Dark matter dominated for most of the universe’s history but in the future, the effects of dark energy will increase.
Ordinary matter such as that in people, planets, stars, and galaxies, comprises only some 5 percent of the universe. Dark matter accounts for roughly a quarter, while dark energy is the largest component of the cosmos.
Are the percentages of dark matter and dark energy stable, or is the ratio of dark matter to dark energy to observable matter changing?
All the atoms and radiation in the universe make up less than 5 percent of its contents. The rest is composed of two invisible, enigmatic entities: dark matter and dark energy. Together they govern the behavior of the universe.
Evidence for dark matter has been accumulating for 50 years. It makes up most of the mass of all galaxies, including the Milky Way, controls the organization of galaxies on the largest scales, and represents 27 percent of the universe. The visible parts of galaxies are outweighed by dark matter, which holds galaxies together. Dark matter exerts gravity but doesn’t interact with light.
Astronomers still don’t know what dark matter is, but they’ve eliminated many possibilities: It can’t be made of black holes, dim stars, free-floating planets, space rocks, or dust particles. That leaves fundamental subatomic particles as the only option. (The alternative is to decide that the law of gravity needs altering, but this is unpalatable to most astronomers.)
Dark energy is causing the universe to expand at an ever-faster rate. It represents 68 percent of the universe. The discovery of dark energy in the 1990s was a surprise, because the expectation in cosmology had been that the gravity of all the matter in the universe would slow down the expansion discovered by Edwin Hubble. Think of the universe as having a brake (gravity) and an accelerator (dark energy), with both being pushed at the same time. Currently, the accelerator is twice as strong as the brake, so the universe is accelerating.
We know far less about dark energy than dark matter, but it seems to be a property of space. Physics tells us that space is not nothing — it has the potential to create energy. Albert Einstein formulated a version of his gravity theory where the energy in empty space is not diluted as space expands. As more space comes into existence, more space energy appears, causing the universe to expand faster and faster. So, the idea that the amount of dark energy grows as the universe expands has been around for a while. But we still lack a physical explanation to test this idea.
The expansion rate of the universe is influenced by competing forces: that of gravity, which slows down expansion, and that of dark energy, which speeds it up. This diagram shows the expansion rate over the universe’s history, with shallower curves representing faster expansion and steeper curves showing times of slower expansion. A noticeable change in the expansion rate occurred about 7.5 billion years ago, when the universe began accelerating.
Are dark matter and dark energy stable and constant? Since we don’t understand their true physical nature, we can’t be sure. But astronomers can see if they vary depending on which direction in space they look. This is a test of whether the universe is lopsided or the same everywhere (the physics term for this is isotropic). It turns out that the amount of dark matter surrounding galaxies is the same in every direction, and the strength of dark energy is also the same in every direction.
To see whether the influence of dark matter and dark energy has changed over cosmic time, astronomers look deep into space. Distant light is old light, so telescopes act as time machines, probing billions of years into the past. By measuring the redshift and brightness of distant objects, astronomers map out the expansion history of the universe. Dark matter dominated for most of that history since the Big Bang. That’s because when the universe was smaller, the gravity exerted by dark matter was stronger, while the force exerted by dark energy has stayed the same. Now is the only time in the entire history of the universe when the two entities’ influences are about equal. In the future, the effects of dark energy will increasingly dominate, and the universe will accelerate forever.
A controversial new paper argues that universes with dark energy profiles like ours do not exist in the “landscape” of universes allowed by string theory.
String theory permits a “landscape” of possible universes, surrounded by a “swampland” of logically inconsistent universes. In all of the simple, viable stringy universes physicists have studied, the density of dark energy is either diminishing or has a stable negative value, unlike our universe, which appears to have a stable positive value.
Introduction
On June 25, Timm Wrase awoke in Vienna and groggily scrolled through an online repository of newly posted physics papers. One title startled him into full consciousness.
The paper, by the prominent string theorist Cumrun Vafa of Harvard University and collaborators, conjectured a simple formula dictating which kinds of universes are allowed to exist and which are forbidden, according to string theory. The leading candidate for a “theory of everything” weaving the force of gravity together with quantum physics, string theory defines all matter and forces as vibrations of tiny strands of energy. The theory permits some 10500 different solutions: a vast, varied “landscape” of possible universes. String theorists like Wrase and Vafa have strived for years to place our particular universe somewhere in this landscape of possibilities.
