The universe’s dark energy may be growing stronger with time, study suggests
Brett Molina, USA TODAYPublished 10:42 p.m. ET Jan. 31, 2019 | Updated 6:14 a.m. ET Feb. 1, 2019
While we aren’t really sure what dark matter and dark energy are, the final data released from ESA’s Planck mission confirms it apparently does exist. Buzz60
Dark energy, a mysterious invisible force believed to play a role in how the universe expands, may be growing stronger over time, according to a new study.
Dark energy, discovered 20 years ago by scientists measuring the distances to supernovas, or exploding stars, is described as an energy of empty space that never changes over space and time. Researchers believe it represents about 70 percent of the total universe.
Using data from NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton observatory, researchers found the expansion rate of the universe is different from the model using supernovas.
“We observed quasars back to just a billion years after the Big Bang, and found that the universe’s expansion rate up to the present day was faster than we expected,” Guido Risalti, a study co-author from the department of physics and astronomy at the University of Florence in Italy, said in a statement. “This could mean dark energy is getting stronger as the cosmos grows older.”
Elisabeta Lusso of Durham University in the United Kingdom said because this technique for assessing dark energy is new, researchers took extra steps to make sure it was a reliable way to measure. “We showed that results from our technique match up with those from supernova measurements over the last 9 billion years, giving us confidence that our results are reliable at even earlier times,” she said.
Researchers say they used quasars to measure because they have a much farther reach compared to supernovas.
Adam Riess, a professor of physics and astronomy at Johns Hopkins University, said while the discovery would be “a really big deal” if confirmed, quasars have not proven to be historically reliable.
“People have not really used them as precision measuring tools for the universe because they have a very large dynamic range,” said Riess. “We don’t have a lot of confidence when we see one, we know how luminous it ought to be.”
Robert Kirshner, a Clowes Research Professor of Science, Emeritus at Harvard University, said that while the results of the study could prove true, there is no other evidence to date showing dark energy has changed with time.
“The thing that’s attractive about (their work) is that quasars are brighter, so you can see them farther back,” said Kirshner. “But you do worry the quasars from the early universe are not quite the same as the ones nearby.”
Aliens May Be Rearranging Stars to Fight Dark Energy, Awesome Study Suggests
By Brandon Specktor, Senior Writer |
How to dominate the universe in three easy steps …
Step 1: Harvest all of your planet’s resources.
Step 2: Harvest all of your nearest star’s energy.
Step 3: Harvest all the energy from all the stars in your local galaxy; then move on to another galaxy.
Congratulations! Your species now has all the elbow room it needs to grow into a universal superpower.
That’s one Russian astronomer’s perspective, anyway. Astrophysicist Nikolai Kardashev first proposed these three phases (called Level I, II and III) of galactic expansion — which he referred to as the three “types” of technologically advanced civilizations — in 1962 as a way to measure the energy consumption of increasingly powerful societies. Recently, a paper posted June 13 to the preprint journal arXiv.org has revived Kardashev’s model and added a new, apocalyptic twist.
According to the author of the paper, Dan Hooper — a senior scientist at the Fermi National Accelerator Laboratory in Illinois and a professor of astronomy and astrophysics at the University of Chicago — harvesting energy from distant stars isn’t just the best way to increase a civilization’s available resources. It’s also the only way to prevent the ever-expanding universe from leaving that civilization totally alone in the vastness of space. (This study has yet to be peer-reviewed.)
“The presence of dark energy in our universe is causing space to expand at an accelerating rate,” Hooper wrote in the new paper. Over the next approximately 100 billion years, the stars beyond our Local Group, or a group of gravitationally bound galaxies that includes the Milky Way, will fall beyond the cosmic horizon, meaning an observer here could never retrieve information from them over the course of the age of the universe.
At that point, “the stars become not only unobservable, but entirely inaccessible, thus limiting how much energy could one day be extracted from them,” Hooper wrote in the paper.
Any advanced civilization worth their starships would understand the grim reality of universal expansion, Hooper wrote, and they wouldn’t just sit around idly while the universe literally passed them by. Rather, they would capture stars from other galaxies, reel them in and harvest their energy first, before those stars (and their energy) became inaccessible forever.
“Given the inevitability of the encroaching horizon, any sufficiently advanced civilization that is determined to maximize its ability to utilize energy will expand throughout the universe, attempting to secure as many stars as possible before they become permanently inaccessible,” Hooper wrote.
