarre features (like having no mass in the Universe or having a static model). Eventually things did get a little bit more lively when two fellows independently came up with pretty much the same model - the prototype for the Big Bang. They were Aleksandr Friedmann and Abbe George LeMaitre - most of the astronomical community didn't really notice their work, which they did in the early 1920s until about 1930. By that time, Friedmann was dead, so LeMaitre became the one who was discussing the model with other astronomers.
|Aleksandr Friedmann (1888-1925) of Russia (left) and Abbe George LeMaitre (1894-1966) of Belgium (right). These two are credited with developing the basics of the Big Bang model.|
The models that LeMaitre and Friedmann came up with had an expanding Universe. The general scheme of the model is that the universe started out much smaller and much hotter. Along the way various things happened that helped produce the Universe that we see today. The details of the model have changed over the years, as observations have caused some parts of the theory to be dropped or altered. One of the foundations of the model is that if the Universe had a really hot temperature in the past, then today it would still have a measurable temperature, though it would be much lower. Of course, the big idea of the model is that it says the Universe is expanding, which is where the whole Hubble Law comes in. I should mention that none of these people called their theory the "Big Bang theory." That term was coined by an astronomer named Fred Hoyle, who was actually sort of making fun of the theory - he never believed that the Big Bang theory was anywhere near correct, in spite of all the observations that support it.
Enough talking about the Big Bang theory - let's get into it. We'll go through a time line, hitting the major events that need to be hit and explaining what is going on at each step. I'll also include what the time is since the creation of the Universe and an estimate of the temperature of the Universe at each point.
Things aren't too good at this point in the history of the Universe, mainly because there is no history yet. At this first point we are pretty clueless as to what was going on in the Universe. There are no current theories of physics that can give us specific information about what the Universe was really like at this point. Why? In part, because we don't understand the laws of physics that exist under these conditions. Why is that? The rules of nature are sort of screwed up under these extreme conditions. In fact, you can't tell the difference between the four normal forces of nature - Gravity, Electro-magnetism, Strong Nuclear and Weak Nuclear. In the earliest history of the Universe, all of these forces were pretty much the same - they weren't distinct. The way that we describe this one merged force is to call it Supergravity. If you were to take a physics class at this point in the history of the Universe, it would be very simple - only one rule at work, so the textbook would be pretty small. Unfortunately, we don't know what is in that textbook since we don't precisely know what supergravity is or how it works. We're pretty clueless here. That's why there is no value for the temperature of the Universe. Guessing that it is infinite is probably not too bad.
You might wonder about the time before time=0 - what was it like before the Universe formed? That question may have no meaning - if there were no Universe, there was no time. Huh? There was no time as we would define it. To try to talk about what the Universe was like before the earliest time, which we can't even precisely describe, is pretty meaningless. There are ideas that the Universe goes through cycles, so that perhaps there was a Universe that existed before this one, or perhaps there just wasn't anything. Let's get back on the time line and tackle a time that we can handle.
Now we can describe something, since we know the rules of physics at this point. Actually, the rules are still sort of unusual, since we still don't have the four normal forces of nature at this point, but we do have two - Gravity and the GUT. Gravity is the same old falling-apple-hits-your-head type of gravity, while the other, GUT, is the Grand Unified Theory. This is where the other three forces are all merged together into one law. If you were to take a physics class during this time in the history of the Universe, you'd have only two rules in the book - a bit more complicated from the previous step, but still not too bad - at least we understand these laws. We understand the rules at this point, so we can write down some numbers to describe the Universe. I can even tell you that we think the Universe was about 10-35 meters in size at this point, but that would be sort of meaningless. It is still just very hot and small. At this point there really isn't much in the Universe except radiation, in part because it is so hot.
