Chapter 146: Multiverse

Although they were not entirely fond of their current state, Hua Feng and Yun Meng had to admit that attending classes and practicing cultivation had become the main rhythm of their daily lives over the past few months. Though they felt bewildered about the path ahead, they both believed that everything they were doing right now was correct. For them, that was enough.

Dark energy accounts for 74 percent of all matter in the universe and acts as the driving force behind the accelerated expansion of the cosmos. The process of the universe's expansion exists in a balance between two opposing forces, much like the mutual opposition of Yin and Yang. One of these forces is gravity, whose effect decelerates the expansion, while the other powerful counter-force is dark energy, which causes the universe to expand at an accelerating rate. As it currently stands, dark energy has won out.

The visible matter in the universe falls far short of being able to hold the cosmos together as a single entity; if not for the presence of a mysterious and invisible substance, galaxies would have long since disintegrated. Scientists refer to this unseen, mysterious substance as "dark matter." Dark matter is the primary type of matter that drove the formation of specific structures under its own gravity during the expansion of the universe.

Astronomical observations indicate that our universe is undergoing an accelerated expansion.

Theoretically, a certain critical density exists. If the average density is less than the critical density, the universe will expand indefinitely, known as an "open universe"; if the average density is greater than the critical density, the expansion process will sooner or later halt and reverse into contraction, known as a "closed universe."

At present, the possibility of an open universe appears more likely.

Accelerated Expansion:
Nobel laureate Brian Schmidt pointed out: "The space between matter is increasing."

In 2011, Brian Schmidt and his colleagues were awarded the Nobel Prize in Physics for discovering the accelerated expansion of the universe by using "supernovae" as "cosmic probes." At the inception of their research, their intention was to measure how the expansion rate of the universe was slowing down due to gravity; however, the ultimate discovery was unexpected. It turned out that the universe's expansion rate is accelerating.

He explained: "We observe the speed at which objects are moving away from us, much like Doppler radar uses Doppler shifts for positioning and measurement. We measure distances and the intervals marked by these movements to calculate the expansion rate of the universe.

Our method of measurement involves observing distant objects, making the same measurements at different times, and drawing conclusions through comparison. From this perspective, it is a very simple experiment. By comparing the different distances measured in the past with those of the present, we have learned that the current expansion rate of the universe is much faster than it used to be."

Researchers have calculated that the current expansion rate of the universe, known as the Hubble constant, is approximately 73.2 kilometers per second per megaparsec. One megaparsec is equivalent to 3.26 million light-years; therefore, for every megaparsec increase in a galaxy's distance from Earth, its receding velocity increases by 73.2 kilometers per second. This means that within 9.8 billion years, the distance between celestial bodies in the universe will double.

Conjectures on the Ultimate Fate of the Universe:
The laws of thermodynamics will not grant the universe eternal life. When new stars can no longer form, the universe will reach the equilibrium point of heat death, returning to a state resembling the soup-like spacetime of its birth. Heat death is the thermodynamic endpoint, where the temperature everywhere in the universe is merely slightly above absolute zero, meaning nothing will survive.

A small fraction of scientists believe that if the ultimate fate of the universe is a big crunch, all matter will eventually revert to an atomic state, and through another accidental quantum fluctuation, a new Big Bang will occur, birthing the next universe.

Cosmologists believe that if the energy density of the universe is equal to or less than the critical density, the expansion will gradually decelerate but will never truly stop. Star formation will eventually cease as the interstellar gas in each galaxy is progressively consumed; stellar evolution will ultimately leave behind only white dwarfs, neutron stars, and black holes.

Extremely slowly, collisions among these dense stellar remnants will lead to the accumulation of mass, successively producing larger black holes. The average temperature of the universe will asymptotically approach absolute zero, thereby reaching what is known as the Big Freeze. Furthermore, if protons are indeed unstable as predicted by the Standard Model, baryonic matter will eventually vanish entirely, leaving only radiation and black holes in the universe, and eventually even the black holes will completely evaporate via Hawking radiation.

The entropy of the universe will increase to its absolute limit, to the extent that no further self-organizing forms of energy can arise, ultimately plunging the universe into a state of heat death. In the $Lambda ext{CDM}$ model, dark energy exists in the form of a cosmological constant. This theory posits that only matter in gravitationally bound systems such as galaxies will clump together, and as the universe expands and cools, they too will reach heat death.

Alternative explanations for dark energy, such as the phantom energy theory, suggest that eventually galaxy clusters, stars, planets, atoms, atomic nuclei, and all matter will be torn apart by the relentless, ongoing expansion—a scenario known as the Big Rip.

The Big Bang Theory: The framework of the Big Bang model is based on Einstein's general relativity, with certain simplifications made in solving the field equations. In 1932, Georges Lemaître first proposed the modern Big Bang theory of the universe, and in 1946, American physicist George Gamow formally put forward the Big Bang theory.

The Big Bang cosmological model posits that the universe originated from a primordial fireball more than ten billion years ago. The inflationary model resolved three major cosmological conundrums: the horizon problem, the flatness problem, and the magnetic monopole problem.

