Weak gravitational waves may originate from primordial faults in space-time

The early universe could have been such a violent place that space-time itself cracked like windowpane. These rips would release gravitational wave currents, and a team of astronomers have discovered that we may have already detected these ripples in the fabric of space-time.

A team that recently reported their results in a paper submitted for publication in the Journal of Computational Astrophysics and published on (will open in a new tab)claim that they saw evidence for the existence 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 linked into one unified force. We do not know what this force looked like or how it operated, but we do know that as the universe cooled and expanded, this combined force split into the four forces we know today. First, gravity appeared, then the strong nuclear force split off, and finally, the electromagnetic and weak nuclear forces separated from each other.

On the subject: History of the Universe: from the Big Bang to the present day in 10 easy steps

With each of these splits, the universe completely remade itself. New particles have emerged to replace those that previously could exist only under extreme conditions. The fundamental quantum fields of space-time, which determine how particles and forces interact with each other, have changed their configuration. We do not know how smoothly or roughly these phase transitions occurred, but it is possible that with each splitting, the Universe immediately became a multiple identity.

This gap is not as exotic as it seems. This happens during all kinds of phase transitions, for example, when water turns into ice. Different parts of the water 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, cracks appear.

Intestinal probing

Physicists are especially interested in the so-called phase transition of our universe. GUT is short for “grand unification theory”, a hypothetical model of physics that combines the strong nuclear force with electromagnetism and the weak nuclear force. These theories are beyond the reach of current experiments, so physicists and astronomers are turning to the conditions of the early universe to study this important transition.

The GUT phase transition, which occurred when the universe was only a fraction of a second old, could very well have left behind domain walls, a network of boundaries between different space-time configurations. However, these defects could not last long. If they persisted for a few seconds or even minutes, their intense energy would slow down the nucleosynthesis process that gave rise to all the primordial hydrogen and helium in the universe or distort our images of the cosmic microwave background (CMB). , the remaining radiation from the Big Bang.

So this interconnected set of domain walls must have decayed into other particles—either ordinary particles like electrons or quarks, or more exotic particles like some form of dark matter. In any case, this decay process, combined with the undulating motion of the domain walls themselves, would unleash a flood of gravitational waves that could persist in the modern universe.

This graph shows the timeline of the universe based on the Big Bang theory and inflation models. (Image credit: NASA/WMAP)

Domain research

These gravitational waves would be incredibly weak and would be impossible to detect with existing ground-based gravitational wave facilities. But for more than a decade, several groups of astronomers around the world have instead been looking for pulsars to map the gravitational waves sloshing around the universe.

Pulsars are incredibly precise timing objects, able to maintain their rhythm to within a millionth of a second. However, if a gravitational wave passes between us and a number of pulsars, this subtly affects the period of the pulsation. By studying large numbers of pulsars over long periods of time, we can hope to find signals of background churning of gravitational waves.

These pulsar timing arrays, such as the NANOGrav experiment and the European Pulsar Timing Array, have already found hints of a signal. Most astronomers believe that this signal is due to the combined action of millions of supermassive black holes colliding 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 the decay of domain walls in the early universe. Their models allow the domain walls to decay fast enough not to disturb other observations such as the CMB, while still providing a strong enough signal to explain the pulsar synchronization array data.

Since the signals in the data are very weak and not confirmed to come from any particular source, there is room for this kind of radical proposal. The team argues that future time measurements of pulsars should be able to distinguish their model of decaying domain walls from the traditional picture of supermassive black hole collisions. In addition, if their model is correct, the domain walls should decay into either normal or exotic particles. In any case, this should be detected by future, much more sensitive measurements of the CMB.

If the result is confirmed, it will be a major victory for physics: we have for the first time found concrete evidence of phase transitions in GUT and the beginning of a new understanding of physics.

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