# The Universe Is Accelerating...and No One Knows Why ## Метаданные - **Канал:** Arvin Ash - **YouTube:** https://www.youtube.com/watch?v=IWfb9FnCTvI - **Дата:** 03.04.2026 - **Длительность:** 20:27 - **Просмотры:** 108,200 - **Источник:** https://ekstraktznaniy.ru/video/51381 ## Описание Support the Research Behind this Channel on Patreon: https://www.patreon.com/arvinash REFERENCES How black holes may be responsible for Dark Energy https://youtu.be/6horr1xNs_M Is Dark Energy made of particles? https://youtu.be/nykTGre_uKA What is Dark Energy made of? https://youtu.be/YQq0VdJApzU CHAPTERS 0:00 The 70% mystery 0:58 How Dark Energy was discovered? 4:26 What could be causing Dark Energy? 6:58 Repulsive Gravity? 10:16 What is the energy made of? 11:56 Evolving Dark energy? Quintesssence 14:18 Could Dark Energy be a particle? 16:43 Could Black Holes cause Dark Energy? SUMMARY Dark energy is one of the greatest mysteries in modern physics. It appears to make up nearly 70% of the universe, yet scientists still do not know what it is. Unlike matter, it does not clump together. Unlike radiation, it does not dilute as space expands. Instead, it causes the expansion of the universe to accelerate, pushing galaxies apart faster over time. The discovery of this acceleration cam ## Транскрипт ### The 70% mystery [] Dark energy is not just another unsolved problem in physics. — It is the dominant component of the universe making up nearly 70% everything that exists. Yet, we still don't know what it is. It doesn't clump like matter. It doesn't dilute like radiation. And instead of slowing the expansion of the universe, it's causing space itself to accelerate outward. So, what exactly is driving this acceleration? Is it a built-in property of empty space? Something Einstein called the cosmological constant? Is it a dynamical field? A new form of energy evolving over time? Possibly even particle-like in nature? Or is it possible that we've misunderstood the data entirely? To answer these questions, we need to carefully examine the leading explanations and the physics behind them. That's coming up right now. ### How Dark Energy was discovered? [0:58] It's one of the biggest mysteries in science. It's the elephant in the room that you can't see. It makes up the majority of the universe, 70%. The other 30% being ordinary and dark matter. But we don't really know what it is. It's as if we're boats sailing on an invisible ocean trying to figure out what the heck we're sailing on. The burning questions we want to ask are, what is dark energy? — What could be causing it? Why is it there? And finally, does it really exist? Or could our measurements be completely wrong? To answer that, we start with the observation that forced dark energy into the conversation. The universe isn't just expanding. Its expansion is accelerating. Since 1929 when Edwin Hubble discovered that the universe was expanding, scientists have sought to find out what this expansion rate was. Most scientists believed that while the universe was known to be expanding, this expansion rate had to be slowing down over time because of the attractive nature of gravity pulling matter together. So, scientists set out to measure the slowing down of the expansion rate. This was not easy to do because in order to make these kinds of measurements, you have to be able to view the brightness and red shifts of galaxies that are hundreds of millions and billions of light-years away. These are objects in the sky that we cannot see with our naked eye. — There was no good way to measure this accurately for most of the 20th century until very large telescopes and the Hubble telescopes were built. In addition, some kind of measurement technique was needed to establish a standard luminosity that these galaxies could be compared to because not all galaxies are equally bright at the same distance. This standard luminosity is called a standard candle. In the late 1980s and early 90s, two groups of astronomers, one led by Saul Perlmutter and Adam Riess in the US and Brian Schmidt in Australia, came up with a clever way to measure the true distances and red shifts of very distant galaxies by trying to find recent supernova explosions, very special ones called type 1a supernova. These provide a standard candle because all type a supernovae explode with the same luminosity because they grow to about the same size. So, by measuring the brightness of these supernovae, scientists can ascertain the distance. — And by measuring the red shift of this same light, they could determine how much space had expanded from the time the light left the supernova. When these two pieces of data were combined from galaxies that were at various distances from Earth, they could get an idea of how much space had expanded over different time frames and thus how fast the galaxies were moving away from Earth. What they found was shocking to most scientists because not only was the universe not slowing down or remaining constant, it was speeding up. The expansion was accelerating. And no one could explain it. Saul Perlmutter, who won a Nobel Prize for this discovery, described