Now things are getting closer to being like the current Universe, though there are still some strange things going on. At this point, there are now three forces in existence - Gravity, Strong Nuclear Force and the Electro-Weak force. This last is a combination of two forces, which, like the GUT, is sort of strange. However, with the Strong Nuclear force out there as a distinct force, there can be some action going on in the Universe that wasn't possible before. One thing that can happen is that the building blocks for atoms can be produced - things like quarks and leptons will be produced (quarks make up protons and neutrons; leptons make up electrons and neutrinos). How was all this matter created? We just go back to Einstein's E=mc2. This law says that a certain amount of matter will create a certain amount of energy, and it also says that a certain amount of energy will create a certain amount of mass - this is the situation that we need to look at. The early Universe was incredibly hot and full of radiation - so full that the radiation (light) could spontaneously produce particles (matter) and their anti-particles (anti-matter). In case you are wondering, anti-matter is not just something that you read about in science fiction but is actually real stuff, though it is sort of an opposite form of the regular matter. One freaky feature of anti-matter is that if it contacts matter, then both are destroyed and their masses are converted back to energy. While it might sound like this is just magic, getting matter out of energy, you should know that this type of event has been observed in high-energy physics experiments, so we do have experimental support for this part of the theory.
Figure 1. The way that matter is produced in the early Universe. Here we have the creation of an electron and anti-electron (positron) out of energy, in this case a photon with enough energy to create the mass of both particles. Of course, matter and anti-matter don't get along, so when they come into contact with one another they destroy one another, and you get a photon out of that. Energy is converted to mass and then mass is converted to energy.
If both a particle and an anti-particle are created, and the Universe is today full of matter, where is all the anti-matter? Conversely, if matter and anti-matter are each created, then why should there be any matter at all; wouldn't all of it be destroyed by the anti-matter that was also created? According to this idea, there should be nothing but radiation in the Universe! I suppose everything that you see must be an illusion (including you).
We get past this problem by, well, basically, um, cheating. There are some rules of physics that allow what might be considered the impossible to actually happen. It is possible that during the production of particles there was a slight excess of matter produced - something like one extra quark for every billion reactions that produced a quark and antiquark. The Universe had so many of these reactions going on at this stage that it is possible for there to be enough extra quarks (and extra leptons) to be produced to provide all the material needed to form all of the mass of the current Universe. While it seems like we are sort of cheating here - well, yes, we are sort of cheating. It is one of the basic rules of quantum physics that - stated simply - sometimes the impossible happens. It actually has a more complicated name than that, but it is completely within the realm of physics for there to be more matter created during the early Universe than anti-matter. Of course, if that weren't the case, you wouldn't exist.
Now here is a case where the original theory of the Big Bang had to be supplemented with some additional work. Why? One of the problems with the original theory was that it could not explain why the Universe looks so flat. In fact, it is so flat looking today that if you wind the clock back to the beginning, it would have to be really flat then as well, since the scales of expansion are so huge that it could not have deviated much from flatness from the beginning. Our models of the Universe say that it doesn't have to be flat - yet it looks really flat. Another problem is how uniform everything appears to be in the Universe, that every part of the Universe has a very similar nature to every other part (remember homogeneity). Yet those parts are very distant and would not have been in contact even in the earliest days of the Universe (since they are so far away). This is sort of like having a pizza that is the same temperature at all parts, even after it is cut up and divided amongst a bunch of people. All parts of the pizza were in contact in the past, so they all had the same temperature and keep that same temperature. What if each part of the pizza as we see it now is 20 miles away? How could all parts have the same temperature now? They're too far apart and can't get back together in enough time. Someone thought that at this point in the Universe (when the quarks and leptons were being made) there was a huge amount of energy around and this energy may have caused the Universe to expand drastically at this time. By drastically I mean it got about 1030-1040 times larger than it was before the expansion.
Inflation helps solve the problem of how flat the Universe looks (just think of suddenly blowing up a gum ball from its normal to a size that is 1013 billion light-years and then standing on it - it would look pretty flat, wouldn't it?). Also, this says that even though the current Universe is really huge, before Inflation it was a lot smaller and the different parts of it would have had a chance to get equal temperatures and other uniform characteristics.
We've finally reached the point where the forces of nature are normal - there are four distinct forces and they stay that way from now on. At this point, some of those quarks are starting to get together to form neutrons and protons.
