Chapter 2
HOW DID THE UNIVERSE BEGIN? HOW WILL IT END?
What Does It Mean to Explain Something?
Planet Earth formed around 4.5 billion years ago. The first primitive forms of life appeared about 4 billion years ago. Natural selection did the rest, giving rise to species increasingly better adapted to their environment. The evidence, as they say, is overwhelming.
Or is it? Imagine that planet Earth began its existence a mere six thousand years ago, with all fossil records in place and stones well weathered. From there on, however, evolution proceeded as scientists’ way. How would you prove this story wrong?
You couldn't.
I am sorry, but I told you it wouldn't be easy!
It is impossible to prove this story wrong, because of the way our current natural laws work. As we discussed in the previous chapter, they work by applying evolution laws to initial states, and we can apply those evolution laws both forward and backward in time. If we want to make a prediction for the path of a celestial object, we measure its present location and velocity and evolve it forward. If we want to know how the universe looked billions of years ago, we use our observations from the present time and then run the equations backward.
This method creates the following problem, however. If I take a present state, like the Earth in the year 2022, and apply an evolution law to it, then that will give me a past state in 3978 BCE. If I then take that past state and evolve it forward in time again, I will correctly get back to the year 2022. Trouble is, I can do that for any evolution law. There is always some state six thousand years ago that, together with the right evolution law, will correctly result in what we observe today.
Indeed, if I wanted to, I could suddenly switch to a different evolution law more than six thousand years in the past, to accommodate a creator, or the construction of a supercomputer that runs the cosmic simulation we all reside in, or really whatever I want. This is why, with natural laws like the ones we currently use, the idea that Earth was created by someone or something with everything in place is impossible to rule out.
Because such creation stories can't be falsified, we can't tell if they are false, but being false is not their problem. The problem with these stories is that they are bad scientific explanations.
The distinction between scientific and nonscientific explanations is central to this book, so it deserves a closer look. Science is about finding useful descriptions of the world; by useful I mean they allow us to make predictions for new experiments, or they quantitatively explain already existing observations. The simpler an explanation, the more useful it is. For a scientific theory, this explanatory power can be quantified in a variety of ways that come down to calculating how much input a theory needs to fit a set of data to a certain level of accuracy. Exactly how one quantifies explanatory power doesn't matter for our purposes. Let us just note that it can be done, and that it's something scientists actually do in some areas of science. Cosmology is one of the cases where this is done frequently.
In other areas of science, like biology or archaeology, mathematical models are not widely used now and therefore explanatory power usually can't be quantified. This is for a variety of reasons, but one is certainly that the observations themselves are often in qualitative, not quantitative, form. Now, a quantification of observations-made, say, by inventing a measure for the evil of war-doesn't necessarily bring more insights, so I'm not saying anything and everything needs to be cast into equations. But quantification can serve to remove doubts that conclusions were biased by human perception. This can be done, for example, to quantify the explanatory power of Darwinian evolution, by developing a mathematical measure of distance between fossils.
Scientific theories greatly simplify the stories we tell about the world, and that simplification embodies what we even mean by doing science. A good scientific theory is one that allows us to calculate the results of many observations from few assumptions. Quantum theory, to name just one, allows us to calculate the properties of the chemical elements. It is an extremely good scientific theory because it explains much from little. The belief that an omniscient being called God made the chemical elements is not a good scientific theory. You might way it is in some sense a simple explanation, and maybe you find it compelling. You may even find it necessary to make sense of your personal experience. However, the God hypothesis has no quantifiable explanatory power. You can't calculate anything from it. That doesn't make it wrong, but it does make it unscientific.
Saying that the world was created six thousand years ago with everything in place is unfalsifiable but also useless. It is quantifiably complicated: you need to put a lot of data into the initial condition. A much simpler, and thus scientifically better, explanation is that planet earth is ages old and Darwinian evolution did its task.
Now that we know what it means to explain something in scientific terms, let us look at one of the cases where physicists currently struggle to find explanations: the beginning of our universe.
Modern Tales of Creation
In the beginning, superstrings created higher-dimensional membranes. That's one story I've been told, but there are many others. Some physicists believe the universe started with a bang, others think it was a bounce, yet again others bet on bubbles. Some say that everything began with a network. Some like the idea that it was a collision of sorts, or a timeless phase of absolute silence, or a gas of superstrings, or a five-dimensional black hole, or a new force of nature.