this as throwing an apple in the air, waiting for it to come down, only to find that it took off like a rocket into outer space. So, the question is, if it took off like a rocket, where did the energy come from to fuel this rocket? This apparent energy that is fueling the accelerating expansion of the universe is called dark energy. So, what could be causing it? ### What could be causing Dark Energy? [4:26] There are three possibilities. The first possibility is that this energy is a property of space itself. Now, this was buried in a version of Einstein's equation of general relativity that contains something called a cosmological constant. It's represented by the Greek letter lambda. Ironically, Einstein had introduced lambda to the counterbalance the effect of gravity to keep the universe static. He abandoned it after Hubble's discovery of the expansion of the universe, calling it his greatest blunder. Lambda can be thought of as the energy density of the vacuum of space, what we perceive as nothing. It comes from a non-perfect cancellation of quantum fluctuations. What do I mean by this? According to Heisenberg's uncertainty principle, quantum fields from which all particles come from cannot sit still. These fields are constantly creating virtual particles in empty space. So, for example, electrons and positrons, protons and antiprotons, quarks and antiquarks come in and out of existence in empty space. This can be seen from this version of Heisenberg's equation. The change or uncertainty in energy times the uncertainty in time is greater than or equal to Planck's constant divided by 2 * pi. This equation also tells you that you can have virtual particles come out of nothing if the change in energy and or the change in time is so small that it is less than Planck's constant over 2 pi. These virtual particles are not something that you can see. — So, is there any experimental evidence that can attest to the fact that this energy exists? — It turns out there is and it's called the Casimir effect. I talked a little bit about it in my warp drive video, but essentially it's a physical effect that you can see and feel due to the fluctuations of the quantum fields in empty space. So, there is some experimental evidence that this actually exists and can have positive energy. But, the biggest problem with this theory is that when you use the known equations of quantum mechanics to calculate what this energy should be, the theoretical energy is almost 120 orders of magnitude higher — than what is actually measured from type 1a supernova and the cosmic microwave background. This is the greatest mismatch between theory and observation in all of physics. The worst prediction of all time. So, lambda is clean and it fits observations, but it leaves us with an absurd theoretical gap. ### Repulsive Gravity? [6:58] That's where repulsive gravity and negative pressure become the key language for what dark energy does in general relativity. Now, let's get back to answering the question of whether repulsive gravity is fundamental. To answer that question, let's first answer the question how can gravity repel? A key difference between Newtonian gravity and general relativity — is that in Einstein's theory the source of gravity is not mass, but energy. This relationship is expressed by the field equations. — The part on the left-hand side describes the local curvature of space-time, while the right-hand side encodes the density and flux of energy and momentum. The equation was summed up in a famous saying by the physicist John Wheeler, "Spacetime tells matter how to move and matter tells spacetime how to curve. " For an expanding universe whose size increases with a scale factor, Einstein's equation can be rewritten in the following way, known as the Friedmann equation. On the left, we have the acceleration of the expansion. On the right side, rho stands for energy density and p for pressure. So, there are two ways to get an accelerated expansion, also known as repulsive gravity. — Negative energy or negative pressure. What does pressure mean in the context of spacetime? It's related to the distribution and flow of energy in space. If we think of masses as dips in spacetime, we can visualize pressure as an effect that modifies the shapes of these — dips. Positive pressure increases curvature, making the dips deeper and steeper. So, it contributes to gravitational pull and slows down expansion. Negative pressure, on the other hand, is like an intrinsic stretching of spacetime that smooths out curvature. This can be counterintuitive because in daily life, when we think of positive pressure, it's pushing things away. For example, blowing more air into a balloon causing it to inflate. But, this is a different kind of mechanical pressure and should not be confused with pressure in the context of spacetime, which — has to do with the gravitational effect of positive energy densities. And here's the crucial part. Normal matter, like stars and planets, — have positive energy and exert a negligible, but positive pressure. They attract and their energy density decreases with expansion. Radiation, like