Astronomy has instant appeal. Most people have looked up at the night sky and marvelled at the beauty of the stars. Twinkle, twinkle little star, how I wonder what you are.... This is the course where you can find out!
Astronomy asks some profound questions. How does the Sun shine ? Is there life on other planets ? How did the Universe begin, and how will it end ?
That there can be answers to such questions is one of the highpoints of human culture. We are all familiar with highpoints of other aspects of human culture - the writings of Shakespeare, the buildings of the pyramids. Yet science can seem less accessible because it is written in a specialist language - maths! But other subjects also have their own specialist languages - I don't need to be able to read Old English to appreciate Beowulf, or Hebrew and Greek to read the Bible. I can read them in translation, and science is just the same. On this course I will act as the translator, to give you an appreciation of some of the immense beauty of the Universe in which we live.
Much of the material can be found in a highly recommended web site by Nick Strobel, Astronomy Notes . The PROVISIONAL title and aims of each lecture are given below, together with links to on-line resources. This page will get added to as term progresses, so keep checking to see what is here (and hit reload to make sure you are getting the most up to date version!).
We've studied the birth, life and death of stars; now we're going on to study the birth, life and death of the Universe. By the Universe I mean everything. So what is everything ? We've already seen that there are lots of stars. How are these distributed in space and what else is there ?
When you look at the night sky the bright stars seem fairly evenly distributed - there are about as many in one direction as another. See the sky chart which I gave out as part of the Night Sky section of the course. But as you look fainter you can see a milky band of light across the sky. Some all sky views of the Milky Way. The first telescopes resolved some of this faint band of light into stars. Photography plus bigger and better telescopes in the 19th century showed that most of the Milky Way was made of stars as in these higher resolution images of the Milky Way.
These observations make sense if the stars are distributed in a disk. Are we at the centre (this had a lot of theological and philosopical implications) ? At first it looked as if we were - the Milky Way is about as bright in all directions. We can be more sophisticated than just looking at the 2 dimensional distribution of stars on the sky - we can look at their 3 dimensional distribution by estimating their distance. Review lecture 4 for parallax (direct way to get the distance but can only be used on relatively few stars which are nearby enough to show a measureable effect) and spectroscopic distances (indirect way - uses the spectrum of the star to classify its spectral type. This then gives the intrinsic luminosity of the star, which can be used together with its apparent brightness and the inverse square law to get its distance). These results again indicated that the Sun being near the centre of the stellar disk.
But spectroscopic distance are indirect measures - they rely on a chain of inference, in this case the use of the inverse square law brings in the ASSUMPTION that the space between the stars is empty. We now know that this isn't true - review lecture 9 on dust and gas in interstellar space. Distant stars have some of their light scattered out of our line of sight by the dust, and they appear dimmer and redder than they would have if the space were truely empty.
Historically this picture of the Sun at the center of Universe consisting of a single disk of stars started to come under serious question at the end of the 19th century. The use of photographic film with large telescopes meant that ever fainter images of the sky could be taken. Looking over the whole sky, the photographs showed that as well as stars (which are point-like) there were fuzzy bits, so they called them nebulae (latin for cloud). The nebulae fell into 2 types, those that could be resolved into clusters of stars, and those which could not! Both these categories can also be split into two. Starclusters can either be open clusters such as The Pleiades which are of irregular shape and tend to be found only in the plane of the Milky Way, or globular clusters such as The Great globular cluster in Hercules, which are spherical and are also seen out of the main disk of stars. The 'fuzzies' split into ones which which looked truely diffuse, which are the clouds of gas and dust from which stars are born such as the Orion Nebula, and ones which have spiral structure (more on these later!). If the Sun is at the centre of our galaxy then we'd expect that the globular clusters should be distributed evenly around the sky. They are not! Its hard to get distances as they are further away than most of the stars seen easily in the galactic disk. But at the stars of the 20th century a class of stars called Cepheids (don't worry about the numerous types) had been identified. These are evolved stars, and are highly luminous so can be seen at large distances. But their key property is that are unstable and pulse, and the period of pulsation gives a very good measure of the absolute luminosity. Again this is a spectroscopic distance measure, and again uses the apparent brightness and the inverse square law (ie assuming space is empty) to get its distance. Getting distances from Cepheids gave a globular cluster distribution which was centred on a point thousands of light years away from the Sun More on location of the sun . Plainly there was something wrong with the star distributions derived from their spectroscopic distances - dust!! Correct for this and we get that the globular clusters and stars are centered on a point 27,000 light years away in the direction of Sagittarius. See here for a nice review of the history of the structure of the Milky Way. Dust affects mainly blue light, so we can see more of the structure of the galaxy if we go to infrared wavelengths - these show the disk and bulge.
