Dark energy - a chapter from the book "Astrophysics for those who don't have time" by Neil deGrasse Tyson

Neil deGrasse Tyson's book, "Astrophysics for those who don't have time" was on the New York Times bestseller list for over 40 weeks, and was recently published in Hebrew by Techelet Publishing

As if we didn't have enough to worry about, in recent decades it has been discovered that the universe is under a mysterious pressure that exerts a force from within the void of space and opposes cosmic gravity. And that's not all, in the end "negative gravity" will win the tug of war, and force the cosmic expansion to accelerate exponentially into the future.

Most of the unconventional ideas in twentieth-century physics are Einstein's fault.

Albert Einstein hardly ever set foot in a laboratory, he did not test phenomena or use sophisticated equipment. He was a theorist who brought to perfection the "thought experiment", in which one deals with nature through the imagination, invents a situation or model and then deduces the consequences according to some physical principle. In pre-World War II Germany, physics based on laboratory experiments was considered far more prestigious by the Armenian scientists than theoretical physics. All the Jewish physicists were pushed into the poor sandbox of the theorists and abandoned there to their own devices. And look what came out of this sandbox.

As was the case with Einstein, if a physicist's model is supposed to represent the entire universe, then manipulating the model is supposed to be like manipulating the universe itself. Observers and experimenters can go out and look for the phenomena that the model predicted. If the model is flawed, or if the theorists got their calculations wrong, observers will find a discrepancy between the model's predictions and the way things happen in the real universe. This is the first clue for the theorist to go back to the metaphorical drawing board, refurbish the old model or create a new model in its place.

One of the most powerful and far-reaching theoretical models ever invented, and already presented here, is Einstein's theory of general relativity. The general theory of relativity published in 1916 details the mathematics relevant to the movement of everything in the universe under the influence of gravity. Every few years, laboratory scientists develop even more precise experiments to test the theory, only to further expand its envelope of accuracy. In 2016, we received a modern demonstration of this amazing introduction to nature that Einstein gave us: gravitational waves were discovered using an observatory designed for this very purpose.^[1] These waves, which Einstein predicted, are ripples that move at the speed of light along the fabric of space-time, and are created by severe gravitational disturbances, such as the collision of two black holes.

And that is exactly what was observed. The gravitational waves of the first detection were created by the collision of black holes in the galaxy 1.3 billion light years ago, at a time when the earth was crawling with simple single-celled organisms. While the ripple moved through space in all directions, after another eight hundred million years complex life developed on Earth, including flowers, dinosaurs, flying creatures, and also a branch of vertebrates known as mammals. A sub-branch of mammals developed frontal lobes and the complex thought capacity that came with them. We call them primates. A single branch of these primates developed a genetic mutation that enables speech, and that branch— Homo sapiens — invented agriculture, civilization, philosophy, art and science. All in the last ten thousand years. In the end, one of the scientists of the twentieth century pulled the theory of relativity out of his head, and predicted the existence of gravitational waves. A century later, technology is finally able to distinguish the waves and close the gap with the prediction, just days before a gravitational wave, which had been on the way for 1.3 billion years, flooded the Earth and was detected.

Yes, Einstein was a cannon.

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Most scientific models are initially presented in an immature state, leaving room for maneuver to adjust the parameters to better fit the known universe. In the "heliocentric" universe, based on the sun, devised by the sixteenth-century mathematician Nicolaus Copernicus, the planets traveled in circles. The part about the sun's orbit was correct, and a great advance in relation to the "geocentric" universe, with the earth at the center, but it turned out that the matter of the circles deviated a little from reality - all the planets circle the sun in flattened circles called ellipses, and this shape is also only an estimate of a more complex orbit. Copernicus' basic idea was correct, and that's what matters most. It just took some tweaking to make it more accurate.

Still, in the case of Einstein's theory of relativity, the founding principles of the entire theory require that everything happen exactly as he predicted. In fact, Einstein built what from the outside looks like a house of cards based on two or three simple basic assumptions that hold the entire structure together. Indeed, when he heard about a book from 1931 called "One Hundred Writers Against Einstein",^[2] He replied that if he was wrong, only one writer would have been enough.

There the seeds were planted for one of the most fascinating blunders in the history of science. Einstein's new gravity equations included a term he called the "cosmological constant", and it was represented by the capital Greek letter for matter: Λ. As an optional mathematical variable the cosmic constant allowed it to represent a static universe.

