24. The Past and Future of the Dark Side - Sean Carroll - Dark Matter, Dark Energy: The Dark Side of the Universe

This last lecture looks at the big picture, a historical overview, and extrapolations for future clues. But first a big pat on the back for everyone; to the scientists for their inspiration and perspiration, to the human race for pursuing such questions, and to the audience for finishing the course!

Yet scientists tend to rest on their laurels when things are quiet. Why do we have a concordance cosmology of 5/25/70? Or is it legitimate to even ask such a question? Most numbers in the universe don't seem finely tuned. Those that are create a Hierarchy Problem, such as the mass of the Higgs particle being 150x the proton mass. It could just as well be many orders of magnitude greater, so why is it so small? Supersymmetry may eventually provide an answer.

Situations in the universe also don't seem finely tuned. The classic gas in a box where all molecules are in one corner does not violate any laws, but just has low entropy. There must be some physical dynamical reason for this to not be a natural situation. Applying this to the situations of the universe such as the horizon and flatness problems, we found the reason to be inflation. This even provided the physical dynamical reason, the perturbation situation. Asking these questions was not just an exercise in philosophy, they were legitimate, as is asking about concordance cosmology.

Dark matter has a natural answer in WIMPs. They are not too difficult to get from the standard model, and we have built detectors for them in space, ground, and underground. Dark energy is not so natural. The vacuum energy fits the data, but has the unnatural value problem of 120 orders of magnitude and the unnatural coincidence of an energy density roughly equal to that of dark matter.

An interesting diagram is displayed with lines and arrows representing interactions between the three 5/25/70 of concordance cosmology. Gravity is at the center, interacting with everything. It is helpful to a certain extent, but in his last comment on it, Sean talks about the very specific interaction possible between axions interacting and photons. I would have liked more general comments such as how this represents our new and so far, best standard model for concordance cosmology. There are many confusing possibilities for more lines and arrows, but I liked Sean pointing out how that is a positive aspect. We are not just resting on our laurels in this respect.

Sean concludes the course with a dramatic speculation and a cautionary tale. Seeking the naturalness in the overall configurations of the universe has worked very well in the dynamical process of inflation. The entropy of the universe is medium sized. Our example of gas in one corner of a box was low, when spread out it was high. The second law of thermodynamics says entropy will increase, so our current medium size would have been quite small long ago. Why would the universe start with such a low entropy? Inflation begs the question with an even smaller patch that becomes dominated by dark energy, with even lower entropy than the big bang. This could mean that in the future, dark energy would not be so quiet. The distant future may even have the quantum jiggles making new baby universes. Or are we a baby universe in yet some other parent universe? We need to examine such configurations as if they are plausible.

Some think we recently we were finished with discovering laws of physics. The early puzzles with electromagnetism and the orbit of Mercury were solved by quantum mechanics and relativity. But today we have the same types of puzzles with vacuum energy, the conflict between quantum mechanics and general relativity, and between dark matter and dark energy. Will there be whole new theories to solve them, or revolutions in our existing theories? The next century promises to be just as interesting and surprising as previous.

At this point in the final lecture of the course, we have a great deal to be proud of. By "we," Sean means scientists who have been trying their best, and put together a picture of what the universe is made of over the last 100 years. Yet he also means the human race, who have been wondering about these questions for the last several thousand years. Also of course, all of us who have been watching these lectures!

We've put together a pretty good picture of everything. Not only do we know that 95% of the universe is this dark sector, where 70% is dark energy that's smoothly distributed through space and time, and 25% is dark matter made of some heavy particle that settles into galaxies and clusters, but we also have good reason to believe that we're not missing anything. There may be 1% or more, here or there, but basically we know everything in the universe. Measuring the total ρ of stuff, through the curvature of space, convinces us that what we know right now, comprises everything there is to know.

However, scientists are notoriously bad at resting on their laurels. We really want to focus on the questions to which we don't know the answer, the puzzles that we're left with from this very successful picture. Fortunately, even though the picture we have of the universe today is very successful, it leaves us with tremendous questions and offers us clues as to where we might find the answers.

The clues reside in the fact that the picture which fits the data so well, is nevertheless extremely unnatural from certain points of view. It's perfectly fair to say that our picture of the universe with 5% ordinary matter, 25% dark matter, and 70% dark energy, makes no sense! Or at the very least, we could have done better if asked to design a simple and elegant universe! So there's something we don't understand about why our universe is like this, versus like something else.