The Big Bang is based on the cosmological principle, meaning the singularity erupted across all of space.

The earliest and most direct observational evidence for the Big Bang includes the Hubble expansion observed via galactic redshifts, precise measurements of the cosmic microwave background radiation, and the abundances of light elements between galaxies. Today, large-scale structure and galactic evolution have also become newly supportive evidence. These four pieces of observational evidence are sometimes referred to as the "four pillars of the Big Bang theory."

The Big Bang model can uniformly account for the following observational facts:
(a) The theory asserts that all stars formed after temperatures dropped; consequently, the age of any celestial body should be shorter than the elapsed time from when the temperature cooled to the present day—meaning it should be less than 20 billion years. Measurements of the ages of various celestial bodies have confirmed this.
(b) Extragalactic celestial bodies exhibit systematic spectral line redshifts, and the redshift is roughly proportional to distance. If explained using the Doppler effect, the redshift is a reflection of cosmic expansion. However, in 2012, it was recognized as cosmological redshift rather than Doppler redshift. In cosmological redshift, the wavelength of light waves changes as space expands during propagation. The redshift of spectral lines is thus a reflection of the expansion of the universe.
(c) The abundance of helium across various celestial bodies is quite substantial, holding steady at around 30%. Stellar nuclear reaction mechanisms alone are insufficient to explain why there is so much helium. However, according to the Big Bang theory, the early temperatures were extremely high, and the efficiency of helium production was very high, which can account for this fact.
(d) Based on the expansion rate of the universe and helium abundance, the temperature of the universe at each historical epoch can be precisely calculated.

According to the Big Bang theory, the universe has no beginning. It is merely a continuous, repeating cycle—a process of cosmic creation, destruction, and re-creation.

Theoretical Basis:
The evidence for the Big Bang theory is as follows:
(a) Redshift: Viewed from any direction on Earth, distant galaxies are moving away from us, from which it can be deduced that the universe is expanding, and the farther a galaxy is from us, the faster it recedes.
(b) Hubble's Law: Hubble's law is a definitive relational formula concerning the recession velocities and distances between galaxies. It further illustrates the motion and expansion of the universe.
$V = H imes D$
Where $V$ ($ ext{km/sec}$) is the recession velocity; $H$ ($ ext{km/sec/Mpc}$) is the Hubble constant, taken as 50; and $D$ ($ ext{Mpc}$) is the distance to the galaxy. $1 ext{ Mpc} = 3.26 ext{ million light-years}$.
(c) Abundances of Hydrogen and Helium: The model predicts abundances of 25% for hydrogen and 75% for helium, which have been confirmed by experimentation.
(d) Abundances of Trace Elements: The abundances of these trace elements predicted by the model match those measured empirically.
(e) The 3K Cosmic Background Radiation: According to the Big Bang theory, the universe cooled as it expanded, and the residual radiation generated at that time should still exist in today's universe. In 1965, the 3K background radiation was detected.
(f) Slight Inhomogeneities in the Background Radiation: This proves that the initial state of the universe was not entirely uniform.
(g) New Evidence for the Big Bang Theory: In the December 2000 issue of the British journal *Nature*, scientists announced the discovery of new evidence that could be used to corroborate the Big Bang theory.

Secondary Inflation Theory:
Some scientists from institutions such as the U.S. Department of Energy's Brookhaven National Laboratory suggest that the early universe may have undergone a relatively brief secondary inflation period. This hypothesis might explain the problem of an excessively large amount of dark matter in the universe.

Houman Davoudiasl, head of the laboratory's High Energy Theory Group, stated that between these major events, there may have been another period of expansion. Unlike the initial explosion, it was not as violent, yet it could "dilute" the dark matter, ultimately bringing the density of dark matter in the universe to what it is today.

Physical Uniformity:
The establishment of the Big Bang theory is based on two fundamental assumptions: the universality of physical laws and the cosmological principle. The cosmological principle states that on a large scale, the universe is homogeneous and isotropic.

The fact that the universe appears the same everywhere (homogeneity) and in every direction (isotropy) is a crucial foundation for the Big Bang theory. The theory posits that the entire universe, and everything we see, originated from a hot, dense, and unified state where the laws and initial conditions were identical everywhere.

The most fundamental principle of cosmology is the mediocrity principle, which suggests that the laws governing Earth are applicable everywhere else in the cosmos.

Conjectures on the Multiverse:
A multiverse is a theoretical collection of infinite or finite possible universes. The individual universes comprising a multiverse are called parallel universes. A tiny minority of theoretical physicists believe in the existence of a multiverse in various states.

According to inflation theory, what is commonly referred to as the Big Bang may not be the absolute beginning of time and space, but rather the beginning of our own observable universe. Prior to that was the inflationary epoch of the universe—an era of exponential expansion filled with energy inherent to the fabric of spacetime. Cosmic inflation is also a theory that has evolved through inheritance and development.