At the time, the idea that our universe was doing something other than just existing was beyond imagination. That is, the sole function of the Lambda was to oppose gravity in Einstein's model, to hold the universe in equilibrium, to oppose the natural tendency of gravity to pull the entire universe into one giant mass. In this way, Einstein invented a universe that neither expands nor shrinks, according to everyone's expectations at the time.

The Russian physicist Alexander Friedman later showed that mathematically Einstein's universe, although in equilibrium, was in an unstable state. Like a ball resting on top of a hill, waiting for the slightest nudge to roll in one direction or another, or like a pencil balanced on its tip, Einstein's universe was perched precariously between expansion and total collapse. Furthermore, Einstein's theory was new, and nothing is made real just because it is called by a name - Einstein knew that lamda, as the negative force of nature's gravity, has no known counterpart in the tangible universe.

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Einstein's theory of general relativity is radically different from all previous conceptions of gravitational attraction. Instead of settling for Sir Isaac Newton's view of gravity as a strange action at a distance (a conclusion Newton himself was uncomfortable with), general relativity sees gravity as a response of mass to a local distortion of space and time caused by another mass or energy field. In other words, the concentration of mass causes distortions—like dimples—in the fabric of space and time. These deformations direct the moving masses along straight geodesic lines,^[3] Although they look like curved paths to us that we call gravitational paths. The American theoretical physicist, John Archibald Wheeler, formulated Einstein's idea in the best way: "Matter dictates to space how to curve; Space dictates to matter how to move."^[4]

In the end, general relativity described two types of gravity. The first is the familiar type, like the attraction between the earth and a ball thrown into the air, or between the sun and the planets. She also envisioned another version—a mysterious, anti-gravity pressure associated with the vacuum of space-time itself. Lamda preserved the almost certain assumption of Einstein, and of every other physicist of his time: the status quo of the static universe - a static unstable universe. But the claim that an unstable state is the natural state of a physical system is tantamount to violating the ten commandments of science. It cannot be argued that the entire universe is a special case that happens to maintain equilibrium forever and ever. Nothing has ever been seen, measured or imagined that behaved like this in the history of science, meaning it is a revolutionary precedent.

Thirteen years later, in 1929, the American astrophysicist Edwin P. Hubble discovered that the universe is not static. He found and collected convincing evidence that the more distant a galaxy is, the faster it is moving away from the Milky Way. In other words, the universe is expanding. Then Einstein, who had missed the opportunity to predict the expansion of the universe for himself because of the cosmological constant—which apparently did not coincide with any known force of nature—felt embarrassed, and he omitted it entirely. He called it "the biggest mistake" of his life. By discarding the lamda from the equation, he assumed that its value is zero, as in this example: Suppose that A = B + C. If we later discover that A = 10 and B = 10, then A is still equal to B plus C, but Only in case C equals 0 and is redundant in the equation.

But that's not the end of the story. For decades, theorists used to exhume the lambda from its grave, and imagine what their ideas would look like in a universe with a cosmological constant. Sixty-nine years later, in 1998, science pulled her out for the last time. At the beginning of that year, two competing groups of astrophysicists made impressive announcements: one led by Saul Perlmutter of the Lawrence Berkeley National Laboratory in Berkeley, California, and the other led by Brian Schmidt of the Mount Stromello and Siding Spring observatories in Canberra, Australia, and Adam Rees of Johns Hopkins University in Baltimore, Maryland. Dozens of the most distant supernovae ever observed appeared considerably dimmer than expected, given the well-documented behavior of this breed of exploding star. The problem could be solved if one of the following two conditions were met: either those distant supernovae behaved differently from their closer sisters, or they were fifteen percent farther away than they were assumed to be by conventional cosmological models. The only known thing that existed to explain "naturally" this acceleration is Einstein's lambda, the cosmological constant. When the astrophysicists dusted it off and returned it to the original equations of Einstein's theory of general relativity, the known state of the universe matched Einstein's equations.

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The supernovae that Perlmutter and Schmidt studied are worth their weight in fissionable atomic nuclei. Within certain limits, each of these stars explodes the same way, ignites the same amount of fuel, releases the same enormous amount of energy in the same amount of time, and therefore reaches the same peak brightness. Therefore, they are used as a kind of scale, or "standard candle", to calculate cosmic distances to the galaxies where they explode, in the most remote areas of the universe.

