Astronomy

Does gravity repel when dark energy is involved?

Does gravity repel when dark energy is involved?


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So I am currently reading the book We have no idea by Jorge Cham and Daniel Whiteson.
In one of their chapters they talk about how gravity is a force that only attracts and doesn't repel, most of the time. Dark energy is what makes up 68% of the universe and the only way we know it exists is from its rapid expansion.

The expansion isn't slowing down, however, it is speeding up. Gravity is a force that is said to be a force that only attracts and doesn't repel, unlike electromagnetism, the weak force and the strong force. Gravity should be slowing down that expansion, and I am wondering if anyone can explain this or is this concept out of our reach.


We do not yet know the answer to this question. We observe the universal expansion, which should be slowing down from mutual gravitational attraction, is not slowing down as much as it should and might not be slowing down at all about now. One explanation for this is that the cosmological constant, an added term in Einstein's equations of General Relativity is non-zero. This term allows for gravity to effectively repel by causing expansion in low density regions. For the entire universe, the expansion resulted from an initial impulse (the Big Bang) and gravity has been working against this, trying to compress it. With the cosmological constant included, as more and more regions of the universe reach sufficiently low densities, gravity reverses and enhances expansion.

Alternatively, some think the expansion is due to some particle or another force that provides a negative pressure in the universe.


In addition to not knowing, there's other reasons to suspect gravity wouldn't reverse. There's no known force that reverses over distance (OK, the strong force, kinda/maybe but over very short distances). Also, the expansion of space explains things that we seem to observe or suspect occurred like galaxies moving away from us faster than the speed of light and the rapid expansion that's suspected to have happened very shortly after the big bang. Gravity can't do either of those things.

There's also the problem of how would gravity know that things are so far apart that they should start repelling, where nearby galaxies are still too close but more distant galaxies the repulsion kicks in? It doesn't make a whole lot of sense that gravity would account for both attraction and expansion of space.

That's not to say it's impossible. We don't know how gravity works or what it is, so, maybe. Never say never, until something is understood.

@eshaya's answer, "We don't know" is probably the more straight forward answer to this question, but I wanted to point out some problems with repulsive gravity theory as well.

In addition (and I don't understand precisely why), but the gravitational waves discovery has ruled out some theoretical gravity/dark energy proposals.


Dark Energy and Gravity Withstand Tests

In the life sciences, researchers are discouraged from challenging certain reigning paradigms, such as the evolutionary explanation for the origin of humans. The opposite is the case in the astrophysical sciences. Astrophysicists are encouraged to critique and test even the best established facts, principles, and laws of physics and astronomy. An example of such a critique and test, one that has enormous implications for our biblical cosmic creation model, was announced just days ago.

It is hard to think of anything better established in physics than the law of gravity. In the other sciences, gravity often is used as an analogy for certainty. Yet, the astrophysical community has been busy trying to build a case for an alternate theory of gravity, one that does away with the need for dark energy.

Dark energy, the self-stretching property of the space surface of the universe, also is a well-established concept in astrophysics. For several decades astronomers insisted that dark energy had to exist to explain a variety of observations of the gross features of the universe. In 1998 and 1999 two teams of astronomers actually observed the cosmic space surface expanding at a progressively faster rate—a clear signature of dark energy. The dark energy signature that they and subsequent observers have measured amounts to about 3/4 of all the stuff that makes up the universe. This discovery won the Nobel Prize for the leaders of the two teams.

Einstein’s theory of general relativity is the most exhaustively tested and best proven principle in the disciplines of astronomy and physics. Some tests of general relativity have proven its reliability to predict the future positions of massive bodies to better than 15 places of the decimal. Yet, it is this very theory of general relativity that many physicists and astronomers are calling into question.

The most seriously proposed alternative to general relativity is something called f(R) theory. In f(R) theory, in addition to the bending or warping of space-time in the vicinity of mass and energy as predicted by general relativity, there is an extra gravity-like force that either attracts or repels. This extra force is referred to as a scalar field or a fifth fundamental force of physics.

In 2007, two theoretical physicists showed that with the just-right choice of the function f(R), the measured expansion history of the universe (a slight deceleration for the first half of cosmic history followed by a slight, ever-increasing acceleration during the second half of cosmic history) could be explained without invoking dark energy. 1 Not until 2015 did anyone attempt to put this special f(R) theory to the test. 2 A large team of astronomers and physicists first determined that this version of f(R) theory predicted that galaxy clusters would form faster and be more numerous than would be the case in the standard cosmological model with dark energy and gravity governed strictly by general relativity. In analyzing the then current databases on galaxy clusters, the team noted that the data favored dark energy and disfavored f(R) theory. However, the galaxy cluster data was neither extensive enough nor of sufficient observational quality to make their deduction conclusive. In particular, the team lacked precision measurements of the total masses of the galaxy clusters.