Here we have big time productions of protons and neutrons. Remember, to make them you need quarks and a Universe that can allow them to come together and make these things, which is what happens at this time. There were also some of the anti-particles still around, but they are gradually decreasing in number.
As the Universe cools down, the radiation's energy decreases and the only particles that can be made are those with low mass - so that's what this era is about. The light weight electrons are being produced here.
We're out of the tiny fractions of seconds eras - we've made it to three minutes since the start of the Universe. At this point, the Universe is like the interior of a star. There are protons all over the place, and they can combine together, just like in the core of star, to make helium nuclei (remember the proton-proton chain?). This is a pretty important stage, since it determines the chemical composition of the Universe. This step helps explain the amount of helium that we see in the Universe today. It would take far too much time for only stars to produce all of the helium we see in the Universe today, so a step in the early Universe where helium is produced in vast quantities is useful. Other things were also made at this point, like deuterium (heavy hydrogen) and possibly some other elements. At this point the composition of the Universe ends up being about 75% hydrogen and 25% helium, which is very close to what we see today. The amount of helium produced by stars is so small that it hasn't changed this ratio a great deal, even after all of this time.
Up to this time, radiation dominated the Universe. What does that mean? It means that whenever matter wanted to do something, like form into atoms or molecules, the radiation would rip it apart - it was still too hot for atoms to form completely. The building blocks were there (protons, electrons, neutrons, and helium nuclei), but they just couldn't get together. By this time the Universe was cool enough for the protons, electrons and the helium nuclei to get together and form complete atoms - and the radiation could not break them up. From this point forth, matter was able to overcome the forces of radiation; it was able to take over domination of the Universe. You know what that means - if matter (mass) is all important now, then that makes gravity also all important.
This would be when the earliest galaxies would have formed. One thing that isn't explained by the Big Bang theory is the order of the galaxy - cluster formation. What the heck does that mean? You could make galaxies, clusters of galaxies and superclusters of galaxies it two basic ways, either via the top-down method or the bottom-up method. The top-down method says that big structures, like superclusters, formed first. These later fragmented into smaller parts that became clusters of galaxies and then broke down into smaller pieces like individual galaxies. The bottom-up method says that the little pieces (galaxies) formed first and then these came together to form the clusters, which then came together to form the superclusters and other large scale structures. Which is it? That sort of depends upon the type of stuff in the Universe - and since most of the matter in the Universe is in the form of dark matter, the way that that stuff acts will help us figure it out. For the top-down method, big things would first form. This would be the case if material was moving too fast to form into small bits (galaxies) but not so fast that the big structures couldn't form. Fast moving stuff is often called "hot," so the top-down method is linked with what is called hot dark matter. Of course, the other method, with the material moving very slowly, would allow the formation of small structures first, so galaxies would form first and then the bigger stuff (clusters and superclusters) later. Slow moving stuff is "cold," so the cold dark matter model is the bottom-up scenario.
Which is it? I'm not telling - at least not yet. I'll get back to it later.
Things progressed with the formation of galaxies, clusters and superclusters, but that formation was likely impacted by several factors. What about the massive black holes in the centers of galaxies? Where did they come from? Did they help or hinder the process of galaxy formation? What about all the energy generated by quasars? Would that influence galaxy formation? What types of galaxies formed? Did they remain in that form so they look like they currently look?
Observations of the characteristics of the early Universe are very difficult, and even observations in the not-to-distant past can be rather perplexing. There are a few things we do find strong evidence for. It appears that massive black holes seemed to have formed early, and quickly in the early Universe, based upon observations by the Chandra telescope. These could have then led to the formation of active galaxies like quasars, Seyferts and Blazars that we see today (of course we see them as they looked in the past due to their great distance). The expulsion of large amounts of gas and energy may also influence the evolution of objects, possibly even hinder the formation of objects (since it is harder for material to come together when it is hot). Also in the early stages of a galaxy's formation, it would have a great deal of star formation and would also release a lot of energy (particularly ultraviolet light) into space. Such a heating event may also influence the environment around galaxies.