In the end, it doesn't matter-the outcome is the same: us, in a universe that looks like the one we see; that it doesn't matter which story you believe is a big warning sign. If this were science, we should have data to tell us which hypothesis is right, or at least an idea for obtaining the necessary data. But it's highly questionable that the data required to falsify any of these origin myths can be obtained, ever. These stories reach back in time so far that data are too sparse for astrophysicists to distinguish one tale from another, and this impasse might be impossible to overcome. For all we know, the beginning of our universe may remain hidden from us forever.
To see why I say this, I need to give you some background on how we develop theories for the early universe. We take all the data we can get, and then we look for a simple explanation. The more patterns in the data we can calculate with it, the better the explanation. For example, the current theory for the universe, the concordance model, is successful not just because, if fed with some initial condition, it gives us the present state. As noted earlier, this can always be done. No, the relevant point is that the initial conditions are simple; they explain a lot from little.
The concordance model is an application of Einstein's theory of general relativity, according to which gravity is caused by the curvature of space-time. I will not go into this in detail here, because you don't need to know the details to follow along; you merely need to know that, according to general relativity, a universe filled with matter and energy will expand, and how fast it expands depends on the types and amounts of matter and energy in the universe. Hence, the concordance model basically keeps track of how much of which stuff is in the universe, from which we deduce the rate of expansion.
In physics we can run our models backward in time, and so, starting with the present state of the universe-expansion with matter clumped in galaxies-we can go back in time and deduce that the matter must have been squeezed together. It must once have been a hot and almost entirely smooth soup of elementary particles, called a plasma.
That the plasma was only almost entirely smooth is important. The plasma had small clumps in which the density was a tiny little bit larger than the average, and in other places, the density was a tiny little bit smaller. But gravity has the effect of drawing matter toward other matter. That is, gravity turns small clumps into bigger clumps. Incredible as it sounds, over the course of billions of years this makes the small irregularities in the plasma grow to entire galaxies. And the distribution of galaxies we observe today is then-through the evolution law-directly related to the distribution of the little clumps in the plasma in the early universe. Therefore, we can use the observations of galaxies today to infer, by running the evolution law backward, what the little clumps in the plasma must have looked like, how large they were, and how far apart from one another they were.
Moreover, the distribution of galaxies is not the only observation we can use to infer what the plasma must have looked like. That's because the spots in the plasma where the density was a little higher were also a little hotter, and the spots where the density was a little lower were a little cooler. Now, as long as the plasma is on average very dense, it is opaque, meaning that light will be swallowed almost immediately after being emitted. However, as the density of the plasma drops, elementary particles can stick together and form the first small atomic nuclei. After some hundred thousand years, there comes a moment-called recombination-when the plasma has cooled sufficiently so the atomic nuclei keep electrons bound to them. ª After that, light is unlikely to be absorbed again. This light from recombination then streams freely through the expanding universe.
As the universe expands, the wavelength of the light stretches and so its vibrational frequency decreases. Because the frequency is proportional to the energy of the light, and the average energy determines the temperature, the temperature of the light drops with the expansion. This light is still around today, though at an extremely low temperature of 2.7 Kelvin (that is, 2.7 degrees Celsius above absolute zero); it makes up the cosmic microwave background. The name derives from the typical wavelength of the light, which is about 2 millimeters and falls into the microwave part of the electromagnetic spectrum. ['One of the countless mysteries of scientific terminology is why it's called recombination rather than just combination, given that it was possibly the first time they were ever combined. My best guess is that this term was borrowed from atomic physics, in which a plasma always first has to be heated before it can cool and recombine. The re probably stuck to combined just because the binding energy was too high to split it off. That's much shorter than the wavelengths used by microwave ovens, which are typically in the range of about 10 centimeters, or 4 inches.]
The temperature of the cosmic microwave background, however, isn't exactly the same in all directions of the sky. The average temperature is 2.7 Kelvin, but around that average there are small deviations of a few hundred-thousandths of a degree Kelvin. This means that the light coming from some directions is a tiny little bit warmer and that from other directions is a tiny little bit colder. These temperature fluctuations in the cosmic microwave background also go back to the density fluctuations in the plasma in the early universe.