So, how do you lasso a star in the first place? Scientists and science-fiction authors alike have pondered this question for decades, and their favored answer is this: Throw a giant net around it, of course.
This net wouldn’t be made of twine or even metal, but of satellites — a swarm of millions of solar-powered satellites known as “Dyson spheres.” Such a colossal cloud of harvesters could permanently hover around a star, beaming energy back to a nearby planet — or, as Hooper proposed in his new paper, actually use that star’s energy to accelerate the whole ball of fire back toward the planet that wanted to use it.
This may seem like a tall order for humans, who are still bumbling around Level I of Kardashev’s scale. (Carl Sagan placed us at about a 0.7 in 1973). But some scientists think there could be alien civilizations thousands, or even millions, of years older than ours who are already well into their Level III, star-harvesting phase.
And if another civilization has indeed begun rearranging the stars, it may not be long before Earthlings notice them, Hooper wrote.
“Those stars that are currently en route to the central civilization could be visible as a result of the propulsion that they are currently undergoing,” Hooper wrote. “Such acceleration would necessarily require large amounts of energy and likely produce significant fluxes of electromagnetic radiation.”
Redecorating the galaxy
Beyond watching for those stars being dragged unceremoniously across distant galaxies, astronomers could also keep an eye out for the unusual galaxies that have had their prime stars ripped away from them, Hooper wrote.
These hypothetical, star-harvesting aliens will probably be picky, Hooper noted: Teeny-tiny stars, hundreds of times smaller than Earth’s sun, wouldn’t produce enough radiation to be useful; significantly larger stars, on the other hand, would likely be too close to going supernova to be used as a viable battery. Only stars with a mass about 20 to 100 times the mass of our sun would be viable candidates for capturing and hauling back to the home galaxy, Hooper said. And because solar objects in that mass range radiate certain wavelengths of light more than others, alien star harvesting would show up in the light signatures from these galaxies.
“The spectrum of starlight from a galaxy that has had its useful stars harvested by an advanced civilization would be dominated by massive stars and thus peak at longer wavelengths than otherwise would have been the case,” Hooper said.
Humans likely don’t have precise enough instruments yet to detect these unusual light signatures beaming from the depths of the universe, Hooper wrote. Hopefully, astronomers will develop them before our sun becomes another flaming marble in some distant civilization’s collection.
Einstein with Edwin Hubble, in 1931, at the Mount Wilson Observatory in California, looking through the lens of the 100-inch telescope through which Hubble discovered the expansion of the universe in 1929. Courtesy of the Archives, Calif Inst of Technology.
In 1917, a year after Albert Einstein’s general theory of relativity was published—but still two years before he would become the international celebrity we know—Einstein chose to tackle the entire universe. For anyone else, this might seem an exceedingly ambitious task—but this was Einstein.
Einstein began by applying his field equations of gravitation to what he considered to be the entire universe. The field equations were the mathematical essence of his general theory of relativity, which extended Newton’s theory of gravity to realms where speeds approach that of light and masses are very large. But his math was better than he wanted to believe—his equations told him that the universe could not stay static: it had to either expand or contract. Einstein chose to ignore what his mathematics was telling him.
The story of Einstein’s solution to this problem—the maligned “cosmological constant” (also called lambda)—is well known in the history of science. But this story, it turns out, has a different ending than everyone thought: Einstein late in life returned to considering his disgraced lambda. And his conversion foretold lambda’s use in an unexpected new setting, with immense relevance to a key conundrum in modern physics and cosmology: dark energy.
The Static Universe Before Hubble
Einstein had what would have seemed a very good reason for ignoring what the math was telling him. Few people know that Einstein was not merely a superb theoretician, but also a physicist skilled in observations and experiments. In 1914, Einstein was wooing a young Scottish-German astronomer, Erwin Finlay Freundlich, to seek proof of relativity through shifts in apparent star locations during a total solar eclipse that was to take place in the Crimea (which ended badly because of the outbreak of World War I). Letters that Einstein wrote to Freundlich during 1913-4 reveal that Einstein had a burgeoning interest in astronomy and understood much about the field, including technical details of lenses and mirrors.* Ironically, his deep knowledge of astronomy would lead Einstein to make the greatest blunder of his entire career….Or not.
