The universe consists of an unknown number of objects. The number of objects seem to defy imagination…it seems the more we see with our grand telescopes and instruments, the more we learn is out there. See The Universe tab in the Header above to get a sense of the numbers and the size. However there appear to be a very limited number of basic types of structures. Sure….we will discover more as we learn more, but there are currently only a handful of basic structures: Starting with the simplest (see further down for more discussion on each of these):
the Local stuff……
- Planets. Earth, of course, is what we are most familiar with.
- Stars. Our Sun is a great example.
- Solar System. A star with planets, moons, asteroids, and/or comets.
out of eye-sight stuff….
- Black Holes. Objects of extreme density and extreme gravity.
- Galaxy. An accumulation of Gas Clouds and Solar Systems.
- Gas Clouds. Mostly Hydrogen…a birthplace for stars.
- Dark Matter. We think we must need it…but what is it? Dark for sure, meaning we can’t detect it…hardly.
- Galaxy Clusters. The largest structure we know of…thousands of galaxies gravitationally bound together, including enormous Gas Clouds and Dark Matter. Note that this is a group of galaxies that are gravitationally bound, i.e., they are connected by gravity and act as an integrated unit….which is why it’s considered a structure.
and then there is the mystery energy…the dark side….
- Dark Energy. The universe not only continues to expand, but the expansion appears to be accelerating. The problem is that there is too much material in the universe and so it should be slowing down…because of gravity. So it is theorized that there is some unknown energy that is pushing it along…..the Dark Energy.
Yes…there is a competing theory which does not require Dark Energy ….. Quintessence. More of that later.
The Local stuff….
Planets, Stars, and Solar Systems These are the objects we can see…some with our unaided eye….others with telescopes or binoculars. Check out the Solar System for notes on the Planets and other objects in our Solar System, and Life of a Star for the life and death of the Stars. Of course we know much about the planets in our Solar System, but we are also learning a great deal about Planets revolving around other suns in our galaxy. Hundreds of Planets around other suns (Exoplanets, i.e., planets external to our solar system) have been discovered……most are very large simply because we are not so good at detecting smaller planets. These large planets most certainly do not support life….they are just too close to their own sun and are very hot. Other planets are also being discovered around white dwarf, brown dwarf, and neutron stars …the leftovers from the death of earlier stars. Unfortunately, planets around these leftovers also are not likely to support life as they are remnants of earlier stars and/or planets in previously active solar systems, and would have been incinerated by the death of their sun. In the dying stage of most stars they become Red Giants and expand to a very large size, incinerating any inner planets and the life they may have.
Out of eye-sight stuff….
Black Holes. A Black Hole is a collection of material so massive that the resulting gravity prevents any particle from escaping, including light itself. It begins life as a dying massive star, grows in size by attracting gas, dust, stars, anything within its gravity field….. and apparently a Black Hole never dies. Most galaxies are believed to have a Black Hole at their center. These Black Holes begin with a mass of 2 to 10 times our sun but soon grow to thousands of solar masses as they consume nearby material.
Galaxy The basic structure we know today in the Universe is the Galaxy. There are untold numbers of galaxies. They are the result of gravitational forces acting since the beginning of time. Each galaxy is made up of millions of stars, with associated planets and meteors and asteroids and dust, and maybe several Black Holes. All of these items within a galaxy are made from the elements we know in the Periodic Table and are observable.
