“The Hubble tension between the early and late universe may be the most exciting development in cosmology in decades,” says Nobel laureate Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University. New Hubble Space Telescope data suggests a faster expansion rate in the modern universe than expected based on how the universe appeared more than 13 billion years ago, strengthening the case that new theories may be needed to explain the dark energy forces that have shaped the cosmos.
In May, 2019, The Daily Galaxy reported on mind-boggling conjectures made by astrophysicists who have found that for the last 7 billion years or so something is pushing the galaxies, adding energy to them. That something they are calling “dark energy,” a force that is real, but so far eludes detection.
The universe is getting bigger every second. The space between galaxies is stretching, like dough rising in the oven. But how fast is the universe expanding? As Hubble and other telescopes seek to answer this question, they have run into an intriguing difference between what scientists predict and what they observe.
The conflict, reports Anil Ananthaswamy for New Scientist, arises from two different ways of measuring the present-day expansion of the universe, the Hubble constant, H0. One method involves measuring it directly by studying stars and supernovae in nearby galaxies. The other involves examining the cosmic microwave background, the universe’s first light that was emitted about 380,000 years after the big bang, and then extrapolating from that data to the present-day universe. The measurements of the early universe come from the European Space Agency’s Planck satellite. The two methods generate significantly different results.
This discrepancy has been identified in scientific papers over the last several years, but it has been unclear whether differences in measurement techniques are to blame, or whether the difference could result from unlucky measurements. “This mismatch has been growing and has now reached a point that is really impossible to dismiss as a fluke. This disparity could not plausibly occur just by chance,” stresses Riess.
The view from the Hubble Constant is that in some distant future of our universe, long after the sun grows to engulf Earth and then shrinks into a dim remnant of its former self, clusters of once-neighboring galaxies will begin zooming away from each other so fast that even light won’t be able to bridge the gap, stars will flicker out and die, and darkness will pervade the cosmos.
But to Harvard string theorist Cumrun Vafa, there is a reason to doubt dark energy—that is, dark energy in its most popular form, called the cosmological constant. The idea originated in 1917 with Einstein and was revived in 1998 when astronomers discovered that not only is spacetime expanding, but the rate of that expansion is picking up. The cosmological constant would be a form of energy in the vacuum of space that never changes and counteracts the inward pull of gravity. But it is not the only possible explanation for the accelerating universe.
An alternative is “quintessence,” an emerging relative of the Higgs field that permeates the cosmos that evolves. “Regardless of whether one can realize a stable dark energy in string theory or not, it turns out that the idea of having dark energy changing over time is actually more natural in string theory,” Vafa says. “If this is the case, then one can measure this sliding of dark energy by astrophysical observations currently taking place.”
So far all astrophysical evidence supports the cosmological constant idea, but there is some wiggle room in the measurements. Upcoming experiments such as Europe’s Euclid space telescope, NASA’s Wide-Field Infrared Survey Telescope (WFIRST) and Chile’s Simmons Observatory being built in the desert will look for signs that dark energy was stronger or gradually diminish over tens of billions of years –the cosmic acceleration is gradually changing, as in quintessence models.
Vafra suggests a dark energy density that instead of being constant, is slowly decreasing with time. If so, that would have profound consequences, says string theorist Timm Wrase at the Vienna University of Technology in Austria: “It certainly has huge implications for the fate of the universe.” Over the coming tens of billions of years, dark energy may go to zero, or even become negative. “And then maybe the universe would end in a big crunch, instead of expanding forever.”
In cosmology, reports Physics World, quintessence is a real form of dark energy distinct from any normal matter or radiation, or even “dark matter”. Its bulk properties – energy density, pressure and so forth – lead to novel behavior and unusual astrophysical phenomena. So far its existence has only been inferred indirectly from a range of observations, but a number of current and planned experiments will make direct searches for this elusive form of energy.
“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, the new field simply switched off, leaving no trace other than a speeded-up universe, says a team of astronomers from Johns Hopkins University led by Riess. 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.”
Reiss’s paper explains that if the new exotic matter takes the form of a cosmological constant (like that required to explain the accelerated cosmic expansion in the universe today), agreement can be achieved between Cosmic Microwave Background (CMB) measurements and theoretical expectations in the standard model using supernovae. In fact, the data seem to fit together slightly better with the early dark energy theory. As the paper shows, more precise measurements of the CMB in the future should further test the newly proposed scenario.
The early dark energy resembles that seen in the universe today, although with a density nearly 10 billion times as large. These observations suggest that the universe may undergo episodic periods where dark energy becomes important, and if so, the dark energy in the current universe may be just the latest incarnation.
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, eventually stop expanding and contract in either a Big Crunch or Big Bounce.
Whereas the de Sitter conjecture requires the dark energy density to get smaller with time, says Wrase, the Hubble constant problem is resolved only if this density increases, suggesting that a big rip, rather than a big crunch, is the ultimate fate of the universe.
“That’s the horror-show version,” says Riess. Eventually, there would be so much dark energy in each bit of space-time that its repulsive force would shred everything: galaxies, planets, molecules, atoms and, eventually, space-time itself. “Resisting dark energy would be futile,” he says.
So we are back to square one, seemingly, with conflicting ideas suggesting dark energy is decreasing, staying constant or increasing with time, and the fate of the universe on the line. And that is where Vafa believes there might be a way of reconciling the de Sitter conjecture with the H0 discrepancy.
“Alongside dark energy, another invisible component of the universe is dark matter, a substance whose gravity is thought to be holding galaxies and clusters of galaxies together. If dark energy is losing strength,” says Vafa, “this will have an impact on dark matter. String theory tells you that there should be an interaction.”
Vafa and his colleagues have found that the interaction causes the mass of dark matter particles to decrease over time, which changes the extrapolated value of the Hubble constant, causing the discrepancy to shrink. “We were not trying to resolve the H0 tension, but it reduced it nevertheless,” he says. “That’s quite a non-trivial thing, implying that, tens of billions of years from now, the universe will be radically transformed.”
“What that typically means in string theory is that a new dimension opens up. So it’s a completely new universe, which is not describable in terms of the language of our current universe,” he says. “We now live in three space dimensions. In this new theory, it might be four spatial dimensions, for example.”
“A completely new phase takes over,” says Vafa. So, while the universe continues to exist, it is unclear what properties it will have and what its subsequent trajectory will be. “More details need to be known to say what may happen,” says Vafa. “We have no way of knowing accurately what this new phase may look like.”
Image at the top of the page shows the galaxy MCG+01-02-015 which has no other galaxies surrounding it for approximately 100 million light-years in all directions. ESA/HUBBLE & NASA AND N. GORIN (STSCI); ACKNOWLEDGEMENT: JUDY SCHMIDT