Astronomical knowledge of the time told Einstein that the universe was unchanging in its size. How could someone think that? Well, this was the second decade of the twentieth century, and telescopes were still relatively small and not very powerful. They were strong enough to allow astronomers to discover all the now-known planets in our solar system, to get good views of “cloudy patches” of the sky such as the Orion nebula, and to view several galaxies, including the Great Andromeda Galaxy—our nearest neighbor at 2.3 million light years’ distance.
But astronomers believed that all these fuzzy objects they were seeing were somehow part of our own Milky Way. (The great Eddington even believed at that time that the Sun was the center of this universe! And an idea about the distances to the most faraway stars only began to emerge through the work of Harlow Shapely on Cepheid variables, conducted at the Mount Wilson Observatory, in 1916.) Since astronomers could detect no expansion of stars or nebulas in the entire cosmos known to them, they assumed that the universe was static.
The Birth of the Cosmological Constant
To force his equations—which theoretically predicted the expansion of the universe—to remain still, Einstein invented the cosmological constant, λ. He multiplied the metric tensor in his equation, g, by the cosmological constant, leading to a term λg, which adjusted his metric tensor acting on space-time. This mathematical trick assured him that his equations would yield a universe that was prevented from expanding or contracting.
Unbeknownst to Einstein, at exactly the time he published his paper on the cosmological equations, across the world in California, the new 100-inch Hooker telescope was being fit in its place at the Mount Wilson Observatory. Within a little over a decade, Edwin Hubble, aided by Vesto Slipher and Milton Humason, would use this, the most powerful telescope on Earth, to study the redshift of distant galaxies and conclude from it definitively that our universe is expanding.
Einstein heard about these results, and in the early 1930s, he traveled to California and met with Hubble. At the Mount Wilson Observatory he saw the massive data set on distant galaxies that had led to “Hubble’s law” describing the expansion of the universe and got angry at himself: had he not forced his equations to stay static with that cosmological-constant invention of his, he could have theoretically predicted Hubble’s findings! That would have been worth a second Nobel Prize for him (he deserved a few more, anyway)—in the same way, for example, that the CERN scientists’ 2012 experimental discovery of the Higgs boson recently won Peter Higgs the Nobel in 2013. In disgust, Einstein exclaimed after his Mount Wilson visit: “If there is no quasi-static world, then away with the cosmological term!” and never considered the cosmological constant again. Or so we thought until recently.
Dark Energy: Lambda Returns
When a genius such as Einstein makes a mistake, it tends to be a “good mistake.” (I am indebted to the mathematician Goro Shimura for this expression.) It can’t simply go away—there is too much thought that has gone into it. So, like a phoenix, Einstein’s cosmological constant made a remarkable comeback, very unexpectedly, in 1998.
That year, two groups of astronomers made an announcement that rocked the world of science. The “Supernova Cosmology Project,” based in California and headed by Saul Perlmutter, and the “High-Z SN Search” group at Harvard-Smithsonian and Australia, announced their results of the shifts of distant galaxies leading to a conclusion that nobody had expected: The universe, rather than slowing its expansion since the Big Bang, is actually accelerating its expansion!
And it turns out that the best theoretical way to explain the accelerating universe is to revive Einstein’s discarded lambda. The cosmological constant (acting differently from how it was designed, as a force stopping the expansion) is the best explanation we have for the mysterious “dark energy” seen to permeate space and push the universe ever outward at an accelerating rate. To most physicists today, lambda, cosmological constant, and dark energy are closely synonymous. But unfortunately Einstein was not there to witness the reversal of his “greatest blunder,” having died in 1955.
And it has been widely assumed that he died without ever reconsidering the cosmological constant. Until now.
Einstein’s Lost Manuscript
The Irish physicist Cormac O’Raifeartaigh was perusing documents at the Einstein Archives at the Hebrew University in Jerusalem in late 2013 when he discovered a handwritten manuscript by Einstein that scholars had never looked at carefully before. The paper, called “Zum kosmologischen Problem” (“About the Cosmological Problem”), had been erroneously filed as a draft of another paper, which Einstein published in 1931 in the annals of the Prussian Academy of Sciences. But it was not. It seems that even with Einstein, old notions die hard: This paper was his stubborn attempt to resurrect the cosmological constant he had vowed never to use again.