Now, a team of astronomers led by Xiangkun Liu has taken advantage of weak gravitational lensing to directly and fairly accurately measure the masses of numerous galaxy clusters. 3 Gravity from a very massive object, or a dense cluster of massive objects, will distort the images of more distant galaxies directly behind it in our line of sight. The featured image for this article shows a nearly complete ring of blue light around a central giant spherical galaxy. The blue ring is the distorted image of a more distant galaxy. The size of the ring and the degree of distortion reveals the mass of the foreground giant galaxy.

The research team led by Liu used observations of 5.5 million galaxies produced by other astronomers using the Canada-France-Hawaii Telescope to develop a catalog of weak gravitational lensing galaxies. Analysis of this catalog strongly agreed with the predictions of dark energy and contradicted the predictions of f(R) theories. In the words of Lui et al.,

Our fits are consistent with general relativity, not requiring a fifth force. 4


Adapted from a release by Bill Schulz, Lawrence Berkeley National Laboratory.

A five-year quest to map the universe and unravel the mysteries of dark energy began officially on May 17, 2021, at Kitt Peak National Observatory near Tucson, Arizona. The Dark Energy Spectroscopic Instrument (DESI) will capture and study the light from more than 30 million galaxies and other distant objects, allowing scientists to construct a 3-D map of the universe with unprecedented detail.

DESI is an international science collaboration that includes physicists from the University of Utah and is managed by the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) with primary funding from DOE’s Office of Science.

Spectra collected by DESI are the components of light corresponding to the colors of the rainbow. As the universe expands, galaxies move away from each other and their light is shifted to longer, redder wavelengths. The more distant the galaxy, the greater its “redshift.” The project will collect and model the data for tens of millions of spectra, which requires training the automated software using a combination of deep observations and visual inspection. Angela Berti, a postdoctoral researcher at the U, is heavily involved in the effort.

“I’ve been doing visual inspection—measuring some redshifts by hand the old-fashioned way—to assess the algorithms we’ll use to measure millions more redshifts automatically. Understanding which spectral features our modeling techniques don’t handle well helps us improve that modeling overall,” said Berti. “You’re never going to have an analysis that gets every single redshift perfectly. But you can optimize and figure out how to reduce your error as much as possible.”

PHOTO CREDIT: DESI collaboration

Photo of a small section of the DESI focal plane, showing the one-of-a-kind robotic positioners. The optical fibers, which are installed in the robotic positioners, are backlit with blue light in this image.

Berti is also working on tools that will identify how the many different types of galaxies congregate together in space.

Allyson Brodzeller, a doctoral student at the U, has been developing new techniques to model data for quasars, bright, distant objects powered by black holes billions of times more massive than the sun. She is using techniques in machine learning to group quasars together by similar features.

“This allows us to classify the objects by their properties in the data and to identify quasar spectra that don’t perfectly fit into the groups,” Brodzeller said. “Establishing these classifications will allow us to get better estimates of their redshift and properties.”

The detailed distribution of galaxies in the map is expected to yield new insights on the influence and nature of dark energy.

“Dark energy is one of the key science drivers for DESI,” said project co-spokesperson Kyle Dawson, a professor of physics and astronomy at U. “The goal is not so much to find out how much there is—we know that about 70% of the energy in the universe today is dark energy—but to study its properties.”

PHOTO CREDIT: DESI collaboration and DESI Legacy Imaging Surveys

The disk of the Andromeda Galaxy (M31), which spans more than 3 degrees, is targeted by a single DESI pointing, represented by the large, pale green, circular overlay. The smaller circles within this overlay represent the regions accessible to each of the 5000 DESI robotic fiber positioners. In this sample, the 5000 spectra that were simultaneously collected by DESI include not only stars within the Andromeda Galaxy, but also distant galaxies and quasars. The example DESI spectrum that overlays this image is of a distant quasar (QSO) 11 billion years old.

“The universe is expanding at a rate determined by its total energy contents,” Dawson explained. “As the DESI instrument looks out in space and time, we can literally take snapshots today, yesterday, 1 billion years ago, 2 billion years ago as far back in time as possible. We can then figure out the energy content in these snapshots—and see how it is evolving.”

The formal start of DESI’s five-year survey follows a four-month trial run of its custom instrumentation that captured more than four million galaxy spectra—more than the combined output of all previous spectroscopic surveys.

The DESI instrument resides at the retrofitted Mayall Telescope at the National Science Foundation’s Kitt Peak National Observatory. The instrument includes new optics that increase the field of view of the telescope and includes 5,000 robotically controlled optical fibers to gather spectroscopic data from an equal number of objects in the telescope’s field of view.

“We’re not using the biggest telescopes,” said Berkeley Lab’s David Schlegel, who is a DESI project scientist. “It’s that the instruments are better and very highly multiplexed, meaning that we can capture the light from many different objects at once.”