So for this lecture, we'll step back a bit, looking at an historical overview of how we got here, and try to go from there to extrapolate where we might go next. We'll try to understand that given what we have learned about the universe, where might we be looking for clues as to what underlies particular configurations in which we find the universe in which we live.

So lets start with how we got here. Just 100 years ago, we knew next to nothing correct about what the universe was like on very large scales. We did know that there were stars, which were different than the sun, that they were further away, but the same basic kind of thing.

We didn't know what those stars were, or how they made their nuclear fuel, the stars or the sun. We didn't know there was such a thing as a nucleus. We knew there were atoms and different chemical elements, yet we hadn't yet decomposed atoms into electrons moving around nuclei. So the basic energy source for these stars was unknown to us.

We believed that the stars were distributed in an island universe configuration. We thought that the stars were moving around each other, in what we now call the Milky Way galaxy, yet we didn't know that there were other galaxies. We knew that there were little fuzzy patches on the sky that were called nebulae, yet we didn't know many of them were in fact galaxies in their own right,with hundreds of billions of stars, all by themselves.

We certainly didn't know about the expansion of the universe. We didn't know that galaxies had an apparent velocity moving away from us, or anything about Einstein's Theory of General Relativity, so we didn't know that space and time could expand. We believed in an absolute Newtonian universe in which space and time were fixed, not changing as a function of time. The dynamic nature of spacetime itself is something we didn't have access to.

So the modern story really begins in 1915 when Einstein puts the finishing touches on his General Theory of Relativity, which says that spacetime can be curved and have a dynamics all of its own. That dynamics is manifested to us as gravity. He very soon thereafter started thinking about cosmology and realized that if the universe was evenly spread all over the place, it would have to be dynamical, either expanding or contracting. Yet as far as he knew in 1917, it wasn't. So he changed his ideas by adding a new term to his equation for General Relativity, called the cosmological constant, something that would keep the universe static, at least in principle.

Then of course, in the late 1920s, it was realized that the universe is not static. Hubble talked about some of the nebulae being separate galaxies all by themselves, and that the universe is expanding. Einstein then tossed away his cosmological constant and realized it was a dynamical universe in which we live.

Very soon thereafter, Fritz Zwicky was looking at clusters of galaxies and found the first evidence for dark matter. He looked in the Coma cluster and realized there was more stuff there than we can possibly account for, just be what we saw. He didn't know at the time, that this stuff, the dark matter, couldn't be explained by ordinary atoms, and that was going to come down the road.

By the 1940s and 50s, we only first realized how the early universe worked. George Gamow had his student, Ralph Alpher and Robert Herman, figured out how to make light elements by nucleosynthesis in the early universe. As a spinoff, they got the CMB, the leftover radiation what we now know provides us a snapshot of what the universe looked like 400,000 years after the Big Bang. This CMB was discovered by accident in the 1960s by Penzias and Wilson. They were very careful radio astronomers who kept on getting these microwaves spread uniformly across the sky, yet didn't realize there was another group down the street in Princeton who were looking for these intentionally. They won the Nobel Prize in the 1970s for their work.

Then in the 70s, physicists put together the standard model of particle physics. So the "cosmological" standard model, crept up on us only gradually! Yet the "particle physics" standard model was put together over the space of only a few years. It's been extremely successful since then. Since the mid-1970s, no particle physics experiment has truly surprised us. Everything we've done has fit together into what we already knew as the standard model.

Also in the 1970s, Vera Rubin found the even better evidence for dark matter, by measuring the masses of individual galaxies by determining their rotation curves. She found that not only are galaxies heavier than they should be, given only the stars, gas, and dust that were in them, but that there was mass out beyond any of the visible matter. So eventually we realized that this dark matter was not able to be accommodated in what we understood about particle physics, so that it had to be a new particle.

By 1980, Alan Guth invents inflation. There were puzzles about the early universe we didn't understand. How was it so smooth? How was the spatial geometry of the early universe so close to being flat? Inflation made all that snap into place, and even though we're still not absolutely sure it's true, it's still the leading candidate to explain why the overall geometry of the universe is what it seems to be.

Then by the 1990s, observational cosmology became a precision science. The one watershed moment was the discovery in 1992 of the tiny fluctuations in temperature of the CMB. This was the result of the COBE satellite of NASA, which enabled us to begin linking the earliest moments of the universe's history, to what we observe today. At early times, the universe was extremely smooth, when only 1 part in 100,000 was the only deviation you could see. Under the force of gravity, those deviations have grown into the galaxies and clusters that we observe today.