But now, Vafa and his colleagues were conjecturing that in the string landscape, universes like ours — or what ours is thought to be like — don’t exist. If the conjecture is correct, Wrase and other string theorists immediately realized, the cosmos must either be profoundly different than previously supposed or string theory must be wrong.
After dropping his kindergartner off that morning, Wrase went to work at the Vienna University of Technology, where his colleagues were also buzzing about the paper. That same day, in Okinawa, Japan, Vafa presented the conjecture at the Strings 2018 conference, which was streamed by physicists worldwide. Debate broke out on- and off-site. “There were people who immediately said, ‘This has to be wrong,’ other people who said, ‘Oh, I’ve been saying this for years,’ and everything in the middle,” Wrase said. There was confusion, he added, but “also, of course, huge excitement. Because if this conjecture was right, then it has a lot of tremendous implications for cosmology.”
Researchers have set to work trying to test the conjecture and explore its implications. Wrase has already written two papers, including one that may lead to a refinement of the conjecture, and both mostly while on vacation with his family. He recalled thinking, “This is so exciting. I have to work and study that further.”
The conjectured formula — posed in the June 25 paper by Vafa, Georges Obied, Hirosi Ooguri and Lev Spodyneiko and further explored in a second paper released two days later by Vafa, Obied, Prateek Agrawal and Paul Steinhardt — says, simply, that as the universe expands, the density of energy in the vacuum of empty space must decrease faster than a certain rate. The rule appears to be true in all simple string theory-based models of universes. But it violates two widespread beliefs about the actual universe: It deems impossible both the accepted picture of the universe’s present-day expansion and the leading model of its explosive birth.
Dark Energy in Question
Since 1998, telescope observations have indicated that the cosmos is expanding ever-so-slightly faster all the time, implying that the vacuum of empty space must be infused with a dose of gravitationally repulsive “dark energy.”
But the new conjecture asserts that the vacuum energy of the universe must be decreasing.
Cumrun Vafa, a prominent string theorist at Harvard University, has been mapping the forbidden “swampland” of impossible universes for 13 years.
Vafa and colleagues contend that universes with stable, constant, positive amounts of vacuum energy, known as “de Sitter universes,” aren’t possible. String theorists have struggled mightily since dark energy’s 1998 discovery to construct convincing stringy models of stable de Sitter universes. But if Vafa is right, such efforts are bound to sink in logical inconsistency; de Sitter universes lie not in the landscape, but in the “swampland.” “The things that look consistent but ultimately are not consistent, I call them swampland,” he explained recently. “They almost look like landscape; you can be fooled by them. You think you should be able to construct them, but you cannot.”
According to this “de Sitter swampland conjecture,” in all possible, logical universes, the vacuum energy must either be dropping, its value like a ball rolling down a hill, or it must have obtained a stable negative value. (So-called “anti-de Sitter” universes, with stable, negative doses of vacuum energy, are easily constructed in string theory.)
The conjecture, if true, would mean the density of dark energy in our universe cannot be constant, but must instead take a form called “quintessence” — an energy source that will gradually diminish over tens of billions of years. Several telescope experiments are underway now to more precisely probe whether the universe is expanding with a constant rate of acceleration, which would mean that as new space is created, a proportionate amount of new dark energy arises with it, or whether the cosmic acceleration is gradually changing, as in quintessence models. A discovery of quintessence would revolutionize fundamental physics and cosmology, including rewriting the cosmos’s history and future. Instead of tearing apart in a Big Rip, a quintessent universe would gradually decelerate, and in most models, would eventually stop expanding and contract in either a Big Crunch or Big Bounce.
Paul Steinhardt, a cosmologist at Princeton University and one of Vafa’s co-authors, said that over the next few years, “all eyes should be on” measurements by the Dark Energy Survey, WFIRST and Euclid telescopes of whether the density of dark energy is changing. “If you find it’s not consistent with quintessence,” Steinhardt said, “it means either the swampland idea is wrong, or string theory is wrong, or both are wrong or — something’s wrong.”
