The universe is the sum total of all the planetary systems, stars, nebulae, galaxies, galactic clusters, black holes, and the atoms and molecules that make all these objects. It also contains all the energy we know about, along with dark matter, which has not been directly observed.
The universe is observed to be 13.7 billion years old and has been expanding since its birth in the Big Bang. Due to this expansion, the universe is now thought to be be around 91 billion light-years across.
Life Cycle of the Universe
The origin and evolution of the universe is the basis for the science of cosmology. There have been many theories about how the universe got started, based on observations of distant galaxies, deductions about the amount of mass in the universe, how quickly it is moving, and the over all expansion rate of the universe.
The Beginning of the Universe: the Big Bang
The formation event of the universe is called the Big Bang. It was the beginning of space and time, as well as the expansion of space. The early universe was in a hot, dense state that cooled as it expanded. The infant universe was shrouded until about 400 million years after the Big Bang, when the first stars began to form and the earliest galaxies developed.
The Expansion and Evolution of the Universe
At its birth, the universe was an expanding soup of sub-atomic particles. As it cooled and continued to expand, the hydrogen (along with some helium and lithium) in the infant universe began to combine under the influence of dark matter to create stars. The stars themselves made up the earliest galaxies. All the stars spent their lives creating heavier elements than hydrogen in their cores. When they died, those elements were scattered to the interstellar space in their galaxies. It became the seed material for the next generations of stars, and helped create planets and life.
The End of the Universe
There are many ideas about how (and if) the universe will end. The ultimate fate of the universe will be influenced by its density (that is, how much mass it has). Currently, cosmologists think that the universe will expand forever, and eventually will spend eternity approaching absolute zero. This means there will be no stars, no heated nebulae, no galaxies, no life. This is what they call the “heat death of the universe”. This will not happen quickly. The best timeframe for heat death is multiple trillions of years in the future.
Facts about the Universe
- The initial ‘state’ of the universe was called the Planck epoch. It lasted a tiny fraction of a second, and at that point all matter of all kinds was condensed into a tiny point.
- The period of “inflation” began at the end of the Planck epoch, and the synthesis of the original elements took place over a period of about 20 minutes. For the next 380,000 years, as the infant universe expanded and cooled, everything was a hot, dense plasma and liight could not pass through it.
- The period of recombination followed, and stable atoms of matter could be formed. This period also allowed radiation to propagate across the expanding universe.
- The initial “glow” of the Big Bang can be seen throughout space today as the cosmic microwave background radiation (CMB).
- Because the universe has now expanded out beyond 13.7 billion light-years, there are areas we will never be able to detect. They are beyond our “visual horizon”.
- The universe is composed of dark energy, dark matter, and ordinary matter (the atoms and molecules that make up everything we can detect).
- The expansion rate of the universe has sped up in “modern” cosmic time.
- It is not clear what the ultimate fate of the universe is. Currently some cosmologists consider that the universe will continue to expand, ultimately cooling off, or “freezing out”. That would result in the heat death of the cosmos.