photons and other particles moving at a very high speeds, has positive energy and pressure. It also attracts and its energy gets diluted faster than that of normal matter, evolving as we see in red shifts in addition to volume dilution. Dark energy has an unusual property, negative pressure. Generalizing the two previous examples, a substance with a negative pressure will get diluted slower than normal matter or not at all. In the latter case, known as the cosmological constant, the energy density remains unchanged throughout cosmic history. In other words, as the volume of space increases, more energy is continuously created. The cumulative repulsive effect then leads to exponentially accelerated expansion. But, what is this energy made of for its density to remain ### What is the energy made of? [10:16] constant with expansion? This mysterious component must be an intrinsic property of space-time, present in equal amounts at every point, and continuously stretching the fabric of the cosmos. The energy of quantum fluctuations, pairs of virtual particles constantly popping into and out of existence, seems like a perfect candidate. But, there's just one issue. The value of the vacuum energy predicted by quantum field theory is a whopping 120 orders of magnitude larger than the measured cosmological constant. This problem, which was dubbed the worst prediction in the history of physics, remains a fundamental open question. A possible solution could be that the vacuum energy cancels out to zero through an unknown symmetry, and the dark energy is made up of exotic particles. This scenario, known as evolving dark energy or quintessence, relies on a scalar quantum field. Just like the Higgs field, vacuum energy is a scalar field and has a single value at every point in space-time. This is unlike, for example, the electromagnetic field, which is a vector field having both a value and a direction. The energy of such a field can vary in time and space, but in the case where these variations are slow, it behaves essentially like a cosmological constant. We know of at least one scalar field in the standard model of particle physics, the Higgs field, which has been proposed in some models as the driving force behind inflation. Other theories include new fields which have not been discovered yet. Interestingly, recent large-scale surveys of the universe have found hints indicating that dark energy may be ### Evolving Dark energy? Quintesssence [11:56] slowly evolving in time in accordance with the quintessence idea, but more data is needed to draw reliable conclusions. If dark energy is exactly constant, it behaves like lambda. If it evolves, you're pushed into a field description. And in quantum theory, fields usually imply particle-like excitations. That brings us to quintessence. In the context of dark energy, such a field is more commonly known as a quintessence field. Now, anytime you have fields, you have particles. This is because excess energy in a quantum field or excitation in the field is what we observe as particles. So, this would point to a particle solution for dark energy. These particles must then interact with matter particles of the standard model to act as a kind of anti-gravity force. This force would be like electromagnetism, but instead of causing repulsion and attraction with charged particles, it would only impart repulsion and to all matter particles. This is called the quintessence model of dark energy. The term quintessence comes from ancient Greeks who believed that the universe was made of five elements: earth, air, — water, fire, and a fifth element that all objects in the heavens were made of. Latin scholars later named this fifth element quintessence. Today, this term in physics is used to describe the hypothetical dark energy field. Now, the question we have to ask is if such a quintessence field exists and its effects are so profound, why didn't we detect it? Well, it could be that the interaction between the standard model particles and dark energy field is so weak that we simply don't have good experiments or sensitive enough instruments to detect it. But, it could also be that such a field does not exist. A particle solution like quintessence would likely affect many things because particles interact with the stuff of the universe. This has an impact. Those interactions should be detectable. Furthermore, depending on the nature of the field, it could possibly also be part of a solution explaining dark matter in addition to dark energy. Since quintessence fields give rise to particles, such a solution may show a connection between the dark matter particles as well as dark energy. But ### Could Dark Energy be a particle? [14:18] we should still remember that theoretical dark matter particles have a different effect in the universe than theoretical quintessence particles. There are stark differences such as dark matter appears to be present in clumps in the universe, whereas dark energy is smoothly distributed. As the universe expands, dark matter gets less dense, but the density of dark energy stays constant. Dark matter has an attractive