The important point now is that the initial conditions for the plasma in the early universe fit to both observations: the distribution of galaxies and the temperature variations in the cosmic microwave background. The concordance model of cosmology, therefore, is a simplification over just collecting the data: it explains why two different types of data fit together in a very specific way. While you can posit an initial condition to any evolution law so the result will agree with observations, you will in general have to put a lot of information into the initial condition to make the calculations come out just right to fit the observations. The concordance model, in contrast, does not need much information-neither in the dynamical law, nor in the Initial condition-to explain several different observations. It makes things fit together. It has, in the words of the previous section, high explanatory power.
I have picked out two specific observations-the distribution of galaxies and the cosmic microwave background-to illustrate what I mean when I say the concordance model is a good explanation, but there are other observations that also fit it, such as the abundance of chemical elements and the way in which galaxies form. These observations strengthen the case for the concordance model.
The concordance model is considered a good scientific theory because it's simple yet it explains such a lot of data. The numerical values that currently fit best to the collected data tell us that only about 5 percent of the universe is made of the same stuff as we are, 26 percent is thinly distributed dark matter, which we can't see, and the remaining 69 percent is attributed to the dark energy of the cosmological constant.
How does the Big Bang fit into this model? The Big Bang refers to a hypothetical first moment in time when the universe began, so it would have happened before the hot-plasma phase we just discussed. If we go purely by the mathematics, then at the time of the Big Bang the matter in the universe must have been infinitely dense. An infinite density makes no physical sense, though, so it probably just signals that Einstein's theory of general relativity breaks down for very high densities. When physicists say "Big Bang," they therefore usually are not referring to the mathematical singularity but to whatever might replace the singularity in a better theory of space-time still to be found. [ Some physicists and science communicators use the term Big Bang to refer to times considerably later in the expansion of the universe. In this case, the Big Bang has nothing to do with the initial singularity. This has caused and continues to cause a lot of confusion, and I will not use the term in this sense here.]
The Big Bang, however, is not part of the concordance model. That's because we have no observation that tells us anything about what happened that far back in time. The problem is, when we run our equations backward in time, the density and temperature of the plasma continue to increase. Eventually, the plasma will be hotter and denser than what we have been able to produce in the world's most powerful particle colliders. And beyond the energy of those colliders, we no longer know what physical processes to expect. We have never tested this regime, and it doesn't occur in any other situation that we have observed. Even inside stars, temperatures and densities do not exceed the ones we have produced on Earth. The only naturally occurring event we know of that can reach higher densities is a star that collapses to a black hole. Alas, in this case, we can't observe what's going on, because the collapse is hidden behind the black hole horizon.
It's not a small gap in our knowledge. The energies at the Big Bang were at least fifteen orders of magnitude higher than the energies we currently have reliable data about. Of course, we can speculate, and physicists have certainly speculated with abandon.
The straightforward speculation is to assume that nothing changes with the evolution equation of the concordance model, so we can just continue to roll it back in time, into the range for which we have no data. Just to give you a sense of what it means to extrapolate over fifteen orders of magnitude, it's comparable to extrapolating from the width of a DNA strand to the radius of Earth-and assuming that nothing new happens in between. It's highly questionable that this extrapolation is any good. In any case, if you do it, then the equations eventually just break down; we get the Big Bang scenario, and that's that. It's rather boring, really.
However, because there's no data to constrain this extrapolation back in time, there is nothing to prevent physicists from changing the equations at earlier times and making up exciting stories about what might have happened. That's much more interesting. For example, it Is very common for physicists to assume that when densities increase beyond the so-far-tested range, the fundamental forces of nature eventually merge to one in an event called grand unification. We have no evidence that something like this ever happened, but a lot of physicists believe it nevertheless. Furthermore, they have come up with hundreds of different ways to change the evolution equations. I cannot possibly go through all of them, but here I'll briefly list the currently most popular ones.
Inflation
According to the theory of inflation, the universe was created from quantum fluctuations of a field called the inflation. The word field here just means that, unlike a particle, it permeates space and time it's everywhere. Emergence from quantum fluctuations means that this creation can happen even in vacuum. The universe starts with vacuum, and all of a sudden, there's a bubble with the inflation field in it, and that bubble keeps expanding. The inflation field causes the universe to undergo a phase of exponentially fast expansion-the inflation that gives the theory its name. Physicists then postulate that the inflation field decays into the particles that we still observe today, [This usually includes the hypothetical particles that make up dark matter.] and from there on, everything continues according to the concordance model.