Technology has now evolved enough to allow us to observe those galaxies and clusters to exquisite precision. We can compare what we see, with the predictions we get from the CMB extrapolated in time, and then put together the ingredients of the cosmological model that is so successful.

The final ingredient we needed was found in 1998, when supernovae used as standard candles allowed discovery of an accelerating universe that can only be explained by dark energy. This is some form of stuff whose ρ doesn't go away as the universe expands. That made up the extra 70% that was predicted by inflation if you wanted to live in a flat universe. Then by 2000, observations of the CMB confirmed that we indeed do live in a flat universe. The fact that 70% of the universe is dark energy, 30% is matter, simultaneously fit the dynamical measurements of galaxies and clusters, the supernovae data that shows the universe is accelerating, and the CMB that shows the universe to be spatially flat.

Since the year 2000, we've been confirming this picture in even more elaborate ways. The WMAP satellite has given us extra information that in fact the ordinary matter is only 5%, that you need some form of dark matter, and that the evolution of structure from then to now is consistent to what we think it would be, in this model where the universe is 70% dark energy.

So we land here at a place where we have a very successful model of everything in the universe. If you think about it, this model has three basic features, not just the three basic ingredients in the inventory, but three aspects that are important to its success. First is the fact that the universe is expanding, and the understanding of it within the context of General Relativity. In the 1915-1917 era, Einstein was realizing that the universe had to be expanding, and we now know that it's true. We can then use his equation to extrapolate all the way back to within a minute of the Big Bang and we get the right answer, as is confirmed by Big Bang nucleosynthesis.

The second source is of course the inventory, the (5%)(25%)(70%) split between ordinary matter, dark matter, and dark energy, which has been tested in multiple ways, and is overdetermined be the observations we have now.

The final aspect is the evolution of structure in the universe. The first two aspects deal with a universe that is spatially smooth, more or less the same everywhere, which is an extremely accurate picture, either at early times or on large scales today. Yet on smaller scales, we see that there are galaxies, there are clusters, there are stars and planets. Unfortunately we're unable to understand where they came from, going from the early universe to now.

So that's an incredibly successful picture. Using data, we can go from one second after the Big Bang, to 14 billion years after the Big Bang. We can sensibly extrapolate with our theories, back to times like 10 to the -30th power seconds after the Big Bang, when we talk about inflation. This inflation acts as a way to generate the perturbations we see today.

So when we are hypothesizing and speculating about what the universe was like at 10 to the -30th power seconds after the Big Bang, we can actually connect those speculations to observations we are making today. We're not absolutely sure of what the universe was like, earlier than one second old, but the very fact that we're trying to go back from one second, to some tiny fraction of a second, is indicative of the progress we've made, compared to 100 years ago when we didn't know the universe was even getting any bigger.

Yet let's step back and ask, given our extremely successful picture, why is it like this? Why is the universe smooth and homogeneous? Why is it that 70% of the universe is dark energy, versus the 30% that is matter? You can ask yourself if this is even a respectable question to be pondering? Should we be people who just look at the universe, describe what it's doing, and think that is our duty as scientists, so that it's not our job to ask why it is like that?

Yet of course as scientists, we are not satisfied with the current state of affairs. We always want a more complete picture. One of the ways to reach that, is to perform more experiments, to collect more data, to take more observations, to find out more about the universe, and we're certainly going to be doing that. In the previous lecture, we got this very diverse smorgasbord of experiments that will be done, a portfolio of different things, both highly speculative and guaranteed to get good results.

Yet in the meantime, there is another way to think about how we can go beyond our current understanding. That's to try and reflect upon whether our current picture of the universe makes sense to us at a deep level, whether it is somehow natural to us. There are no absolutely strong contradictions between the different elements of the standard model of cosmology, like Einstein was lucky enough to have a deep contradiction between Newtonian gravity and Special Relativity, when he was developing General Relativity.

Yet there are tensions between the different elements. So we're going to try and ask if it's somehow a natural universe in which we live, and if not, is the fact that the universe appears natural or unnatural to us, the sign that there's some deeper underlying forces with respect to what it would seem much more natural? So we as scientists have ways to think about this concept of naturalness, ways to take this more or less fuzzy human idea and turn it into something a little bit more quantitative. Both for theories that we have and for situations that we find in the world, science can tell you whether or not that particular thing sounds natural to us. For a theory like the standard model of particle physics for example, we say it's natural if everything that might be happening in that theory, is happening and doing so with the right strength?

