In 1970, Stephen Hawking and Roger Penrose published a famous paper in which they proved that if time had been turned back, the opening of the cosmic story would have been a Big Bang singularity.
Today, most people have heard that the universe began with the Big Bang about 13.8 billion years ago, and if that's the case, where did the Big Bang come from?
The first substances
Let's first look at how the so-called physical matter was first produced.
If our goal is to explain the origin of stable matter made of atoms or molecules, then it is true that such a thing did not appear at the time of the Big Bang.
Since the Big Bang, the universe has expanded and cooled. As the universe continues to cool, once the conditions are right, the first atoms form from simpler particles, which in turn coalesce into heavier elements within the star. We already have a detailed understanding of these processes, but this understanding does not solve the problem of the universe "out of nothing."
So let's think further. The earliest long-lived particles of matter were protons and neutrons, which together make up the nucleus of an atom. These particles appeared about a tenth of a second after the Big Bang. Until then, there really wasn't anything in the sense of what we were familiar with.
But physics allows us to continue to travel back in time, back to the physical processes that preceded any stable matter.
Period of great unification
This brings us to the so-called period of great unification. Now, we have entered the realm of speculative physics because we have not yet been able to generate enough energy in experiments to detect the kind of processes that happened at that time.
But a reasonable assumption is that the physical world is made up of a soup of elementary particles with short lifespans. The amounts of matter and antimatter are roughly equal, and each particle of matter (such as a quark) has a "mirror image" partner of antimatter, which is almost identical, differing in only one aspect, namely the opposite charge. When matter and antimatter meet, they annihilate in a flash of energy, which means that these particles are constantly being created and destroyed.
But how did these particles first exist?
Quantum field theory tells us that even a vacuum, the "empty" space-time, is also filled with physical activity in the form of energy fluctuations. These fluctuations can produce particles, but they disappear quickly. This has been discovered in countless experiments.
Simulation of vacuum quantum fluctuations in quantum chromodynamics. | Image credit: Wikimedia/Ahmed Neutron
Particles in the vacuum of space-time are constantly being created and destroyed, which is obviously "made out of nothing" in some sense. But perhaps what all this tells us is that the quantum vacuum is "something", not nothing.
Planck period
Suppose we ask further, where did space-time itself arise from? Then we can continue to turn the clock forward and go back to the really ancient Planck period. This was a period in the history of the universe that was as early as the collapse of our best physical theories, existing only in one trillionth of a trillionth of a trillionth of a second after the Big Bang.
At that moment, space and time itself began to be affected by quantum fluctuations. Physicists usually use quantum mechanics, which governs the microscopic world of particles, and general relativity, which governs the microscopic world of particles, respectively, and general relativity, which applies to the enormous cosmic scale. But to truly understand planck's time, we need a complete theory of quantum gravity that combines the two into one.
We still don't have a perfect theory of quantum gravity, but there have been some attempts, such as string theory and loop quantum gravity.
In these attempts, conventional space and time are often seen as emerging, like waves on the surface of the deep sea. The space and time we experience are the products of quantum processes that operate on a deeper microscopic level, and these processes can be extremely disruptive for those of us who are rooted in the macroscopic world.
In planck's time, our conventional understanding of space and time was broken, and we could not continue to rely on our conventional understanding of causality. Nonetheless, all the candidate theories of quantum gravity describe some of the physical phenomena that occurred during the Planck period, namely some "quantum precursors" of conventional space and time. But where did that come about?
Unfortunately, until now, until we continue in the direction of the theory of everything, our best physics still can't give any definitive answers.
A cycle almost from scratch
In order to truly answer the question of "something out of nothing," we need to explain the quantum state of the entire universe at the beginning of planck's time. All attempts to do this are still highly speculative.
Penrose proposed an interesting but controversial model called conformal cyclic cosmology (CCC).
Penrose was inspired by an interesting mathematical connection between the hot, dense, tiny states of the universe (such as those at the Time of the Big Bang) and extremely cold, empty, and expanding states (such as the state of the distant future of the universe).
His radical theory of explaining this correspondence is that when these states reach their limits, they are mathematically identical. While this may seem contradictory, he argues that the state of complete absence of matter may have managed to produce all the matter we see in the universe.
Image credit: Roger Penrose
In this view, the Big Bang arose from (almost) "nothing." That's what's left when all the matter in a universe is swallowed up into a black hole, and the black hole evaporates into photons. But no matter how empty it is, it is still a physical universe.
Why is the same state a cold and empty universe from one point of view, but a hot and dense universe from another? The answer, in a complex mathematical process, is called conformal rescaling, a geometric transformation that actually changes the size of an object but keeps its shape unchanged.