We have no evidence for the existence of the inflation field or for the idea that today's particles were produced in its decay. Some physicists have claimed that inflation theory makes predictions that may be falsified by upcoming observations. However, you can always choose the properties of the inflation field so they match whatever we will observe, which means the hypothesis has no explanatory power. The reason inflation is popular with physicists is that it's believed to simplify the initial conditions, but leaving aside that this claim has been contested, this simplification comes at the cost of complicating the evolution equation.
That the inflation field gives rise to a universe where previously there was only vacuum is, on occasion, interpreted as creation ex nihilo, "out of nothing," as, for example, in physicist Lawrence Krauss's book A Universe from Nothing. A quantum vacuum, however, is not nothing. It is definitely something with very specific mathematical properties. Also, in the common version of inflation theory, space and time existed before the creation of our universe, so it is clearly not creation ex nihilo.
New Forces
Physicists currently count four fundamental forces: gravity, the electromagnetic force, and the strong and weak nuclear forces. All other forces we know of-van der Waals forces, friction, muscle forces, and so on-arise from those four fundamental forces. Physicists call any hypothetical new force a fifth force. This name doesn't (yet) refer to any specific force but to a large number of different forces that have been conjectured for different reasons, one of which is to alter the hypothetical conditions in the early universe.
I'll just pick out one for illustration, the force created by a field, the cuscuton, that supposedly existed in the early universe. It has since disappeared, but back then it allowed fluctuations to travel faster than the speed of light. The cuscuton is not named after couscous, and not after the marsupial species cuscus either, but rather after the plant genus Cuscuta. This parasite grows on plants and bushes and looks somewhat like a fuzzy green wig. Cuscuta is found almost exclusively in tropical and subtropical regions, which is my excuse for never having heard of it before. The cuscuton field is so named because, like the parasite, the field "grows" on the dynamic law of the concordance model.
The force created by the cuscuton has a similar consequence for the distribution of matter in the universe as the exponential expansion of inflation theory, and it suffers from the same problem-namely, that it is unnecessary to explain any existing observation and provides no simplification over the concordance model.
The cuscuton was first proposed in 2006, and I have to admit it's somewhat of a niche idea. I am mentioning it here because it has been shown that as far as current observations are concerned, the cuscuton can't be distinguished from inflation. This drives home my point that these hypotheses are ambiguous and make a simple story more complicated, the opposite of what scientific theories should do.
Bounces and Cycles
This class of theories has it that the current expansion of our universe was preceded by a contraction phase; they replace the Big Bang with a Big Bounce: that is, a smooth transition from an earlier universe into ours. In some variants of these theories, our universe will eventually end in yet another bounce, part of an infinite cycle. There are various versions of such cycles, depending on just how you change the evolution equation around the Big Bang singularity.
The most popular cyclic models are conformal cyclic cosmology, proposed by Roger Penrose, and the ekpyrotic universe, originally proposed by Justin Khoury and collaborators. Penrose glues the late phase of the universe to the early phase of the next universe, whereas Khoury and friends imagine that the universe was created in an extradimensional collision of high-dimensional surfaces, which can happen repeatedly. A Big Bounce without a cycle also happens in some approaches that aim to unify gravity with quantum mechanics, like loop quantum cosmology.
The problem with these ideas-you probably guessed it-is that they have no explanatory power. They do not simplify the calculation of any observation; instead, they make things more complicated, and it is highly questionable that there is any observation that can ever be uniquely attributed to one of them.
The No-Boundary Proposal
The no-boundary proposal avoids the Big Bang singularity by replacing time with space outside the early universe. I say outside because it makes little sense to use before if there was no time. Imagine a paper with a circle drawn on it. The circle is our universe as we know it. It has space and time. The area outside the circle has no time. It is not before anything, but next to everything. In the no-boundary proposal, our universe is embedded into space just like that.
This idea was originally proposed by Stephen Hawking and JimI lartle, but a similar disappearance of time has appeared more recently in some versions of loop quantum cosmology. Yes, that's the same approach to quantizing space-time that, according to other people, might give rise to a bounce. This ambiguity doesn't appear merely because the math is difficult, though it is, but also because there are different ways to turn ideas into math but no data to tell us which is the right way.
