Repeating itself incessantly From that vantage point, we are now in a "boom" phase that will ultimately (somehow) slow down, halt, reverse, and crush back down to extremely high temperatures and pressures. The cosmos will then miraculously rebound and re-ignite in a new big bang phase. This may sound like science fiction, but it is not: it is an accurate description of what happened after the first few moments of our universe.
The laws of physics as we know them today did not exist at the beginning of the universe. Instead, there was a huge explosion, called the Big Bang, which is when the universe started expanding at a rapid rate. As time passed, it cooled down enough for atoms to form, which created stars and galaxies. These objects create gravity which causes them to collapse in on themselves, creating even more heat and light in a process called nucleosynthesis. This will continue indefinitely until something stops it!
The universe has been expanding since its inception over 13 billion years ago, but it will one day come to a stop and start collapsing under its own weight. This event is known as cosmic inflation because the universe suddenly bursts forth into infinite size within a fraction of a second. This explosive expansion reduces the overall density of the universe, making it possible for certain particles to emerge from the chaos unharmed. Scientists believe that these particles are responsible for the origin of life on Earth, so they cannot be ignored when trying to understand how life began.
The cosmos goes through an eternal cycle of contracting in a huge crunch and re-emerging in an expanding big bang, with trillions of years of evolution in between. The universe's temperature and density do not reach infinity at any time during the cycle. Instead, they go through periodic increases and decreases as it repeats itself.
This cosmic expansion and contraction is called "the oscillation of the universe." It starts when the energy that was driving expansion becomes sufficiently weak that its effect can no longer prevent gravity from pulling everything together into a giant ball. This occurs when the energy density in the universe drops low enough for gravity to overcome it and cause a collapse.
As this happens, light particles called photons are emitted by every object in the universe due to the increase in their relative speed as they escape the gravitational pull of matter. They then begin to spread out across space, eventually reaching galaxies far away from the collapse site. As they travel through space, they encounter other galaxies which contain stars like our sun. When a photon hits a star, it may be absorbed by the star, causing it to heat up and emit radiation. Other photons pass through the star without affecting it in any way.
However, if there is too much material, the universe's expansion will slow and eventually cease. The cosmos will thereafter begin to contract. A contracting universe will become smaller and smaller as it gets hotter and denser, finally culminating in a fantastically compact inferno known as the Big Crunch, a type of reverse Big Bang. This scenario would be disastrous for any life that might exist in the universe.
The most common explanation for why the expansion of the universe is currently accelerating is that there is some unknown form of energy with negative pressure contributing about 75% of the total energy density of the universe. This energy causes the expansion of the universe to accelerate instead of slowing down as expected from gravity alone.
An alternative explanation for why the expansion of the universe is currently speeding up is called "quintessence". Here we will focus on the standard cosmological model since it provides a consistent description of our universe at all scales from the largest to the smallest ones. At the very large scale, we know that the universe is isotropic and homogeneous because all galaxies are found in clusters or not clustered at all. At the small scale, we know that the universe is smooth and has little structure except for galaxy clusters. Between these two extremes, there are all kinds of structures from huge superclusters down to individual stars.
At early times, the universe was extremely hot and dense and expanded rapidly due to quantum effects.