OK, that didn't seem much less fuzzier than the original idea of naturalness, but what we mean quantitatively is that when we look at the numbers that appear afterward, the numbers that characterize the strength of different forces, and the frequency of different interactions, those numbers don't seem very finely tuned. In other words, if we were to change those numbers just a little bit, you would get the same basic kind of physics within your theory. Well when we look at some of the different aspects of the standard model, we see that there are fine tunings there.

One example is what particle physicists call the hierarchy problem. They look at the mass of the Higgs boson. We haven't detected it yet, but it's probably something like 100-150 times the mass of the proton. Yet if you look at it, there's no reason why that number, the mass of the Higgs boson, shouldn't be very much larger, shouldn't be up there near the Planck scale, some 10 to the 18th power times the mass of the proton.

Why is the mass of the Higgs boson so small compared to the natural scale it would like to have? That's an example of the kind of naturalness argument that the particle physicists use. What they will then do, is to try to find some underlying reason. In fact, in the form of supersymmertry, there is an underlying dynamic explanation for why the Higgs boson might be so much lighter than the much heavier scales characterized by the rest of particle physics.

For situations in the universe, it's a slightly different aspect of naturalness that we are concerned about. A configuration of stuff in the universe is natural if it's robust. If you move some particles around, you get the same basic idea. This is the kind of thing you might have been exposed to in some physics class, some time ago. If you have a box full of gas, and all that gas happens to be stuck in one corner of the box, that's not a very natural configuration. You let it go, and it begins to fill the box. We say that all the air molecules being stuck in one corner has a very low entropy, and as you let it go, it's going to naturally fill the box.

If you start with a gas filling the box, it will not naturally curl up in one corner. The configuration in which stuff is all spread out, is just much more robust. We can move a few particles around, yet it's still gas spread out smoothly throughout the box. So we have these notions of naturalness and unnaturalness. It is not that is you observe a gas with all the particles in one corner of a box, you would say that it violates the laws of physics. That is completely consistent with the laws of physics, but it's unnatural. We say there must be some reason why it's there. There must be some dynamical physics mechanism for why it looks like that.

So we want to apply that same kind of reasoning to the whole universe. This is a successful strategy that has been pursued before. For example, the horizon and flatness problems were inspirations for the development of the model of inflation. These problems were first, the idea that the universe is smooth on scales larger than it has any right to be. If we look at the universe in the CMB era, there were points with almost the same ρ and temperature, yet they shared no common points in their pasts. There was no way for them to know to be the same temperature, and yet they are. That's a naturalness problem.

Space, meanwhile, is very close to geometrically flat, yet tends to become less and less so, as time goes on. Why was it so close to flat in the early universe? That's another naturalness problem. By thinking about these problems, scientists were able to invent the model of inflation, which not only solves these problems, but as a bonus provides and explanation for the perturbations that we now think, grow into large-scale structure.

So this kind of reasoning is not just philosophizing, but it can lead us to very explicit scenarios that help us explain something that we don't otherwise understand. Well what can we do? What is our current universe like? Does it seem natural to us? Do we have possible mechanisms that make it fit into a bigger picture?

Well the two things we're concerned about in these lectures are dark matter and dark energy. For these two cases, the situation is actually very different. The dark matter case has scenarios that make it seem very natural to us. We have first and foremost the WIMP scenario, where if there exists in nature, some particle that is heavy, electrically neutral so we can't observe it very easy, yet that feels the weak nuclear force. As long as that particle is stable, it will naturally give rise to a density of particles that is very similar to what we observe in dark matter today.

In other words, you don't have to work very hard to extend the standard model of particle physics, so that naturally a dark matter candidate appears, with naturally the right amount of stuff in the universe. We don't necessarily know that this is the right scenario. Yet it's one that fits in with our understanding of what probably would make sense to us. Therefor, we take seriously this idea that the dark matter is a WIMP, seriously enough that we build very expensive experiments to go look for it, on the basis of that hypothesis. We build particle accelerators to try to produce it directly, we build satellites to try and look for the annihilation of WIMPs and anti-WIMPs in space, and we build underground detectors to find it directly in our own labs.

Meanwhile, the dark energy case does not have any model that would pass the naturalness test. The simplest model for dark energy, since we know it to be smoothly distributed through space and nearly constant as a function of time, is vacuum energy. This is something that is absolutely the same ρ in every cm³ of space and time. An absolutely minimum energy that doesn't evolve or fluctuate in any way, through time or space.