Like the other theories for the early universe, this one, too, works by replacing the evolution equation with a different one. The no-boundary proposal suffers from the same problem as all other theories for early universe: it is unnecessary to explain any observation, it does not result in any simplification, and its predictions are ambiguous.
Geometrogenesis
The idea of geometrogenesis ("birth of geometry") is that space was created along with the universe. In such an approach, scientists typically describe the prenatal phase of the universe as some kind of network that has too many connections to lend itself to a meaningful geometric interpretation. This network then changes with time or with temperature and eventually takes on a regular, geometric shape that approximates the space of Einstein's theory.
Geometrogenesis is inspired by the observation that every surface we think of as smooth and continuous-like paper or plastic-upon close inspection is actually made of smaller things and has holes in it. The problem with geometrogenesis is, once again, that it isn't actually necessary to describe anything we observe. It is filling a story into a gap in our knowledge because scientists are unwilling to accept that the answer is "We don't know."
Let me be clear that I am not saying these models make no predictions. Physicists have all read their Karl Popper, and they usually try to predict something. The problem is that the models are malleable, and if an observation doesn't fit a prediction, that can easily be remedied by amending the models. If physicists hadn't dropped their philosophy of science course after Popper, they'd see the problem with this method. But they don't, which is why we now have hundreds of stories about the beginning of our universe, none of which is actually necessary to explain anything we have observed.
My intention here is not to trash cosmology. OK, maybe a little bit. But we should keep in mind that we have learned some truly amazing facts about the universe from research in cosmology. A century ago, we knew neither that there are galaxies besides our own nor that the universe expands, and I certainly do not want to belittle these achievements. Neither do I want to argue that cosmology is finished. The best current model of the universe, the concordance model, will almost certainly not be the last word. It is foreseeable that data will continue to get better for a long time. This will rule out some models-maybe the concordance model among them-and new, better ones will be put forward and become established. These better models will have good chances to extend further back in time than the concordance model.
Nevertheless, cosmological research is limited by two different problems. First, all these hypotheses about the early universe-the ones I've listed and many others you may have heard about-are pure speculation. They're modern creation myths written in the language of mathematics. Not only is there no evidence for them, but also, it’s hard to conceive of any evidence that could settle the debate regarding which one is correct, because they are all so flexible they can plausibly be made to accommodate any data thrown at them.
Second, when it comes to explaining the early universe, physicists are faced with a fundamental problem that might be impossible to overcome. All our current theories rely on simple initial conditions. This isn't optional; it's essential for our mode of explanation to work. If you have to make the initial conditions complicated, even the simplest evolution law will not give your theory explanatory power. If the universe went through an earlier phase that is more difficult to describe than that hot plasma from which galaxies formed, then our entire scientific methodology would stop functioning. Even if this hypothesis were right, we'd have no rationale that would allow us to add a more difficult story before a simple one.
The only way I can think of to overcome this impasse is to eventually develop theories that do not require initial conditions but instead apply to all times at once. There isn't any such theory at the momentous that, too, is pure speculation.
In the End
If we take our current theories of the universe and extrapolate them Into the distant future, the result, in one word, is dark. In about four billion years, our neighboring galaxy Andromeda is projected to collide with the Milky Way. Our own Sun will have spent its nuclear fuel and burned out in about eight billion years, and so will, eventually, all other stars. While matter cools and clumps, with much of it ending up in black holes, the expansion of the universe will happen faster and faster, making it more and more difficult to see the faint glow of other galaxies as they recede from us. Night skies will go black.
But no one will be around to see them anyway. The universe can support life only in the limited, blessed window of time we currently find ourselves in. That's regardless of how flexibly you define life, because the supply of useful energy will inevitably run out. Even if we imagine forms of life very different from ours (Freeman Dyson, for example, speculated that life might form in interstellar clouds of gas), they will all ultimately fall victim to the same problem: life requires change, and change requires free energy, and there's a limited supply of it. Another way to say this is that entropy cannot decrease. We will talk more about entropy in chapter 3. For now, let us just have a critical look at how much one should trust these extrapolations into the far future.
Let me begin by noting that we don't know whether the laws of nature will remain the same even tomorrow. In science, it's often an unwritten article of faith that the laws of nature will remain what they are and not suddenly change.