“DESI is the most ambitious of a new generation of instruments aimed at better understanding the cosmos—in particular, its dark energy component,” said project co-spokesperson Nathalie Palanque-Delabrouille, a cosmologist at France’s Alternative Energies and Atomic Energy Commission. She said the scientific program—including her own interest in quasars—will allow researchers to address with precision two primary questions: What is dark energy? And, to which degree does gravity follow the laws of general relativity (which form the basis of our understanding of the cosmos)?

DESI is supported by the DOE Office of Science and by the National Energy Research Scientific Computing Center, a DOE Office of Science user facility. Additional support for DESI is provided by the U.S. National Science Foundation, the Science and Technologies Facilities Council of the United Kingdom, the Gordon and Betty Moore Foundation, the Heising-Simons Foundation, the French Alternative Energies and Atomic Energy Commission (CEA), the National Council of Science and Technology of Mexico, the Ministry of Economy of Spain and by the DESI member institutions.

The authors are honored to be permitted to conduct astronomical research on Iolkam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation.


Dark Energy vs. Dark Matter

Our universe may contain 100 billion galaxies, each with billions of stars, great clouds of gas and dust, and perhaps scads of planets and moons and other little bits of cosmic flotsam. The stars produce an abundance of energy, from radio waves to X-rays, which streak across the universe at the speed of light.

Yet everything that we can see is like the tip of the cosmic iceberg &mdash it accounts for only about four percent of the total mass and energy in the universe.

About one-quarter of the universe consists of dark matter, which releases no detectable energy, but which exerts a gravitational pull on all the visible matter in the universe.

Because of the names, it’s easy to confuse dark matter and dark energy. And while they may be related, their effects are quite different. In brief, dark matter attracts, dark energy repels. While dark matter pulls matter inward, dark energy pushes it outward. Also, while dark energy shows itself only on the largest cosmic scale, dark matter exerts its influence on individual galaxies as well as the universe at large.

In fact, astronomers discovered dark matter while studying the outer regions of our galaxy, the Milky Way.

A ring of possible dark matter highlights this Hubble Space Telescope image of a distant galaxy cluster. [NASA/ESA/M.J. Jee/H. Ford (Johns Hopkins)]

The Milky Way is shaped like a disk that is about 100,000 light-years across. The stars in this disk all orbit the center of the galaxy. The laws of gravity say that the stars that are closest to the center of the galaxy &mdash which is also its center of mass &mdash should move faster than those out on the galaxy’s edge.

Yet when astronomers measured stars all across the galaxy, they found that they all orbit the center of the galaxy at about the same speed. This suggests that something outside the galaxy’s disk is tugging at the stars: dark matter.

Calculations show that a vast "halo" of dark matter surrounds the Milky Way. The halo may be 10 times as massive as the bright disk, so it exerts a strong gravitational pull.

The same effect is seen in many other galaxies. And clusters of galaxies show exactly the same thing &mdash their gravity is far stronger than the combined pull of all their visible stars and gas clouds.

Are dark matter and dark energy related? No one knows. The leading theory says that dark matter consists of a type of subatomic particle that has not yet been detected, although upcoming experiments with the world’s most powerful particle accelerator may reveal its presence. Dark energy may have its own particle, although there is little evidence of one.

Instead, dark matter and dark energy appear to be competing forces in our universe. The only things they seem to have in common is that both were forged in the Big Bang, and both remain mysterious.

Resources

Copyright ©2008-2015 McDonald Observatory. HETDEX is a collaboration of The University of Texas at Austin, Pennsylvania State University, Texas A&M University, Universitats-Sternwärte Munich, Leibniz Institute for Astrophysics (AIP), Max-Planck-Institut für Extraterrestrische Physik, Institut für Astrophysik Göttingen, and University of Oxford. Financial support is provided by the State of Texas, the United States Air Force, the National Science Foundation and the generous contributions of many private foundations and individuals.


To explain away dark matter, gravity would have to be really weird, cosmologists say

Dark matter, the invisible stuff whose gravity is thought to hold galaxies together, may be the least satisfying concept in physics. But if you want to get rid of it, a new study finds, you’ll need to replace it with something even more bizarre: a force of gravity that, at some distances, pulls massive objects together and, at other distances, pushes them apart. The analysis underscores how hard it is to explain away dark matter.

Concocting such a theory of gravity “is so complicated that it seems very unlikely that anyone could come up with a scenario that would work,” says Scott Dodelson, a theoretical physicist at Carnegie Mellon University, who wasn’t involved in the new work. Still, some theorists say it may be possible to pass the test.

According to cosmologists’ prevailing theory, dark matter pervades pretty much every galaxy, providing the extra gravity that keeps stars from swirling out into space, given the speeds at which astronomers see the galaxies rotating. A vast web of clumps and strands of the stuff served as the scaffolding on which the cosmos developed. Yet, after of decades of trying, physicists haven’t spotted particles of dark matter floating around, and many would happily dismiss the idea—if it didn’t work so well.

