Mammals, as a kingdom, are curious creatures, always exploring and investigating the unknown. Humans, with their advanced intellect and technological innovations take this curiosity to new levels. We have an intense desire to know everything. We are always searching for the answers to how things work and why things are the way the are. One key we have found to answering the questions of today is to learn what happened in the past. It is easier to understand where you are going if you already know where you've been.

Cosmologists take our study of the past as far back as they can. There are limits to our knowledge however (more likely we just haven't figured out how to pass those limits yet). We can only travel so far back in time before everything we have learned about our universe no longer applies. This is what we call the "beginning." Not because nothing existed before that beginning, but rather because we have no way of describing it. Before our universe was created, space and time did not exist as we know it. If we were somehow able to peer past our beginnings, none of our laws of physics would even apply. It is unlikely that we would even be able to comprehend what we saw.

It would seem that matter flung out into space in such a way would remain fairly uniform in size and shape. This is clearly not what our universe looks like today. Nowhere can there be seen galaxies or solar systems. Clearly the matter was not uniformly distributed. For the most part, the matter was uniform in density as it expanded in to space. It was not, however, perfectly uniform. The fluctuations in density around the creation of the universe were probably no larger than a very small fraction of a percent. In fact, nearly 500,000 years after the Big Bang, our own Milky Way galaxy was only estimated to be about ½% more dense than the surrounding regions. This does not seem to be a significant amount but in reality it is. You see, the more dense regions of the universe had stronger gravitational attraction to each other than their surrounding areas. This gravitational attraction would have slowed their expansion down ever so slightly. With the universe expanding at such an astounding rate, the slower expanding areas were basically left behind. Relative to the areas around them, the densities of the slower expanding areas became much larger.

Galaxies consist mostly of cold clouds of gas and dust. These clouds are called nebula and are mostly made up of hydrogen. For the most part the cloud remains stable and floats along within the galaxy. It does not take much, however, to turn this inactive cloud into a glowing ball of fire known as a star. All the cloud needs is for some of the atoms that make it up to be disturbed or agitated. As the first few atoms begin moving they come closer to other atoms. Their proximity to these atoms increases the gravitational attraction between the two atoms. As more atoms become attracted to each other, their mass increases and their ability to attract even more atoms increases. Fairly soon, you have a giant ball of hydrogen forming in space. As the atoms come closer together, they collide more often, generating heat. The heat generated from these collisions causes the ball of hydrogen to begin to glow faintly. This is a protostar. For more information on stars and how they form please visit the section of this web site entitled A Star's Life.

 

 

The planets, by far, make up the majority of the remaining 2% of mass. There are two distinct types of planets, the terrestrial planets (inner planets) and the jovian planets (outer planets.) The terrestrial planets (Mercury, Venus, Earth, and Mars) are relatively small, rocky, and composed of heavier elements. The jovian planets (Jupiter, Saturn, Uranus, and Neptune) are considerably larger than the terrestrial planets, gaseous, and composed of much lighter elements. Since the jovian planets are mostly made of gas they are also referred to as the "gas giants." It should be noted that Pluto, although considered a planet, is not classified with either type of planets. This is because Pluto is clearly an outer planet (Pluto is the farthest planet from the sun) yet it shares none of the features of the outer planets. Astronomers believe that Pluto was formerly a moon of Neptune that was somehow knocked out of orbit. This could explain why Pluto's features are much more similar to a moon of one of the outer planets instead of one of the outer planets themselves.

 

 

Perhaps we have the capability to understand what happened before the beginning and our knowledge just has not reached that point yet. Perhaps we shall never truly understand what happened before time was created. We can, however, travel back approximately 15 billion years (give or take five billion years or so), to where the universe was simply a point in space. This is where our journey begins, at a point in time now known as the Big Bang.

Fifteen billion years ago everything in our universe existed in singularity. A singularity is a point in space that is infinitely small but can have an extremely large amount of mass in it. It is unclear what happened next but something caused this singularity to explode, hurtling all the mass that was in it outward at an incredible rate. The energy from this explosion caused reactions in some of these particles, known as quarks, causing them to combine and form the first protons. Later, neutrons and electrons would form and start the basis for all of the elements that we know.

Again, returning to our own Milky Way galaxy, approximately 1.2 billion years after the Big Bang, the Milky Way's relative density was closer to twice that of its surrounding areas. It was somewhere around this time, cosmologists believe, that the inner portions of the galaxy began to form.

For reasons not yet clearly understood, all of the mass from the nebula does not always go into the formation of the star. Sometimes, other bodies are formed with the matter left unused by the forming star. These bodies are what we call the planets, of which our solar system has nine. Our sun takes up about 98% of all the mass in our solar system. The remaining 2% went into the formation of every other body in our solar system including all of the planets, moons, comets, and asteroids.

 

 

The reason for the distinction between the two types of planets has much to do with their proximity to the sun. The energy released from the sun during its formation made the areas closer to the sun considerably warmer than those farther away. This meant that the lighter elements, like hydrogen and helium, would exist in a gaseous form. You'll remember that gases are much more active than liquids or solids. Because of their more active state, it was harder for them to be caught by the forming planet's gravitational fields. Only the heavier elements remained in a solid form where they could be captured by the forming planets. The planets closer to the sun would not have access to hydrogen during their formation. Where the outer planets formed, they were far enough away that hydrogen existed in a solid form and could be captured by the planet's gravitational field. Without access to hydrogen, the most abundant element in our solar system, the inner planets could not become nearly as large as the outer planets. They did not have the resources to make themselves that large. You may wonder why we refer to the outer planets as the gas giants if hydrogen exists as a solid that far away from the sun. The planets are heated as they form, just like a star.

The reason they do not produce energy like a star is because they do not have enough mass to reach the required temperature to perform nuclear fusion (this is discussed more fully in A Star's Life.) As the planets warm, hydrogen can move from a solid state to a gaseous one. Now that it is caught so deeply within the planet's gravitational field, however, it cannot escape.

One thing we have not mentioned at all yet is the end, be it of a planet, star, or even the universe itself. Besides the simple answer that this is a section on the beginnings there is good reason for this. The death of a star will be discussed in A Star's Life. The universe, however, is a different story all together. When will the universe end? Will it ever end? The real answer is that we do not know. The answer is dependent upon the exact amount of mass that is in the universe, which is something we have yet to determine. If enough mass exists, then eventually the gravitational force of that mass will stop the expansion of the universe and cause it to contract and implode upon itself. However, if there is not enough mass, it is likely that the universe will continue expanding forever. We do not know what will happen. In any event, if the universe is coming to an end, it certainly won't be any time soon.

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