In a paper just filed on the electronic physics repository ArXiv, O’Raifeartaigh and colleagues show that in the early 1930s (the assumed date is 1931, but this is uncertain), Einstein was still trying to return to his 1917 analysis of a universe with a cosmological constant. Einstein wrote (the authors’ translation from the German):
“This difficulty [the inconsistency of the laws of gravity with a finite mean density of matter] also arises in the general theory of relativity. However, I have shown that this can be overcome through the introduction of the so-called “λ–term” to the field equations… I showed that these equations can be satisfied by a spherical space of constant radius over time, in which matter has a density ρ that is constant over space and time.”
But he was now aware of Hubble’s discovery of the expansion of the universe:
“On the other hand, Hubbel’s [sic**] exceedingly important investigations have shown that the extragalactic nebulae have the following two properties 1) Within the bounds of observational accuracy they are uniformly distributed in space 2) They possess a Doppler effect proportional to their distance” (Quoted in O’Raifeartaigh, et al., 2014, p. 4)
And so Einstein proposed a revision of his model, still with a cosmological constant, but now the constant was responsible for the creation of new matter as the universe expanded (because Einstein believed that in an expanding universe, the overall density of matter had to still stay constant):
“In what follows, I would like to draw attention to a solution to equation (1) that can account for Hubbel’s facts, and in which the density is constant over time.” And: “If one considers a physically bounded volume, particles of matter will be continually leaving it. For the density to remain constant, new particles of matter must be continually formed in the volume from space.”
Einstein achieves this property by the use of his old cosmological constant, λ:
“The conservation law is preserved in that by setting the λ-term, space itself is not empty of energy; as is well-known its validity is guaranteed by equations (1).” (Quoted in O’Raifeartaigh, et al., 2014, p. 7.)
So Einstein keeps on using his discarded lambda—despite the fact that he invented it for a non-expanding universe. If the universe expands as Hubble showed, Einstein seems to be saying, then I still need my lambda—now to keep the universe from becoming less dense as it expands in volume.
Almost two decades later, a similar “steady state” universe would be proposed by Fred Hoyle, Hermann Bondi, and Tommy Gold, in papers published in 1949. But these models of the universe are not supported by modern theories. In fact, a tenet of modern cosmology is that as the universe will expand a great deal (after an unimaginably long period of time), it will become very thinly populated, rather than dense, with stray photons and electrons zipping alone through immense expanses of emptiness, all stars having by then died and disappeared.
Views of the Cosmos, Old and New
As for why Einstein was so intent on maintaining the use of his discarded lambda, the constant represents the energy of empty space—a powerful notion—and Einstein in this paper wanted to use this energy to create new particles as time goes on.
Today we view the same energy of the vacuum as the reason for the acceleration of the universe’s expansion. Einstein presciently understood that the energy of the vacuum, unleashed by his cosmological constant, was too important to let die.
Einstein was far from the only person to wonder about the universe and whether it has always existed or was born at some point in the past and would die at a future time. This question has been pondered by people ever since the dawn of civilization. The origin and ultimate fate of the universe are highly interlinked with its overall geometry—the actual shape of the space-time manifold. In a closed geometry, the universe was born and will someday recollapse on itself. In an open geometry, it was born and will expand forever, and the same happens in a flat (Euclidean) geometry. Based on modern theories supported by satellite observations of the microwave background radiation in space, space-time is nearly perfectly Euclidean, meaning that the universe was born in a Big Bang and will expand forever, becoming less dense with time. Eventually, matter may decay into few kinds of elementary particles and photons, the distances among them growing to infinity.
Cosmology in Context
Between 1917 and 1929—the year Hubble and his colleagues discovered the expansion of the universe, implying the possibility of a beginning for the cosmos—Einstein and most scientists held that the universe was “simply there” with no beginning or end. But it’s interesting to note that creation myths across cultures tell the opposite story. Traditions of Chinese, Indian, pre-Colombian, and African cultures, as well as the biblical book of Genesis, all describe (clearly in allegorical terms) a distinct beginning to the universe—whether it’s the “creation in six days” of Genesis or the “Cosmic Egg” of the ancient Indian text the Rig Veda.