Some scientists have tried to kick the dark matter habit. In 1983, Israeli physicist Mordehai Milgrom found he could account for the high speeds of stars swirling around the peripheries of galaxies by modifying Isaac Newton’s famous second law of motion: force equals mass times acceleration. That insight suggested the need for dark matter could be obviated by changing the law of gravity, at least on the scale of individual galaxies. But theorists labored for decades to turn the idea into a coherent theory of gravity akin to Albert Einstein’s general theory of relativity, and to do so, they had to add new fields, cousins of the usual gravitational field.

But to do away with dark matter, theorists would also need explain away its effects on much larger, cosmological scales. And that is much harder, argues Kris Pardo, a cosmologist at NASA's Jet Propulsion Laboratory, and David Spergel, a cosmologist at Princeton University. To make their case, they compare the distribution of ordinary matter in the early universe as revealed by measurements of the afterglow of the big bang—the cosmic microwave background (CMB)—with the distribution of the galaxies today.

The evolution of the universe is a tale of two fluids: dark matter, which doesn’t interact with light, and ordinary matter, which does. The big bang left ripples in the dark matter, which under its own gravity began to coalesce into the denser spots. Ordinary matter—then, a hot soup of free-flying protons and electrons—also began to fall into the dark matter clumps. However, those charged particles themselves generated radiation that pushed them back out, creating sound waves known as a baryon acoustic oscillations. The waves continued to spread until the universe cooled enough to form neutral atoms, 380,000 years after the big bang, when the CMB was born. The sound wave left its imprint on the CMB and, faintly, in the distribution of the galaxies.

Or could that evolution be explained with only ordinary matter interacting through modified gravity? To explore that possibility, Pardo and Spergel derived a mathematical function that describes how gravity would have had to work to get from the distribution of ordinary matter revealed by the CMB to the current distribution of the galaxies. They found something striking: That function must swing between positive and negative values, meaning gravity would be attractive at some length scales and repulsive at others, Pardo and Spergel report this week in Physical Review Letters . “And that’s superweird,” Pardo says.

The strange behavior is required to explain how the larger baryon acoustic oscillation faded over cosmic time while the smaller galaxies emerged, Pardo says. Just as Milgrom did with individual galaxies, the new work shows how, without dark matter, gravity would have to change to explain the universe’s large-scale structure, Dodelson says. But that change would have to be radical, he says. “They’re demonstrating that to do that you have to jump through these 13 hoops,” he says.

However, theorists already seem prepared to jump through those hoops. In a paper posted in June to the preprint server arXiv, theoretical cosmologists Constantinos Skordis and Tom Złosnik of the Czech Academy of Sciences present a dark matter – less theory of modified gravity they say jibes with CMB data. To do that, researchers add to a theory like general relativity an additional, tunable field called a scalar field. It has energy, and through Einstein’s equivalence of mass and energy, it can behave like a form of mass. Set things up just right and at large spatial scales, the scalar field interacts only with itself and acts like dark matter.

The team hasn’t explicitly shown that the theory, which isn’t meant to be a fundamental theory of gravity, passes Pardo’s and Spergel’s particular test. But because it’s designed to mimic dark matter, it ought to, Skordis says. “We engineered it to have that behavior.”

Skordis’s and Złosnik’s paper is “very exciting,” Pardo says. But he notes that in some sense it merely replaces one mysterious thing—dark matter—with another—a carefully tuned scalar field. Given the complications, Pardo says, “dark matter is kind of the easier explanation.”


Dark Matter

In the 1930s, Swiss-born astronomer Fritz Zwicky studied images of the roughly 1,000 galaxies that make up the Coma Cluster — and he spotted something funny about their behavior. The galaxies moved so fast that they should simply fly apart. He speculated that some kind of “dark matter” held them together.

Decades later, astronomers Vera Rubin and Kent Ford found a similar phenomenon when they studied the rotation rates of individual galaxies. The stars at a galaxy’s outer edge should circle slower than stars near the center. That’s the way planets in our solar system orbit. Instead, they noticed that the stars on a galaxy’s outskirts orbit just as fast — or faster — than the stars closer in. Rubin and Ford had found more evidence that some invisible form of matter is apparently holding the universe together.

“Even stars at the periphery are orbiting at high velocities,” Rubin once explained in an interview with Discover . “There has to be a lot of mass to make the stars orbit so rapidly, but we can’t see it. We call this invisible mass dark matter.”

Astronomers now have many other lines of evidence that suggest dark matter is real. In fact, the existence of dark matter is so widely accepted that it’s part of the so-called standard model of cosmology , which forms the foundation of how scientists understand the universe’s birth and evolution. Without it, we can’t explain how we got here.

But that lofty status puts pressure on cosmologists to find definitive proof that dark matter exists and that their model of the universe is correct. For decades, physicists all over the world have employed increasingly high-tech instruments to try and detect dark matter . So far, they’ve found no signs of it.


