This video was produced by David Butler.\(^{[1]}\) For the transcript of this video, visit: https://www.youtube.com/watch?v=JpNaIzw_CZk
Fundamental questions which have been asked since the beginning of humanity are: How did structure arise in the cosmos? Why is there something rather than nothing? Why is there “clumpyness” in the cosmos (in other words, why is there more stuff here than there)? For a long time, answering such questions would have been impossible. But in the past few decades cosmologists, by using the scientific method and advanced technology, have unraveled these deep and profound mysteries which humans have long pondered since their origin. Let’s start out by answering the second question: Why is there something rather than nothing? The fundamental principles of quantum mechanics predict a myriad of mathematical and physical consequences. One of them is the time-energy uncertainty principle which predicts that over any given time interval, the observed energy of a particle is always uncertain and will be within a range of values because you must give it a “kick” to observe it. The energy of something can never be exactly zero (because then \(ΔE=0\) and the time-energy uncertainty principle would be violated). Therefore, not even the vacuum of empty space can have no energy. This energy of empty space can be transformed into mass (a property of matter) according to Einstein’s mass-energy equivalence principle and the rules of particles physics. Since the possible energy values of any region of space are random within a certain range of values, it follows that the distribution of energy, mass, and matter also must be random throughout space. But if the fundamental principles of quantum mechanics and the time-energy uncertainty principle in particular explain the origin of matter and energy in the universe, then how did the cosmos attain structure and clympyness? Over the next few paragraphs, we’ll begin a discussion which starts with analyzing the predicament (namely, applying quantum mechanics alone predicts that the universe should have been too uniform for galaxies and other structures to arise); then, after that, I’ll take you through a very brief account of how the universe developed structure and clumpyness.
In the earliest epochs of our universe, all of the matter and energy existed in the form of virtual particles popping in and out of existence. If the distribution of this matter was random then, on average, there would be just as many virtual particles over here as over there. But in order for structure to arise in the cosmos, the distribution of matter must start out clumpy and not almost completely uniform as the uncertainty principle predicts. The uncertainty principle predicts that the distribution of matter and virtual particles throughout space starts out more or less uniform. This seems to contradict today’s observations since if all of the matter density is initially uniform, it’ll more or less stay uniform. The Big Bang theory is widely regarded as one of the greatest triumphs in science of the 20th century. It successfully predicted an enormous range of things: the CMBR; where the light elements came from as well as the correct amounts and proportion of each of them; and the observed redshifts and recessional velocities of distant galaxies. But there were many things which the Big Bang theory could not account for such as how the universe got to its present size, how the recorded temperatures of the CMBR are so uniform, and how the CMBR has slight fluctuations to one part in \(10^5\). Cosmologists generally agree that we must postulate the existence of a physical mechanism which correctly predicts all of these observed features. This is precisely what inflationary theory accomplishes. Many have criticized the physical mechanisms postulated by inflationary theory as ad hoc despite the success in inflationary theory of predicting what has been experimentally observed.
The universe is presently 13.8 billion years old and we are causally connected with all of the matter and energy within a sphere whose radius is 13.8 billion light years. When the universe was only one second old, its temperature was \(10^{10}K\); the photons buzzing around back then were 3.7 billion times hotter than their present temperature today of \(2.725K\). By Boltzman’s relationship \(E∝T\), Plank’s relationship \(E∝λ\), and the relationship between wavelength and the scaling factor (\(λ∝a^{-1}\)), it follows that the entire observable universe must have been 3.7 billion times smaller than it is today—or about only 3.7 light years in radius. Back in that distant epoch, since the universe was only one second old, photons emitted by charged particles could, at most, have travelled a distance of one light second. The gravitational field generated by energetic particles propagates at the speed of light and, therefore, would have had enough time to only spread out a distance of one light second. Any signal emitted by an energetic particle therefore could not travel farther than one light second and could not communicate with other energetic particles more than one light second away. Therefore, any pair of energetic particles separated by more than one light second from one another were causally disconnected.
The causal disconnection between matter and energy in a region of the universe 3.7 light years in radius when it was only one second old is significant because it implies that the energy density of regions separated by more than one light-second should be radically different. Therefore, the CMBR should not be so uniform. However, it successfully explains the origin of the observed fluctuations in matter and energy density: all of the particles within very small regions move towards one another due to gravity without effecting distant regions. This is how the first fluctuation in matter density originated. The clumpyness of matter was initially very slight; but over enormous time intervals (billions of years), gravity amplified this clumpyness to form the cosmic web, filaments, galaxy superclusters and clusters and groups, starts, planets, moons, comets, asteroids, and all of the large-scale structure and clumpyness we see in the universe today. On a medium size scale, the force of electromagnetism pulls and subsequently binds together atoms and molecules to form complex chemistry—the basis of all living organisms. This is the origin of structure and clumpyness on a medium size scale. The strong nuclear force binds together protons and neutrons in order to keeps atoms held together and form structure and clumpyness on an even smaller scale. The slight non-uniformities in matter density imparted by quantum fluctuations eons ago and three fundamental forces (gravity, electromagnetism, and the strong force), together, seeded and eventually developed all of the structure and clumpyness in the cosmos we see today.
This article is licensed under a CC BY-NC-SA 4.0 license.
References
1. David Butler. "Classroom Aid - Cosmic Inflation". Online video clip. YouTube. YouTube, 11 September 2017. Web. 11 November 2017.
2. Gott, Richard. The Cosmic Web: Mysterious Architecture of the Universe. Princeton University Press, 2016.
3. Goldsmith, Donald; Tyson, Niel. Origins: Fourteen Billion Years of Cosmic Evolution. Inc. Blackstone Audio, 2014.