Cosmologists speculate based on hints of twisted light that offer clues to an exotic substance called “quintessence”that could be the unknown force accelerating the expansion of the universe, with implications not only for cosmology, but also for fundamental physics — the standard model of particle physics does not predict its existence. This hypothetical form of dark energy –a relative of the Higgs Field that permeates the cosmos– could be an exotic unknown form of dark energy distinct from any form of or normal matter, radiation, or even dark matter.
“A Fifth Force–Quintessence Field?”
“They suggest,” reports Davide Castelvecchi in Nature, “that the twisting of light, which they identified in data on the cosmic microwave background (CMB) the glow of “relic radiation” collected by the Planck space telescope, and the acceleration of the Universe could be produced by a cosmic ‘quintessence’, an exotic substance that pervades the cosmos — “a quintessence field, after the fifth element, or ether — the name that ancient Greek philosophers gave to an invisible material thought to fill all the empty space in the universe– a ‘fifth’ force beyond the four already known – gravitational, electromagnetic, and the strong and weak nuclear forces. However, researchers think this fifth force may be ‘screened’ or ‘hidden’ for large objects like planets or weights on Earth, making it difficult to detect.”
“The discovery of dark energy has greatly changed how we think about the laws of nature,” said Edward Witten, one of the world’s leading theoretical physicist at the Institute for Advanced Study in Princeton, N.J. who has been compared to Newton and Einstein, about the unknown force that is causing the universe to expand at an accelerating rate– a ‘fifth’ force that acts on matter.
Persistence of Dark Energy
“The relevant fact about dark energy isn’t its pressure,” says Caltech physicist Sean Carroll in his Preposterous Universe blog, “is it’s that it’s persistent. It doesn’t dilute away as the universe expands. And this is even a fact that can be explained, by saying that dark energy isn’t a collection of particles growing less dense as space expands, but instead is (according to our simplest and best models) a feature of space itself. The amount of dark energy is constant throughout both space and time: about one hundred-millionth of an erg per cubic centimeter. It doesn’t dilute away, even as space expands.”
Enter Dynamic Quintessence
This new, controversial quintessence theory suggests that this dark energy might be getting stronger and denser, leading to a future in which atoms are torn asunder and time ends. In stark contrast, the accepted theory described above by Sean Carroll, is that dark energy is an intrinsic property of space, which would mean that the amount of dark energy per unit volume of space is fixed as a ‘cosmological constant’. In other words, the density of dark energy is constant, which means the curvature of spacetime is constant, which means that the universe expands at a fixed rate.
Quintessence, which posits that the vast majority of the energy in the universe is in the form of a hitherto undiscovered substance, reports Physics World, has the striking physical characteristic that it causes the expansion of the universe to speed up. Most forms of energy, such as matter or radiation, cause the expansion to slow down due to the attractive force of gravity. For quintessence, however, the gravitational force is repulsive, and this causes the expansion of the universe to accelerate.
Strange New Energy Field Switches On
“Long, long ago, when the universe was only about 100,000 years old — a buzzing, expanding mass of particles and radiation — a strange new energy field switched on,” writes Dennis Overbye for New York Times Science. “That energy suffused space with a kind of cosmic antigravity, delivering a not-so-gentle boost to the expansion of the universe.”
Then, after another 100,000 years or so, observes Overbye, the new field simply switched off, leaving no trace other than a speeded-up universe, according to a team of astronomers from Johns Hopkins University led by Adam Riess, a Bloomberg Distinguished Professor and Nobel laureate who is an expert in the Hubble constant. In a bold and speculative leap into the past, the team has posited the existence of this field to explain a baffling astronomical puzzle: the universe seems to be expanding faster than it should be.
“A growing mystery about the universe, known as the ‘Hubble Tension,’ is that it appears to be expanding much faster now than predicted even with our latest understanding of its initial conditions and contents,” says Riess. Their research is the first to provide a possible explanation—”that the early universe received an infusion of dark energy soon after the Big Bang giving it a boost—which better matches all observations. This theory shows how this ‘tension’ may actually be revealing a new feature of the universe. It also makes predictions which can be tested so that more measurements should tell us if it is correct.”
“We now know that the accelerated expansion of the universe did not start until sometime in the last 10 billion years,” says Robert Caldwell, a cosmologist at Dartmouth College, one of the first researchers to propose the existence of quintessence as dark energy.
“Such a discovery would require a major revision of current theories,” writes Castelvecchi. “But physicists warn that the evidence is tentative, failing to meet the ‘5 sigma’ threshold used to determine whether a signal is a discovery.”
“We Have Zero Idea about How the Universe is Going to End”
“If dark energy is a quintessence, its push on the expansion could slowly wither or disappear, or could even reverse to become an attractive force, causing the Universe to collapse into a ‘big crunch’, says Sean Carrol. “We’re back to a situation where we have zero idea about how the universe is going to end.”
The first direct evidence that an unknown force was pushing cosmic expansion to accelerate emerged in 1998, from two separate surveys of supernovae. A host of other studies have since confirmed the presence of this force, dubbed dark energy, but have provided precious little information about its nature.