Dark Energy FAQ

In honor of the Nobel Prize , here are some questions that are frequently asked about dark energy, or should be. What is dark energy? It's what makes the universe accelerate, if indeed there is a "thing" that does that. (See below.) So I guess I should be asking. what does it mean to say the universe is "accelerating"? First, the universe is expanding : as shown by Hubble , distant galaxies are moving away from us with velocities that are roughly proportional to their distance. "Acceleration" means that if you measure the velocity of one such galaxy, and come back a billion years later and measure it again, the recession velocity will be larger. Galaxies are moving away from us at an accelerating rate. But that's so down-to-Earth and concrete. Isn't there a more abstract and scientific-sounding way of putting it? The relative distance between far-flung galaxies can be summed up in a single quantity called the "scale factor," often written a ( t ) or R ( t ). The scale factor is basically the "size" of the universe, although it's not really the size because the universe might be infinitely big -- more accurately, it's the relative size of space from moment to moment. The expansion of the universe is the fact that the scale factor is increasing with time. The acceleration of the universe is the fact that it's increasing at an increasing rate -- the second derivative is positive, in calculus-speak. Does that mean the Hubble constant, which measures the expansion rate, is increasing? No. The Hubble "constant" (or Hubble "parameter," if you want to acknowledge that it changes with time) characterizes the expansion rate, but it's not simply the derivative of the scale factor: it's the derivative divided by the scale factor itself. Why? Because then it's a physically measurable quantity, not something we can change by switching conventions. The Hubble constant is basically the answer to the question "how quickly does the scale factor of the universe expand by some multiplicative factor?" If the universe is decelerating, the Hubble constant is decreasing. If the Hubble constant is increasing, the universe is accelerating. But there's an intermediate regime in which the universe is accelerating but the Hubble constant is decreasing -- and that's exactly where we think we are. The velocity of individual galaxies is increasing, but it takes longer and longer for the universe to double in size. Said yet another way: Hubble's Law relates the velocity v of a galaxy to its distance d via v = H d . The velocity can increase even if the Hubble parameter is decreasing, as long as it's decreasing more slowly than the distance is increasing. Did the astronomers really wait a billion years and measure the velocity of galaxies again? No. You measure the velocity of galaxies that are very far away. Because light travels at a fixed speed (one light year per year), you are looking into the past. Reconstructing the history of how the velocities were different in the past reveals that the universe is accelerating. How do you measure the distance to galaxies so far away? It's not easy. The most robust method is to use a "standard candle" -- some object that is bright enough to see from great distance, and whose intrinsic brightness is known ahead of time. Then you can figure out the distance simply by measuring how bright it actually looks: dimmer = further away. Sadly, there are no standard candles. Then what did they do? Fortunately we have the next best thing: standardizable candles. A specific type of supernova, Type Ia , are very bright and approximately-but-not-quite the same brightness. Happily, in the 1990's Mark Phillips discovered a remarkable relationship between intrinsic brightness and the length of time it takes for a supernova to decline after reaching peak brightness. Therefore, if we measure the brightness as it declines over time, we can correct for this difference, constructing a universal measure of brightness that can be used to determine distances. Why are Type Ia supernovae standardizable candles? We're not completely sure -- mostly it's an empirical relationship. But we have a good idea: we think that SNIa are white dwarf stars that have been accreting matter from outside until they hit the Chandrasekhar Limit and explode. Since that limit is basically the same number everywhere in the universe, it's not completely surprising that the supernovae have similar brightnesses. The deviations are presumably due to differences in composition. But how do you know when a supernova is going to happen? You don't. They are rare, maybe once per century in a typical galaxy. So what you do is look at many, many galaxies with wide-field cameras. In particular you compare an image of the sky taken at one moment to another taken a few weeks later -- "a few weeks" being roughly the time between new Moons (when the sky is darkest), and coincidentally about the time it takes a supernova to flare up in brightness. Then you use computers to compare the images and look for new bright spots. Then you go back and examine those bright spots closely to try to check whether they are indeed Type Ia supernovae. Obviously this is very hard and wouldn't even be conceivable if it weren't for a number of relatively recent technological advances -- CCD cameras as well as giant telescopes. These days we can go out and be confident that we'll harvest supernovae by the dozens -- but when Perlmutter and his group started out, that was very far from obvious. And what did they find when they did this? Most (almost all) astronomers expected them to find that the universe was decelerating -- galaxies pull on each other with their gravitational fields, which should slow the whole thing down. (Actually many astronomers just thought they would fail completely, but that's another story.) But what they actually found was that the distant supernovae were dimmer than expected -- a sign that they are farther away than we predicted, which means the universe has been accelerating. Why did cosmologists accept this result so quickly? Even before the 1998 announcements, it was clear that something funny was going on with the universe. There seemed to be evidence that the age of the universe was younger than the age of its oldest stars. There wasn't as much total matter as theorists predicted. And there was less structure on large scales than people expected. The discovery of dark energy solved all of these problems at once. It made everything snap into place. So people were still rightfully cautious, but once this one startling observation was made, the universe suddenly made a lot more sense. How do we know the supernovae not dimmer because something is obscuring them, or just because things were different in the far past? That's the right question to ask, and one reason the two supernova teams worked so hard on their analysis. You can never be 100% sure, but you can gain more and more confidence. For example, astronomers have long known that obscuring material tends to scatter blue light more easily than red, leading to "reddening" of stars that sit behind clouds of gas and dust. You can look for reddening, and in the case of these supernovae it