The sun must then be orbiting the centre, because of gravity. Assuming the globular clusters orbit randomly and add their velocities together. Anything left over then is the velocity of the centre of the galaxy with respect to the Sun. So reverse it and get the velocity of the sun with respect to the center of the galaxy - its 220 km/sec. We know the distance (27000 light years equals 2.5x1017 km) and we know the speed, so we can work out that the orbital period is 240 million years. This seems a long time. But the Sun is ~5 thousand million years old. So its gone around the center 20x!
But we can use our knowledge of gravity to translate this orbital period and distance to get the mass inside the suns orbit. Comes out to be 1011Msun. So if its all in stars like the Sun there are one hundred thousand million of them!!
The galaxy shows spiral arms - the O and B stars are not distributed randomly around the sun.
Getting the distances to other galaxies is difficult because they are so far away. It only became possible in the 1920's when Cepheid variables were resolved in nearby 'spiral nebulae'. At the time there was a lot of debate as to the nature of these object - were they associated with our Galaxy or were they external star systems, separate Galaxies. The distances obtained for the nearby 'spiral nebulae' from the Cepheid period-luminosity relation were much larger than anything proposed for the size of our own Galaxy. Knowing the distance, the diameters of the 'spiral nebulae' can be found from their apparent size on the sky - they turned out to be similar in size to our own galaxy - 100,000 light years across the stellar disk. Again, knowing the distance, we can use the apparent brightness of the galaxy to work out its intrinsic luminosity from the inverse square law (being careful of dust!). These turned out to be approximately 1011x brighter than the luminosity of the sun. Thus if they are made up of stars like the sun then these galaxies contain approximately 1011 stars!!! The 'spiral nebulae' are then separate star systems comparable in size to our own galaxy i.e. they are galaxies in their own right. The Universe consists not just of our own galaxy, the Milky way, but of many many other galaxies too. See an introduction to galaxies.
Galaxies come in all sorts of shapes and sizes. We can classify these into several types, ellipticals, spirals (and barred spirals), and irregular galaxies. The nearest galaxy of similar size to the Milky Way is the Andromeda Galaxy - it is 2.2 million light years away, so the light we see from it left the stars in that galaxy 2.2 million years ago!! We are looking at it, not as it is now, but as it was 2.2 million years ago - we are looking back in time! And this is the NEAREST galaxy like our own.
We can currently (just!!) see Cepheids with the Hubble Space Telescope out to ~100 million light years (more details here - click onto the individual images line to see the Cepheids). But what can we use after that ? We need a something where we know its absolute luminosity (called a 'standard candle'), so we can combine this with the apparent brightness via the inverse square law to get the distance. But we want this 'standard candle' to be much brighter than the Cepheids so we can use it out to larger distances. Using the galaxies where the distances are already known via Cepheids gives that the brightest globular clusters all have much the same absolute luminosity. Alternatively, the very brightest stars in each galaxy have much the same absolute luminosity. ASSUMING that the faroff galaxies are similar to the nearby ones, then we can use these to determine the distance.