Standard candles greatly simplify the calculations: since all supernovae have the same spec, the dim ones are far and the bright ones are close. After measuring their brightness (a simple task), it is possible to say exactly how far they are from us and how far they are from each other. If each supernova had a different brightness, we could not use brightness alone to determine the distance of one supernova compared to another. A dim supernova could have been a distant high-powered bulb or a nearby low-powered bulb.

Everything is good. But there is another way to measure the distance to galaxies: the speed at which they are receding from our Milky Way, receding which is an integral part of the expansion of the universe. Hubble was the first to show that as the universe expands, distant objects escape us faster than nearby objects. Therefore, by measuring the galaxy's speed of receding (another simple task), the distance of a galaxy can be deduced.

If two tested methods find different distances from the same object, something must be wrong. Either supernovae are poor standard candles, or our model of the expansion rate of the universe as measured by the velocities of galaxies is wrong.

Well, something Was Erroneous. Supernovae turned out to be great standard candles, surviving the rigorous testing of many skeptical researchers, and so astrophysicists were left with a universe that expanded faster than they thought, and with galaxies located further away than their receding velocities should have placed them. And there was no easy way to explain the additional expansion without evoking Lambda, Einstein's cosmological constant.

Here was the first direct evidence that a force of repulsion does exist in the universe, a force that opposes gravity, and thus the cosmological constant came back from the dead. Meda suddenly got a physical entity that needed a name, and thus the "dark energy" got the center stage of the cosmic drama. The name aptly describes both the mystery and our ignorance of its cause. Perlmutter, Schmidt, and Riess rightfully shared the 2011 Nobel Prize in Physics for this discovery.

The most accurate measurements so far reveal that dark energy is the most important thing in the city, it is currently responsible for 68% of all mass energy in the universe; Dark matter makes up 27% of it, while normal matter only makes up 5%.

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The shape of our four-dimensional universe was determined by the ratio between the amounts of matter and energy that exist in the universe and the rate at which the universe is expanding. The agreed-upon mathematical measure is omega: Ω, another large Greek letter with a firm hold on the universe.

If you take the matter-energy density of the universe and divide it by the matter-energy density exactly required to stop the expansion (it is called the "critical" density), you get omega.

Since both mass and energy warp or twist spacetime, omega tells us about the shape of the universe. If omega is less than 1, the actual mass-energy is below the critical value, and the universe expands forever in all directions and time, taking the shape of a saddle, where initially parallel lines move away from each other. If omega equals 1, the universe is expanding forever, but very slowly. In this case its shape is flat, and it fulfills all the geometry laws we learned in high school about parallel lines. If omega is greater than 1, the parallel lines converge, the universe curves back in on itself, and eventually collapses into the fireball from which it came.

At no point since Hubble discovered that the universe is expanding has any team of all the teams of observers who have reliably measured omega found it to be even close to 1. When they added up all the mass and all the energy their telescopes could see, including extrapolation to the range beyond their limits, and including the dark matter, the highest values ​​of the best observations barely reached the result Ω = 0.3. As far as observers were concerned, the universe was "open" for business, riding a one-way saddle into the future.

Meanwhile, from 1979 onwards, the American physicist Alan H. Gott of the Massachusetts Institute of Technology, as well as others, developed an adjustment to the Big Bang theory that removed some vexing problems about the formation of a matter- and energy-dense universe like the one we know. A fundamental byproduct of the revision of the Big Bang Theory is the push of Omega in the direction of the value one. Not half way. Not in direction two. Not in the direction of a million. in one direction.

Almost no theorist in the world saw a problem with this requirement, as it helped to place the responsibility for the global characteristics of the known universe on the Big Bang. However, another small problem emerged: the update predicted three times more mass-energy than the observers were able to find. The theorists did not hold back and said that the observers simply do not put enough effort into their observations.

At the end of all accounting, the visible material alone explained no more than 5% of the critical density. And what about the mysterious dark matter? They added him too. No one knew what he was, and we still don't know what he was, but there is no doubt that he contributed to the whole. From these calculations we get five or six times more dark matter than visible matter. But it is still too little. The observers were at a loss, and the theorists replied: "Keep looking."

Each of the camps was sure that the other camp was wrong - until the discovery of dark energy. This single element, added to normal matter, normal energy, and dark matter, raised the mass-energy density of the universe to the critical level, satisfying observers and theorists at the same time.

This was the first time that the theorists and observers kissed and reconciled. Both camps, in their own way, were right. Omega is equal to one, just as the theorists demanded of the universe, although we can't get there even if we add all the matter - dark or not - as they naively assumed. Today there is no other matter circulating in the universe that has not been estimated by observers.