We see evidence for many of these things, with galaxies changing color over time as the stars within them evolve, as they transition through various active galaxy phases, and as they undergo mergers and collisions. The end result is that we have a Universe which was initially energy rich, to one that has matter in it (70% hydrogen, 28% helium and 2% other stuff), and is still changing.
This Big Bang thing is all fine and dandy, but is any of it correct? Did the Universe really form this way? Where is the evidence? We have the stuff that Hubble originally came up with - that we see the Universe expanding. We also have the concept that comes from Olbers's Paradox, that the Universe can't be infinitely old; it had to have some initial time of origin. There is also the way that we perceive the Universe - that the concepts of the Cosmological Principle and Universality should be correct (or else we're in big trouble). There is also the observed helium and deuterium abundance of the Universe - how else but through a phase of large scale nucleosynthesis could this much stuff be created? While this evidence does support the idea behind the Big Bang, there is still no smoking gun - we always need more evidence, not just circumstantial evidence but incriminating evidence (oh, dear, I've been watching too much Law & Order again).
When the Big Bang theory was being worked on in the 1940s, astronomers determined that there should be some radiation left over from the Big Bang. After all, the Universe was thought to have originated from a very hot, radiation filled space. That radiation should still be around everywhere in the Universe, but it would be diminished by the expansion and cooling of the Universe. The temperature of the Universe was estimated in 1948 to be around 5 K. At that time there wasn't a high enough level of technology available to search for it, so the estimate was sort of forgotten. According to Wien's Law, an object with a temperature of about 5 K would be producing radiation at a wavelength that corresponds to microwave light, and in 1948 there were no microwave detecting telescopes out there.
A strange thing happened in 1965. Two radio astronomers (Arno Penzias and Bob Wilson) working for Bell Labs (the once nation-wide phone company) wanted to do some basic radio measurements of the sky using a telescope that was originally designed for satellite communications. They started using this rather strange looking radio telescope by first checking how well it worked. They noticed right away that there was an annoying static noise coming from all directions of the sky. This is not normal. Radio signals should just come from objects that give off radio light and these would be only found in certain locations of the sky, not all over the place. They thought that there was something wrong with the telescope and decided to check out its internal workings. One thing they found were some pigeons roosting in the telescope and that these pigeons had produced "a layer of white, sticky, dielectric substance coating the inside of the antenna." This is just science-talk for pigeon poop. After cleaning up the pigeon poop and double and triple checking the electronics they were still amazed to find that the annoying buzz was still there. They knew that it was coming from space, but they had no idea what was causing it. Further tests on the radiation told them that it corresponded to an object which had a temperature of about 3.5 K (the exact temperature wasn't known; this was really just an estimate they made).
Figure 3. Robert Wilson (left) and Arno Penzias (right) shown in front of the telescope they used to discover the Cosmic Background Radiation (in 1965). Copyright Lucent Technologies.
Just down the road (literally) at Princeton University, physicists Robert Dicke and P. J. E. Peebles were going to try to find the radiation left over from the Big Bang, but they didn't have the equipment on hand to detect it. They knew that the radiation of the Big Bang should have been cooled down to a pretty low value, less than 10 K, but they didn't have a telescope to view it and they were working hard to make one that would detect this radiation. Eventually a mutual colleague of the Princeton guys (a fellow named Bernie Burke), who had heard what the Bell Lab guys had done, suggested to the Princeton guys that they talk to the Bell Lab guys. They did. The basic upshot were two papers that were published in 1965 (one from the Princeton group and one from the Bell Labs group) in the Astrophysical Journal, and in 1978 a Nobel Prize for Physics went to the Bell Lab guys. Why? What the two folks at Bell Labs discovered was the energy left over from the Big Bang - the radiation that was created from the early, hot Universe that has been stretched out and cooled down to a temperature of around 3 K today.
The 3 K radiation is called various things, like the Cosmic Background Radiation, the Cosmic Microwave Background Radiation, the 3 K background radiation or all other sorts of variations of these words. Let's just call it the CBR (Cosmic Background Radiation). This radiation is one of the strongest supports for the Big Bang theory. The radiation is seen in all directions of the sky and has the same temperature everywhere, so it is an indication about the smoothness of the Universe.
