The Known Universe

On large scales like the Universe, the most important force is gravity. Between any two objects the gravitational attraction is proportional to the product of the masses divided by the square of the distance between them. Gravity is the force responsible for keeping the Earth and other planets in our solar system in orbit around the Sun. Gravity also governs the motions of the Sun and nearly all the stars you can see in the sky, which are orbiting about the centre of the Milky Way Galaxy. The Milky Way is part of a gravitationally bound collection of galaxies which includes Andromeda, and is called the Local Group. Apart from observing that objects large and small are gravitationally attracted to each other, astronomers also observe that the Universe is expanding: an after-effect of the birth of the Universe in the Big Bang.


Up until a year or two ago it was thought that the Universe's expansion rate was decreasing, due to gravity pulling back on the material exploding form the Big Bang. However, more recently scientists have been able to measure how fast the Universe was expanding, and the data indicate that it is actually speeding up. This measurement was made by looking at distant supernovae. If distant supernovae are different in brightness than nearby supernovae (e.g. if there is more dust dimming the light than we think) then the measurement could be wrong. However most astronomers think that the measurements are strong.

The observable Universe is about 10 billion light years in radius. That number is obtained by multiplying how old we think the Universe is by the speed of light. The reasoning there is quite straightforward: we can only see out to that distance from which light can have reached us since the Universe began. 

We determine the age of the Universe in a number of ways. One is to estimate the age of the oldest stars we see. Our knowledge of how stars of a given size evolve with time is very good (based on what we know about atomic and nuclear physics) so the major uncertainty here is usually measuring how far away (and so how big) such stars are. The standard method is to look for very small changes in the apparent positions of the stars as the Earth moves around the Sun. (This effect is called parallax). A second way to get an age for the Universe is to try to figure out the time of the big bang itself. Here the method is to use a series of techniques (based on how bright things appear to be - like Cepheid variable stars - that we think we know the true brightness of) to determine first the distance of the nearby galaxies, then increasingly distant galaxies, until we have estimated distances for many galaxies for which relative velocity measurements have been made (using the Doppler red shift of features in their spectra). The relative velocities we observe for distant galaxies have been largely determined by the expansion of the Universe begun with the 'big bang'. So, once we've determined how expansion velocity correlates with distance for some range of distances, it's possible to extrapolate back (with some assumptions) to calculate the instant of the big bang, when all the matter in the Universe was at a single point.

We also observe that massive objects attract each other through the gravitational force. This force tends to contract matter locally (for example, a gas cloud condenses to form a star). On the large scale you can think of the expansion of the Universe acting to separate galaxies from one another, and the gravitational force acting to attract them toward one another.

The end of time depends on just how much mass there is in the Universe. We talk about this in terms of the density of the Universe, and compare densities to the critical density. If the density is greater than the critical density, then eventually gravity will overtake the expansion. The expansion will slow down and eventually reverse, so that the Universe will be contracting. Eventually it will end in a collapse (or a bounce) called the Big Crunch. If the density is less than the critical density then the Universe will continue to expand forever, with the gravitational force never overtaking the expansion. An ongoing area of research is to measure the density of the Universe. Currently, some observations (and some theories) indicate that the density of the Universe is very close to the critical density. In this case the expansion will slow down so that it is approaching zero expansion as time approaches infinity.

It has been said that because our universe creates its own space and time it is expanding into pure nothing. An infinitely dense point of matter appears in an otherwise perfect vacuum state. An event of some sort causes that point to begin to expand very rapidly, overcoming whatever initial gravitation pull would keep the point of mass together. As the new universe continues to expand, there is less and less gravitational pull to bring all of this mass back to its origin. If the pulling force outward is constant and the gravitational pull continues to decrease, the expansion rate will continue to increase. Apart from that potentially damaging argument, there is the issue of the definition of universe. Or universe, by one definition, is everything. There is nothing beyond or outside of it, not even the empty space-time we can conceive of as perfect space, so there would be no vacuum into which the universe could expand. This may seem a bit of a paradox, as we can always imagine something outside of our house or our solar system, but then it really becomes a question of philosophy as much as science.

The second law of thermodynamics states, that entropy will increase with time, where entropy = the amount of disorder in a system. With increasing disorder, there is inherently less energy that can be used to do useful work. With this inherent lack of useful energy, at some point in time, the universe will reach a state of thermal equilibrium, where there is nothing more than a collection of protons evenly spaced apart and all moving at the same speed. That state is called the heat death of the Universe. (The protons will all have decayed, but the Universe will consist of smaller particles all drifting at random and getting more and more distant from each other as the Universe expands).


Dark Matter
Dark matter means just that; matter of whatever type that does not shine brightly (in visual light, X-rays or at any other wavelengths). Even though we do not see dark matter directly, its gravitational influence can be seen in the motion of gas and stars in galaxies, and in the motion of hot gas and galaxies within clusters of galaxies. Dark matter means matter of an unknown type that astronomers and cosmologists believe must make up the majority of the mass in the universe. Its existence was deduced from the relative amounts of light elements and isotopes produced in the Big Bang, from the properties of high-temperature gas located in clusters of galaxies, and from the high speeds at which galaxies are moving in such clusters. This means that there must be 10 to 20 times as much dark matter, by mass, as ordinary matter, which scientists call baryonic matter. There is recent evidence from microlensing observations that at least some of the dark matter in our own galaxy is in the form of MACHOS, or MAssive Compact Halo ObjectS. These are planets or stars, made up of ordinary (baryonic) matter, that are too faint to be observed directly, but can act as a gravitational lens and magnify the brightness of brighter stars in the background. There is nothing weak in the observational proof for dark matter in this sense.