Gas Clouds Gas Clouds are found throughout the universe….some in our Milky Way…some in the galaxies and beyond. After all, this is the stuff Stars are made out of and, in a sense, is the basic building block for the heavens. If we think about it, we should see gas clouds all over the universe, and in fact, they are quite common. These Gas Clouds are primarily hydrogen and are normally very cool (but they can be very hot…millions of degrees). When cool they begin to “coalesce” and form stars. Gas Clouds are located throughout galaxies and, in addition, there are large gas clouds of Hydrogen orbiting around most galaxies, beyond the visible portion of the galaxy. These large gas clouds move at a higher than expected velocity around the galaxy…. to continue…
Dark Matter Since the rotational speed of galaxies is not super fast, less than 0.1% the speed of light, it is widely accepted that Newtonian Laws of motion apply. And these Laws predict that, given the known observable mass of a galaxy like our own Milky Way and the gas cloud velocity, the gas cloud should be flying off into space. Obviously that is not the case. What could possibly be preventing that from happening? ….enter Dark Matter More mass than can be detected must be present within the Galaxy. And I mean more mass…like several times more mass than is observed It is now suspected that an additional component of mass must be present. This has been given the name Dark Matter. “Dark” because, not only can we not see it, but we can’t detect it by any means! For the present it is only a theory. It might be made up of weird particles that so far have escaped detection. The particle might be very heavy and not common….or it might be very light and undetectable. (Or, maybe, the Newtonian Laws of Motion don’t work at the huge distances of galaxies!) Dark Matter is needed to keep the galaxy and its outlying gas cloud together. It is also needed to keep the Galaxy Cluster together. Galaxy Clusters Groups of several hundred Galaxies in the same region of space can be tied together (gravitationally) by a common cloud of gas, from the earliest days of the cosmos. These galaxy groups are called Galaxy Clusters. The cluster is a group of many galaxies, all of which are enveloped by a hot gas cloud. The Galaxy Cluster is thought to be the largest single structure in the universe. Note that the Cluster is gravitationally bound together and this feature is what makes it a genuine structure. There may be thousands of galaxies within a cluster. The cluster itself is similar to a city at the intersection of several major highways each highway leading to other cities, i.e., other clusters. Just as the city is connected to other cities, the cluster is yet connected to other clusters. This is the largest component we know of for the universe.
Dark Energy. During the late 1990’s astronomers were discovering that the universe is expanding at a faster rate than realized. For many years there was the ongoing debate among astronomers on whether the universe was slowing down and soon to stop or whether it was expanding. If the former, then it was thought that the universe would soon stop, go in reverse, collapse on itself, and the Big Bang would be created again. Basically the Big Bang in that case would become the Big Bounce. However, the latter case, that the universe would continue to expand, became the dominate view and as observations became more complete it became evident that the universe expansion is not only not slowing down, but in fact is accelerating….it is going faster and faster! At the same time astronomers are discovering that there is a huge amount of matter in the universe. In fact, so much matter that the universe really should be slowing down instead of expanding. The mutual attraction of all this matter….gravity….should in fact be slowing down the expansion! ….yet the expansion accelerates! Why? Scientists have no clue. It is a mystery. So around 1998 physicists had literally invented the concept of a mysterious energy field which cannot be detected, but which is pushing the various parts of the universe apart. This mysterious energy is called Dark Energy, Dark because it cannot be detected by any known means. To be sure, Albert Einstein had originally included a cosmological constant in his theory of general relativity which was meant to be a property of space which could supplement gravity, but which he soon abandoned. Dark energy represents this cosmological constant, but while the cosmological constant can vary over time, Dark energy will remain constant for all time.10 Dark energy is a property of space. It allows the model of the universe to continue its accelerating expansion in spite of the fact that gravity should be slowing us down.
Dark Matter and Dark Energy Now here is an interesting thought…..notice that these astrophysicists and scientists have found that more matter (Dark Matter) is needed to keep the galaxies and galaxy clusters from flying apart…then they discover that there is too much matter for the universe to be expanding as fast as it apparently is. So now we need more energy (Dark Energy) to make it expand faster. Seems like they’re digging a pretty deep hole. (As a wise man once said….”when you find yourself in a deep hole and it’s getting deeper ….it’s best to stop digging”). Anyway, no need to solve that oddity now….just understand what this ongoing discussion of Dark stuff is all about.
So, to summarize….the structure of the Universe is made of those particles which we can detect and which we know of. No secret here. Then there is this mysterious Dark Matter and Dark Energy which needs to make up the missing mass and the missing energy which we think we need to make all the theories work…..?? Yes….I think I would leave that to the physicists to discuss. It is enough for us to know that some very smart people are losing sleep over this. But not I….whatever is there, is there….we only need to figure it out. (That’s one of the great things about this universe…it doesn’t depend on us humans to save it….we’re only observers!)