Inflation Under Siege
No less dramatically, the new swampland conjecture also casts doubt on the widely believed story of the universe’s birth: the Big Bang theory known as cosmic inflation. According to this theory, a minuscule, energy-infused speck of space-time rapidly inflated to form the macroscopic universe we inhabit. The theory was devised to explain, in part, how the universe got so huge, smooth and flat.
Maybe string theory doesn’t describe the world. [Maybe] dark energy has falsified it.
Matthew Kleban
But the hypothetical “inflaton field” of energy that supposedly drove cosmic inflation doesn’t sit well with Vafa’s formula. To abide by the formula, the inflaton field’s energy would probably have needed to diminish too quickly to form a smooth- and flat-enough universe, he and other researchers explained. Thus, the conjecture disfavors many popular models of cosmic inflation. In the coming years, telescopes such as the Simons Observatory will look for definitive signatures of cosmic inflation, testing it against rival ideas.
In the meantime, string theorists, who normally form a united front, will disagree about the conjecture. Eva Silverstein, a physics professor at Stanford University and a leader in the effort to construct string-theoretic models of inflation, thinks it is very likely to be false. So does her husband, the Stanford professor Shamit Kachru; he is the first “K” in KKLT, a famous 2003 paper (known by its authors’ initials) that suggested a set of stringy ingredients that might be used to construct de Sitter universes. Vafa’s formula says both Silverstein’s and Kachru’s constructions won’t work. “We’re besieged by these conjectures in our family,” Silverstein joked. But in her view, accelerating-expansion models are no more disfavored now, in light of the new papers, than before. “They essentially just speculate that those things don’t exist, citing very limited and in some cases highly dubious analyses,” she said.
Matthew Kleban, a string theorist and cosmologist at New York University, also works on stringy models of inflation. He stresses that the new swampland conjecture is highly speculative and an example of “lamppost reasoning,” since much of the string landscape has yet to be explored. And yet he acknowledges that, based on existing evidence, the conjecture could well be true. “It could be true about string theory, and then maybe string theory doesn’t describe the world,” Kleban said. “[Maybe] dark energy has falsified it. That obviously would be very interesting.”
Mapping the Swampland
Whether the de Sitter swampland conjecture and future experiments really have the power to falsify string theory remains to be seen. The discovery in the early 2000s that string theory has something like 10500 solutions killed the dream that it might uniquely and inevitably predict the properties of our one universe. The theory seemed like it could support almost any observations and became very difficult to experimentally test or disprove.
In 2005, Vafa and a network of collaborators began to think about how to pare the possibilities down by mapping out fundamental features of nature that absolutely have to be true. For example, their “weak gravity conjecture” asserts that gravity must always be the weakest force in any logical universe. Imagined universes that don’t satisfy such requirements get tossed from the landscape into the swampland. Many of these swampland conjectures have held up famously against attack, and some are now “on a very solid theoretical footing,” said Hirosi Ooguri, a theoretical physicist at the California Institute of Technology and one of Vafa’s first swampland collaborators. The weak gravity conjecture, for instance, has accumulated so much evidence that it’s now suspected to hold generally, independent of whether string theory is the correct theory of quantum gravity.
The intuition about where landscape ends and swampland begins derives from decades of effort to construct stringy models of universes. The chief challenge of that project has been that string theory predicts the existence of 10 space-time dimensions — far more than are apparent in our 4-D universe. String theorists posit that the six extra spatial dimensions must be small — curled up tightly at every point. The landscape springs from all the different ways of configuring these extra dimensions. But although the possibilities are enormous, researchers like Vafa have found that general principles emerge. For instance, the curled-up dimensions typically want to gravitationally contract inward, whereas fields like electromagnetic fields tend to push everything apart. And in simple, stable configurations, these effects balance out by having negative vacuum energy, producing anti-de Sitter universes. Turning the vacuum energy positive is hard. “Usually in physics, we have simple examples of general phenomena,” Vafa said. “De Sitter is not such a thing.
The KKLT paper, by Kachru, Renata Kallosh, Andrei Linde and Sandip Trivedi, suggested stringy trappings like “fluxes,” “instantons” and “anti-D-branes” that could potentially serve as tools for configuring a positive, constant vacuum energy. However, these constructions are complicated, and over the years possible instabilities have been identified. Though Kachru said he does not have “any serious doubts,” many researchers have come to suspect the KKLT scenario does not produce stable de Sitter universes after all.
