One of the most inviolable laws of the universe is the second law of thermodynamics. It tells us that in any physical system, nothing (no particles and no energy) is exchanged with the external environment, and entropy always increases. This applies not only to a closed and isolated system in our universe, but to the entire universe itself. If you look at the universe today and compare it to any earlier point in time, you will see that entropy has been rising and continues to rise in all of our cosmic history, without exception.
But what if we go all the way back to the earliest times: the first moments of the big **? And even earlier: before the Thermo ** and established the era of the expansion of the universe? If entropy always increases, does this mean that the entropy of the universe starts from zero at some initial time and reaches what we might consider to be the "maximally organized" state at some point?
Surprisingly, perhaps, the answer is no. Not only was the universe not maximally organized at the beginning of the Great **, but even in the earliest stages that we can describe, during the Thermal Great ** and even earlier, there was considerable entropy. Also, "organized" is not a reasonable way of thinking, although we use "disorder" as a casual way to describe entropy. Let's find out what it all means.
Our universe, from the blazing big ** to today, has undergone tremendous growth and evolution and will continue to do so. About 13.8 billion years ago, our entire observable universe was about the size of an ordinary monolith, but today it has expanded to a radius of 46 billion light-years. The complex structures that have emerged must have accounted for at least 0 percent of the average density from early times003% of the seeds are grown from defects. **nasa/cxc/m. weiss
When we think about the universe in the early stages of the thermo-big **, we are imagining that all the matter and radiation we have today – currently distributed over a sphere about 92 billion light-years in diameter – is crammed into a volume about the size of the world's largest pumpkin. The universe at that stage was very hot and dense, with about 1090 particles, antiparticles and radiation quanta all possessing enormous amounts of energy: billions of times greater than even the energy that CERN's Large Hadron Collider could achieve. This includes:
All matter particles of the Standard Model, all their antimatter counterparts, gluons, neutrinos.
Photon. Whether it's the culprit of dark matter, plus particles of any alien species that may be present, all of this is packed in that tiny volume and at the same time has a huge amount of kinetic energy. One-tenth of this hot, dense, expanding, and uniform state of 30,000 will grow into the observable universe we inhabit today within the next 13.8 billion years. However, when you think about what we looked like in the beginning, it does look like a state of utter disorder.
The early universe was filled with matter and radiation, and was so hot and dense that it prevented all complex particles, such as protons and neutrons, from forming steadily in the first fractions of a second. Only one quark-gluon plasma, along with other particles such as charged leptons, neutrons, and other bosons, are flying around at nearly the speed of light. This primordial soup is made up of particles, antiparticles, and radiation, and although the entropy is lower than that of our modern universe, it still has a lot of entropy. **Model and Data Analysis Program at Duke University.
But what does the appearance of disorder actually mean when it comes to entropy? We usually talk about entropy as if it were a measure of disorder:
A broken egg on the floor has more entropy than an unbroken egg on the countertop, a dollop of cold cream next to a cup of hot coffee has less entropy than a well-mixed combination of the two, and a messy pile of clothes has a higher entropy than a neat set of dresser drawers, all folded and put away in an orderly manner.
While these examples correctly identify high-entropy and low-entropy states, it is not precisely "ordered" or "disordered" that allows us to quantify entropy.
On the contrary, for all the particles, antiparticles, etc., that exist in the system, what we should consider is what the quantum states of each particle are, and which quantum states are allowed taking into account the energy and energy distribution. Entropy is actually a measure of, not a fuzzy feature like what humans call disorder, but only: the number of possible permutations of quantum states of the entire system.
A system that is set up under the initial conditions on the left and allows evolution will have less entropy if the gate remains closed (left) than if the gate is open (right). If the particles are allowed to mix, then there are more ways to arrange twice as many particles at the same equilibrium temperature than to arrange half of the particles at two different temperatures, resulting in a much greater entropy of the system on the right than the system on the left. **htkym & dhollm/wikimedia commons
For example, consider the two systems above. On the left, a box with a partition in the middle, with cold gas on one side and hot gas on the other; On the right, the divider has been opened, so the entire box is now filled with gas of the same (equilibrium) temperature. Which system has more entropy? The answer is the one on the right that mixes well, because when all particles have the same property, there are more ways to arrange (or exchange) quantum states than when half have one set of properties and half have another set of different properties.