David Hume, in the eighteenth century, called it the problem of induction: when we infer the probability of a future event from past observations, we implicitly assume nature is uniform, constant, and reliable in its proceedings. The laws of nature don't suddenly change. If they did, we wouldn't call them laws.
But we may be mistaken in our assumption that nature is uniform. Bertrand Russell, in his 1912 book The Problems of Philosophy, com-pared Hume's argument to a chicken's attempt at inferring the laws of living on a farm. The chicken is fed reliably every morning at 9:00 a.m., until one day the farmer chops off its head. "More refined views as to the uniformity of nature would have been useful to the chicken, “Russell mused.
Hume's eighteenth-century problem is still a problem today, and it might be an unsolvable problem. The uniformity of nature itself is certainly an expectation based on our past observations, but we can’t use an assumption to confirm itself. It's impossible to predict that nothing unpredictable will happen.
In case you were hoping that requiring the laws of nature to be mathematical is a way out: sorry, but that doesn't help. It isn't difficult to come up with mathematical laws that will look indistinguishable from the ones we have confirmed so far but will blast apart the solar system tomorrow. It's not that anything speaks for this, but nothing speaks against it either. A smarter chicken might have been able to infer the farmer's intentions, but it would still not have been able to Infer that its inference would work.
What is going on? For 97 percent of all Wikipedia articles, if you click on the first link and repeat this in each subsequent article, you will eventually get to an entry about philosophy. Philosophy is where our knowledge ends, and the scientific method is no exception. Does the scientific method work? Yes. Why does it work? Ultimately, we don’t know. And because we don't know why it works, we can't be sure it'll continue to work.
Why then do science at all? Why, indeed, do anything when the universe might fall apart any moment? When I first learned about Hume’s problem of induction, as an undergrad, I was stumped. I felt that someone had pulled the carpet of reality out from under me, to reveal a big, gaping void. Why hadn't anyone warned me of this?
But then I thought, "Well, what difference does it make?" The laws of nature will either continue to do what they've been doing so far, or they won't. If they continue, the scientific method will serve us well and will help us decide which course of action best suits our needs. If the laws don't continue, there isn't anything we can do about it, and no course of action will prepare us for it, so why bother thinking about it? I rolled back the carpet. There's still a void under it, but I can live with that. I guess I wasn't meant to be a philosopher.
I have the same reaction to scary stories about the demise of our universe. If we can't do anything about it anyway, it's pointless to fret about it.
Take, for example, the risk that the universe might undergo spontaneous vacuum decay, which means the vacuum might suddenly fall apart into particles that come out of nowhere. If that happens, an enormous amount of energy will be released into what was previously empty space. All matter will be ripped apart instantaneously. We cannot rule out this possibility, because observations merely tell us that the vacuum has not decayed so far. This means we cannot tell a truly stable vacuum from one that is merely very long-lived, or meta-stable, as the physicists say. It's Russell's chicken for vacuum-expectation values rather than food-expectation values.
Stickers that glow in the dark, for example, work with metastable states. The paint used for them contains atoms capable of phosphorescence. If you shine light onto these atoms, they temporarily store it by moving electrons to higher, metastable energy levels. When the electrons decay back to the lower level, the atoms release the energy again in the form of light, hence the glow.
Like one of those phosphorescent atoms, our vacuum might also undergo decay. And because this is a quantum process, it's not as if it starts slowly, so we'd see it coming. It just happens with a certain probability within a certain amount of time, with no advance warning.
Whether or not our vacuum can decay depends on a couple of parameters whose values we don't know exactly. The best current estimates say that, yes, the universe can decay, but its average lifetime is something like 10500 years. That's a number so big, it doesn't even have a name. But that's only the average lifetime. It means the probability is small that the vacuum will decay much earlier than that. But the vacuum can decay earlier; it's just very unlikely.
In my opinion, though, this and similar estimates are meaningless, because they require an extrapolation over more than a dozen orders of magnitude of unknown physics, down to distances of about 10-35meters, whereas the best current experiments reach down to only about 10-20 meters. ['A distance of 10. * meters are the so-called Planck length, the scale at which quantum gravity is expected to become important, and 10 " meters is approximately the distance probed by the. urgently largest particle collider in the world, the 1arge Hadron Collider at CERN.] If there is anything we don't yet know of in this range (which we have good reason to think is the case), the estimate Is wrong. Hence, the brief summary is that we don't know.