Vafa thinks a concerted search for definitely stable de Sitter universe models is long overdue. His conjecture is, above all, intended to press the issue. In his view, string theorists have not felt sufficiently motivated to figure out whether string theory really is capable of describing our world, instead taking the attitude that because the string landscape is huge, there must be a place in it for us, even if no one knows where. “The bulk of the community in string theory still sides on the side of de Sitter constructions [existing],” he said, “because the belief is, ‘Look, we live in a de Sitter universe with positive energy; therefore we better have examples of that type.’”
His conjecture has roused the community to action, with researchers like Wrase looking for stable de Sitter counterexamples, while others toy with little-explored stringy models of quintessent universes. “I would be equally interested to know if the conjecture is true or false,” Vafa said. “Raising the question is what we should be doing. And finding evidence for or against it — that’s how we make progress.”
Over ten years ago, the Dark Energy Survey (DES) began mapping the universe to find evidence that could help us understand the nature of the mysterious phenomenon known as dark energy. I’m one of more than 100 contributing scientists that have helped produce the final DES measurement, which has just been released at the 243rd American Astronomical Society meeting in New Orleans.
Dark energy is estimated to make up nearly 70% of the observable universe, yet we still don’t understand what it is. While its nature remains mysterious, the impact of dark energy is felt on grand scales. Its primary effect is to drive the accelerating expansion of the universe.
The announcement in New Orleans may take us closer to a better understanding of this form of energy. Among other things, it gives us the opportunity to test our observations against an idea called the cosmological constant that was introduced by Albert Einstein in 1917 as a way of counteracting the effects of gravity in his equations to achieve a universe that was neither expanding nor contracting. Einstein later removed it from his calculations.
However, cosmologists later discovered that not only was the universe expanding, but the expansion was accelerating. This observation was attributed to the mysterious quantity called dark energy. Einstein’s concept of the cosmological constant could actually explain dark energy if it had a positive value (allowing it to conform to the accelerating expansion of the cosmos).
The DES results are the culmination of decades of work by researchers around the globe and provide one of the best measurements yet of an elusive parameter called “w”, which stands for the “equation of state” of dark energy. Since the discovery of dark energy in 1998, the value of its equation of state has been a fundamental question.
This state describes the ratio of pressure over energy density for a substance. Everything in the universe has an equation of state.
Its value tells you whether a substance is gas-like, relativistic (described by Einstein’s theory of relativity) or not, or if it behaves like a fluid. Working out this figure is the first step to really understanding the true nature of dark energy.
Our best theory for w predicts that it should be exactly minus one (w=-1). This prediction also assumes that dark energy is the cosmological constant proposed by Einstein.
Subverting expectations
An equation of state of minus one tells us that as the energy density of dark energy increases, so the negative pressure also increases. The more energy density in the universe, the more repulsion there is – in other words, matter pushes against other matter. This leads to an ever-expanding accelerating universe. It might sound a bit bizarre, as it is counterintuitive to everything we experience on Earth.
The work uses the most direct probe we have on the expansion history of the universe: Type Ia supernovae. These are a type of star explosion and they act as a kind of cosmic yardstick, allowing us to measure staggeringly large distances far into the universe. These distances can then be compared to our expectations. This is the same technique that was used to detect the existence of dark energy 25 years ago.
The difference now is in the size and quality of our sample of supernovae. Using new techniques, the DES team has 20 times more data, over a wide range of distances. This allows for one of the most precise ever measurements of w, giving a value of -0.8
Facilities such as the Vera Rubin Observatory will make further measurements.
The detection of the Higgs Boson subatomic particle in 2012 at the Large Hadron Collider required odds of a million to one chance of being wrong. However, this measurement may signal the end of “Big Rip” models which have equations of state that are more negative than one. In such models the universe would expand indefinitely at a faster and faster rate – eventually pulling apart galaxies, planetary systems and even space-time itself. That’s a relief.
As usual, scientists want more data and those plans are already well underway. The DES results suggest that our new techniques will work for future supernova experiments with ESA’s Euclid mission (launched July 2023) and the new Vera Rubin Observatory in Chile. This observatory should soon use its telescope to take a first image of the sky following construction, giving a glimpse into its capabilities.