It's a simple model, yet it's not natural in terms of what we know about dark energy, because it doesn't have the right value. You can estimate how big the vacuum energy should be, according to our current understanding of gravitation and particle physics, the vacuum energy should be enormously larger than what we observe. it should be 10 to the 120 times what you actually get. So we don't know what is going on. We have a theory that fits, yet only at the cost of an extremely unnatural parameter.

The other thing that we know about the vacuum energy, is that it's victim to something called the coincidence scandal. If you'll notice, in the current universe the dark energy is bout 70% of the total energy budget, while matter is about 30%. Now by cosmology standards, 70% and 30% are equal. Those are very similar numbers. The reason why this is a little bit of a surprise, is because dark energy and matter evolve very differently with respect to each other. Matter dilutes away as the universe expands, so the ρ in matter when the CMB was formed, was a billion times bigger than the ρ of matter now. In the early universe when things squeezed together, the densities are much higher.

Dark energy meanwhile, has a ρ that is constant. The number of ergs per cm³ is a tiny number that is fixed, it's just as small in the early universe, as it is today, as it will be in the future. So back when the was formed, the ρ of dark energy is the same as it is today. In other words, it was a billionth of the ρ of matter. These two numbers, the ρ in dark energy and the ρ in matter, change rapidly with respect to each other as the universe expands. Yet today, when we happen to be here looking for them, they are approximately equal. Why is that?

We don't know the answer to this question, and it might be explained by environmental selection. It might be that most observers in some large ensemble of a multiverse, are going to be born at a time in their regions of space history when the ρ of dark energy is similar to the ρ of dark matter. That is a hypothesis, the kind of thing that we're driven to contemplate when thinking about these naturalness arguments. It may or may not be right, but these give us a clue for the directions in which to move next.

So here is a picture that we can return to, of where we might go in future years after we study the dark sector in greater detail. We right now have a very successful understanding of an ordinary model from the standard model of particle physics. We have dark matter and dark energy which are consistent with the data if they only interact with us through gravity. Yet that doesn't mean that this is the final word. It could be much more rich than that. So there's plenty of possibilities for new physics here that we're looking for in future experiments.

The dark energy could interact with itself. In other words, it might not be absolutely constant everywhere, but might evolve. It might change slowly with time in one way or another, yet might also change slowly with space. There could be perturbations in the ρ of dark energy. Likewise, the dark matter might interact with itself. In fact, in every realistic model of dark matter, there are some interactions between the dark matter and itself. There's some way for the dark matter to be produced in the first place.

More interestingly, there could be interactions between ordinary matter and the dark sector, in one way or another. There could be interactions between dark matter and ordinary matter, like the kinds we are looking for. It could very well be that the dark mater feels the weak interactions of the standard model of particle physics. That would enable us to create the dark matter in accelerator experiments, and in deep underground labs.

Even if not, even if the dark matter is not weakly interacting, it might interact through some other force. The axion is an example of a particle that is electrically neutral, yet indirectly interacts with photons. So we have separate experiments that are dedicated for looking for the interactions of axions with photons. It could be any day now that people begin to actually discover the dark matter directly, because of those interactions. We're looking for them because they make the dark matter theories more natural in the context of other things we understand about particle physics.

Best of all, maybe, would be if dark energy interacted with ordinary matter. If the dark energy is just vacuum energy, just a constant amount of stuff in every cm³, then it will not interact. It's just a background feature of spacetime, and all we'll ever be able to do, is measure its affects on the curvature of spacetime, on the expansion of the universe.

Yet if the dark energy is something different, if it's dynamical, something that changes from place to place, then dynamical fields tend to interact. We can look for interactions between the dark energy in photons, or the dark energy in nuclear matter, and experiments are ongoing to do exactly that. It could even be that the dark energy has similar kinds of interactions with the dark matter.

It could be that the dark sector all by itself, forms some interestingly interacting set of things, and we, the ordinary matter, sit off by the sides. So the dark matter could be affected by the dark energy, and have its mass change. Or it could have its interactions change in some interesting way. We don't know, yet it's not bad that we don't know. It's good that there are so many possibilities that we'll be pursuing, looking for observational evidence for anyone of these things happening.

