The Big Bang Theory is the most accepted cosmological model explaining the origin and evolution of the universe. The theory says that all the energy and spacetime in the universe were initially contained in an infinitely dense, hot, and incredibly tiny singularity. A massive expansion is believed to have started out from that super hot and super dense speck about 13.8 billion years ago.
The event is often described as an explosion, although it was actually an expansion of space itself, rather than material being flung out into existing space faster than the speed of light. However, it did release a tremendous amount of matter and radiation. As this dissipated, subatomic particles and atoms were created. Gravity eventually pulled them together to form stars and galaxies across millions of years, while the space itself kept stretching out — as it still does, according to the Hubble-Lemaître's law of the expansion of the universe.
The Big Bang Theory is supported by this law, which states that galaxies are drifting apart at a rate that increases as the distance between them increases (galaxies are moving away from Earth at a speed that is proportional to their distance). Astronomers know this due to a noticeable shift in the light that galaxies emit toward the end of the visible spectrum —a phenomenon called "galactic redshift." In other words, the further away a galaxy is, the more red-shifted its light is, and the faster it is moving.
We've said before that the Big Bang Theory has a high level of acceptance in the scientific community. But why is that? First things first, the aforementioned Hubble-Lemaître's law constitutes an important piece of empirical evidence for the Big Bang and the correlated idea of an expanding universe. But there are others.
The existence of cosmic microwave background radiation (CMBR) is one of them. The CMBR is an electromagnetic signal in the microwave band that can't be linked to any object in the universe (that is why it is called "background radiation"). It was discovered by accident in 1964 when two scientists from the Bell Telephone Laboratories, Arno Penzias and Robert Wilson, were testing a microwave antenna intended for satellite communication. This microwave radiometer continually detected an "excess radio noise" that was uniform in all directions and eventually was found to come from beyond our galaxy.Temperature variations in the cosmic background radiation measured with the satellite COBE. Source: Large Norwegian Encyclopedia
In 1948, George Gamow, Ralph Alpher, and Robert Herman studied the nucleosynthesis of light elements in the Big Bang. They had theorized that in order for that process to occur, the universe had to be extremely hot, and due to the continuous expansion, there could be remnants of this extreme heat in the form of microwave wavelengths. What Arno Penzias and Robert Wilson observed in the Bell Telephone Laboratories was then concluded to be this leftover heat of the expansive process that gave birth to the universe.
Other evidence is that the amounts of helium, hydrogen, deuterium, tritium, lithium, and other trace elements in the universe are exactly what it has been theorized that they should be if the Big Bang took place. In other words, the Big Bang Theory predicts these chemical elements should be found in particular abundance due to the "explosion" of the singularity, and scientists have found it. For example, the theory indicated that if there was a Big Bang, the amount of helium in the universe would be about 25 percent. And in fact, helium accounts for 25 percent of atoms in space.
According to simulations, galaxy formation and evolution can also be considered evidence for the Big Bang Theory, mainly because of the way that they've organized themselves in large structures, such as clusters and superclusters. There are other lines of evidence, but the observation of redshift in space, CMBR, high quantities of light elements, and galaxy evolution are what scientists call "The Four Pillars of the Big Bang."
There is no empirical evidence that contradicts the Big Bang Theory. But like all theories, Big Bang is not perfect, and astronomers developed other explanations for the birth of the universe.
One of them is the steady-state model, which explains the expansion of the universe by positing an eternal creation of matter, maintaining its density over time. In this model, the universe is ageless and infinite. It has no beginning, no end, and no evolution. It only changes because the continuous expansion of the universe is always producing new matter (especially hydrogen), and the new matter gives birth to new stars.
The steady-state model was first challenged in the 1950s when radio galaxies were spotted at such large distances away and in such states that they didn't fit in the steady-state model. In a Big Bang universe, due to the travel time of light, astronomers can see distant galaxies as they were in the past; therefore, galaxies farther away should be more densely crowded together than nearby galaxies. Under the steady-state model, you would expect to find the same average density of galaxies everywhere (and at every time) — but in fact, there are more radio galaxies at great distances than nearby. This demonstrates that the universe has changed over time.
The steady-state model fell as the other pillars of the Big Bang Theory were found, and especially after the discovery of quasars and of the CMBR, the steady-state model was abandoned in favor of the Big Bang Theory in the 1960s.
Another alternative is Eternal Inflation. This theory posits that the inflation that occurred in the period immediately after the Big Bang never stopped and that even now, new universes are coming into existence, possibly with different physical laws.
There is also the Oscillating model, which states that there is an endless series of Big Bangs, followed by Big Crunches that restart the cycle. This theory also has a number of variations.
And there are other, more esoteric theories that have come from work in string theory and quantum gravity, such as the holographic theory, which states that the universe is a two-dimensional hologram projected onto three-dimensional space.
Using the Big Bang Theory, the universe can be divided into several stages of development.
First, there was an initial singularity where all the energy and spacetime of the universe were "trapped" in an extremely dense, hot speck. At this point, the universe is theorized to have spanned just 10-35 meters (1 Planck length) with a temperature of over 1032°C (the Planck temperature). Quantum fluctuations led to a period of ultra-hot cosmic inflation, considered the beginning of the ultra-fast, exponential expansion of the universe.
Cosmic inflation also founded the initial properties of the universe. It is in these stages that quarks combined to form hadrons, electrons, and protons collide to form neutrons and neutrinos, neutrons and neutrinos re-formed into new proton-electron pairs, etc.Source: NASA/WMAP Science Team
As the universe cooled further, protons and neutrons were bound into light atomic nuclei of elements such as hydrogen, helium, and lithium. This is called Big Bang Nucleosynthesis (BBN), and it happened in the period between approximately 10 seconds to 20 minutes after the Big Bang. Neutral atoms and photons of the CMBR originated a bit later, in a period named "recombination."
As the temperature and density of the universe continued to fall, ionized hydrogen and helium atoms captured electrons to form neutral atoms. With the electrons now bound to atoms, the universe finally becomes transparent to light. At the same time, photons are released from their interaction with electrons and protons and can move freely. It is these photons that we can detect in the CMBR.
Then, there is a period often referred to as the "dark ages" because, at this point, the first atoms had been formed, but they had not yet coalesced into stars. Although photons existed, there were no stars to give off visible light. It remained like this until the formation of the first stars, roughly 400 million years after the Big Bang. This period is also called re-ionization.
At this time, denser regions of gas collapsed under their own gravity and became dense and hot enough to trigger nuclear fusion reactions between hydrogen atoms and form stars and galaxies. The emitted ultraviolet light from this star formation re-ionized the surrounding neutral hydrogen gas, causing the universe to become transparent to ultraviolet light.
And then we reach the present times, in which we have an accelerating expansion of the universe, a period of cosmic acceleration where more distant galaxies are receding faster. According to some calculations, we entered this period about 5 billion years ago, and we don't really know where it will take us in the future.
Now it's time to talk about the ultimate fate of the universe. Some theories include the Big Crunch, which states that the universe will eventually deflate and re-collapse; and the Big Bounce, which states that after this "deflation", there will be another Big Bang, implying that the universe is cyclical and the Big Bang might not be the birth of the universe but the beginning of a new cycle.
However, most observations indicate that the expansion of the universe will continue forever. The problem is that the universe keeps cooling down as it expands, so it could eventually reach a state of thermodynamic equilibrium where there is not enough energy left to sustain processes that increase entropy. In other words, no more work can be extracted from the universe. This is called the heat death of the universe or the Big Freeze.