But there are more faint galaxies which seem to be at still further distances. How can we get a good distance estimate to these ? We could assume that the Galaxy was like the Milky Way, and has the same absolute luminosity - this is a bit of a wild guess as we already know that galaxies come in all sorts of luminosities. Ditto if we assumed that the diameter of the faint galaxies was the same as that for the Milky Way. We need something better to go further. If we take the spectra of galaxies as well then the amount of doppler broadening of the spectral lines tells us how fast the stars orbit in the galaxy, which is proportional to its mass and so to its luminosity. These can be calibrated separately to get the absolute luminosity for spiral galaxies (Tully-Fisher relation) and for elliptical galaxies (Faber-Jackson relation). More on using these to get distances to galaxies.
But theres something far more obvious in the spectra of galaxies than the breadth of the absorption lines and thats their position! In the 1920's it was already known that most galaxies had absorption lines which were doppler redshifted. Plotting the redshift versus the distance of the galaxy (found by Cepheids or any of the other methods above) gave a linear relation - the redshift that we measure in the spectrum is directly proportional to the distance. This is Hubbles Law. We can easily take a spectrum and then use the redshift to get the distance. The current record holder for the furthest galaxy whose distance can be determined by this technique is 12.2 thousand million light years away.
But what does this all mean ? The Universe must be expanding.
Is our galaxy at the center of this expansion ? At first glance it
seems so since we see most of the galaxies rushing away from
us ? But its not true.
Suppose we can see a galaxy receeding from us - from
that galaxy an observer would see
But what else is there in the Universe ? Astronomers in the 1950's started to look at the sky in radio wavelengths. They found some radio sources seemed to be associated with rather blue objects which looked like stars (could not be resolved into fuzz as expected for a galaxy). But it was already known that normal stars produced very little energy at radio wavelengths. Taking an optical spectra of these quasi-stellar objects (or quasars for short) gave even more problems. The spectra looked NOTHING like the spectra from stars - there were strong emission lines rather than the absorption lines seen in stars and the wavelenghts of these lines were NOTHING like the wavelengths for the known atomic transitions, and the lines were very broad. Here are various types of Quasar and active galaxy spectra compared to a standard galaxy spectrum. In 1963 they realised that these emission wavelengths would line up with the known atomic transitions if the quasar was highly redshifted, implying recession velocities which were as big as those derived from the most distant galaxies then known (many galaxies with much larger redshifts are now known).
If the redshift can be interpreted as part of Hubbles law, then we can use it to get the distance, and hence the luminosity - about 1013 Lsun. i.e. the object is 100x brighter than the whole Milky Way galaxy. But it doesn't look fuzzy like a galaxy and its spectrum isn't the spectrum you expect from lots of stars! Even more startlingly, looking back on old optical photographic plates, it became clear that the optical light varied on timescales of years, months and even weeks. This sets some stringent conditions on the size of the object through light travel time arguments, leading to the conclusion that we have more optical luminosity (with a bizzare spectrum) than that from even a large galaxy, produced in a region the size of our own solar system!!! More on Quasars
Is this reasonable ? Not at first glance, certainly. A huge luminosity from a tiny region means that there must be a huge density of photons streaming out from the source. Photons interact with matter (especially electrons) and with enough of them they can blow away any matter in the region. Yet there must be matter in the region, as Einstein said E=mc2 - we have lots of energy and ultimately that must have come from matter. The only way to hold the matter in the region against the pressure of the outgoing radiation is with lots of gravity. Doing the numbers, this means that for the brightest quasars we need a mass of around 100-1000 million times bigger than the sun. The only way to get this much mass into a region the size of our solar system is in a black hole.
We saw in lecture 8 how we can get ultraviolet and X-ray emission from matter heating up as it falls into a black hole of a few solar masses. These supermassive black holes can also produce ultraviolet and X--ray radiation from material falling into them - in fact they produce a LOT more luminosity in the X-ray and ultraviolet than in the optical. And ironically, only about 10 per cent produce much radio emission - the ones where there is a jet of material shooting out from the center at velocities close to the speed of light! Some nice pictures of quasars with jets.