So for the last part in this set of lectures, we'll end with one dramatic speculation, and one cautionary tale, just to bring us back to reality at the end. The dramatic speculation has to do with, one again, the naturalness of our picture of cosmology. In this case, we're not talking about the naturalness of the theory that we have, the theory with the parameters that describe dark matter and dark energy, but rather the configuration in which we find our universe today. Remember that this already worked once, this kind of reasoning where we said, "Why is the universe in this kind of situation in which we find it?"

This question worked with inflation. Inflation was able to successfully explain why the universe is homogeneous and isotropic, and why it's very close to spatially flat, invoking a dynamical mechanism based in physics, located in the early universe. It turns out that there is a deeper question that inflation doesn't quite answer. This is very closely related to the question of the gas in a box, the question of the entropy of the universe.

Our current universe has what you might call a medium sized entropy. This basically tells you how disordered things are. So if your gas in a box is stuck in one corner, that's a very low entropy. It's very highly ordered since someone cleaned it up and put it there. Yet if the gas is spread out all over the place, that's high entropy, and is disordered. We understand on the basis of physics, that stuff in the universe like to go from being low entropy, to being high entropy. Disorder tends to increase, just because there are many more ways of being disordered, than being ordered.

So this is a deep feature of fundamental physics, called the second law of thermodynamics. Things tend to become more disordered with time. Our current universe is semi-ordered. In the far past, things were much more highly ordered, so that entropy was lower. In the far future, it will be much more disordered, and the entropy will be much more higher. The entropy is growing with time, as the universe goes from the Big Bang, to the cold heat death, where everything is spread out and not radiating.

The question then is, why did the universe ever start out in some phase where the entropy was so low? You might, and many contemporary cosmologist do, think this has something to do with inflation. Yet in fact, inflation is begging the question. It imagines that a tiny patch of the universe was dominated by the dark energy, an even lower entropy state than the ordinary Big Bang. The truth is we don't yet now know, why the universe had such a low entropy.

It might, however, have something to do with dark energy. Dark energy is telling us that in the future of our universe, it will not be completely quiet. Dark energy means that even in empty space, there is some energy that keeps propelling the universe and keeps giving a little quantum jiggle to all the fields in the universe. If you wait long enough, much longer than the current age of the universe, It could be that those quantum jiggles come together in exactly the right configurations to make what is called a baby universe. To start a little patch with incredibly high ρ, ready to inflate and separate off from the universe that we know. Furthermore it could be that our universe is somebody else's baby, started off in some other universe that was cold and filled with nothing than dark energy.

In other words, the dark energy that we've discovered, though we're obviously just speculating here, might play some role in helping us understand why the configuration of our current universe looks so unnatural, looks like it started in such a low entropy state. That's the kind of thing we're led to think about, by thinking about dark matter and dark energy.

Finally, the cautionary tale is, how close are we to being done? In 1900, a lot of physicists though we were close to being done with the laws of physics. We had a really good understanding on the basis of Newtonian mechanics, of how matter and energy worked. There were just a few little things that were a tiny bit bothersome. For example, if you calculated the lifetime of an atom, you found it was an incredibly tiny fraction of a second. It should be the case that an electron zooming around a nucleus, should collapse really quickly.

Why was that number so much smaller than the observations said it was supposed to be? There was also an inconsistency in 1900 between successful theories of physics. Electromagnetism and Newtonian mechanics, didn't quite have the same set of symmetries, and that was a puzzle. Finally, if you looked at the sky, you saw that some celestial bodies weren't moving as they should. The orbit of Mercury was discrepant. It was moving in the wrong way, a different way than predicted by Newtonian mechanics.

So as it turns out that all of these tiny discrepancies led to revolutions. They led to quantum mechanics, the Special Theory of Relativity, and to General Relativity. It could have been that you just cleaned up something here and there, or it could have been a completely different picture of the universe.

The same thing is true today. We have a number that is wrong, and that number is the vacuum energy. Our number is much smaller than it has any right to be, since we observe a vacuum energy of 120 orders of magnitude smaller than we predict. We have an inconsistency between successful theories. We have General Relativity and quantum mechanics, both of which fit the data, but are mutually incompatible. Finally, we have celestial bodies moving in a way that doesn't make sense to us. We see things orbiting faster than they should, and attribute these to dark matter and dark energy.

It may be that we're almost done. It may be that a couple of things will fall into place, and we'll have a complete understanding of the fundamental laws of nature. Or it could be that a revolution is in the offing, similar to what we had 100 years ago. We don't know, and that's the fun of science. The only guarantee is that the next 100 years of exploration, will be equally interesting and surprising as the last 100 years have been.