Back in the very young days of the universe, there were a certain number of particles in it, and these particles had a specific energy distribution. In these early stages, almost all entropy is due to radiation; If we calculate it, then we find that the total entropy is about s = 1088 kb**kb is the Boltzmann constant. However, every time an energy emission reaction occurs, such as:
Forming a neutral atom, fusing a lighter nucleus into a heavier nucleus, gravitational pulling to collapse a cloud of gas into a planet or star, or creating a black hole, the overall entropy of the system – in this case the entire universe – will have to increase.
This fragment from a structural formation simulation, as the universe expands, represents billions of years of gravitational growth in a universe rich in dark matter. The entropy of the universe, at every step forward, is always increasing, even though the entropy density (including expansion) may decrease. **Ralf Kaehler and Tom Abel (Kipac) Oliver Hahn
Today, we are able to quantify the entropy of the universe to a certain degree of precision, and we find that the largest contributor to the entropy of the universe is the black hole. Taking into account the number and mass of black holes in the universe today, entropy has reached modern values, about trillion times that of the early stages of the Great **: s = 10103 kb.For black holes, entropy is proportional to the surface area of the black hole, and for heavier black holes, the surface area is larger. The Milky Way's relatively mild, supermassive black holes, have an entropy of about s = 1091 kb, or about 1,000 times more than the entire universe in the early stages of the thermally large **.
Over time, as the cosmic clock continues to tick, the universe continues to form and develop more and more massive black holes, and the heaviest black holes attract the greatest mass to them. About 10 people in 20 years, entropy will reach its maximum, because it could be as high as 1% of the mass of the universe (up from 0. today04%) will eventually be locked in the form of black holes, giving us an entropy of 119 kb to s = 10121 kb in the range of s = 10121, and this entropy will (probably) only be conserved, not created or destroyed, because these black holes will eventually decay through Hawking radiation, which is an entropy-conserving (adiabatic) process.
The bits of information encoded on the surface of a black hole can be proportional to the surface area of the event horizon. When a black hole decays, it decays to a state of thermal radiation. When matter and radiation fall into a black hole, the surface area becomes larger, enabling the information to be successfully encoded. When a black hole decays, entropy does not decrease, but remains the same because Hawking radiation is an entropy-conserving (adiabatic) process. **t.b. bakker/dr. j.p.Van der Schaar, University of Amsterdam.
However, this assumes that the entropy of our universe only applies to the observable universe, which expands dramatically over time. If we were to compare the entropy density of the universe – in other words, the entropy of the observable universe divided by its volume – we would find that this amount, the entropy density, would ultimately tell us a very different story.
A giant pumpkin representing the size of the observable universe shortly after the start of the thermo-large **, with a radius of about 13 meters, with a volume of about 10 cubic meters, which means that the entropy density of the early universe is slightly higher than 1087 kb m3, which is huge. Perhaps an instructive comparison is that the black hole at the center of the Milky Way itself occupies a volume of about 10 cubic meters of 40 cubic meters, making its entropy density only 10 or so 51 kb m3, which is still very large, but with a coefficient of about 10 36 The entropy density of less than the entropy density is back in the initial stages of thermal greatness.
In fact, if we look at the universe today, even though the overall entropy is enormous, the fact that the volume is so large drives the entropy density to a relatively small number: in the range of 10 kb m3 to 1028 kb m3
This simulation shows the cosmic network of dark matter and the large-scale structure it forms. Normal matter exists, but only 1 6 of the total substance. At the same time, matter itself accounts for only about 2 3 of the entire universe, and the rest is dark energy. Although the entropy of our entire universe is enormous, dominated by supermassive black holes, the entropy density is very small. Although entropy always increases, in an expanding universe, the entropy density does not increase. **the millennium simulation， v. springel et al.
While it is instructive that entropy density has dropped dramatically over time, it is also important (and perhaps more important) to note that the actual calculable entropy of the universe has increased dramatically, despite the relentless expansion of the universe. In fact, when we compare the entropy that existed in the early universe, there was about 15 to 16 orders of magnitude difference compared to today's entropy at the very beginning of the hot world. Over the cosmic history of the universe, even as expansion diluted entropy density – or the amount of entropy per unit volume – total entropy increased by a factor of more than a trillion times.
Moreover, if we decide to distinguish between the observable universe that we can see and measure today and the unobservable universe, this difference will only intensify, and we still remain largely unaware of it. While we can currently see all directions at 46 billion light-years, more expanding universes will eventually be revealed to us over time. We don't know its true size or extent; We can only set a lower limit on the size of the universe, beyond what we can observe. As far as we know, space can really be infinite, beyond what we can see of its components.