For the stellar remnant black holes the source of infalling matter is a companion star. What is the source of the matter for the supermassive black holes in quasars ? If they are in the center of a galaxy, then this is a very dense environment - giant molecular clouds, star clusters etc. And it only needs something like the total mass of the sun to fall in every 10-100 years to power all the activity that we see.
This model (supermasssive black holes in the centers of galaxies) can also easily explain the bizzare spectrum seen - material close to the black hole will be illuminated by the intense radiation and will produce emission lines. But it will also be moving at high velocities because of the strong gravity of the black hole - so we get strongly broadened emission lines, which then get redshifted by the Hubble law (review the material in lecture 11) according to the distance the quasars are from us.
But for such a theory we want to see lots of tests - we'll trust it if keeps on passing all of the tests and there is no other obvious answer. We know that there are nearby galaxies with bright, pointlike nuclei which have broad emission line spectra - these are called active galaxies. These can fit into the same picture but with a less massive black hole (maybe only 1-10 million times the mass of the sun!). We'd expect less massive black holes to be more common than the very large ones. So the chances are that there would be some in the nearby galaxies, where we can see the galaxy as well as the nucleus. Also, for the quasars, (look at end of this section) we are now able to use the Hubble Space Telescope to take very deep images, and the fuzzy underlying galaxy can be seen. Clearly this is activity which is associated with the centers of galaxies. But is it a supermassive black hole ? We have some direct ways to estimate the mass, by looking at material very close to the center - using the excellent resolution of the Hubble Space telescope or going into the radio spectrum where we can get higher resolution than in the optical. If we can look very close to the center and measure the orbital velocities of gas and stars (by their red and blue shifts) then we can use the image to get a size and our knowledge of gravity to get a mass. So far the closest in that we can get shows that there can be a mass of 10-100 million x that of the sun in a region thats just a few light years across Here is an HST image and spectrum showing the doppler shift of a gas disk in an active galaxy called M87, while here is some radio data from the center of an active galaxy NGC 4258.
So what about our own galaxy ? Whats in the middle ? Its hard to look in optical light because of the gas and dust which obscure our view. Radio is less affected by dust, so we can see into the center, and this shows an intense radio source (so its not stars). Instead it seems to be emission from energetic particles trapped in a magnetic field. More on the galactic centre. Infrared light is also less affected by dust so we can see a central star cluster. The velocities of the stars are high that they are consistent with orbiting a central mass of about 2 million times the mass of the sun. More here. But it doesn't show activity like the active galaxies and quasars. The most probable explanation is that the black hole currently doesn't have much material falling onto it.
We saw in lecture 11 that the galaxies had spectra which are generally redshifted, implying that they are receeding from us with a velocity proportional to their distance (the Hubble law). This is a nice way to find distance, but what does it mean ?
The redshifts are often talked of in terms of a doppler shift, to give a recession velocity. If everything is receeding from everything else then the universe must be expanding. The general analogy used is that of blowing up a balloon. but this can be a little misleading as it looks as if the balloon expands into space. Einstein's general theory of relativity tells us that space (or rather space-time) is the same thing as a gravitational field. So space-time can't exist apart from the matter and energy that creates the gravitational field. So we can't talk about the universe expanding into empty space, because space apart from the universe doesn't exist! So what we are seeing is not a universe expanding through space, but space itself expanding.
So if its expanding, then at some time in the past the universe must have been much smaller than it is now. As things expand they cool, so the early Universe must also have been much hotter, as well as much denser. Taking this to its logical conclusion then the Universe must once have been incredibly hot and dense, expanding explosively outwards. This is the hot big bang model. We can look at the rate of expansion now (the hubble law), and use this to get an estimate for the time when the universe was created - the age of the universe! (for those who want to see the maths). For those who don't it comes out to about 10-20 billion years
But whats the evidence! In general we trust a theory if several (the more the better) independant lines of evidence come up with the same story. Where is the corroborating evidence for this ? The wilder the story, the more evidence we'll need to justify it, and this is pretty wild.