doesn't appear to be important. More crucially, by now we have a lot of independent lines of evidence that reach the same conclusion, so it looks like the original supernova results were solid. There's really independent evidence for dark energy? Oh yes. One simple argument is "subtraction": the cosmic microwave background measures the total amount of energy (including matter) in the universe. Local measures of galaxies and clusters measure the total amount of matter. The latter turns out to be about 27% of the former, leaving 73% or so in the form of some invisible stuff that is not matter: "dark energy." That's the right amount to explain the acceleration of the universe. Other lines of evidence come from baryon acoustic oscillations (ripples in large-scale structure whose size helps measure the expansion history of the universe) and the evolution of structure as the universe expands. Okay, so: what is dark energy? Glad you asked! Dark energy has three crucial properties. First, it's dark: we don't see it, and as far as we can observe it doesn't interact with matter at all. (Maybe it does, but beneath our ability to currently detect.) Second, it's smoothly distributed: it doesn't fall into galaxies and clusters, or we would have found it by studying the dynamics of those objects. Third, it's persistent: the density of dark energy (amount of energy per cubic light-year) remains approximately constant as the universe expands. It doesn't dilute away like matter does. These last two properties (smooth and persistent) are why we call it "energy" rather than "matter." Dark energy doesn't seem to act like particles, which have local dynamics and dilute away as the universe expands. Dark energy is something else. That's a nice general story. What might dark energy specifically be? The leading candidate is the simplest one: "vacuum energy," or the "cosmological constant." Since we know that dark energy is pretty smooth and fairly persistent, the first guess is that it's perfectly smooth and exactly persistent. That's vacuum energy: a fixed amount of energy attached to every tiny region of space, unchanging from place to place or time to time. About one hundred-millionth of an erg per cubic centimeter, if you want to know the numbers. Is vacuum energy really the same as the cosmological constant? Yes. Don't believe claims to the contrary. When Einstein first invented the idea, he didn't think of it as "energy," he thought of it as a modification of the way spacetime curvature interacted with energy. But it turns out to be precisely the same thing. (If someone doesn't want to believe this, ask them how they would observationally distinguish the two.) Doesn't vacuum energy come from quantum fluctuations? Not exactly. There are many different things that can contribute to the energy of empty space, and some of them are completely classical (nothing to do with quantum fluctuations). But in addition to whatever classical contribution the vacuum energy has, there are also quantum fluctuations on top of that. These fluctuation are very large, and that leads to the cosmological constant problem. What is the cosmological constant problem? If all we knew was classical mechanics, the cosmological constant would just be a number -- there's no reason for it to be big or small, positive or negative. We would just measure it and be done. But the world isn't classical, it's quantum. In quantum field theory we expect that classical quantities receive "quantum corrections." In the case of the vacuum energy, these corrections come in the form of the energy of virtual particles fluctuating in the vacuum of empty space. We can add up the amount of energy we expect in these vacuum fluctuations, and the answer is: an infinite amount. That's obviously wrong, but we suspect that we're overcounting. In particular, that rough calculation includes fluctuations at all sizes, including wavelengths smaller than the Planck distance at which spacetime probably loses its conceptual validity. If instead we only include wavelengths that are at the Planck length or longer, we get a specific estimate for the value of the cosmological constant. The answer is: 10 ^120 times what we actually observe. That discrepancy is the cosmological constant problem. Why is the cosmological constant so small? Nobody knows. Before the supernovae came along, many physicists assumed there was some secret symmetry or dynamical mechanism that set the cosmological constant to precisely zero, since we certainly knew it was much smaller than our estimates would indicate. Now we are faced with both explaining why it's small, and why it's not quite zero. And for good measure: the coincidence problem, which is why the dark energy density is the same order of magnitude as the matter density. Here's how bad things are: right now, the best theoretical explanation for the value of the cosmological constant is the anthropic principle. If we live in a multiverse, where different regions have very different values of the vacuum energy, one can plausibly argue that life can only exist (to make observations and win Nobel Prizes) in regions where the vacuum energy is much smaller than the estimate. If it were larger and positive, galaxies (and even atoms) would be ripped apart if it were larger and negative, the universe would quickly recollapse. Indeed, we can roughly estimate what typical observers should measure in such a situation the answer is pretty close to the observed value. Steven Weinberg actually made this prediction in 1988, long before the acceleration of the universe was discovered. He didn't push it too hard, though more like "if this is how things work out, this is what we should expect to see…" There are many problems with this calculation, especially when you start talking about "typical observers," even if you're willing to believe there might be a multiverse. (I'm very happy to contemplate the multiverse, but much more skeptical that we can currently make a reasonable prediction for observable quantities within that framework.) What we would really like is a simple formula that predicts the cosmological constant once and for all as a function of other measured constants of nature. We don't have that yet, but we're trying. Proposed scenarios make use of quantum gravity, extra dimensions, wormholes, supersymmetry, nonlocality, and other interesting but speculative ideas. Nothing has really caught on as yet. Has the course of progress in string theory ever been affected by an experimental result? Yes: the acceleration of the universe. Previously, string theorists (like everyone else) assumed that the right thing to do was to explain a universe with zero vacuum energy. Once there was a real chance that the vacuum energy is not zero, they asked whether that was easy to accommodate within string theory. The answer is: it's not that hard. The problem is that if you can find one solution, you can find an absurdly large number of solutions. That's the string theory landscape , which seems to kill the hopes for one unique solution that would explain the real world. That would have been nice, but science has to take what nature has to offer. What's the coincidence problem?


