There’s a crisis brewing in the cosmos. Measurements over the past few years of the distances and velocities of faraway galaxies don’t agree with the increasingly controversial “standard model” of the cosmos that has prevailed for the past two decades. Astronomers think that a 9 percent discrepancy in the value of a long-sought number called the Hubble Constant, which describes how fast the universe is expanding, might be revealing something new and astounding about the universe.
The cosmos has been expanding for 13.8 billion years and its present rate of expansion, known as the Hubble constant, gives the time elapsed since the Big Bang. However, the two best methods used to measure the Hubble constant do not agree, suggesting our understanding of the structure and history of the universe – called the ‘standard cosmological model’ – may be wrong.
There was, writes Dennis Overbye in New York Times Science, a disturbance in the Force: “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. 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 winked off, leaving no trace other than a speeded-up universe.”
This reports Overbye, is the strange-sounding story being promulgated by a handful of astronomers from Johns Hopkins University. In a bold and speculative leap into the past, the team has posited the existence of this field to explain an astronomical puzzle: the universe seems to be expanding faster than it should be.”
Adding to the current scrum, there already is a force field — called dark energy — making the universe expand faster. A new, controversial report suggests that this dark energy might be getting stronger and denser, leading to a future in which atoms are ripped apart and time ends. Amen.
The concept of dark energy emerging in phases hints at a link to, or between, two mysterious episodes in the history of the universe, with the first episode occurring when the universe was less than at Planck scale a trillionth of a trillionth of a second old. At that moment, a fraction of a trillionth of a second of the Big Bang, this event — named “inflation” by the cosmologist Alan Guth, of M.I.T. — smoothed and flattened the initial chaos into the universe observed today.
But, Overbye observes: “Nobody knows what drove inflation.”
The second episode is unfolding today: cosmic expansion is speeding up. But, as Dr. Adam Riess, Professor of Astronomy and Physics at the Johns Hopkins University and a Senior member of the Science Staff at the Space Telescope Science Institute, said, “This is not the first time the universe has been expanding too fast.” But why? The issue came to light in 1998, when Riess led a study for the High-z Team which provided the first direct and published evidence that the expansion of the universe was accelerating and filled with dark energy.
“Dark energy is incredibly strange, but actually it makes sense to me that it went unnoticed,” said the Noble Prize winning Riess in an interview. “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.”
The two competing 1998 teams asked whether the collective gravity of the galaxies might be slowing the expansion enough to one day drag everything together into a Big Crunch. What they discovered, however, was the opposite: the expansion was accelerating under the influence of an anti-gravitational force later called dark energy. The two teams won a Nobel Prize.
“Until the 1990s, there were few reliable observations about movement at the scale of the entire universe, which is the only scale dark energy effects. So dark energy could not be seen until we could measure things very, very far away.”
Before his and his colleagues’ discovery, many scientists had posited the rate at which the universe was expanding was decreasing. Riess was awarded the Nobel Prize in conjunction with Brian Schmidt, who like Riess was a member of the High-Z Supernova Search Team, and Saul Perlmutter, head scientist of the Supernova Cosmology Project, a competitor to Riess’ team which published a paper in 1999 corroborating the results of Riess’ 1998 paper.
It so happens, adds Overbye, that this increase in dark energy also would be just enough to resolve the discrepancy in measurements of the Hubble constant.
“The bad news,” says Robert Caldwell, a Dartmouth theoretical physicist, is that, “if this model is right, dark energy may be in a particularly virulent and — most physicists say — implausible form called phantom energy. Its existence would imply that things can lose energy by speeding up, for instance.” Caldwell’s research addresses questions about the basic properties of the universe, dark energy, dark matter, the cosmic microwave background, gravitational waves, and the fate of the Universe.
As the universe expands, Overbye adds, the push from phantom energy would grow without bounds, eventually overcoming gravity and tearing apart first Earth, then atoms.
“If it is real, we will learn new physics,” said Wendy Freedman of the University of Chicago, who has spent most of her career studying the expansion rate of the universe and the nature of dark energy A decade ago she led a team of 30 astronomers who carried out the Hubble Key Project to measure the current expansion rate of the universe. The project’s final results determined the age of the universe as approximately 13.7 billion years, resolving a longstanding debate regarding previously wide-ranging estimates.
Elsewhere, an international team including University College London (UCL) and CL and Flatiron Institute cosmologists say that measurements of gravitational waves from ~50 binary neutron stars over the next decade will definitively resolve this increasingly intense debate.
The study, published today in Physical Review Letters, shows how new independent data from gravitational waves emitted by binary neutron stars called ‘standard sirens’ will break the deadlock between the measurements once and for all.
“The Hubble Constant is one of the most important numbers in cosmology because it is essential for estimating the curvature of space and the age of the universe, as well as exploring its fate,” said Professor Hiranya Peiris (UCL Physics & Astronomy).
“We can measure the Hubble Constant by using two methods – one observing Cepheid stars and supernovae in the local universe, and a second using measurements of cosmic background radiation from the early universe – but these methods don’t give the same values, which means our standard cosmological model might be flawed.”
The team developed a universally applicable technique which calculates how gravitational wave data will resolve the issue.
Gravitational waves are emitted when binary neutron stars spiral towards each other before colliding in a bright flash of light that can be detected by telescopes. Indeed, UCL researchers were involved in detecting the first light from a gravitational wave event in August 2017.
Binary neutron star events are rare but invaluable in providing another route to track how the universe is expanding.
This is because the gravitational waves they emit cause ripples in space-time which can be detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo experiments, giving a precise measurement of the system’s distance from Earth.
By additionally detecting the light from the accompanying explosion, astronomers can determine the system’s velocity, and hence calculate the Hubble constant using Hubble’s Law.
For this study, the researchers modeled how many such observations would be needed to resolve the issue in measuring the Hubble constant accurately.
“We’ve calculated that by observing 50 binary neutron stars over the next decade, we will have sufficient gravitational wave data to independently determine the best measurement of the Hubble constant. We should be able to detect enough mergers to answer this question within 5-10 years,” said lead author Dr Stephen Feeney of the Center for Computational Astrophysics at the Flatiron Institute in New York City.
“This in turn will lead to the most accurate picture of how the universe is expanding and help us improve the standard cosmological model,” concluded Professor Peiris.
Image Credit: NASA Dark Energy