No one anticipated the dominant presence of cosmic dark energy, and no one imagined that it would be the great bridging of the gaps.

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So what is this thing? No one knows. The closest hypothesis that has been put forward is that dark energy is a quantum phenomenon - that is, the void in space is not empty, but teeming with compatible matter and antimatter particles. They jump in pairs between existence and non-existence, and do not last long enough to be measured. Their moniker reflects their ephemeral existence: virtual particles. The amazing legacy of quantum physics—the science of small things—demands that we give the idea serious attention. Each pair of virtual particles exerts a little outward pressure, as if in the blink of an eye it elbows its way into space.

Unfortunately, when we estimate the amount of "vacuum pressure" rejection that comes from the short lifetime of the virtual particles, the result is greater than 10 times¹²⁰ The array of the experimentally determined cosmological constant. This is an insanely large multiplier, resulting in the largest discrepancy between theory and observation in the history of science.

Indeed, we have no idea. But this is not a shameful lack of understanding. Dark energy doesn't just drift away without any theory to anchor it. Dark energy resides in one of the safest harbors we can imagine: Einstein's equations of general relativity. This is the cosmological constant. This is Lamda. Whatever dark energy is, we already know how to measure it and how to calculate its effect on the universe's past, present and future.

Undoubtedly, the biggest mistake Einstein made was declaring Mada to be the biggest mistake he ever made.

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And the chase continues. Now that we know dark energy is real, teams of astrophysicists have hatched ambitious plans to measure distances and measure the growth of the universe's structure using ground-based and space-based telescopes. These observations will examine in detail the effect of dark energy on the expansion history of the universe, and will no doubt keep theorists busy. They desperately need atonement for what came out of their embarrassing dark energy calculations.

Do we need an alternative to general relativity? Does the marriage of general relativity and quantum mechanics need a comprehensive overhaul? Or is there some dark energy theory waiting to be discovered by a wise man yet to be born?

A notable feature of Lamda and the accelerating universe is that the force of repulsion comes from within the void, and not from something material. As the void increases, the (known) density of matter and energy within the universe decreases, and the relative influence of Meda on the cosmic state of affairs increases. Simultaneously with greater repulsion pressure comes more emptiness, and with more emptiness comes even greater repulsion pressure, which forces an infinite and exponential acceleration of cosmic expansion.

As a result, anything that is not gravitationally attracted to the region of the Milky Way galaxy will move away at an increasing speed, as part of the accelerated expansion of the space-time fabric. Distant galaxies currently visible in the night sky will eventually disappear beyond the unreachable horizon, moving away from us faster than the speed of light. This is an achievement that is possible not because they move through space at such speeds, but because the fabric of the universe itself carries them at such speeds. No physical law prevents this.

It's possible that in a trillion years anyone living in our galaxy will know nothing about other galaxies. Our observable universe would just be a system of nearby, long-lived stars within the Milky Way. And beyond this starry night there will remain emptiness and endless darkness into the depths.

Dark energy, a fundamental property of the universe, will eventually detract from the ability of future generations to understand the universe they have been given. If contemporary astrophysicists across the galaxy do not keep meaningful records and bury a huge trillion-year time capsule, post-apocalyptic scientists will know nothing about galaxies—the primary form of organization of matter in our universe—and thus will be denied access to essential information about the cosmic drama of our universe.

And here is my recurring nightmare: Are we also missing some fundamental parts of the universe that once were? On which part of the book of cosmic history are the words "no access" already written? What is missing from our theories and equations even though it should have been there, leaving us searching for answers we will never find?

Comments

^[1]    The LIGO laser detector for detecting gravitational waves, two identical detectors in the United States, one in Hanford, Washington, and the other in Livingston, Louisiana.

^[2]    R. Israel, E. Ruckhaber, R. Weinmann, et al., Hundert Autoren Gegen Einstein (Leipzig: R. Voigtlanders Verlag, 1931)

^[3]    "Geodesy" is an unjustifiably inflated word describing the shortest distance between two points on a curved surface—which in this case has been extended to the shortest distance between two points in the four-dimensional curved texture.

^[4]    As part of my graduate studies, I took a general relativity course with John Wheeler (where I met my wife), and he used to say this often.

See more on the subject on the science website:

• The dark energy puzzle
• New research reveals that the early universe expanded much faster than today
• Is dark energy responsible for accelerating the expansion of the universe?
• Nobel Prize for discovering the acceleration of the expansion of the universe
• Is the universe being judged faster than we thought?