Big Bang Theory: Overview

Introduction

The Big Bang theory is the current dominant scientific theory explaining the evolution of the observable universe. While there are many variants of the theory, the version most commonly accepted as part of the standard cosmological model (SCM) suggests that the universe came into being and expanded about 13.7 billion years ago out of an extremely dense, high-temperature state, or singularity. In a phase of increased inflation, time, space, and matter as we know them came into existence. Since the 1960s, the Big Bang theory has been the generally accepted theory of how the universe emerged, as it aligns with the general theory of relativity and the cosmological principle. A handful of scientists refute the claim that the universe began with a Big Bang, however, often positing instead that the universe had no beginning and has no end.

Ironically, the term "Big Bang" was originally coined as a term of derision by astronomer Fred Hoyle, who found the idea of the universe exploding from a single atom or "cosmic egg", as was suggested by George Lemaître, preposterous. Opponents of the Big Bang theory claim that it has become incontestable in the field of cosmology, despite being based on what they say is untenable observational evidence.

The popular image of the Big Bang as an "enormous explosion" in space is technically inaccurate; prior to the Big Bang there could not have been three-dimensional space in which the reaction could occur, since the Big Bang is credited with creating three-dimensional space. The question of when the Big Bang occurred and what was happening prior to the Big Bang is also in a sense misplaced, since the Big Bang also created time. The question of "when" and "before" cannot be answered in the terms of physics as it is commonly understood.

Time, in particular, is a difficult problem for scientists; it defines our existence as linear beings who are clearly moving forward on some sort of timeline that has existed for as long as humans have, and presumably could not end without ending our existence, as well. Only by examining the evidence can cosmologists hope to even tentatively favor a particular model for the creation of the universe.

Understanding the Discussion

Astronomy: The study of celestial objects, apart from Earth, and their composition and evolution.

Cosmology: A branch of astrophysics (in turn a branch of astronomy) that deals with the large-scale structure of the universe and studies its composition and evolution.

Inflation Theory: A widely accepted theory that resolves certain very complex cosmological problems. The inflation theory claims that, right after the Big Bang, the expansion of space and matter from the point of the original singularity occurred at an acceleration in excess of the speed of light, thereby accounting for the observed shape of the universe today, which deviates from the shape it should have according to a linear expansion in the Big Bang Theory.

Theory: In the scientific sense, a theory is a factual model that explains some aspect of the natural world, by incorporating and referencing other facts, laws, inferences, and tested hypotheses to form a larger picture of the aspect under inspection. Scientific theories are never considered absolute, and are judged on the evidence to be more or less likely than other competing theories. A theory such as the Big Bang theory is judged on how likely it is to be true, by analyzing the verifiable evidence.

Redshift: Redshift is the degree to which a source of electromagnetic radiation increases in wavelength, or shifts toward the red end of the light spectrum. Astronomer Edwin Hubble observed that the further a galaxy is from our own, the redder its light becomes, and used this observation to conclude that the universe is expanding.

Singularity: A singularity is a point in space-time that has an infinite density but no volume. In the Big Bang theory, it is assumed that the universe began with such a singularity that circumscribed space-time itself.

History

The Big Bang theory has its roots in astronomical observations and theories that emerged in the early twentieth century and began to indicate the universe was expanding, including work by astronomer Vesto Slipher and mathematician Alexander Friedmann. Georges Lemaître, a Roman Catholic priest and astronomer, proposed the expansion of the universe in 1927. In the following years he began to formulate a theory of the origin of the expansion. In the scientific journal Nature, Lemaître eventually outlined in 1931 what he called his "hypothesis of the primeval atom," which relied on Albert Einstein's laws of relativity.