“Dark energy is incredibly strange, but actually it makes sense to me that it went unnoticed,” said Noble Prize winning physicist Adam Riess, who was not part of the current study, in an interview with The Atlantic. “I have absolutely no clue what dark energy is. Dark energy appears strong enough to push the entire universe – yet its source is unknown, its location is unknown and its physics are highly speculative.”
“Quintessence is Dynamic, Time-Evolving, and Spatially Dependent”
Unlike the cosmological constant, quintessence “is a tangible medium and it has fluctuations of its own”, says Robert Caldwell, a cosmologist at Dartmouth College in Hanover, New Hampshire, who was one of the first researchers to propose the material’s existence in a 1998 paper by Robert R. Caldwell, Rahul Dave and Noble Prize laureate Paul Steinhardt. “It differs from the cosmological constant explanation of dark energy,” says Caldwell, “in that it is dynamic, and changes over time. Quintessence can be either attractive or repulsive depending on the ratio of its kinetic and potential energy, becoming repulsive about ten billion years ago, about 3.5 billion years after the Big Bang.”
“Quintessence,” says Caldwell, “is a dynamic, time-evolving, and spatially dependent form of energy with negative pressure sufficient to drive the accelerating expansion. Whereas the cosmological constant is a very specific form of energy vacuum energy quintessence encompasses a wide class of possibilities.”
Quintessence could have properties that are intermediate between those of matter and of a cosmological constant, Caldwell adds. “As the universe expands, a cosmological constant would maintain a constant density, whereas the density of quintessence would decrease — although not as fast as the density of matter, which drops as galaxies spread out.”
In 1998, reports Castelvecchi, “Carroll proposed an experimental test for quintessence, based on the prediction that it alters how light propagates in space. A group led by the theoretical physicist Marc Kamionkowski, now at Johns Hopkins University, then calculated how this effect could be measured in the CMB, the primordial radiation often described as the afterglow of the Big Bang. The researchers suggested that it would be possible to detect signs of quintessence by looking at maps of polarized light across the CMB. Light is polarized when its electric field ‘wiggles’ in a particular direction, rather than in a random one. The theory says that quintessence twists the direction in which the polarization points, in a way that could be detected by looking at polarization across the whole sky.”
Fast forward to today, two cosmologists — Yuto Minami at the High Energy Accelerator Research Organization (KEK) and Eiichiro Komatsu at the Max Planck Institute for Astrophysics — have identified that CMB signature in data from the European Space Agency’s Planck mission to map tiny variations in the CMB’s temperature across the sky, and measure the radiation’s polarization, which concluded in 2013.
Minami and Komatsu were able to detect signs of quintessence using a new technique that they reported in 2019. Their results, reports Nature, “differ from those of other groups, which have looked at CMB polarization maps — including Planck’s — and found no twist,” says physicist Suzanne Staggs at Princeton University in New Jersey, whose team measures CMB radiation using the Atacama Cosmology Telescope (ACT) in Chile. Staggs’s team plans to try out Minami and Komatsu’s technique on ACT data. “We are interested in exploring it,” she says.
“If it’s Real, it’s Big”
The paper is “quite a nice analysis”, but noise in the Planck signals could be a complicating factor, says George Efstathiou, a leading Planck cosmologist at the University of Cambridge, UK. “If it were real, it’s big,” says Carroll. But he notes that the statistical significance — only 2.5 sigma — of the result is weak, and says that such results often fade away on further scrutiny.
“The rock that they’re standing on is the cosmological constant. If you change that rock, that could have an effect on everything else,” says Caldwell.
“I think we’ll probably want to be going through all that very carefully before getting too worked up,” says Kamionkowski. He adds that “the existence of quintessence would have implications not only for cosmology but also for fundamental physics: the standard model of particle physics does not predict any kind of quintessence.”
Other efforts, reports Nature, are in the works to map the CMB polarization with greater accuracy than ever before, and will put a stringent test on quintessence. These projects include the Simons Observatory, another CMB experiment now being set up in the Atacama Desert, and a future Japanese-led space probe called LiteBIRD.
To end on more speculative, fun note, Caleb Scharf at Columbia University takes a hint from Albert Einstein who was fond of saying that “Imagination is everything. It is the preview of life’s coming attractions.” Perhaps some hyper-advanced extraterrestrial life five billion years ago figured out how to switch on the Higgs field. Scharf, who works in the fields of exoplanetary science and astrobiology, proposes in Is Physical Law an Alien Intelligence? that part of the fabric of the universe is a product of intelligence or is perhaps even life itself. Perhaps, Scharf concludes, “hyper-advanced life isn’t just external. Perhaps it’s already all around. It is embedded in what we perceive to be physics itself, from the root behavior of particles and fields to the phenomena of complexity and emergence. In other words, life might not just be in the equations. It might be the equations.”
Impossible? Far fetched? Perhaps not, said Princeton’s great quantum physicist, John Archibald Wheeler, who, near the end of his life, suggested that when we finally unravel the secret of the universe, of human existence, we will be astounded by its simplicity.
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