The Big Bang Theory is the most widely accepted theory for how the universe started. At its most basic level, it states that the universe as we know it began with a small singularity and grew over 13.8 billion years to become the cosmos we see today.
Because current instruments do not allow astronomers to peer back in time to witness the birth of the universe, we have to rely on mathematical formulas and models to explain the Big Bang Theory. The "echo" of the expansion, however, can be seen by astronomers thanks to a phenomenon known as the cosmic microwave background.
While the majority of astronomers accept the theory, some theorists propose alternate explanations such as eternal inflation or an oscillating universe in addition to the Big Bang.
The term "Big Bang Theory" has been popular among astrophysicists for decades, but it became popular in the general public in 2According to NASA, the surrounding temperature was about 10 billion degrees Fahrenheit (5.5 billion degrees Celsius) in the first second after the universe started. The universe was full of fundamental particles like neutrons, electrons, and protons. As the universe cooled, these decayed or combined.
Because light couldn't pass through it, this early soup would have been impossible to look at. According to NASA, "the free electrons would have caused light (photons) to scatter in the same way that sunlight scatters from water droplets in clouds." However, over time, the free electrons collided with nuclei, forming neutral atoms. About 380,000 years after the Big Bang, this allowed light to shine through.
The cosmic microwave background is the name given to this early light, which is sometimes referred to as the "afterglow" of the Big Bang (CMB). Ralph Alpher and other scientists predicted it in 1948, but it was discovered almost 20 years later by chance. [Images: Looking Back at the Big Bang and the Beginning of the Universe]
According to NASA, in 1965, Arno Penzias and Robert Wilson of Bell Telephone Laboratories in Murray Hill, New Jersey, were building a radio receiver and picking up higher-than-expected temperatures. They initially assumed the anomaly was caused by pigeons and their feces, but even after cleaning up the mess and killing pigeons who attempted to roost inside the antenna, the anomaly persisted.
Many missions have observed the cosmic microwave background. NASA's Cosmic Background Explorer (COBE) satellite, which mapped the sky in the 1990s, is one of the most well-known space missions.
The BOOMERanG experiment (Balloon Observations of Millimetric Extragalactic Radiation and Geophysics), NASA's Wilkinson Microwave Anisotropy Probe (WMAP), and the European Space Agency's Planck satellite have all followed in COBE's footsteps.
Planck's observations, first published in 2013, showed that the universe was older than previously believed, at 13.82 billion years old rather than 13.7 billion years old. [Related: What Is the Age of the Universe?] (The research observatory's mission is ongoing, and new CMB maps are published on a regular basis.)
However, the maps raise new questions, such as why the Southern Hemisphere looks slightly redder (warmer) than the Northern. According to The Big Bang Theory, the CMB would look mostly the same no matter where you looked.
Astronomers can learn about the universe's composition by studying the CMB. Researchers believe that the majority of the universe is made up of matter and energy that cannot be "sensed" with traditional instruments, hence the terms "dark matter" and "dark energy." Planets, stars, and galaxies account for just 5% of the universe's total mass.
While astronomers were able to see the universe's beginnings, they were also looking for evidence of its rapid expansion. According to one theory, our universe expanded faster than the speed of light in the first second after its birth. That, by the way, does not violate Albert Einstein's speed limit, as he stated that the maximum speed that anything can travel within the universe is light. That was not the case with the expansion of the universe.
Astronomers announced in 2014 that they had discovered evidence in the CMB for "B-modes," a type of polarization that occurred as the universe grew larger and caused gravitational waves. The team spotted evidence of this using an Antarctic telescope called "Background Imaging of Cosmic Extragalactic Polarization", or BICEP2.
In March 2014, lead researcher John Kovac of the Harvard-Smithsonian Center for Astrophysics told Space.com, "We're very confident that the signal that we're seeing is genuine, and it's on the sky."
However, by June, the same team claimed that galactic dust had obstructed their field of view, causing their findings to be skewed.
In a press conference reported by the New York Times, Kovac said, "The underlying takeaway has not changed; we have high confidence in our findings." "New data from Planck suggests that pre-Planckian dust forecasts were too low," he added.
In September, the Planck findings were pre-published on the internet. By January 2015, researchers from both teams had "confirmed that the Bicep signal was mostly, if not entirely, stardust," according to another New York Times article.
Separately, gravitational waves have been confirmed when discussing the movements and collisions of black holes a few tens of millions of times the mass of our sun. Since 2016, the Laser Interferometer Gravitational-Wave Observatory (LIGO) has detected these waves several times. As LIGO's sensitivity improves, it's expected that finding black hole-related gravitational waves will become more common.
Not only is the universe expanding, but it is expanding at a greater rate. This means that no one will be able to see other galaxies from Earth or any other location within our galaxy in the future.
In a March 2014 Space.com article, Harvard University astronomer Avi Loeb said, "We can see distant galaxies moving away from us, but their speed is increasing with time."
As a result, if you wait long enough, a distant galaxy will eventually exceed the speed of light. That means that not even light will be able to bridge the chasm that is forming between that galaxy and us. Once their galaxy moves faster than light in relation to us, extraterrestrials on that galaxy will have no way to communicate with us or send any signals that will reach us."
Some physicists believe that the universe we see is only one of many. Different universes will coexist with each other like bubbles lying side by side in the "multiverse" model. According to the theory, different parts of space-time grew at different rates during the first great push of inflation. This could have resulted in the creation of different sections — universes — with potentially different physics laws.
"It's difficult to construct models of inflation that don't lead to a multiverse," Alan Guth, a theoretical physicist at the Massachusetts Institute of Technology, said at a press conference about the gravitational wave discovery in March 2014.
"It's not impossible, so I think there is still some research to be done. However, most inflation models lead to a multiverse, and evidence for inflation will force us to consider [the idea of a] multiverse seriously."
While we understand how the universe we see came to be, it's possible that the Big Bang was not the universe's first inflationary time. Some scientists think that we live in a universe that goes through periodic cycles of inflation and deflation, and that we are currently experiencing one of these cycles.