If the Universe was originally incredibly hot and dense, then it would radiate like a blackbody, with photons and electrons interacting. Electrons couldn't recombine with nuclei to form atoms because another photon would come along and knock it out of the atom. But as the universe cools, these blackbody photons would cool along with it. Eventually the Universe would get to a temperature where photons no longer had enough energy to immediately knock any electron out of an atom - this is the era of recombination. The blackbody photons then don't interact with the electrons any more. So the blackbody cools with the expansion of the universe. If you take the expansion from the Hubble law, then this predicts that this cooling blackbody radiation left over from the hot early universe should be at temperatures of only a few Kelvin i.e. that this peaks in the microwave region of the spectrum. This cosmic microwave background radiation was found experimentally at much the same time as it was predicted theoretically!
Wind this back even further, and the universe can be even hotter and denser at earlier times (the first 3 minutes!). So hot that we can have nuclear fusion reactions - but only for a very short time as the universe is expanding and cooling so rapidly. So some (but not all!!) hydrogen undergoes fusion reactions. Thus there is some helium which is NOT produced in stars, but in the very early universe. If we look at very old stars which were formed before there was much chemical enrichment from supernovae then we see these primordial abundances.
This also gives an answer to a very old question - why is the sky dark at night ? This is called Olbers paradox. Firstly the expansion of the universe means that the light from distant objects is redshifted. But secondly and more importantly, because the universe is not infinitely old, light from very distant objects hasn't had time to reach us yet!
We saw in lecture 11 that the galaxies had spectra which are generally redshifted, implying that the Universe is expanding. Lecture 13 traced this expansion back to the big bang, the beginning of the Universe. Now we are going to trace the expansion forward, and look at the ultimate fate of the Universe.
Will the Universe keep on expanding forever ? What can stop it ? Gravity! The gravitational force between all the matter must be slowing the expansion down. Can it slow it down enough to stop the expansion, and bring all the galaxies back together again ? Obviously this depends on the balance is between how fast the Universe is currently expanding and how much mass (i.e. gravity) there is in the Universe to pull it back. The average density of matter which forms the borderline between these two outcomes is called the critical density - its TINY, corresponding to 5-6 protons (or hydrogen nuclei) per cubic metre! The ratio between the actual density of matter and this critical density is called Omega. For Omega < 1 then the Universe will expand forever, the stars will all eventually die out - a dark and cold death of everything. For Omega > 1 then there is enough matter to eventually halt the expansion, and recollapse the Universe into a big crunch. Perhaps this could be a cyclical process, seeding a new big bang ? More on ultimate fate of the Universe .
If the Universe has enough gravity to recontract then space must curve back in on itself - nothing can ever escape. Its very hard to visualise this, so we tend to use 2 dimensional analogies so we can draw it. If we were creatures who lived in 2 dimensions, then Omega > 1 would be a space which curved in on itself like a sphere. We would live on the surface of the sphere, and see no centers or edges, and all light paths eventually come back to where they started. Suppose instead that the Universe has Omega = 1, the critical density. Then photons (just!) never come back on themselves, and space is flat. Now suppose Omega > 1. Then the expansion makes photons get even further away from each other as well as never coming back on themselves, so then we need space shaped like a saddle (hyperbolic geometry). More on the curvature of space (NB the 'scale factor' talked of here is the average distance between galaxies), the geometry of the Universe and the fate of the Universe.
So, what is the density of matter in the Universe ? We know more or less how massive stars are, and we know more or less how luminous they are, so we can look at the apparent brightness of a galaxy, get its distance using any of the ways described in lecture 11, so get the luminosity of the stars and so get the mass of stars. Do this for all the galaxies we can see within a certain distance, and then work out the density! And its WAY WAY too small to close the Universe, giving something like Omega ~ 0.01!!