Einstein himself felt that Lemaître had misunderstood his math, and publicly criticized Lemaître's hypothesis. Lemaître's theory took hold, however, particularly because it coincided with astronomer Edwin Hubble's observations apparently confirming the universe is expanding, which supported the existence of the primeval atom. "Hubble's law," which is often cited when explaining the Big Bang theory, states that the amount of light coming from a distant galaxy is proportional to its distance from our own. Still, many scientists rejected the idea of a Big Bang, with some arguing that the suggestion of a definitive beginning of the universe was too similar to common religious views of creation.

Despite there being over one hundred known elements in the universe, approximately 99.99 percent of the universe is made up of only two: hydrogen and helium. With this figure in mind, Ralph Alpher and George Gamow attempted during the 1940s and 1950s to explain the existence and synthesis of these two important atoms. Gamow and Alpher determined that there should have been approximately one helium nucleus for every ten hydrogen nuclei at the end of the Big Bang; this hypothesis was confirmed by contemporary astronomy, which found the one-to-ten ratio present in the modern universe.

During the 1950s, Fred Hoyle and Geoffrey Burbidge studied how heavy elements formed. Most cosmologists, physicists, and astronomers claimed that the Big Bang explained their formation as easily as it explained the formation of the lighter elements, such as hydrogen and helium. Hoyle claimed that heavier elements seemed to have been formed at extremely high temperatures, and proposed instead that they were formed inside stars, and that heavier elements were created from the combination of lighter elements. This idea eventually led Hoyle to conclude that the Big Bang theory was inaccurate, and he posed a different cosmological model that had no beginning and no ending. This new model, the "classical steady-state theory," has not garnered the same following as the SCM, but its adherents claim the evidence is on their side.

One of the strongest arguments in favor of the Big Bang and the SCM came in 1965, with the discovery of cosmic microwave background radiation, or CMB. CMB is believed to be the remains of the Big Bang. The discovery and subsequent observations of the radiation essentially discredited previous steady-state theories.

However, one challenge to the Big Bang theory came with observations that the CMB was relatively flat or smooth, without any of the wide variations in temperature the accepted laws of physics would predict. In the 1980s researcher Alan Guth developed a revision that would explain these observations: the early universe expanded exponentially at first, in a so-called inflationary epoch, but later expansion continued without acceleration. This theory of cosmic inflation, as it became known, suggested the existence of a hot, dense, but finite state rather than a singularity. Many variants of this idea, as well as competing theories, would be developed over the next few decades.

In one of the best-known competitors to the inflationary theory, Margaret and Geoffrey Burbidge revised Hoyle's classical steady-state theory into a "quasi steady-state" model in the 1990s, based on their observations of cosmic particles called quasars. Most experts claim that quasars are very far away from Earth, basing their conclusions on Hubble's law governing the relationship between light and distance, and leading them back to the theory of an expanding universe. The Burbidges' observations seemed to show that quasars are indeed quite close to Earth, but move so fast that their light appears to originate from very far away. Astronomer Halton Arp agreed with the Burbidges' assessment. His observations of high-redshift quasars attached to low-redshift galaxies suggested that quasars are ejected from these galaxies, and travel to our own at high speeds, implying that redshift may not determine distance. Arp's observations, like the Burbidges', were dismissed by most astronomers and cosmologists as visual anomalies, but continued to hold sway for some opponents of the Big Bang theory as discrediting the central premise of the SCM.

Research on the Big Bang theory and related concepts was aided by improved observational technology in the 1990s. Instruments such as the Cosmic Background Explorer (COBE) and the Hubble Space Telescope brought more precise measurements, many of which supported the basic concepts of the SCM. However, other data presented new challenges, such as the discovery of the apparent acceleration of the universe. Complex—and often controversial—concepts such as dark energy and dark matter became central to cosmological investigations.

Big Bang Theory Today

The Big Bang, and more broadly the SCM, remained the most widely accepted theories of the universe's evolution into the early twenty-first century. Most scientists argue that some variant of the Big Bang best explains available observational evidence as well as theoretical findings. Measurements from advanced instruments such as the Wilkinson Microwave Anistropy Probe (WMAP) satellite, which gathered data on the CMB in the 2000s, helped advance scientists' understanding of the early universe.