In a universe dominated by dark energy, there are four zones: one where everything in it is palpable and observable, one where everything is observable but unreachable, and one where things are observable one day and one where things can never be observed. The marked numbers correspond to our consensus cosmology as of 2024. **andrew z. colvin/wikimedia commons;Notes: e siegel
However, it is important to remember that the big **, while it is the origin of the universe as we know it, is not the first thing we can talk about wisely. As far as we know, the Great ** is not at all the beginning of our universe, but describes a series of conditions – hot, dense, almost completely homogeneous, expanding, full of matter, antimatter, radiation, etc. – that existed very early. However, in order to establish the Great **, the best evidence we have points to another state before the Great **: the expansion of the universe.
According to inflation, before the Great **, the universe was filled with a form of energy similar to dark energy: energy inherent in the sites that permeate the structure of space or the structure of space itself. It is this form of energy, not particles, antiparticles, or radiation, that dominates the universe. As the universe expands, it expands exponentially: inexorably, not at a constantly decreasing rate determined by the decreasing density of matter and radiation. During this time, no matter how long inflation lasted, every 10-32 or so that passed, a region of Planck's length size, the smallest scale at which the laws of physics would not collapse, were stretched to the size of the universe currently visible today.
The exponential expansion that occurs during inflation is so powerful because it is relentless. Every 10 -35 seconds (or so), the volume of any particular area of space doubles in each direction, resulting in any dilution of particles or radiation, and causing any curvature to quickly become indistinguishable from flat. This also helps to keep the entropy constant, but it reduces the entropy density considerably. **e. siegel （l）;ned wright'S cosmology tutorial (R) (English).
During inflation, the entropy of our universe must be much lower: about 1015 kb in volume equivalent to the size of the observable universe at the beginning of our thermal **: this value can be obtained by calculating the unruh radiation produced by the existence of the cosmic horizon. But the most important aspect of this story is this: the actual entropy of anything that existed in the universe didn't change much during the inflationary process before the inflation; It's just diluted. The initial entropy density changes dramatically and is rapidly diluted until it is negligible, as any pre-existing entropy that existed in the universe before the inflation is still present but is gradually stretched into larger and larger volumes.
This is essential for understanding what happened in the early stages of our universe. We don't need some miraculous low-entropy state to start our universe or start the inflationary process. All we need is for the expansion of a certain volume of the universe** to appear, even if it is a small volume, and the space within that volume begins to expand. In a short period of time – no more than a fraction of a second later – the initial entropy is now dispersed over a larger volume, regardless of how much entropy there was initially. Entropy may always be increasing, but the entropy density, or the amount of entropy that will one day be included in the volume of our entire observable universe, is driven by unruh radiation to drop to this extremely low value: about 10 nanojoules per Kelvin, distributed over the volume of the universe at that time.
During inflation (highlighted in green), the world line is elongated by exponential expansion (horizontal axis), resulting in a significant drop in entropy density (the amount of entropy present within the blue circle), even though the overall entropy of the universe never decreases. When the inflation ends, the field energy trapped in the inflation is converted into particles, resulting in a huge increase in entropy: in fact, the largest increase in the history of the universe. Ned Wright's Cosmology Tutorial ESiegel's note.
When inflation ends, the energy that drives the exponential expansion is converted into matter, antimatter, and radiation: the hot, dense, almost uniform, expanding but cooling state that marks the beginning of the thermal maximum. Converting field energy into particles causes entropy in our observable universe to rise dramatically at that moment: from the final stage of inflation to the beginning of thermal hyperemia, entropy rises by about 73 orders of magnitude. Over the next 13.8 billion years, as our universe expanded, cooled, fused, gravitationally pulled, atoms, stars, galaxies, black holes, planets, and humans, our entropy "only" rose by 15 or 16 orders of magnitude and continues to rise today.
In contrast to the largest spurt of entropy growth ever recorded, what has happened and will happen throughout the history of the universe is peanuts: the end of inflation and the beginning of a hot one. However, even in this inflationary state, where entropy is surprisingly low, we still don't see a reduction in entropy in the universe; It's just that the entropy density decreases as the volume of the universe increases exponentially. In the distant future, when the universe expands to 10 billion times its current radius, the entropy density will again be as small as it was a long time ago in the era of inflation. While the entropy in the volume of our universe will continue to increase, the entropy density will continue to decline and will never be as large as it was at the beginning of the thermal maxima**: about 13.8 billion years ago.
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