This is an interesting example of scientists being dead wrong (for a time) and primitive ancient observers having an essentially correct intuition about nature. And with the present explosion of models of the universe and sometimes outrageous “scientific speculations” about its origin and future, some commentators are clearly overstating what science has done. One recent example is the book by the physicist Lawrence M. Krauss, A Universe From Nothing, which claims that science has shown that the universe somehow sprang out of sheer nothingness.***
A century ago, Einstein’s powerful field equations of gravitation showed the way forward. His uncanny intuition about the universe prevailed despite temporary reversals, and his decades-old insights are now at the cutting edge of modern physics and cosmology, helping us shed light on the greatest mysteries of all: the nature of matter, gravity, time, space, and the mysterious dark energy pushing it all outwards.
But in the new study, Massimo Villata, an astrophysicist at the Observatory of Turin in Italy, suggests the effects attributed to dark energy are actually due to a kind of “antigravity” created when normal matter and antimatter repel one another.
“Usually this repulsion is ascribed to a mysterious dark energy that would uniformly permeate the cosmos, but nobody knows what it is nor why it behaves this way,” Villata said in an email.
“We are replacing an unknown force caused by an unknown element with the repulsive gravity of the well-known antimatter.”
Antimatter Hiding in “Holes” in the Universe?
According to Villata, the keys to accelerated expansion lie in large-scale voids that are seen scattered throughout the cosmos.
These holes in our map of the universe—which can each be millions of light-years wide—are inexplicably empty of galaxies and galaxy clusters. The nearest hole to us is called the Local Void, bordering the Virgo supercluster of galaxies.
Villata thinks these voids harbor vast quantities of antimatter, which could even be organized into antimatter galaxies, complete with antimatter stars and planets.
All this antimatter doesn’t emit radiation that can be detected by current sensors, making it effectively invisible, Villata said.
“There can be various reasons why antimatter in voids should be invisible, but we do not know which of them is the right one,” Villata said. “Moreover, antimatter in laboratories could have different behavior, since it is ‘immersed’ in a world of matter.”
While we can’t see antimatter superstructures, we can observe their effects on our visible universe, Villata argues, because antimatter must repel the normal matter in galaxies, pushing them farther from one another.
According to standard physics, matter and antimatter particles should have been created in equal amounts during the big bang. Yet the visible universe appears to be dominated by structures made up of normal matter.
To determine how much antimatter might be contained in the Local Void, Villata calculated how much would be needed to create a repulsive force strong enough to explain the so-called Local Sheet. This collection of normal matter, which includes our Milky Way and other nearby galaxies, is all moving at the same speed.
“If each void contains a mass of antimatter similar to that calculated for our Local Void … then our universe would host an amount of antimatter equivalent to that of matter, and [there] would finally be a matter-antimatter symmetric universe,” Villata said.
But Do Matter and Antimatter Repel?
While Villata’s theory doesn’t require mysterious forces created from nothing, it does rely on the untested assumption that matter and antimatter are mutually repulsive.
There is as yet “no [experimental] evidence that antimatter repels matter,” said physicist Frank Close of the University of Oxford in the U.K., although, he added, plans are underway at the European Organization for Nuclear Research (CERN) in Switzerland to test the idea.
In fact, Dragan Hajdukovic, a physicist at CERN, recently proposed a separate antigravity theory that also relies on repulsion between matter and antimatter to explain dark energy and dark matter.
Hajdukovic called Villata’s theory “an interesting idea,” be he added that he disagrees with the hypothesis of a matter-antimatter symmetric universe.
“The major problem is why [such] big quantities of antimatter in the voids are not observed,” Hajdukovic said.
In Hajdukovic’s theory, antimatter particles spontaneously pop in and out of existence in the quantum vacuum, which is the name physicists give to seemingly empty space.
“I use the reality of the quantum vacuum. For a physicist, it is more natural and plausible,” Hajdukovic said.
“In order to explain the invisibility of antimatter, proponents of a matter-antimatter symmetric universe would be forced to invoke an additional hypothesis”—such as the emission by antimatter of so-called advanced photons, which travel backward in time and so wouldn’t be detectable to current instruments.
“It is not a good sign for a theory if one hypothesis immediately demands introduction of other hypotheses.”
But study author Villata argues that the assumptions in his theory—including matter-antimatter repulsion and advanced photons—are predicted by well-established theories in physics.
In that sense, he said, there is “no introduction of other hypotheses.”