What Was There Before the Big Bang?
Unraveling the mysteries of the universe's earliest moments, scientists have made significant strides in understanding the origins of the cosmos.
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The Big Bang theory explains the evolution of the universe from a starting density and temperature that is currently beyond humanity's capability to replicate. As a result, the most extreme conditions and earliest times of the universe are speculative, and any explanation for what caused the Big Bang should be taken with a grain of salt. Nevertheless, it's natural to ask questions like what was there before the Big Bang. However, another answer is that the very question may not make as much sense as we initially thought. We know how to parse that sentence, and we know what it means to talk about what came before the Big Bang, but it could be that when it comes to the Big Bang, the sentence actually doesn't mean anything. It could be that the Big Bang was the point where time itself began to exist, roughly 13.8 billion years ago. If that's the case, then we can't go further back in time than the origin of time itself.
To understand how the universe came to be, scientists have combined mathematical models and observations to develop workable theories that explain the evolution of the cosmos. The Big Bang Theory, which is built upon the equations of classical general relativity, indicates a singularity at the origin of cosmic time. However, the physical theories of general relativity and quantum mechanics, as currently realized, are not applicable before the Planck Epoch, which is the earliest period in time in the history of the universe. To correct this would require the development of new treatments of quantum gravity. Certain quantum gravity treatments imply that time itself could be an emergent property, leading some physicists to conclude that time did not exist before the Big Bang. Others are open to the possibility of time preceding the Big Bang.
Einstein's equations actually allow gravity to also be repulsive, meaning it can push outward rather than just pulling inward. This is something we have never experienced, because the gravity created by a rocky object like Earth or a compact object like a star is always attractive. However, Einstein's math shows that if you have energy uniformly spread throughout a region of space, rather than a rocky object isolated in space, it can yield repulsive gravity. If the very early universe was filled with a uniform bath of this energy, known as the inflaton field, it would have been subject to repulsive gravity. Repulsive gravity would have pushed everything apart, causing everything to rush outward.
The Big Bang theory suggests that the "bang" may have been a spark of repulsive gravity, operating within a tiny region of space that pushed everything apart. One common misconception about the Big Bang model is that it fully explains the origin of the universe. However, the Big Bang model does not describe how energy, time, and space came into being, but rather it describes the emergence of the present universe from an ultra-dense and high-temperature initial state. It is misleading to visualize the Big Bang by comparing its size to everyday objects, as the size of the universe at the Big Bang refers to the size of the observable universe, not the entire universe.
To better understand the universe at its earliest state and subsequent evolution, cosmologists have developed a chronology of the universe, which describes the history and future of the universe according to Big Bang cosmology. The first pico-second of cosmic time, which is 1,000,000,000,000th of a second, includes the Planck Epoch, during which currently established laws of physics may not apply. This period is followed by the emergence and stages of the four known fundamental forces: gravitation, electromagnetism, weak interactions, and strong interactions. The expansion of space itself and supercooling of the still immensely hot universe occur during this period. Tiny ripples in the universe at this stage are believed to be the basis for large-scale structures that formed much later.
If we were to look at the universe one second after the Big Bang, we would see a 10 billion-degree sea of neutrons, protons, electrons, anti-electrons (or positrons), photons, and neutrinos. At this point, neutrinos decouple and form the cosmic neutrino background. If primordial black holes exist, they are also formed around one second after the Big Bang. Composite subatomic particles emerge around two minutes after the Big Bang, including protons and neutrons. Conditions become suitable for nucleosynthesis around 25% of the protons and all of the neutrons fuse into heavier elements, initially deuterium. This deuterium itself quickly fuses into mainly an isotope of helium. By 20 minutes after the Big Bang, the universe is no longer hot enough for nuclear fusion but still too hot for neutral atoms to exist or photons to travel far.
The universe is still an opaque plasma, with electrons combining with helium nuclei around 18,000 years after the Big Bang. As the universe cools, it becomes dominated by matter rather than radiation, and neutral helium atoms form. Later, hydrogen and helium hydride react to form molecular hydrogen, which is the fuel needed for the first stars. Around 370,000 years after the Big Bang, neutral hydrogen atoms finish forming and become transparent for the first time, allowing photons to escape. These photons, known as the cosmic microwave background or CMB, are still detectable today and provide a glimpse into the universe's earliest moments.
While we may not know what came before the hot dense state of the early universe or how it originated, scientists have proposed various hypotheses and explanations. One possibility is that the Big Bang was not the first event in reality, but rather a localized event within a larger universe or multiverse. This could mean that there was a grander realm of space that existed before our own part of space, and that our universe is just one small part of a larger cosmic landscape. This idea is supported by some theories that propose multiple Big Bangs or eternal inflation, where the universe cycles through periods of expansion and contraction.
Some models suggest that the Big Bang was caused by quantum fluctuations or the absence of perceived time before the Big Bang. The Hartle-Hawking no boundary condition proposes that the whole of space-time is finite, with no singularity at the beginning of time. Other models, such as brain cosmology and pre-Big Bang models, propose that the universe has been cycling through periods of expansion and contraction, with collisions between brains or other particles leading to the creation of new universes. Eternal inflation proposes that universal inflation ends locally in random places, creating bubble universes that expand and create new Big Bangs.
While we may not have a definitive answer to what happened before the Big Bang, scientists will continue to search for answers. Theories and models will continue to evolve as new evidence emerges and our understanding of the universe grows. Ultimately, the mystery of what came before the Big Bang remains one of the greatest unsolved questions in science.
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