The smoothness of the CBR is, however, a very bad thing! Remember that radiation at one time dominated the early Universe and under those circumstances matter would have been "controlled" by radiation. What this really means is that matter would not have been able to clump together unless the radiation was also clumping together. When we look at the Universe, we see that matter is not uniformly spread out across the Universe like the background radiation appears to be. There are all those large scale structures, clustering of superclusters and so forth. If the radiation were perfectly uniform in all directions, then the matter should also be perfectly uniform and stuff would not have come together to form galaxies, stars, people, etc. It looks like we're in trouble with the Big Bang theory.
Astronomers realized that the radiation appears very smooth when measured from the surface of the Earth. As you know, the view from Earth is limited due to the fuzziness and blurring caused by the atmosphere. The best way to measure the radiation would be to launch a satellite to measure it from space. In 1989 the COBE - Cosmic Background Explorer - satellite was launched. Unlike observations from the ground, the CBR could be observed by COBE in all directions and at very high precision. Within about one month of launch, the COBE satellite measured the temperature of the CBR and found it to be 2.735 K (I'll still call it 3 K, since that's easier to remember). Actually, the temperature that it measured was so precise that all graphs showing the readings at different wavelengths have such small uncertainties (errors in measurements) that you can't even plot those uncertainties - they are just too small. In January of 1990 (less than two months after the launch), the results showing the radiation temperature were presented at the American Astronomical Society meeting in Washington D.C. When the plot showing the radiation was displayed, it received a standing ovation - those crazy astronomers.
Figure 4. The data from the COBE satellite showing that the Cosmic Background radiation has a temperature of about 2.73 K (based upon the shape of the curve and the location of the peak - Wien's Law) and is pretty much a black body. The boxes show where the satellite data are plotted and the line is the theoretical line for a black body. Data from the COBE mission.
One thing that I should point out about that graph - there is a line drawn through the data points. This is the line a true, perfect black body would have if it has a temperature of 2.735 K. WOW! Do you get what I just said? The radiation from the Big Bang is the closest thing out there to a real black body - a perfect radiation source! It seems sort of appropriate that the only thing in the Universe that is close to being a perfect black body is the Universe itself. After some additional work it was shown that the temperature is really about 2.728 (with a 0.004 uncertainty) K. I think I'll still stick to calling it 3 K.
The radiation is that of a black body, but what about the clumpiness of the Universe problem? It took some time, but a thorough examination of the COBE results showed that the CBR doesn't have exactly the same value everywhere. Also, by examining the data very carefully, they found that the radiation had a temperature variation of around 0.0001 K. This is sort of a small temperature variation, but it is enough to do the job. The radiation is clumpy enough to lead to a clumpy Universe.
Figure 5. A map of the entire sky based upon the COBE data. The different colors correspond to slightly different temperatures, indicating that the Cosmic Background Radiation is not of a uniform temperature but does have some small variations. Image from the COBE mission.
There have been many other CBR experiments that have improved upon the COBE data. Many of these projects are based in Antarctica, since the atmosphere there is rather dry and observing conditions are good. Some of the current projects include BOOMERanG, BEAST, ACBAR, and many more. Even though these experiments are all unique in how they measure the CBR, they tend to give the same results - that the radiation is clumpy on very small scales, which indicates that the early Universe had very small scale structures very early in its history. This is evidence that supports the cold dark matter formation theory or the bottom-up scheme. Actually, similar results (supporting cold dark matter) were also obtained by the Hubble Telescope observation of distant galaxies (2004) as well as other visible light surveys of galaxies. Most of the CBR experiments mentioned above were rather limited in the areas of they sky they surveyed. Another recent survey from space was completed by the WMAP spacecraft (operating 2001-2010), which provided the best data yet.
