The present-day answer is simple, if somewhat frustrating: we don't know!
We don't know WHY the space in our Universe began to expand about 13.7 billion years ago, and we also don't know why this expansion began to speed up about five billion years ago. We only know that both statements are true.
In that way we are like Johannes Kepler, who figured out that planets are going around our Sun in eliptical orbits. He couldn't say WHY this is occuring, he only knew that it IS. A future scientist, Isaac newton, was able to provide the WHY.
If it turns out that one and only one future scientist does for expansion what Newton did for eliptical orbits -- give the WHY -- then s/he will go down in history as another Newton or Einstein. Most such breakthroughs today, however, are accomplished by several people working together, and incrementally on earlier work.
It is not so much that the universe is expanding, but rather the rationalization for its expansion that provides evidence to support the Big Bang Theory. The Big Bang supports interpreted observational evidence of the Cosmic Microwave Background Radiation (CMBR) barrier that there is an evolutionary expansion of the universe which promotes a finite age for the universe.
expansion of the universe as shown by the Hubble constant3K universal microwave background radiationetc.
The spectrum of galaxies, including the distribution of their light, shows evidence of the expansion of the universe, as predicted by the Big Bang theory. The redshift of galaxy spectra indicates that galaxies are moving away from us, suggesting that the universe is expanding. This cosmic expansion provides crucial support for the Big Bang model of the universe's origin.
Evidence. See related question.
The theory that suggests for every big bang there is a crunch is known as the "Big Crunch" theory. It proposes that the expansion of the universe will eventually stop and reverse, leading to a contraction of the universe back to a singular point, followed by another big bang.
It is not so much that the universe is expanding, but rather the rationalization for its expansion that provides evidence to support the Big Bang Theory. The Big Bang supports interpreted observational evidence of the Cosmic Microwave Background Radiation (CMBR) barrier that there is an evolutionary expansion of the universe which promotes a finite age for the universe.
It is not so much that the universe is expanding, but rather the rationalization for its expansion that provides evidence to support the Big Bang Theory. The Big Bang supports interpreted observational evidence of the Cosmic Microwave Background Radiation (CMBR) barrier that there is an evolutionary expansion of the universe which promotes a finite age for the universe.
expansion of the universe as shown by the Hubble constant3K universal microwave background radiationetc.
Other evidence supporting the Big Bang theory includes the cosmic microwave background radiation, the abundance of light elements in the universe, and the large-scale structure and distribution of galaxies in the universe. Additionally, observations of the universe's expansion and the Hubble law provide further support for the Big Bang theory.
Inflation theory. This theory suggests that in the first moments after the Big Bang, the universe underwent a rapid and exponential expansion, causing it to grow exponentially in size. This concept helps explain certain observations of the universe, such as its large-scale structure and uniformity.
The big bang theory states that the galaxies in the universe are continuously moving away from each other due to the expansion of space. This expansion suggests that the universe began from a hot, dense state approximately 13.8 billion years ago.
The spectrum of galaxies, including the distribution of their light, shows evidence of the expansion of the universe, as predicted by the Big Bang theory. The redshift of galaxy spectra indicates that galaxies are moving away from us, suggesting that the universe is expanding. This cosmic expansion provides crucial support for the Big Bang model of the universe's origin.
Evidence. See related question.
They do not, per se. The perpetual expansion of the universe does.
Some refer to it as the Great Expansion, because the Universe didn't really "bang", it just began a very rapid expansion.
The big bang theory which is a theory of cosmology holding that the expansion of the universe began with a gigantic explosion between 12 and 20 billion years ago
The theory that suggests for every big bang there is a crunch is known as the "Big Crunch" theory. It proposes that the expansion of the universe will eventually stop and reverse, leading to a contraction of the universe back to a singular point, followed by another big bang.