Cosmology is the study of the overall structure of the universe.
And just what is the Universe?
Quite simply, it is everything that exists.
However, we cannot observe everything in the Universe from earth as some things are dark such as brown dwarf stars, planets, and dark matter and we just cannot see them, additionally there are parts of the universe whose light has not yet reached us in this part of the Universe.
The observable universe is the universe that reveals itself through electromagnetic radiation that can be detected on Earth.
Because the radiation travels at a finite speed we actually look back in time when we look into the cosmos.
Astronomers observe some rather interesting and perplexing structure in the Current Universe.
That structure can tell us much about the History of the Universe.
It can also tell us what we can expect for the Future of the Universe...
A computer simulation depicting a large chunk of our universe
Image is the work of G. L. Bryan, M. L. Norman, UIUC, NCSA, GC3 and sourced from the The Window of the Universe.
When Astronomers probe the deepest regions of space they are actually looking back in time, this is simply because of the finite speed of light.
Light moves at the speed of 300,000,000 meters/second (186,000 Miles/second).
At short distances, like from satellites in orbit of Earth, the light travel time is only a fraction of a second. However, the Sun is so distant from Earth (150,000,000 Kilometers) that its light takes 8 minutes to reach us.
So when you look at the sun in the sky (never look at it directly, you'll go blind) you see it as it was 8 minutes ago.
As distances get larger so does the time frame "look-back time", our closest stellar neighbor, Alpha Centauri (also known as Rigil Kentaurus), is so far away that its light travels for 4.3 years before reaching us.
When we look at the closest spiral galaxy to our own Milky Way, the Andromeda galaxy, we see it as it was 2 million years ago (when Homo Sapiens first began walking the Earth).
Picture of the position of Alpha Centauri and sourced from wikipedia
The theory that best explains the currently observed state of the universe is the Big Bang theory.
This theory states that in the beginning, the universe was all in one place.
All of its matter and energy were squished into an infinitely small point, a singularity.
The laws of physics at that instant are not understood at all.
But something caused the universe to explode, and thus began the expansion that we witness today.
The early universe was small, so everything happened very quickly compared to the timescales on which events happen for the present universe.
At the start, the universe was very small, dense, and very hot.
This stage was called the primordial fireball. For the first second, only elementary particles, such as protons, neutrons and electrons, could exist.
But the universe quickly cooled and expanded.
For about the next 500,000 years electromagnetic radiation was the most important thing in the universe and hence this time was known as the radiation era.
Once the universe had cooled to the point where the simplest atoms (hydrogen) could form, radiation no longer dominated and matter took over, begining the matter era.
The cosmic microwave background radiation was produced at this time, as light that had been trapped by free electrons escaped when the electrons combined with protons to form hydrogen.
So how old is the universe?
In principle, that's an easy question to answer.
With the rate at which the universe is expanding, called the Hubble constant, astronomers can determine how long ago the universe was at size zero - the age of the universe. In practice, it is not so easy.
Despite its name, the Hubble constant is not constant in time.
It changes as gravity takes hold of the universe and slows the expansion.
How much it changes depends on the density of the universe.
To determine this density, astronomers need to measure the distances to very distant galaxies, which is a very difficult task.
Although there is much debate over the current age of the universe among astrophysicists, they do agree that it is somewhere between 10 and 20 billion years old, which is still a pretty good estimate in astronomical terms.