However, the Big Bang theory has also continued to face obstacles as scientific inquiry has advanced and new information has come to light. For example, inflation theory led to many potential complications, including the possibility of multiple universes. Subjects such as dark energy and dark matter remain poorly understood, driving further cutting-edge research that has often conflicted with established ideas. Some findings that are expected under the prevailing theory have also defied researchers' efforts, as with the struggle to find gravitational waves (a landmark 2014 detection of the phenomenon was later invalidated). Issues such as these continue to drive cosmologists to explore alternative theories, and have even led to proclamations that many longstanding beliefs must be rethought. Prominent opponents of the Big Bang theory included Jim Peebles, winner of the 2019 Nobel Prize in Physics, who argued that popular conceptions of the Big Bang as a beginning from a singularity had essentially no supporting evidence.

In June 2006, the first Crisis in Cosmology conference was held in Monção, Portugal, to discuss the practicality of continued reliance on the SCM. One of the alternative theories proposed was Geoffrey and Margaret Burbidge's quasi steady-state theory, which says that the universe is infinite, alternately expanding and contracting, in a series of "little bangs" that occur approximately every 100 billion years. Their theory appeared to be supported by Hubble's original observations, which would place our current universe in the middle of one of these cycles. However, most scientists believe that alternative models such as these rely too heavily on anomalous observations that fail to take into account all of the evidence. Some also find that steady-state models rely on the existence of the SCM in order to refute it, and thus contradict themselves.

Throughout the 2010s and 2020s, new research attempted to gain further insights into the origins of the universe and further clarify aspects of the Big Bang Theory. For example, by the early 2020s, some researchers had advanced a cyclic model of the Big Bang. This model addressed some logical and scientific inconsistencies in prevailing theories of the origins of the universe. Rather than a single moment of explosion and expansion, as had been traditionally accepted, the cyclic model of the Big Bang argued that there was a phase before the moment of the Big Bang. This preliminary phase included multiple repeating cycles of contraction and expansion, and also defined the shape of the universe. Some scientists referred to this long, preliminary period of slow contraction as the "Big Crunch." These hypotheses paved the way for some scientists to begin embracing the idea of the Big Bang as a somewhat gradual process consisting of multiple phases.

Many researchers at that time focused on concepts which were not well understood, such as dark matter, which has been proposed as a hypothetical type of matter that apparently does not interact with electromagnetic fields or light. Scientists had not been able to determine what percentage of the universe consisted of dark matter, and wide disagreement existed on what percentage of the universe was made up of dark matter; some scientists claimed this number was less than a third, while other research suggested that as much as 99 percent of the universe consisted of dark matter. To account for this massive amount of dark matter dispersed throughout the universe, by the early 2020s some scientists had proposed that, shortly after the first Big Bang occurred, a second similar event took place. This second, "dark" Big Bang, some scientists claimed, created dark matter and dispersed it throughout the universe.

Some of the difficulties inherent in astronomical theories rest within scientists' inability to observe cosmic phenomena up close. All cosmological and astronomical observations, until this time, have been made from Earth or very nearby, and objects (and the relationships between them) often look different up close than they do from a distance. Many physicists also claim that the laws of physics as they are understood today are different than they were when the Big Bang occurred, making it even more difficult to grasp the processes that created the universe. American physicist Richard Feynman observed that the study of physics and the laws of physics have so far focused on the "what" as in: "What are the laws of physics?" As more is understood about the universe, other questions are beginning to be asked, such as: "Where did the laws of physics come from?" and "Why do we have the physical laws that we do?" While most physicists agree on the basic laws of physics, their origin is ultimately still a mystery. And indeed, research continues to challenge the absolute nature of these laws, perhaps putting the very underpinnings of the Big Bang theory—and all other elements of cosmology—in question.

These essays and any opinions, information, or representations contained therein are the creation of the particular author and do not necessarily reflect the opinion of EBSCO Information Services.

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