|The BOOMERanG project is shown at left - a mapping of the sky using a balloon from Antarctica. This is similar to the COBE project, except that the BOOMERanG instrument is about 35 times more sensitive. A part of the map it produced is shown at right. Click on the image at right to see a larger version of the map and how it compares to the COBE data. Clicking on the image at left takes you to the BOOMERanG website. Images courtesy the BOOMERanG project.|
Currently most astronomers who are trying to figure out what the Universe is like have been using the cold dark matter theory to model the growth of structures in the Universe. This has been done by various groups, including some that have made some rather nifty images. You can check out the views of the early Universe according to a group from Princeton (movie by Paul Bode and collaborators), which shows the early clumping of structures. This view shows a region of space 100 million light years in size - pretty big! Another animation from a group at Los Alamos, Cal Tech, and Mount Stromlo shows the quick clumping of material early in the history of the Universe. In both cases, material clumps together rather quickly, and the clumps grow in size as they pull in more and more stuff. These views of the Universe are being constantly adjusted due to discoveries and clearer results from all of the CBR mapping projects. For the most part there is good agreement between our theories and our observations on the origin of the Universe. Now let's get to the other end of the Universe - its end!
Now that we think we know how the Universe started, how do we think it will end? Will it end? Is there an end? Traditionally, we believed that the fate of the Universe depended only on one force. That force is... (all together now) Gravity. Why? Gravity is the only thing that we knew of that could stop the expansion of the Universe and possibly bring it all back together. It is the only force in nature with enough power to do such a feat. Remember, there are no limits over how far gravity extends. You are pulling on and being pulled on by all the objects in the Universe that have mass. Even though you don't feel the pull (and neither do they), the effect is still there. When the influence of all of the matter of the Universe is accounted for, we need to know if it just might be enough to overcome the expansion of the Universe.
If it all depends upon gravity, what does gravity depend on? If you just look at the formula for gravity, there are two values that can be changed, and they are mass and distance. As you know, astronomers have a really difficult time measuring distances and our measurements of masses are also sort of dubious. To determine individual values for mass and distance is quite tricky. Actually, we don't do that - we sort of combine mass and distance together into one value - density. All we have to do is figure out the average density of the Universe. If it is dense enough, then there is enough material in a small enough area to force the Universe to stop its expansion and collapse down again. If it isn't dense enough, then the expansion won't stop.
What is "dense enough"? We define the critical density as the density needed to stop the expansion of the Universe. To complicate matters, the value of the critical density depends on the value of the Hubble constant, Ho, and the shape (curvature) of the Universe. Even though we don't know the value of the Hubble Constant precisely, we do have a rather good idea of what the critical density is. For current estimates of the Hubble Constant, the critical density is close to 8 x 10-27 kg/m3, or about 5 protons per cubic meter of space. This isn't very dense compared to stuff that you are familiar with, and actually it is an incredibly low density. What is the actual density of the Universe? How does it compare to the critical density? Unfortunately, we don't know (yes, that is a pretty lame answer, but there is a reason). When we do measure the density of the Universe, we get a value for the density that is nowhere close to the critical density. We always get very low values, sometimes up to 100 times less than the critical density. I guess that means there isn't enough matter in the Universe to stop the expansion. No, it just means we get a density that is pretty low. You must remember, the density that we get is based upon our accounting for all of the stuff we can see. What about all of that stuff that we can't see? What about all of that dark matter? What affect would that have on our value of the measured density of the Universe? If there is a huge amount of dark matter then the value for the density that we get would be much greater, perhaps even greater than the critical density. We don't know precisely how much dark matter there is out there, so we don't know what we are missing. I guess we can't use density of the Universe to determine the fate of the Universe.
