Here, in simplified form, is the latest thinking on the subject.
The Universe was created by a big bang which occurred about 13 billion years ago. Prior to the big bang, all the matter which exists in the universe as we know it today was compressed into an area perhaps no larger than the head of a pin.
In the years after the big bang, the universe rapidly expanded. The universe may be thought of as a balloon blowing up. As air is pumped into a balloon, the dots on the surface get further and further apart. Similarly, as the universe grew older, the objects in it got further and further apart.
Nevertheless, some objects randomly came together and fused into larger objects. As time went on, more particles unified and bigger objects formed. Eventually, at some point, the relatively large objects of electrons, neutrons and protons were created. These then came together to form the smallest atom, which is the hydrogen atom, which consists of one proton, one neutron and one electron.
When enough hydrogen atoms gathered together, stars were formed. The first stars were small, but some of them came together to form bigger stars. These stars became giant furnaces, very hot inside. These giant stars became nuclear reactors in which smaller hydrogen atoms were cooked and fused together under enormous pressure to become heavier elements, including especially iron and nickel.
Because the universe was smaller then, we can reason that the stars were bigger and closer together than the stars we have now. It requires a large star to sustain a thermo-nuclear reaction. Eventually, the first generation of stars all exploded. They spewed the products of their thermo-nuclear furnaces all over the universe. It is from the debris of the explosion of one of these first generation of stars that our own solar system was formed. Scientists believe that our sun is a second generation star, although it could be a third or even a fourth generation star.
Our planet Earth is made primarily of iron and nickel. Although water is on the surface, deep inside the Earth is a hot molten core of iron. This iron is constantly swirling around. This creates the gravitational field which holds us down on Earth.
Other planets such as Jupiter we believe does not have a core of iron as the Earth does or else the core is too small to create much of a gravitational field, so Jupiter does not have solid ground as Earth does. Jupiter is primarily a ball of vapor and gas.
It has always been assumed that the universe consists primarily objects we can see, such as the sun and the stars, many of which presumably have planets. However, we now realize that actually most of the universe cannot be seen at all. At the original formation of the universe, everything consisted of very small particles. Some of these particles came together and formed atoms and later stars. However, the vast majority did not. Those particles did not hit anything. They did not fuse together and form larger objects. Instead, those particles just sped outwardly in all directions.
The type of objects we are talking about include gamma rays and other objects too numerous to mention. We do not yet know what these objects really are, but we do know that they are in great abundance. We do not know whether they constitute 99% of the matter in the universe or merely 90%, but we can say with confidence that most of the matter in the universe consists of these very small particles, too small to see.
One reason we would like to know how many of these objects there are is that by knowing this, we can determine the ultimate destiny of the universe. If there is a lot of this so-called "dark matter", then there may enough matter in the universe to create a gravitational field to bring everything in the universe back together into one black hole as it was when the big bang occurred. If there is not so much dark matter or if it does not produce sufficient gravitational attraction, then the universe may go on expanding forever.
Without knowing the ultimate answer, logic tells us that there is enough matter in the universe to bring about an ultimate collapse. This would mean that there is an endless cycle in which the universe collapses, blows up and then collapses again, and so on forever. Otherwise, our universe would be much older than it is today.
The universe consists of all things that we know about or could possibly know about. The universe has limits. However, the cosmos is infinite. This means that there might be other universes out there, too far away to be detected. We had better hope that we never detect those other universes, because they might consist of anti-matter, which would cancel out our matter and cause our universe to vanish.
Our sun, which is a second generation star, was formed about 5 billion years ago. Our earth, which grew out of the same material which was created by the explosion of a previous star which formed our sun, was formed about 4.5 billion years ago. At first, our earth was molten and resembled a small star. However, Earth was too small to sustain the thermo nuclear reactions which power the sun, so eventually the earth cooled. A solid surface formed about one billion years ago. Comets from outer space bombarded the earth with water. The seas were formed. The earliest that Earth could have sustained life was about one billion years ago, but life probably actually arose more recently.
How was life created? This is the most intriguing question. Life as we know it requires water and, until one billion years ago, the earth was too hot to sustain liquid water, so we know that life on Earth must have formed within the last one billion years.
Actually, the evidence we have shows that the oldest life we can detect occurred 500 million years ago. How did that life form?
We now understand that all life on Earth contains DNA, which is a long complex carbon-based strand consisting of billions of molecules. Every plant and animal on Earth has DNA. This DNA contains the instructions of how to grow and replicate itself, thereby creating more life.
A single strand of DNA is unbelievably complicated, even in the smallest organisms. How is it possible that such a complicated thing was created in the short span of only one billion years?
Scientists have long theorized that life on Earth was created in some primordial soup. Somehow a soup formed consisting of water, carbon and other elements and by chance eventually some of these elements came together in a form which reproduced itself. The key feature of life it is that is able to create a duplicate of itself.
The problem that scientists have faced with this theory is that they have never been able to replicate this primordial soup. They have cooked all kinds of concoctions, mixing carbon and lots of other things together and have heated, mixed and stirred and so on under pressure and never have they come close to creating anything like life.
The conclusion is that life was created under conditions which do not exist on Earth today. Life was either created at the early stages when the Earth was still hot but cool enough to have water on the surface, or else life itself came from outer space.
Just about everybody now believes that life came from outer space. We are not talking about little green men. We are talking about relatively simple forms of life, very small, but containing DNA.
While it is possible that DNA was formed on Earth, the probabilities are that it came from elsewhere. The Earth has only been cool enough to sustain life for the last one billion years. However, the universe is 13 billion years old and for the last 12 billion of those years at least some spots of the universe must have been cool enough to allow for the creation of the first germs of life.
Then, how did life get here? That is the easy part. Every day, the earth is bombarded with hundreds of comets, asteroids and other objects from outer space. It was only necessary for one of these objects to contain, perhaps sealed deep inside, a single cell or virus capable of eating the carbon, iron and other elements already available on earth. Once that one cell arrived on Earth, it could quickly make two and then four and it would not take long before the entire Earth would be covered with life.
Probably what happened is this: All matter in our solar system today comes from a first generation star, which was created shortly after the big bang and lived for seven or eight billion years. Life was created on one of the planets of that star. When that star blew up, the seeds of life were scattered. Those seeds, spores or whatever contained a molecular structure of DNA which survived even while flying around in outer space for several billion years. Eventually, less than one billion years ago, those seeds of life landed on earth and became reactivated when mixed with the water which also arrived or liquefied at about the same time. From these seeds of life which came from outer space, grew all the living creatures that exist on Earth today.
The problem with seeding of DNA on earth by "outer space" is that ALL of the material located in the solar disk (including objects in the kupier belt i.e. comets and like) originated from the same source as the rest of the solar system and is about 4.5 billion years old.
But really, if DNA couldn't conceivable be formed in 1 billion years, how can one resolve that it could after X billion years???
Seems pretty imaginative if you ask me.
Which you didn't but thanks for the read it was fun.
john mitchell ( firstname.lastname@example.org )