The theory has its flaws. For instance, the mass of the Higgs boson — the component of the Standard Model that determines the masses of other particles — is frustratingly “unnatural.” It appears arbitrary, and puzzlingly small compared to the far greater energy scales of the universe. Moreover, the Standard Model offers no explanation for dark matter, nor for the mysterious dark energy that drives the accelerating expansion of space. Another problem is that antimatter and matter behave exactly the same under the three forces of the Standard Model — which obviously isn’t the full story, since matter dominates the universe. And then there’s gravity. The Standard Model completely ignores the fourth fundamental force, which must be described using its own bespoke theory, general relativity.

“So a lot of theorists like me have been trying to squeeze the Standard Model a little bit and try to make extensions of it,” said Pierre Auclair, a theoretical cosmologist at the Catholic University of Louvain in Belgium. But without experimental evidence with which to test them, these extended theories remain, well, theoretical.

Auclair is a theorist. “But still, I’m trying to be linked with experiments as much as I can,” he said. That’s one reason he was drawn to LISA. “These extensions usually lead to different extreme events in the early universe,” he said.

Garcia Garcia likewise said that LISA’s promise of observational evidence for high-energy physics led her to rethink her career — gravitational waves could “probe the early universe in a way no other experiment can,” she said. A few years ago, she started studying gravitational waves and how physics beyond the Standard Model would leave fingerprints detectable by LISA.

Last year, Garcia Garcia and her colleagues published work on the gravitational wave signature of bubble walls — energetic barriers between pockets of space that got trapped in different states as the universe cooled. This cooling happened as the universe expanded. Just as water boils and turns into steam, the universe went through phase transitions. In the Standard Model, the phase transition during which a single, “electroweak” force split into separate electromagnetic and weak forces was relatively smooth. But many extensions of the theory predict violent events that left the cosmic soup frothy and disturbed, said Dunsky, who also studies topological defects like bubble walls.

Quantum fields that permeate our universe have minimum-energy states, or ground states. And as the universe cooled down, new, lower-energy ground states developed, but a given field didn’t always immediately land in its new ground state. Some got trapped in local energy minima — false ground states that only appear stable. Sometimes, though, one little piece of the universe would quantum-tunnel into the true state, nucleating a rapidly expanding bubble of true vacuum with a lower energy than the universe outside.

“These bubbles are very energetic; they’re moving very close to the speed of light due to this pressure difference between their interior and exterior,” Dunsky said. “So when they collide, you get this violent collision between these two very relativistic objects, somewhat similar to how black holes emit strong gravitational waves right before colliding.”

Strings and Walls

More speculatively, phase transitions in the early universe could also have created structures called cosmic strings and domain walls — enormous strands and sheets, respectively, of dense energy.

These structures arise when a quantum field’s ground state changes in such a way that there is more than one new ground state, each equally valid. This can result in high-energy defects along the borders between pockets of the universe that happened to fall into different, but equally favorable, ground states.

The process is a bit like the way certain rocks develop natural magnetism as they cool, said Dunsky, who has studied the observable fingerprints of the process. At high temperatures, atoms are randomly oriented. But at cool temperatures, it becomes energetically favorable for them to magnetically align — the ground state changes. Without some external magnetic field to orient the atoms, they’re free to line themselves up any which way. All “choices” are equally valid, and different domains of the mineral will, by chance, make different choices. The magnetic field generated by all the atoms bends dramatically at the borders between domains.

Similarly, the quantum fields in different regions of the universe “must change rapidly at the boundary” of these domains, he said, resulting in large energy densities at these boundaries that “signify the presence of a domain wall or cosmic string.”

These cosmic strings and domain walls, if they exist, would have stretched out to span practically the entire universe as space expanded. These objects produce gravitational waves as kinks propagate along them and as loops oscillate and form cusps. But the energy scales of these waves were mostly set as the objects formed in the first moments of the universe. And LISA could detect them, if they exist.

Echoes of Creation

The gravitational waves reaching us from the very early universe will not arrive in neatly packaged chirps, like the signals of black hole collisions. Because they happened so early in time, such signals have since been stretched out across all of space. They’ll echo from every direction, from every point in space, all at once — a background gravitational hum.

“You turn on your detector, and it’s always there,” Garcia Garcia said.

Patterns in this background would probably “just look like noise to the average person,” Sundrum said. “But secretly, there’s a hidden code.”

One important clue will be the background signal’s spectrum — its strength at different frequencies. If we think of a gravitational wave signal as sound, its spectrum would be a plot of pitch versus volume. Truly random white noise would have a flat spectrum, Auclair said. But gravitational waves unleashed during phase transitions or cast from cosmic strings or domain walls would be loudest at specific frequencies. Auclair has worked on calculating the spectral signatures of cosmic strings, which throw out gravitational waves at characteristic wavelengths when their kinks and loops evolve. And Caprini studies how violent phase transitions would leave their own mark on the gravitational wave background.

Another approach, which Sundrum and his colleagues outlined in 2018 and recently elaborated, would be to try to map the overall intensity of the background across the sky. This would make it possible to look for anisotropies, or patches that are just a tiny bit louder or quieter than average.

“The problem,” Caprini said, “is that this kind of signal has practically the same characteristics of the instrument noise. So the entire question is how to be able to distinguish it once we detect something.”

LISA is more like a microphone than a telescope. Instead of peering in a particular direction, it will listen to the whole sky at once. It will hear primordial gravitational waves if they are present. But it will also hear the chirps and howls of merging black holes, neutron stars and the many pairs of white dwarf stars within our galaxy. In order for LISA to detect a background of primordial gravitational waves, all other signals will need to be carefully identified and removed. Filtering out the true signal from the early universe will be like picking out the sound of a spring breeze at a construction site.

But Sundrum chooses to be hopeful. “We’re not crazy to be doing the research,” he said. “It’ll be hard for experimentalists. It’ll be hard for the public to pay for the various things that need to get done. And it’ll be hard for theorists to calculate their way past all of the uncertainties and errors and backgrounds and so on.”

But still, Sundrum added, “it appears to be possible. With a little luck.”