In a cluster of galaxies, we can estimate the masses of stars in the galaxies and the hot gas that fill the cluster. We can also infer the total mass of the cluster that is needed to keep it gravitationally bound. The latter is typically found to be ~5 times the combined mass of the stars and the hot gas; an analogy with our Galaxy suggest that only some of the dark matter can be MACHOS. Although circumstantial, such results point strongly to the presence of non-baryonic dark matter in the clusters of galaxies. When it comes to deciding what kind of exotic particles may make up the non-baryonic dark matter, however, there may be a hint of weakness, in that different particle physicists favor different exotic particles. Moreover, as far as I know, there has not been a direct detection of these exotic particles.

Two likely possibilities for the dark matter in our own galaxies are MACHOs (MAssive Compact Halo Objects) and WIMPs (Weakly Interacting Massive Particles). MACHOs are low mass stars, brown dwarfs, neutron stars and white dwarfs. If MACHOs make up most of the dark matter, the distribution is not smooth on the scale of the Solar system, but it is smooth on a much larger scale. If the Galactic dark matter consists of WIMPs, then they are dispersed throughout the Galaxy, with a distribution somewhat different from that of the stars that we can see. Since gravitational pulls of WIMPs from different directions tend to cancel out, the orbit of planets in our solar system is not affected by the presence of WIMPs. However, since there are more WIMPs towards the centre of our Galaxy than away from it, the motion of the Solar system (and other stars) in the Galaxy is strongly affected. This is how astrophysicists infer the presence of the dark matter.

One of the best ways of determining the mass of a bound system, such as a cluster of galaxies, group of galaxies or a massive elliptical galaxy , is to measure the X-ray temperature and gas profiles. If the gas is in hydrostatic equilibrium, the total mass and the gas mass can be directly determined from the X-ray data alone. Use of this technique has shown that clusters of galaxies are gas and baryon rich, that is, the mass in gas exceeds the mass in stars by factors of 3-5 and that the total baryonic mass is ~10-30% of the total mass of the cluster. This discrepancy is called the baryon catastrophe. For galaxies and groups, the X-ray data have often indicated very extended dark matter halos far beyond the radius at which one sees starlight or galaxies. The total inferred dark matter mass is often 10 times that in the visible galaxies alone


Dark Energy
Dark Energy appears to be based on the brightness of the most distant type-Ia supernovae, a mysterious force that is accelerating the expansion of the universe. These recent discoveries have provided good evidence that there is such an outward force on the universe (variously called the cosmological constant, quintessence, or dark energy). Data about the rotation of galaxies shows us that the outer parts rotate as fast as the inner parts. This only makes sense if there is a spherical distribution of matter in each galaxy, which is not what we see. Therefore we infer that there is a certain amount of Dark Matter in each galaxy. This could be some exotic particles, or just lots of stars too small to have ignited. Aside from this, there is also the Dark Matter that we think is there, based on theoretical arguments. This is something we can measure by looking at the cosmic microwave background and distant supernovae. These are the measurements (recently made) that imply the existence of both Dark Matter and Dark Energy


The Great Attractor
The Great Attractor is far bigger than a galaxy. In the terminology of astronomers, there are clusters of galaxies containing maybe hundreds of galaxies, and superclusters containing many clusters. The Great Attractor is a supercluster or something even bigger. The gravity of the Great Attractor has been pulling the Milky Way in its direction, the motion of local galaxies indicated there was something massive out there that are pulling the Milky Way, the Andromeda Galaxy, and other nearby galaxies towards it. For a while, nobody could see what it was, because it lies behind the plane of our Galaxy. That means the gas and dust in our Galaxy obscures the light from the Great Attractor, and it is outshone by the stars and other objects in our Galaxy. X-ray observations with the ROSAT satellite then revealed that Abell 3627, a previously known cluster of galaxies, was much more massive than originally suspected, containing many more galaxies. Optical astronomers had missed a great number of galaxies, because of the obscuration, but with hindsight (and with better observations), could spot many more galaxies. It is now thought that the Great Attractor is probably a supercluster, with Abell 3627 near its centre.


Solar Systems
A solar system is created when a rotating cloud of gas and dust in space start to coalesce. They are pulled together and towards the centre of the gas/dust cloud by their gravitational attraction to each other. As they condense, the particles collide faster and more often, which causes the gas and dust to heat up. The gas and dust at the centre collapses to form the central star of the solar system; the heat generated by the colliding particles starts nuclear fusion in its core. If there was enough angular momentum in the system at the very beginning, then not all of the dust and gas will go into the central star. The rest will remain in a flattened disk around the star. The planets form from this disk of rotating material as it clumps together because of gravity.


As of August 24th 2006 the International Astronomical Union decided that to be called a planet an object must have three traits. 

1) It must orbit the sun, 

2) be massive enough that its own gravity pulls it into a nearly round shape, and 

3) be dominant enough to clear away objects in its neighbourhood. 

To be admitted to the dwarf planet category an object must have only two of those traits -- it must orbit the sun and have a nearly round shape. And no, moons don't count as dwarf planets. 

In addition to Pluto, Ceres and 2003 UB313 the astronomical union has a dozen potential 'dwarf planets' on its watchlist. What's to become of the other objects in our solar system neighbourhood, the ones that are not planets, not dwarf planets and not moons? The organization has decided that most asteroids, comets and other small objects will be called 'small solar-system bodies.' Despite the establishment of these three distinct categories, there are bound to be grey areas. As technologies improve and more objects are found the International Astronomical Union will set up a process to decide which of the three categories are most appropriate for specific objects.


The Universe Black and White Holes The Stars
Our Sun The Inner Planets The Middle Planets
The Outer Planets