How the Universe we know come into existence? And how do we explain its origin? These are some of the questions cosmologists and physicists have been trying to unravel for decades.
Undoubtedly, every other piece of evidence and data collected over the years by cosmologists points toward the possibility that it all might have started with a ‘big bang.’ But what if there is more?
What Is The Big Bang Theory? A Brief Introduction
In 1927, Belgian astronomer Georges Lemaitre propose the theory of an expanding universe (later confirmed by Edwin Hubble). He theorized that an expanding universe could be traced back to a singular point, which he termed “primeval atom,” back in time. It laid the foundation for the modern Big Bang theory.
The Big Bang Theory is an explanation, based mostly on mathematical models, of how and when the Universe came into existence.
The cosmological model of the Universe described in the Big Bang theory explains how it initially expanded from a state of infinite density and temperature, known as the primordial (or gravitational) singularity.
This expansion was followed by cosmic inflation and a massive temperature drop. During this phase, the Universe ballooned at a much faster rate than the speed of light (by a factor of 1026).
Subsequently, the Universe was reheated to a point where elementary particles (quarks, leptons, and so on) before a gradual decrease in temperature (and density) led to the formation of the first protons and neutrons.
A few minutes into the expansion, protons and neutrons combine to form primordial hydrogen and helium-4 nuclei. The estimated radius of the observable Universe during this phase was 300 light-years. The earliest stars and galaxies appeared about 400 million years after the event.
A crucial piece of the Big Bang model is the cosmic microwave background (CMB), which is the electromagnetic radiation left from the time when the Universe was in its infancy. CMB remains the most definitive proof of the Big Bang.
While the theory remains widely accepted across the scientific spectrum, a few alternative explanations — such as steady-state Universe and eternal inflation, have gained attraction over the years.
Below, we have discussed seven of the most popular alternatives of the Big Bang, explaining the origin of the Universe.
7. Theory of Eternal Inflation
The concept of the inflationary universe was first introduced by cosmologist Alan Guth in 1979 to explain why the Universe is flat, something that was missing from the original big bang theory.
Though Guth’s idea of inflation explains the flat Universe, it created a scenario that prevents the Universe from escaping that inflation. If it were the case, reheating of the Universe wouldn’t have taken place, and neither is the formation of stars and galaxies.
This particular problem was solved by Andreas Albrecht and Paul Steinhardt in their “new inflation” model. They argued that rapid inflation of the Universe happened just for a few seconds before ceasing. It demonstrated how the Universe can go through rapid inflation and still end up getting heated.
Based on the previous works of Steinhardt and Alexander Vilenkin, Andrei Linde, a professor at Stanford University, proposed an alternative of Guth’s inflation theory called chaotic inflation or ‘eternal inflation theory.
The theory argues that the inflationary phase of the Universe goes on forever; it didn’t end for the Universe as a whole. In other words, cosmic inflation continues in some parts of the Universe and ceases in others. This leads to a multiverse scenario, wherein space is broken into bubbles. It’s like a universe inside a universe.
In a multiverse, different universes may have different laws of nature and physics at work. So, instead of a single expanding cosmos, our Universe might be an inflationary multiverse with many small universes with varying properties.
However, Paul Steinhardt believes that his ‘new inflation’ theory doesn’t lead to or predict anything and argues that the multiverse notion is a “fatal flaw” and unnatural.
Reference Sources –
Eternal inflation and its implications – Guth, Alan H.
Inflationary paradigm in trouble after Planck2013 – Anna Ijjasa, Paul J. Steinhardt, Abraham Loeba
6. Conformal Cyclic Model
The conformal cyclic cosmological (CCC) model speculates that the Universe goes through repeated cycles of the big bang and subsequent expansions. The general idea is that the ‘big bang’ was not the beginning of the Universe but rather a transition phase. It was developed by renowned theoretical physicist and mathematician Roger Penrose.