Even though we don't know the density of the Universe, we can still see what the results of its value would be for the fate of the Universe. If the density of the Universe is less than the critical density, then the Universe does not have enough mass (gravity) to stop the expansion and it will continue to expand forever. In this case the Universe is OPEN. If the Universe were to expand forever, then clusters of galaxies would be getting further apart from one another, and eventually all the matter in galaxies that could go into making stars would be used up. Finally all the stars would die and the Universe would be a very dark, cold place - large, but dark and cold. If the density of the Universe is greater than the critical density, eventually the force of gravity will slow down the expansion and cause it to stop. In this case the Universe is CLOSED. In the case of a closed Universe, not only would the expansion be stopped but also eventually reversed. The Universe will start heading toward a collapse after the expansion is stopped. This is sort of how a ball that you throw in the air goes first away from the Earth (expansion of the Universe) and then returns to the Earth (collapse of the Universe). Gravity will bring everything back together again. Galaxies would start getting closer, and they would all head for a Big Crunch. It is also possible that a closed Universe could experience another Big Bang after the Big Crunch. This is due to the way that material would have been destroyed as it was crunched - it would have been turned to radiation, and there would be a lot of that. A lot of radiation is what you would have when you start out with a Big Bang. Of course, it isn't guaranteed that there would be another Big Bang following the Big Crunch, but you never know.
There is also the very unlikely possibility that the density of the Universe equals the critical density, in which case the Universe is FLAT. It has to be exactly, precisely, completely EQUAL to the critical density, no more or less. In that case, the Universe will keep expanding but slow down as it does so. In a way this is very similar to the Open Universe case, except here the Universe will keep slowing down, slowing down, and slowing down, until, far into the future, the expansion is so slow that it isn't observable. The odds of the density of the Universe being exactly, precisely equal to the critical density made many astronomers think that this could not be the case. However, our ideas about this have been changing.
The different, traditional scenarios for the fate of the Universe, the shapes and the densities are presented in a table here. You may wonder why I say "traditional" - or maybe you don't. Either way, our "traditional" view of the future of the Universe has recently been thrown out the window - just keep reading.
In 1997, the stuff that I have been describing would have been about as far as I needed to go when talking about the Universe. This changed in 1998 when some very strange and unexpected observations were made. Several groups had been observing very distant galaxies containing supernovae to see what the expansion of the Universe was like long ago. Remember, when you look further out in space, you are looking at objects that used to be at those locations and looking at how the Universe was behaving long ago. If the expansion of the Universe was slowing down due to the force of gravity, then the distances of these supernovae would be different than what you would get if the expansion never changed speed - by just using the Hubble Law and a current Hubble Constant. If the rate of the expansion changes over time, then the Hubble Constant will change over time. Actually, calling it a "constant" isn't really a good idea, since it was different in the past and will probably be different in the future. The astronomers making the observations of the distant supernovae expected to find that the Universe is slowing down its expansion due to gravity, since we thought that the Universe was either flat or possibly even closed. And as you know by now, gravity is the all important force, and we (astronomers) thought it was the only thing that could control the fate of the Universe.
What did the supernovae observers see? They determined that the expansion of the Universe was not slowing down but instead was speeding up. The Universe is ACCELERATING!?!?!?! This is such a drastic result that a lot of people have a hard time believing it. But observations by even more astronomers have done nothing more than support this conclusion, and they can't all be wrong, can they? It is unlikely, since they are using different methods and measuring different things (not just supernovae). Why is an accelerating Universe such a bad thing? Go back to Newton's Laws of Motion. You have F=ma. To accelerate something (a), you need to exert a force on it (F). Something is pushing on the Universe? Not really, but something has to be not only overcoming the slowing down effects of gravity but exceeding them. Dang, this is weird stuff. Also, by having a force fighting the effects of gravity and overcoming them, worrying about the amount of dark matter in the Universe is kind of not so important - the density of the Universe doesn't seem to make much of a difference to its fate. The density and dark matter are still important in helping us determine the structure of the Universe and how that came about, but it kind of ruins all that stuff I was saying about Closed and Open Universes. It's time to turn to the mysterious force that is overcoming gravity.
What is going on with the Universe? What can overcome gravity? Believe it or not, something that can be best described as anti-gravity has been proposed. The way that this anti-gravity is included in Einstein's General Relativity formulas is by adding in that term that Einstein stuck in his formulas when he thought that there was no motion to the Universe - the Cosmological Constant, . Einstein thought he screwed up when he stuck this term in his formulas. Now it seems he might have been correct to include it - but not for the reasons he thought it was needed.