Penrose shows how the cold, empty state and the hot, dense state are linked by this heavy scale, so that they match in terms of the shape of space-time, although the two are of different sizes. Admittedly, it's hard to grasp how two objects are identical on this level when they have different dimensions, but Penrose argues that in this extreme physical environment, size is no longer a meaningful concept.
In the CCC, the direction of interpretation is from the ancient cold, to the young hot, hot dense state that exists because of the cold and empty state. But the "because" here is not the cause and effect with which we are familiar, that is, the kind of cause and effect that precedes its effect in time.
In these extreme states, not only is size no longer meaningful, but so is time. The cold, empty state and the hot, dense state are actually located on different timelines. From the observer's point of view, the cold emptiness will last forever in its own time geometry, but the hot, dense state it produces is actually located on a new timeline alone.
The beginning of the loop
The CCC offers some detailed, but speculative, answers to the question of where the Big Bang of our universe came from. But even if Penrose's vision is ultimately proven by future cosmological advances, we might argue that we still won't answer a deeper question, a question about where physical reality itself comes from, in other words, how did the entire circulatory system come about?
There are roughly three broad options for the physical explanation of the deeper question of how the loop began: it may have no physical explanation at all; or there may be an endless cycle of repetition, each of which is itself a universe, and the initial quantum state of each universe is explained by certain features of the previous universe; or there may be a single loop, and a single repeating universe, the beginning of which is explained by certain features of its own end.
Penrose envisions an endless series of new cycles, in part because of his own preference for interpretations of quantum theory. In quantum mechanics, a physical system has the superposition of many different states at the same time, and when we measure it, we can only "pick" one at random.
For Penrose, each cycle involves a different outcome of a random quantum event, meaning that each cycle will be different from the previous and subsequent cycles.
This is actually good news for experimental physicists, as it could allow us to peek into the ancient universe that produced us by giving us faint traces or anomalies in the radiation left over from the Big Bang as seen through the Planck satellite.
Cosmic microwave background radiation. | Image credit: ESA and the Planck Collaboration
Penrose and his collaborators even think they may have found these traces, attributing the patterns in planck's data to radiation from supermassive black holes in the previous universe. However, their claimed observations have been questioned by other physicists. So far, the academic community has not yet reached a conclusion.
Endless new cycles are key to Penrose's vision. However, there is a natural way to convert CCC from a multi-cycle to a single-cycle form. Physical reality, then, consists of a single cycle through the Big Bang to an extremely empty state of the distant future, and then looping back to the same Big Bang, recreating the same universe.
The latter possibility is consistent with another interpretation of quantum mechanics, known as the multi-world interpretation. The multi-world interpretation tells us that every time we measure a system in a superposition state, the measurement is not a random selection of a state. Rather, the measurements we see are only a possibility, the kind that plays out in our own universe. Other measurements are taking place in other universes in the multiverse, and they are isolated from our universe. So no matter how small the odds of something happening, as long as it has a non-zero chance, it will happen in some quantum parallel world.
Some argue that this multiverse could also be observed in cosmological data as an imprint left by another universe when it collides with ours.
While Penrose doesn't agree with the idea, multi-world quantum theory provides a new turning point for the CCC. Our Big Bang could be the rebirth of a single quantum multiverse that contains an infinite number of universes, and they all happen together. Everything that could have happened happened, and then it happened again and again.
From nothing to nothing, from nothing to nothing
For philosophers of science, Penrose's views are fascinating. It opens up new possibilities for explaining the Big Bang, making our explanations beyond conventional cause and effect. Even for lovers of mythology, Penrose's vision is equally beautiful, implying a picture of an endless new world born from the ashes of antiquity.
The last star will slowly cool and fade away. As it goes out, the universe will once again become a void, with no light, no life, and no meaning.
Physicist Brian Cox says this in the documentary Universe.
And the disappearance of the last star will be just the beginning of a long dark age. All matter will eventually be swallowed up by a huge black hole, which in turn will evaporate into its faintest light. Space continues to expand outwards until even those faint rays of light become too dispersed and can no longer interact. The event will eventually cease.
Will the distant future of the universe be like this? And is this also the source of the Big Bang?
We don't have the answer yet.
#创作团队:
Original author: Alastair Wilson (Professor of Philosophy, University of Birmingham)
Compile: Takeko
#参考来源:
https://theconversation.com/how-could-the-big-bang-arise-from-nothing-171986
#图片来源:
Cover: NASA
Beginning: Wikiart