Similar considerations apply to other stories about the end of the universe. We can certainly take the laws of nature that we know and extrapolate them, and that's a fun exercise. But even leaving aside the problem of induction, the further we look ahead, the more uncertain our predictions become. If there are any physical processes that are so slow or rare that we haven't observed them so far, they might become relevant in the distant future.
For example, a lot of physicists have speculated that protons, one of the constituents of atomic nuclei, might be unstable but are just so long-lived that we haven't seen one decaying yet. Maybe so, maybe not. Black hole evaporation, too, happens so slowly that we can't measure it-if it happens at all, for which we have no evidence.
We also don't know what dark energy will do in the distant future. We haven't found evidence that its amount changes, but if it changes rally slowly, we won't be able to measure it. Yet even an exceedingly slow change in the amount of dark energy would have a large effect on the expansion rate. Indeed, when the universe was five billion years younger-a time when our planet hadn't been born but life was already possible on other planets-we probably wouldn't have been able to measure dark energy at all. Back then, the influence of dark energy was much smaller, not large enough to cause the universe’s expansion to accelerate.
Lawrence Krauss has joked that he makes predictions only trillions of years into the future, because no one will be around to check if he’s correct. It seems to me that the more reliable but less funny prediction is that Krauss won't be around in case it turns out to be wrong that no one will be around. In any case, you shouldn't trust physicists ‘predictions for the end of the universe. You might as well ask a fruit fly for a weather forecast.
>> THE BRIEF ANSWER
We improve scientific theories by simplification. When it comes to the early universe, there may be a limit to how much we can possibly simplify our explanations. It could therefore be that we will never be able to tell which one of many possible theories for how the universe began is correct. This is certainly presently the case for theories about the beginning of the universe. For possible ways the universe could end, the problem is that we don't know anything about processes that are so rare or slow we wouldn't yet have been able to observe them. So don't take these stories too seriously, but feel free to believe them if you want.
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The Beginning According to the Bible: Genesis 1 (American Standard Version)
1 In the beginning God created the heavens and the earth. 2 And the earth was waste and void; and darkness was upon the face of the deep: and the Spirit of God [a]moved upon the face of the waters. 3 And God said, Let there be light: and there was light. 4 And God saw the light, that it was good: and God divided the light from the darkness. 5 And God called the light Day, and the darkness he called Night. And there was evening and there was morning, one day.
6 And God said, Let there be a [b]firmament in the midst of the waters, and let it divide the waters from the waters. 7 And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so. 8 And God called the firmament Heaven. And there was evening and there was morning, a second day.
9 And God said, Let the waters under the heavens be gathered together unto one place, and let the dry land appear: and it was so. 10 And God called the dry land Earth; and the gathering together of the waters called he Seas: and God saw that it was good. 11 And God said, Let the earth put forth grass, herbs yielding seed, and fruit-trees bearing fruit after their kind, wherein is the seed thereof, upon the earth: and it was so. 12 And the earth brought forth grass, herbs yielding seed after their kind, and trees bearing fruit, wherein is the seed thereof, after their kind: and God saw that it was good. 13 And there was evening and there was morning, a third day.
14 And God said, Let there be lights in the firmament of heaven to divide the day from the night; and let them be for signs, and for seasons, and for days and years: 15 and let them be for lights in the firmament of heaven to give light upon the earth: and it was so. 16 And God made the two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also. 17 And God set them in the firmament of heaven to give light upon the earth, 18 and to rule over the day and over the night, and to divide the light from the darkness: and God saw that it was good. 19 And there was evening and there was morning, a fourth day.
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The End According to the Bible: Revelation 21 (American Standard Version):
21 And I saw a new heaven and a new earth: for the first heaven and the first earth are passed away; and the sea is no more. 2 And I saw [a]the holy city, new Jerusalem, coming down out of heaven from God, made ready as a bride adorned for her husband. 3 And I heard a great voice out of the throne saying, Behold, the tabernacle of God is with men, and he shall [b]dwell with them, and they shall be his peoples, and God himself shall be with them, [c]and be their God: 4 and he shall wipe away every tear from their eyes; and death shall be no more; neither shall there be mourning, nor crying, nor pain, any more: the first things are passed away.
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