That's how we deal with the anti-gravity in formulas, but what is this anti-gravity thing really? A concept known as dark energy has been proposed as the form that the anti-gravity force takes, energy that exists in a "vacuum". If you remember back to the Big Bang discussion, matter and anti-matter can be made from energy (radiation). If this is the case, then this "vacuum" would have some energy, which can be translated into a pressure, which can help to push the expansion of the Universe. I know that this sounds like magic and other silliness, but various projects have shown that this may actually be the case. This dark energy is not too unusual since it is sort of like the energy that went into driving the expansion of the Universe during the Inflation stage. Recent work by the Hubble Space Telescope shows indications of the influence of this repulsive force as far back as 9 billion years ago! Observations by the ultraviolet telescope GALEX also confirm the presence of dark energy. Observations by the Chandra telescope indicate that dark energy may have even influenced the formation of early galaxies (in a negative way). So it now appears that we have a Universe that is made up mainly of dark matter (which works to pull it together), being overwhelmed by dark energy (which works to expand it). A cosmic tug-of-war is going on.
Here's another interesting thing you should ponder, E=mc2. I'm sure you remember that interesting little formula, but what does it have to do with the Universe? This formula shows the relationship between energy and matter. So if you want to take an inventory of the stuff in the Universe, you have to include not only regular stuff (matter), but also dark matter, energy and dark energy! So how much of that stuff is out there? I'll get to that in a second.
Another result that has come about which is equally unusual is that the most likely curvature (shape) for the Universe based upon these and other observations is that it is perfectly Flat. This result is seen when comparing the BOOMERanG, WMAP and distant supernovae results (and many other studies) to see what is the most likely shape for the Universe, along with determining what the expansion of the Universe is doing. Ultimately this means that the Universe isn't just Open, but WIDE OPEN! If this dark energy keeps driving the expansion of the Universe, then there will be no closing the Universe, no collapse. The Universe will just get bigger and bigger at a faster and faster rate. How's that for mind blowing?
So does this mean that the Universe is really never ending? Some have proposed that if the acceleration rate of the expansion increases, it will be harder to keep things together, since gravity will have to fight the ever accelerating expansion "force". First superclusters of galaxies would break apart. Then clusters of galaxies would break up. Eventually it would be difficult for galaxies to hold themselves together. Star clusters and then solar systems would be torn apart. Eventually it would difficult for individual objects like stars and planets to stay together. Ultimately all matter would be broken apart, since even the forces that hold atoms together would fail. Models that follow this scenario predict an end to the Universe (as we know it) in about 20 billion years. This theory is known as the Big Rip. Will it happen? Not likely. While it may seem interesting to have things ripped apart down to the smallest levels, it doesn't appear that a Big Rip will happen. At least we don't think so at this time (but remember, our ideas about what the Universe was doing have changed not so long ago, so stay tuned for new findings that may change all of this - again).
Currently the most accurate measurements of the Universe have come from the WMAP mission that was launched in June 2001 and finished operations in 2010. It was designed, like COBE, to view the entire sky, but with a much greater precision than COBE. In February 2003, many results from WMAP were released and they provide us with the best information yet about the Universe. These results have been revised as more and more data has been collected by WMAP, and they will no doubt be revised in the future with even better satellites. While these are the best measurements we have available today, they are of course not exactly 100% precise, though they seem to be pretty well accepted by the astronomical community. Here are some findings based upon not only WMAP results but also those of other studies (updated January 2013) -
Figure 6. Map of the sky from the WMAP satellite. The different colors indicate the variations in the CMR. This telescope has a better resolution than COBE (Figure 5) and was able to see even small scale clumpiness in the radiation. Image courtesy NASA/WMAP Science Team.
That pretty much answers it, right? Of course it doesn't. There is still a lot of stuff to be worked out, mainly details and finding out more about what happened in various parts of the Universe's past. There is quite a bit more to do yet and I'm sure there will be more newspaper headlines about stuff like this in the future. I'm sure some of you don't believe any of the stuff I've said about the creation of the Universe, or any of the results that have been observed about the origin and fate of the Universe, but that's fine; you're entitled to your opinion. Of course, this is the stuff that will be on the test.