New insights on dark energy

A representation of the evolution of the universe over 13.8 billion years. Different methods of studying cosmic expansion yield slightly different results, including for the age of the universe. Astronomers have calculated that these discrepancies could be reconciled if the dark energy that drives cosmic acceleration were not constant in time. Credit: NASA and the WMAP consortium

The universe is not only expanding - it is accelerating outward, driven by what is commonly referred to as "dark energy." The term is a poetic analogy to label for dark matter, the mysterious material that dominates the matter in the universe and that really is dark because it does not radiate light (it reveals itself via its gravitational influence on galaxies). Two explanations are commonly advanced to explain dark energy. The first, as Einstein once speculated, is that gravity itself causes objects to repel one another when they are far enough apart (he added this "cosmological constant" term to his equations). The second explanation hypothesizes (based on our current understanding of elementary particle physics) that the vacuum has properties that provide energy to the cosmos for expansion.

For several decades cosmologies have successfully used a relativistic equation with dark matter and dark energy to explain increasingly precise observations about the cosmic microwave background, the cosmological distribution of galaxies, and other large-scale cosmic features. But as the observations have improved, some apparent discrepancies have emerged. One of the most notable is the age of the universe: there is an almost 10% difference between measurements inferred from the Planck satellite data and those from so-called Baryon Acoustic Oscillation experiments. The former relies on far-infrared and submillimeter measurements of the cosmic microwave background and the latter on spatial distribution of visible galaxies.

CfA astronomer Daniel Eisenstein was a member of a large consortium of scientists who suggest that most of the difference between these two methods, which sample different components of the cosmic fabric, could be reconciled if the dark energy were not constant in time. The scientists apply sophisticated statistical techniques to the relevant cosmological datasets and conclude that if the dark energy term varied slightly as the universe expanded (though still subject to other constraints), it could explain the discrepancy. Direct evidence for such a variation would be a dramatic breakthrough, but so far has not been obtained. One of the team's major new experiments, the Dark Energy Spectroscopic Instrument (DESI) Survey, could settle the matter. It will map over twenty-five million galaxies in the universe, reaching back to objects only a few billion years after the big bang, and should be completed sometime in the mid 2020's.


Repulsive gravity as an alternative to dark energy (Part 2: In the quantum vacuum)

(PhysOrg.com) -- During the past few years, CERN physicist Dragan Hajdukovic has been investigating what he thinks may be a widely overlooked part of the cosmos: the quantum vacuum. He suggests that the quantum vacuum has a gravitational charge stemming from the gravitational repulsion of virtual particles and antiparticles. Previously, he has theoretically shown that this repulsive gravity can explain several observations, including effects usually attributed to dark matter. Additionally, this additional gravity suggests that we live in a cyclic Universe (with no Big Bang) and may provide insight into the nature of black holes and an estimate of the neutrino mass. In his most recent paper, published in Astrophysics and Space Science, he shows that the quantum vacuum could explain one more observation: the Universe’s accelerating expansion, without the need for dark energy.

“The quantum vacuum was predicted theoretically more than 60 years ago,” Hajdukovic told PhysOrg.com. “Today, there is significant experimental evidence that the quantum vacuum exists. I have decided to combine one reality (the quantum vacuum) with one hypothesis (the negative gravitational charge of antiparticles) and to study the consequences. The hypothesis of the gravitational repulsion between matter and antimatter is older than half a century, but before me no one has used it in the combination with the quantum vacuum. . The results are surprising there is potential to explain [the Universe’s accelerating expansion] in the framework of the quantum vacuum enriched with the gravitational repulsion between matter and antimatter.”