These next-generation telescopes could find thousands more supernovae, helping us make new measurements of the equation of state and shedding even more light on the nature of dark energy.
A study using the new generation of super accurate strontium clocks supports physicists’ theories about time dilatation and Lorentz invariance.
A new generation of super accurate clocks are proving that Einstein’s idea that time is not absolute is correct. This is not great news for anyone looking for a unifying theory for everything from Einstein’s work to Quantum Mechanics, but it’s decent news for the rest of us who are relying on precision timepieces.
There is a highly regarded theory in physics that any two observers moving at a constant speed relative to each other will experience the same exact laws of physics. This symmetry of special relativity is called Lorentz invariance. Each of the symmetrical speeders would see themselves as stationary, but they would look over and see the other person’s clock running slowly. This is the time dilation effect, and Einstein’s theory of General Relativity adds gas to the fire by saying that if the speeders experience different gravitational forces, their clocks will run differently.
So far, we have confirmed these ideas by comparing atomic clocks on GPS satellites with their Earthbound cousins. However, it has always been clear that deviations from relativity would be minuscule, and so it remained possible that we simply lacked tools with the sufficient precision to detect any deviations.
TIME MATTERS
Enter the next-generation strontium clocks, which are at least three times more precise than the caesium-133 models, neither losing nor gaining more than a second over the course of 15 billion years. Pacôme Delva of the Paris Observatory and his team used fiberoptic links to test time dilation between these strontium clocks in London, Paris, and Braunschweig, Germany. Because of their positions on the Earth’s surface, the clocks would tick at slightly different rates. If the theory of relativity was accurate, it would correctly predict those differences.
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The team took their measurements and then calculated a parameter called alpha. No violation of the Lorentz invariance would produce an alpha of zero, and the results produced a near-zero alpha of less than 10-8. This result supports the Lorentz invariance and is twice as accurate as the best limit from previous work, and two orders of magnitude better than previous caesium clock results.
As far as strontium clocks go, things are looking good for relativity. Things are also looking good for advanced technologies that demand precision time measurement, such as GPS systems and autonomous vehicles.
Still, fans of the Lorentz invariance shouldn’t get too comfortable. Although there is not yet any evidence of a violation, physicists all over the world are going to keep looking. A confirmed violation of the Lorentz invariance would have tremendous implications for quantizing gravity and our understanding of the nature of Dark Energy and Dark Matter.
Dark energy is an unknown form of energy that is proposed to drive the accelerated expansion of the universe. A new study by University of Georgia professor Edward Kipreos suggests that changes in how people think about time dilation—the slowing of time predicted by Albert Einstein—can provide an alternate explanation of dark energy.
In the recent Hollywood film “Interstellar,” a team of scientists travel through a wormhole in space to access planets with promising conditions to sustain life on Earth. One of the issues the team must grapple with is time dilation: each hour spent collecting data on a given planet is equal to seven years on Earth.
Einstein’s general theory of relativity indicates that time dilation in response to gravity is directional in that an object in high gravity will have slower time than an object in low gravity. In contrast, Einstein’s theory of special relativity describes reciprocal time dilation between two moving objects, such that both moving objects’ times appear to be slowed down relative to each other.
The new paper makes the case that instead of being reciprocal, time dilation in response to movement is directional, with only the moving object undergoing time dilation.
The study, “Implication of an Absolute Simultaneity Theory for Cosmology and Universe Acceleration,” was published Dec. 23 in the journal PLOS ONE.
A molecular geneticist whose lab works on cell cycle regulation, Kipreos became interested in cosmology and the theory of special relativity several years ago. He says the phenomenon can be easily understood in the context of how Global Positioning System satellites work.
“The satellites, which travel in free-fall reference frames, are moving fast enough, in relation to the Earth, that you have to correct for their time being slowed down, based on their speed,” he said. “If we didn’t correct for that, then the satellites’ GPS measurement would be off by a factor of two kilometers per day.”
This simple example—GPS satellites sending out the time, which is then detected back on Earth, where the distance between the two is measured—is based on the theory ofspecial relativity and the Lorentz Transformation, a mathematical map that describes how measurements of space and time by two observers are related.
