Following the inflationary epoch after the singularity of the Big Bang, the universe continued to expand, but at a slower rate. The acceleration of the expansion due to dark energy began after the universe was already over nine billion years old (four billion years ago). The weakest of the Standard Model forces, gravity, may provide the solution to this unsolved enigma –the cosmic elephant in the room–in astronomy.
There’s Something a little off about the Standard Theory of the Universe
“There’s something a little off about our theory of the universe. Almost everything fits,” report University of Chicago physicists, “but there’s a fly in the cosmic ointment, a particle of sand in the infinite sandwich” –something is making the universe not only expand, but expand faster and faster over time—and no one knows what the unknown force –cryptically labeled “dark energy”–is that’s causing the universe to expand at an accelerating rate.
“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,” said Noble-Prize winning physicist Adam Riess, in an interview with The Atlantic.
“Particle of Sand in the Infinite Sandwich”
Some scientists think the culprit might be gravitational waves. These subtle ripples in the fabric of space-time distort the very geometry of space itself and may provide a glimpse of our universe less than a trillionth of a trillionth of a second after the Big Bang. Gravitational waves could help us find the “particle of sand in the infinite sandwich.”
Just as the heat of the Big Bang left our universe filled with a background of cosmic light, the epoch of inflation created a background of gravitational waves, disturbances in the curvature of spacetime, that still ripple throughout all of space and time today. But whereas the cosmic microwave background has enabled us to learn about our universe as it was 380,000 years after the Big Bang, writes U of Chicago physicist, Dan Hooper, in The Edge of Time, “this background of gravitational waves carries information about a far more primordial epoch.”
Massive Objects Would Change the Signature of the Wave
A new paper co-authored by University of Chicago astrophysicist Jose María Ezquiaga, a NASA Einstein postdoctoral fellow in the Kavli Institute for Cosmological Physics, and co-author Miguel Zumalácarregui, Marie Curie Global Fellow at the Berkeley Center for Cosmological Physics, argues that if gravitational waves hit a supermassive black hole or cluster of galaxies on their way to Earth, the signature of the ripple would change. If there were a difference in gravity compared to Einstein’s theory, the evidence would be embedded in that signature.
Some physicists propose that the discovery of a link between gravity and quantum theory could turn out to be as important as the discovery by Maxwell in the nineteenth century that a single theory connects electricity, magnetism and light.
“The Perfect Messenger”
Scientists have proposed all kinds of theories for what the missing piece might be. “Many of these rely on changing the way gravity works over large scales,” said paper co-author Ezquiaga. “So gravitational waves are the perfect messenger to see these possible modifications of gravity, if they exist.”
Since 2015, humanity has been able to pick up these ripples using the LIGO observatories. Whenever two massively heavy objects collide elsewhere in the universe, they create a ripple that travels across space, carrying the signature of whatever made it—perhaps two black holes or two neutron stars colliding.
The Existence of an Extra Particle?
The detailed particle physics mechanism responsible for inflation is unknown. One theory for the missing piece of the universe is the existence of an extra particle. Such a particle would, among other effects, generate a kind of background or “medium” around large objects. If a traveling gravitational wave hit a supermassive black hole, it would generate waves that would get mixed up with the gravitational wave itself. Depending on what it encountered, the gravitational wave signature could carry an “echo,” or show up scrambled.
Why Additional Particles are Critical
“In theories beyond the standard cosmological model, additional particles are postulated in order to explain the observed accelerated expansion,” Ezquiaga replied to an email from The Daily Galaxy about the significance of this extra particle. “When this extra particle is associated to alternative gravity theories,” he adds, “it accumulates around massive objects. This ‘screening’ mechanism serves to hide modifications of gravity at short distances. This is important because we know very well how gravity works near us, at least up to Solar System scales. However, when gravitational waves travel across this screened region, they can be sensitive to this background or medium. The resulting signal can be distorted or split in two, leading to a scrambled wave or echoes.”
First LIGO Detection –50 Times the Power of All the Stars in the Observable Universe
“This is a new way to probe scenarios that couldn’t be tested before,” Ezquiaga observed, referring to conditions for how to find such effects in future data. The next LIGO run is scheduled to begin in 2022, with an upgrade to make the detectors even more sensitive than they already are. On 11 February 2016, the LIGO-Virgo collaborations announced the first detection of gravitational waves from the merger of two black holes about 1.3 billion light-years away, releasing more than 50 times the power of all the stars in the observable universe combined.
Occurring Once Every Five Minutes
“In our last observing run with LIGO, we were seeing a new gravitational wave reading every six days, which is amazing. But in the entire universe, we think they’re actually happening once every five minutes,” Ezquiaga said, which make it more likely that one or more wave will have traveled through a massive object, and that scientists will be able to analyze them for clues to the missing components. “In the next upgrade, we could see so many of those—hundreds of events per year.”
Source: Jose María Ezquiaga et al. Gravitational wave lensing beyond general relativity: Birefringence, echoes, and shadows, Physical Review D (2020). DOI: 10.1103/PhysRevD.102.124048
Image credit, top of page: In 2018, astronomers have witnessed the birth of a colossal cluster of galaxies. Their observations reveal at least 14 galaxies packed into an area only four times the diameter of the Milky Way’s galactic disk. ESO/M. Kornmesser.