According to Hajdukovic, gravity in the quantum vacuum arises from the gravitational repulsion between the positive gravitational charge of matter and the (hypothetical) negative gravitational charge of antimatter. While matter and antimatter are gravitationally self-attractive, they are mutually repulsive. (This part is similar to Massimo Villata’s theory from part 1, in which negatively charged antimatter exists in voids rather than in the quantum vacuum.) Although the quantum vacuum does not contain real matter and antimatter, short-lived virtual particles and virtual antiparticles could momentarily appear and form pairs, becoming gravitational dipoles.

“If particles and antiparticles have gravitational charges of the opposite sign, a sufficiently strong gravitational field can convert a virtual pair into a real one,” Hajdukovic explained. “It is not a new hypothesis but a consequence of the Schwinger mechanism, well known in quantum field theories.”

In the new paper, Hajdukovic calculates that the energy density of the gravitational dipoles in the quantum vacuum is the correct order of magnitude to act as the cosmological constant, or the force causing the Universe’s accelerating expansion. While this agreement may not seem that remarkable at first, it becomes impressive in the context of the much less agreeable predictions of quantum field theory, which predicts the energy density of the quantum vacuum to be at least 30 - and up to 120 - orders of magnitude larger than the observed dark energy density. Hajdukovic’s calculations also estimate that the Universe’s expansion began accelerating when the Universe was about half of its present size, which is only slightly earlier than the prediction of standard cosmology.

Interestingly, one significant difference between Hajdukovic’s quantum vacuum model and standard cosmology is that the former predicts that the acceleration is decreasing, while the latter predicts it is increasing. Very different predictions for the fate of the Universe result from these differences.

“The series of publications shows that the quantum vacuum, enriched with the hypothesis of the negative gravitational charge for antiparticles, has the potential to explain the observed phenomena in astrophysics and cosmology without invoking dark matter and dark energy and mysterious mechanisms for inflation and matter-antimatter asymmetry,” Hajdukovic said. “If antimatter really has negative gravitational charge (which could be revealed by the AEGIS experiment at CERN), the above papers have started a new scientific revolution. But the papers are important even if antimatter has no negative gravitational charge, because they encourage reconsidering the quantum vacuum as a key for the understanding of the Universe.”

In addition to the AEGIS experiment in CERN, which is designed to reveal the gravitational properties of antihydrogen, Hajdukovic said that other experiments are also investigating the gravitational properties of antimatter. For instance, physicists at the University of California, Riverside, have recently begun studying the gravitational properties of positronium (an electron-positron pair).


Harvard Study Provides New Insights on Dark Energy

Different methods of studying cosmic expansion yield slightly different results, including for the age of the universe. In a new study, astronomers from the Harvard-Smithsonian Center For Astrophysics have calculated that these discrepancies could be reconciled if the dark energy that drives cosmic acceleration were not constant in time.

The universe is not only expanding – it is accelerating outward, driven by what is commonly referred to as “dark energy.” The term is a poetic analogy to label for dark matter, the mysterious material that dominates the matter in the universe and that really is dark because it does not radiate light (it reveals itself via its gravitational influence on galaxies). Two explanations are commonly advanced to explain dark energy. The first, as Einstein once speculated, is that gravity itself causes objects to repel one another when they are far enough apart (he added this “cosmological constant” term to his equations). The second explanation hypothesizes (based on our current understanding of elementary particle physics) that the vacuum has properties that provide energy to the cosmos for expansion.

For several decades cosmologies have successfully used a relativistic equation with dark matter and dark energy to explain increasingly precise observations about the cosmic microwave background, the cosmological distribution of galaxies, and other large-scale cosmic features. But as the observations have improved, some apparent discrepancies have emerged. One of the most notable is the age of the universe: there is an almost 10% difference between measurements inferred from the Planck satellite data and those from so-called Baryon Acoustic Oscillation experiments. The former relies on far-infrared and submillimeter measurements of the cosmic microwave background and the latter on spatial distribution of visible galaxies.

CfA astronomer Daniel Eisenstein was a member of a large consortium of scientists who suggest that most of the difference between these two methods, which sample different components of the cosmic fabric, could be reconciled if the dark energy were not constant in time. The scientists apply sophisticated statistical techniques to the relevant cosmological datasets and conclude that if the dark energy term varied slightly as the universe expanded (though still subject to other constraints), it could explain the discrepancy. Direct evidence for such a variation would be a dramatic breakthrough, but so far has not been obtained. One of the team’s major new experiments, the Dark Energy Spectroscopic Instrument (DESI) Survey, could settle the matter. It will map over twenty-five million galaxies in the universe, reaching back to objects only a few billion years after the big bang, and should be completed sometime in the mid 2020’s.

Publication: Gong-Bo Zhao, et al., “Dynamical Dark Energy in Light of the Latest Observations,” Nature Astronomy 1, 627–632 (2017) doi:10.1038/s41550-017-0216-z



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