But what we can see isn't all of what there is! When we estimated the mass of our Galaxy we did it two ways, first by looking at the numbers of stars, and secondly by looking DIRECTLY at the amount of gravity by looking at the motion of stars orbiting around the center - with a velocity and a distance we can use our understanding of gravity to get the mass within the orbit. Stars are OK to do this with, but gas is better as the gas extends out a fair bit further than the stars and we want now to look at the mass of as much of a galaxy as possible. Dust is always a problem, so we want to go to long wavelengths. And we want a spectral line which is very strong so we can easily track it. Hydrogen has a line at 21cm (radio wavelenghts) which is ideal for this. Get an edge on spiral and map the rotation velocity of the 21cm line at all distances from the center. As we go towards the edge of a galaxy the starlight falls off (definition of the 'edge' of a galaxy!), so we expect that this is all the mass, so the rotation curve should fall away too. It doesn't!!! There must be large amounts of DARK mass in a halo around the galaxy in order to match the observed rotation curves. A nice site on including a java amimation of rotation curves. Masses measured in this way show 10-100x more mass than that which we can see in stars!
Gravity pulls galaxies together into groups and into clusters of galaxies. We can look at the mass of these by looking at the motions of individual galaxies in the clusters. The more mass in the cluster, the bigger the velocities of the galaxies. Here is a nice java animation of motions of galaxies in clusters. Also, clusters of galaxies are a big gravitational potential well, and trap gas inside. The gas falls into the big gravitational potential of the cluster and so heats up - to X-ray temperatures! The temperature tells us about how much gravitational energy it has, and so how much mass. More on X-ray hot gas in clusters.
All these assume that the masses are bound and in orbits - a fairly good assumption as if galaxies were not gravitationally bound then the materila would dissipate. But it'd be nice to have a way to measure mass without having to make this assumption. Mass = curvature of space, and curving space will bend light. This is gravitational lensing. The amount of lensing tells us the mass. A cluster is a big mass concentration, and its gravity will distort the images of background galaxies - see this nice animation . Cluster masses derived in this way agree with the masses derived above from galaxy velocities and the X--ray hot gas in saying that there is much more mass than we can see in the stars. The mass of the Universe is dominated by DARK MATTER. But even including this DARK MATTER we get Omega ~0.3 i.e. not enough to close the Universe
But what is this dark matter ? Maybe its failed stars, or dead stars (white dwarfs, neutron stars, black holes), or black holes left over from the big bang, or.... But there is a problem. Nucleosynthesis in the big bang puts a fairly stringent constraint on how much of this can be in the form of protons/neutrons (collectively known as baryons) - and its something like Omega~0.06. So, a small faction of this dark matter can be normal stuff, but the majority of it has to be something else entirely!! More about dark matter. The least exotic guess is if neutrinos have some (small) mass. Nasty isn't it - at least 90% of the mass in the Universe is in the form of something which we don't yet understand!
We saw in the lecture 13 how we could get some observational constraints on the hot early Universe. Recombination of protons and electrons lets the cosmic microwave background radiation freely escape. This requires temperatures of ~3000K, maybe 300,00 years after the big bang. Even better constraints come from primordial nucleosynthesis because that needs higher temperatures - bigger than 109 K, corresponding to times less than 3 minutes after the big bang! Can we look earlier than this ?
At earlier times the Universe would have been hotter still. The protons would smash together, but the Universe was so hot that the blackbody photons would break the nuclei apart as soon as they were formed. At even earlier times, when the Universe was less than a millisecond old and its temperature was 1012K then the Universe would be so hot that the blackbody photons crashing into each other would make particles and their antiparticles (remember E=mc2 - matter and energy are the same thing!). Protons must then slightly have outnumbered antiprotons (by only 1 in a billion!) in order to have the Universe made of protons as we see it today. Why ? Plainly there are some observable constraints here!
Back even further and the Universe getts hotter and hotter. At 10-10 of a second after the big bang then the temperature was more than 1015 K. At this point the electromagnetic force and weak nuclear force (the one that the neutrino interacts by) become one and the same thing - called the electroweak force (like water and ice are the same thing - but look very different!). We have experimental evidence backing up these theories from particle accelerators - the theories prediced some new particles (W and Z bosons), and these have been seen!
