The early universe may have been such a violent place that spacetime itself shattered like a pane of glass. These ruptures would have released floods gravitational wavesand a team of astronomers has discovered that we may have already detected these ripples in the fabric of spacetime.
The team, which recently published their findings in a paper pending publication in the Journal of Computational Astrophysics and published on arXiv.org (opens in new tab)claim that they saw evidence of so-called domain walls in the early universe.
When our universe was incredibly young, it was also incredibly exotic. The four forces of nature were combined into a single, unified force. We don’t know what that force looked like or how it worked, but we do know that as the universe cooled and expanded, that combined force broke into the four familiar forces we have today. First came heavinessthen the strong nuclear power splintered off, and eventually the electromagnetic and weak nuclear forces split off from each other.
Related: The History of the Universe: Big Bang to Date in 10 Easy Steps
With each of these splits, the universe completely reshaped itself. New particles emerged to replace those that previously could only exist under extreme conditions. The fundamental quantum fields of spacetime that govern how particles and forces interact have been reconfigured. We don’t know how smooth or rough these phase transitions were, but it’s entirely possible that the universe settled into multiple identities at the same time with each split.
This breaking is not as exotic as it sounds. It happens in all kinds of phase transitions, like when water turns to ice. Different water spots can form ice molecules with different orientations. No matter what, all water turns to ice, but different domains can have different molecular arrangements. Where these domains meet walls or imperfections, fractures occur.
Examination of the GUT
Physicists are particularly interested in the so-called GUT phase transition of our universe. GUT is short for Grand Unified Theory, a hypothetical model of physics that merges the strong nuclear force with electromagnetism and the weak nuclear force. These theories are just beyond the reach of current experiments, so physicists and astronomers are turning to conditions in the early Universe to study this important transition.
The GUT phase transition that took place when the universe was only a split second old may very well have left behind domain walls, a network of boundaries between different configurations of spacetime. However, these shortcomings could not last long. Had they paused for a few seconds or even minutes, their intense energies would have disrupted, or distorted our images of, the process of nucleosynthesis that produced all of the primordial hydrogen and helium in the universe cosmic microwave background (CMB), the residual radiation from the Big Bang.
So this interconnected set of domain walls had to decay into other particles – either normal particles, like electrons or cottage cheeseor more exotic particles, like a form of Dark matter. In any case, this decay process, coupled with the undulating motion of the domain walls themselves, would have unleashed a deluge of gravitational waves that could persist into the Universe today.
surveying the domain
These gravitational waves would be incredibly faint and impossible to detect with existing ground-based gravitational wave facilities. But for over a decade, multiple teams of astronomers around the world have been looking for it instead pulsars Map gravitational waves sloshing through the universe.
Pulsars are incredibly precise timing devices, capable of keeping their rhythm accurate to less than a millionth of a second. However, if a gravitational wave passes between us and a series of pulsars, it will subtly affect the pulsation period. By studying large numbers of pulsars over long periods of time, we can hope to find signals of gravitational-wave background foam.
These pulsar timing arrays, like the NANOGrav experiment and the European Pulsar Timing Array, have already found evidence of a signal. Most astronomers believe this signal is due to the combined effect of millions supermassive black holes collide with each other over billions of years.
But the new study paints a different picture. The team argues that the signal could also be explained by domain wall decay in the early Universe. Their models allow the domain walls to decay fast enough not to affect other observations like the CMB, while still providing a strong enough signal to explain the pulsar timing array data.
Since the signals in the data are very weak and not confirmed to be from any particular source, there is room for this type of radical proposition. The team argues that future pulsar timing measurements should be able to distinguish their model of crumbling domain walls from the traditional picture of colliding supermassive black holes. If their model is accurate, the domain walls should also decay into normal or exotic particles. In any case, this should be detectable with future, much more sensitive CMB measurements.
If the result holds, it will be a major win for physics: the first time we have found concrete evidence for GUT phase transitions and the beginning of a new understanding of physics.
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