“Special relativity is supposed to be reciprocal, where both parties will experience the same time dilation, but all the examples that we have right now can be interpreted as directional time dilation,” Kipreos said. “If you look at the GPS satellites, the satellite time is slowing down, but according to the GPS satellites, our time is not slowing down—which would occur if it were reciprocal. Instead, our time is going faster relative to the satellites, and we know that because of constant communication with the satellites.”
An alternative theory, the Absolute Lorentz Transformation, describes directional time dilation. Kipreos found that this theory is compatible with available evidence if the “preferred reference frame” for the theory, relative to which directional time dilation occurs, is linked to centers of gravitational mass. Near the Earth, the preferred reference frame would be the “Earth-centered non-rotating inertial reference frame,” which is currently used to calculate the time dilation of GPS satellites.
“A strict application of the Absolute Lorentz Transformation to cosmological data has significant implications for the universe and the existence of dark energy,” Kipreos said.
As the universe gets larger, cosmological objects, such as galaxies, move more rapidly away from each other in a process known as Hubble expansion. The Absolute Lorentz Transformation indicates that increased velocities induce directional time dilation. Applying this to the increased velocities associated with Hubble expansion in the present universe suggests a scenario in which the present experiences time dilation relative to the past. The passage of time would therefore be slower in the present and faster in the past.
Supernovas that explode with the same intensity are used as “standard candles” to measure cosmological distances based on how bright they appear. Supernovas that are relatively close to the Earth line up on a plot of distance (based on the redshift of light) and brightness. However, in 1998 and 1999, the observation that supernovas at greater distances are fainter than would be expected provided evidence that the rate of universe expansion has accelerated recently.
“The accelerated expansion of the universe has been attributed to the effects of dark energy,” Kipreos said. “However, there is no understanding of what dark energy is or why it has manifested only recently.
“The predicted effects of time being faster in the past would have the effect of making the plot of supernovas become linear at all distances, which would imply that there is no acceleration in the expansion of the universe. In this scenario there would be no necessity to invoke the existence of dark energy.”
The universe has no center and no edge, no special regions tucked in among the galaxies and light. No matter where you look, it’s the same—or so physicists thought. This cosmological principle—one of the foundations of the modern understanding of the universe—has come into question recently as astronomers find evidence, subtle but growing, of a special direction in space.
The first and most well-established data point comes from the cosmic microwave background (CMB), the so-called afterglow of the big bang. As expected, the afterglow is not perfectly smooth—hot and cold spots speckle the sky. In recent years, however, scientists have discovered that these spots are not quite as randomly distributed as they first appeared—they align in a pattern that points out a special direction in space. Cosmologists have theatrically dubbed it the “axis of evil.”
More hints of a cosmic arrow come from studies of supernovae, stellar cataclysms that briefly outshine entire galaxies. Cosmologists have been using supernovae to map the accelerating expansion of the universe (a feat that garnered this year’s Nobel Prize in Physics). Detailed statistical studies reveal that supernovae are moving even faster in a line pointing just slightly off the axis of evil. Similarly, astronomers have measured galaxy clusters streaming through space at a million miles an hour toward an area in the southern sky.
What could all this mean? Perhaps nothing. “It could be a fluke,” says Dragan Huterer, a cosmologist at the University of Michigan at Ann Arbor, or it could be a subtle error that has crept into the data (despite careful efforts). Or, Huterer says, perhaps we are seeing the first signs of “something amazing.” The universe’s first burst of expansion could have lasted a little longer than we thought, introducing a tilt to it that still persists today. Another possibility is that at large scales, the universe could be rolled up like a tube, curved in one direction and flat in the others, according to Glenn D. Starkman, a cosmologist at Case Western Reserve University. Alternatively, the so-called dark energy—the bewildering stuff accelerating the universe’s expansion—might act differently in different places.
For now, the data remain preliminary—subtle signs that something may be wrong with our standard understanding of the universe. Scientists are eagerly anticipating the data from the Planck satellite, which is currently measuring the CMB from a quiet spot 930,000 miles up. It will either confirm earlier measurements of the axis of evil or show them to be ephemera. Until then, the universe could be pointing us anywhere.
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