As a basis for his model, Penrose used multiple FLRW (Friedmann–Lemaître–Robertson–Walker) metric sequences. He argued that the conformal boundary of one FLRW sequence could be attached to the boundary of another.
The FLRW metric is the closest approximation of the nature of the Universe and a part of the Lambda-CDM model. Each sequence begins with a big bang, followed by inflation and subsequent expansion.
The cyclic or oscillating model, wherein the Universe reiterates over and over in an indefinite cycle, first came into the spotlight in the 1930s, when Albert Einstein investigated the idea of an ‘everlasting’ universe. He considered that after reaching a certain point, the Universe starts collapsing and ends with a Big Crunch before going through the Big Bounce.
Right now, there are four different variations of the cyclic model of the Universe and one of which is the Conformal Cyclic Cosmology.
5. Black Hole Mirage
A study conducted by a group of researchers in 2013 speculated that our Universe might have originated from the debris spewed out of a collapsed four-dimensional star or a black hole.
According to the cosmologists associated with the research, one of the limitations of the Big Bang theory is to explain the temperature equilibrium found in the Universe.
While most scientists concur that the inflationary theory gives an adequate explanation of how a small patch with uniform temperature would rapidly expand to become the Universe we observe today, the group found it implausible due to the chaotic nature of the Big Bang.
To solve this problem, the team proposed a model of the cosmos, in which our three-dimensional Universe is a membrane and is floating inside a four-dimensional ‘bulk universe.’
They argued that if the 4-D ‘bulk universe’ has 4-D stars, it’s likely they will collapse into 4-D black holes. These 4-D black would have a 3-D event horizon (just like the 3-D ones have 2-D event horizon), which they named ‘hypersphere.’
When the team simulated the collapse of a 4-D star, they discovered that the ejected debris from the dying star is likely to cast a 3-D membrane around that 3-D event horizon. Our Universe might be one such membrane.
The ‘4-D black hole’ model of the cosmos does explain why the temperature is almost uniform throughout the Universe. It may also give valuable insights into exactly what triggered the cosmic inflation a few seconds after its genesis.
However, a recent observation by ESA’s Planck satellite has uncovered small variations in the cosmic microwave background (CMB) temperature. These satellite readings differ from the proposed model by about four percent.
4. Plasma Universe Theory
Image Courtesy: Luc Viatour
Our current understanding of the Universe is mostly influenced by gravity, specifically Einstein’s General Theory of Relativity, through which cosmologists explain the nature of the Universe. Coincidentally, just like most other things, an alternative to gravity has also been entertained by scientists over the years.
The plasma cosmology (or plasma universe theory) speculates that electromagnetic forces and plasma play a much more important role in the Universe instead of gravity.
Although the approach has many different flavors, the basic idea remains the same; every astronomical body, including the sun, star, and galaxies, results from some electrical process.
The first prominent plasma universe theory was proposed by Nobel laureate Hannes Alfvén in the 1960s. He was later joined by Swedish theoretical physicist Oskar Klein to develop the Alfvén–Klein model.
The model is built around the assumption that the Universe sustains equal amounts of matter and antimatter (that’s not the case according to modern particle physics). The boundaries of these two regions are marked with cosmic electromagnetic fields. And thus, interactions between the two would produce plasma, which Alfvén named ‘ambiplasma.’
According to the theory, such plasma would form large sections of matter and antimatter throughout the Universe. Furthermore, it theorized that our current location in the cosmos must be in a section where the matter is much more abundant than the antimatter – hence solving the matter-antimatter asymmetry problem.
3. Slow Freeze Theory
Image Courtesy: NASA
Decades of mathematical modeling and research have led cosmologists to a valid conclusion that our Universe started from a single point of infinite density and temperature called the singularity. The subsequent expansion of the cosmos allowed it to cool, which led to the formation of galaxies, stars, and other astronomical objects.