effect to slow the expansion of the universe, whereas dark energy has a repulsive effect accelerating the expansion of the universe. Having said that, it is possible that both particles are part of the same quantum field theory connected perhaps at a higher level in as yet an undiscovered grand unified theory. If they are related, the relationship is hidden from our current understanding of the universe and is probably quite subtle. An interesting consequence of a dynamical dark energy solution like quintessence is that the repulsion could get stronger and stronger as the universe expands. Meaning the universe would have a runaway acceleration. It would become so strong that it would eventually rip everything apart. First, all galaxies would move away from each other so that it would appear as if our Milky Way galaxy is the only galaxy in the universe — because we would not be able to see the light of any other galaxy. Then our galaxy would be ripped apart. All the stars, including our sun, would then be ripped apart. Eventually, the repulsion becomes so strong that the cells and later molecules, then atoms, would be ripped apart until everything in the universe becomes nothing — but fundamental particles. This is called the Big Rip. And of course, in such a universe, no life could likely exist. Finally, space-time itself would rip apart and all distances in the universe will be infinite. This might be scary, but we can't rule out the possibility. Fortunately for us, this would happen hundreds of billions of years in the future, so no need to lose sleep over it tonight. So far, quintessence remains hypothetical. The key point is this, it's testable because if dark energy changes, it should leave signatures in large-scale structures and expansion history. In February of 2023, a fascinating new study was published that's generating huge buzz in cosmology ### Could Black Holes cause Dark Energy? [16:43] circles and with the general public. A group of researchers from nine countries claimed to have found a solution to dark energy, which is one of the biggest mysteries in cosmology. The research seems to show a connection with dark energy and supermassive black holes. If what they propose is true, it would be the biggest discovery since the discovery of dark energy itself 25 years ago and would be a turning point in our understanding of the universe. The group of scientists in this paper connected the mysterious dark energy to another somewhat mysterious object, black holes. The overall claim is that black holes, and specifically supermassive black holes, couple to the universe's expansion on the largest cosmological scales. And that the specific way that they must couple could potentially explain some or even all of the dark energy we observe. The astonishing thing that they found is that over a period of 9 billion years, the supermassive black holes in galaxies seem to grow 7 to 20 times their own mass. Something which we would not expect. So, it raises the question, from where are these black holes getting the mass or energy to grow? What are they eating? This is where dark energy comes into the picture because the scientists propose that black holes have a connection to dark energy. And that they grow because of it. In fact, dark energy may be contained inside black holes. Okay, so now let's look at how this growth in black holes is connected to dark energy. They present a model where they assume some coupling, K, between black holes and dark energy. In very simple terms, if the evolution of the growth in dark energy matches of supermassive black holes mass, — then depending on how good the correlation is, you can hypothesize that there must be some connection. So, in practice, it means that if K equals 0, then there's no connection between dark energy and black holes. If K equals 3, then it's a perfect match. Well, lo and behold, their result shows that K equals 3. 11 plus or minus 1. 1 with a 90% confidence. So, there's a significant uncertainty in the result. But, they also showed, with a 99. 98 certainty, that K is not zero. Meaning, it is very unlikely that there's no connection. Now, 99. 98 significance level is about 4 sigma, but in physics, a 5 sigma correlation is needed for acceptance of a new theory. If this result is correct, it would mean that black holes could be full of dark energy. That's where it resides. What do I think? Well, I'm a natural skeptic and I think that this is an extraordinary claim and we need extraordinary evidence to confirm it. I don't think the evidence is extraordinary yet. So, I think we need more data to confirm its correlation. But, we also need a theoretical basis to connect exactly how the correlation would occur. So, where does that leave us? Dark energy is a measured effect with multiple candidate explanations. A constant vacuum-like component, a dynamical field like quintessence, or something deeper we haven't unified yet. And until we tighten the measurements and test these models harder, dark energy remains one of the biggest open questions in physics. Hope you enjoyed this video. I'll see you in the next video, my friend.