Back even further, and we are into temperatures/energies which are higher than we can currently attain in particle accelerators. Theories have been developed which unify the electroweak and strong nuclear force into a single force at very high energies - maybe something like 10-35 seconds after the big bang (these are called grand unified theories or GUT's). Its in these theories that we have to look for the slight imbalance between matter and antimatter that could result in the fact that we and the rest of the Universe is made of matter. And there are GUT theories can do this, which is interesting and makes them potentially believable even if we can't yet test them directly in particle accelerators. And they might also make some odd, heavy particles which make up the dark matter.
And most physisics believe that if you go to high enough energies then all the forces (ie GUT+gravity) unify into a single force. But that is on timescales of 10-43 seconds (called the Planck time) after the big bang. We don't yet have even a theory of how to unify gravity with the other forces, so we can't get anywhere closer to the origin of the big bang than this planck time (though its pretty close!). Here is a nice overview, and some stuff about unification of forces.
So, the fact that we are made of matter gets us into the GUT era which isn't well understood. Since we are, can we work the expansion of the Universe forward from say 10-10 seconds after the big bang to the present day and get a Universe like the one we see ? Actually, no! there are some nasty problems in there. The cosmic microwave background is incredibly uniform over all of the sky - it has the same temperature everywhere to about 1 part in 105. Yet diametrically opposite bits of the sky are not close enough together to ever have exchanged photons so its very surprising that they are at such similar temperatures. This is called the horizon problem. And even worse, why do we see Omega of order unity ? Both the density and critical density of the universe change rapidly with time as the universe expands, but they change in different ways. For Omega to be so close to unity now means that it must have been very very close to unity very soon after the big bang. This is the flatness problem.
Once way to solve all these simultaneously is to postulate a period in the very early universe when the expansion was dramatically larger than it is now - a period of inflation. And it'd better be in an era which we don't understand (because there isn't any reason to get it in anything we do understand!). One idea is that at the end of the GUT era, when the strong force parted company from the electroweak then that released a tremendous amount of energy, powering inflation. Anyway, if we do this way way back, then we start off with a universe which is much much smaller, so things on opposite sides of our current sky DO get time to exchange photons and get into equilibrium. The expansion dramatically flattens out the curvature of the Universe and the size of the fluctuations! Nice! And from this we CAN get to the sort of Universe we see today (as long as we use Omega~0.3 which is mostly made from dark matter). See the overview of the beginning of the Universe. And there are some more good pages doing a similar review on the web site for the cambridge cosmology group, and the Microwave anisotropy probe
But this is a backwards test - we have the observational problems and so we postulate something to fix them. To test a model rather than just say that its consistent with what we see then we want it to predict something! And these inflationary models make a fairly strong prediction that space is flat, ie Omega=1. But the amount of matter (including dark matter) really doesn't look like this - it looks much more like Omega~0.3. But recent observations using supernovae to get distances to remote galaxies show an accelerating expansion - see here for a nice introduction (you need to have java enabled) - you can start at The High-Z SN Search page as you already know a lot of the background information! If this truely what the data are telling us then it is completely at odds with our ideas about how the expansion must be slowing down because of gravity! This acceleration (termed the cosmological constant) is currently interpreted as the vacuum itself having some energy. Its very similar to what we needed for inflation, but scaled down! But however it gets interpreted it should be including in our calculations of Omega. And it looks like it would give a effecive contribution of Omega~0.7 so the total Omega ~ 1. But having this odd acceleration breaks the nice picture whereby whether or not the Universe will expand forever is the same as asking about the curvature of the Universe. These accelerating models are spatially flat, but dramatically expand forever! For those of you who are brave have a quick glance at this page on the cosmological constant - ignore the maths, just have a quick read of the text and pictures!
So, here we are, at the very end of our current knowledge. An accessible list of articles on cosmology is kept at the astronomy cafe For the truely brave (or foolhardy!) here is a link to frequently asked questions in cosmology.