However, as we know, the standard Big Bang model has not gone unchallenged, and one such challenging theory was proposed by Christof Wetterich, a professor at Germany’s Heidelberg University.
Wetterich argued that the Universe we know today might have actually started as cold and sparse, awaken from a long freeze. Over time, the fundamental particles in the early Universe became heavier while the gravitational constant decreased.
Furthermore, he explained that if masses of the particles have been increasing, radiation from the early Universe could make space appear hotter and moving away from each other even if it wasn’t the case.
The basic idea of Wetterich’s Slow Freeze cosmic model is that the Universe has no beginning and no future. Instead of a hot Big Bang, the theory advocates for a cold and slowly evolving Universe.
According to Wetterich, the theory explains density fluctuations in the early Universe (primordial fluctuations) and why our current cosmos is dominated by dark energy.
2. Hindu Cosmology
Religion and science have been the best of enemies since at least the time of Copernicus and Galileo. There is perhaps no room for science when we talk about religion and vice-versa. However, there is one religion whose cosmological beliefs sit well with the current model of the Universe.
Creation theories in Hindu mythology are widely considered as one of the most ancient and significant of all other religious counterparts.
Over the years, prominent physicists and cosmologists, including Carl Sagan and Niels Bohr, have admired Hindu cosmological beliefs for its close similarity with the timelines in the standard cosmological model of the Universe.
According to Hindu mythology, the Universe follows an infinite cyclic model. It means that our current Universe will be replaced by an endless number of universes. Each iteration of the Universe is divided into two phases, ‘Kalpa’ (or the day of Brahma) and ‘pralaya’ (the night of Brahma), and each is 4.32 billion years long.
According to Hindu mythology, the age of the Universe (8.64 billion years) is more than the currently estimated age of the solar system.
1. Steady State Universe
The constant creation of matter in the Steady-State model as opposed to the Big Bang Theory
The Steady-State model asserts that the observable Universe remains the same at any place and time. In the Universe, which is forever expanding, matter is continuously created to fill the space.
The idea of the steady-state theory was first proposed in 1948 by cosmologists Hermann Bondi, Fred Hoyle, and Thomas Gold. It was derived from the perfect cosmological principle, which itself states that the Universe is the same no matter where you look, and it will always be the same.
According to the model, galaxies and other large astronomical bodies near us should appear similar to those that are far away. However, the Big Bang tells us that distant galaxies should look younger than those at close proximity (when observed from the earth) since light takes much longer to reach us.
The Steady-State theory gained widespread popularity in the early and mid-20th century. However, by the 1960s, it was mostly discarded by the scientific community in favor of the Big Bang after the discovery of the cosmic microwave background.
Interestingly, according to a 1931 manuscript that was discovered by researchers in 2014, Albert Einstein was working on an alternative to the Big Bang theory. It was identical to Fred Hoyle’s Steady State model proposing that the universe has expanded steadily. However, the idea was shortly abandoned by Einstein.
More to Know
How Old is The Universe?
The universe is nearly 14 billion years old (13.78 billion, to be exact). Scientists have reached this conclusion after extensive studies of the cosmic microwave background and analyzing data from the Plack Space Observatory, WMAP, and other space probes.
However, a team of researchers in 2019 calculated the age of the universe to be a couple of billion years younger than the age predicted by the Plank study. The team used the movement of galaxies and stars to estimate how fast the universe is expanding. A higher expansion rate means that the universe reached its current size faster, and thus it must be younger.
To put an end to the discrepancy, an international team of astronomers analyzed the data from Atacama Cosmology Telescope (ACT) in Chile in 2020 to find out the approximate age of the universe.
The team determined the age of the universe to be 13.77 billion years, give to take 40 million years, which is in line with the estimate from Plank research team.
What is the Age of the Sun?
The estimated age of the Sun is about 4.56 billion years. How did they reach this number? Well, it is a combination of nuclear cosmochronology and simulations of the stellar evolution model.