Seeing the 'real' Big Bang through gravitational waves

The earliest and most momentous epoch in the history of the universe released a flood of gravitational waves, tiny ripples in the fabric of space-time. 

Now, new research shows how future detections of these primordial gravitational waves can reveal the inner workings of a process called reheating, which may be the origin of the "real" Big Bang.

Astronomers aren't exactly sure what happened when the universe was less than a second old, but they do know it was something eventful.

Related: The 1st few seconds of the Big Bang: What we know and what we don't

Cosmological observations have revealed that the universe is too smooth and too uniform on the very largest of scales given the standard picture of the Big Bang and the known age of the universe. Distant regions of the universe have roughly the same temperature despite being separated by billions of light-years. There just hasn't been enough time in the history of the cosmos for those regions to exchange heat.

To account for these observations, cosmologists suspect that at one point long ago, those distant regions of the universe were much closer together, allowing them to reach a common temperature before being flung apart. To do the work of "flinging apart," cosmologists have theorized an event known as inflation. 

Prior to inflation, the universe was very small, and temperatures and densities equalized. But then inflation happened, sending everything flying away from everything else in a very brief amount of time, explaining why very distant patches of the universe look pretty much the same.

Inflation happened well before the universe was even 1 second old, and the event lasted less than a billionth of a billionth of a billionth of a second. But in that extraordinarily brief amount of time, the universe became billions upon billions upon billions (and there are probably a few more billions in there) larger than it had been before.

Astronomers have seen evidence for inflation in the particular pattern of light observed in the cosmic microwave background, the radiation left over from when the universe was only 380,000 years old. But beyond that, astronomers aren't sure what triggered inflation, what kept it running and what made it stop.

A little heat

One of the biggest puzzles in modern cosmology is what happened at the end of the inflationary epoch. You see, the process of inflation (whatever it is) is really good at smoothing things over, making things big and chilling everything out. So, shortly after inflation ended, the universe was big, cool and empty — which are most definitely not the conditions we observe in the early universe. 

Instead, shortly after inflation, the universe was filled with a hot, frenzied soup of particles. Cosmologists think that, to make that happen, whatever drove inflation decayed, releasing energy and reheating the (now much larger) universe with a flood of particles that we know and love today.

It would be entirely fair to consider this reheating process the "real" Big Bang — the mechanism by which an expanding universe became filled with particles and radiation.

Observing this reheating process, and inflation itself, is incredibly difficult. That's because, when the universe was younger than 380,000 years old, it was a plasma and opaque. We simply can't see any further into the past than that — at least not with light.

Feeling the Earth move

But the processes of inflation and reheating didn't just flood the universe with energy and radiation; they also released tremendous amounts of gravitational waves. Gravitational waves are ripples in the fabric of space-time that travel at the speed of light. Right now, gravitational waves are washing over you from a variety of sources, including merging black holes, supernovas, and all sorts of other high-energy, high-density phenomena in the cosmos.

Also washing over you right now are gravitational waves left over from inflation and reheating. These processes generated so much energy that they were capable of shaking space-time itself, and those gravitational waves — called "primordial" gravitational waves — are still rippling through the cosmos today.

For many years, cosmologists have been investigating the possible gravitational wave signals released by inflation. But new research, which was posted recently to the preprint database arXiv, looked more closely at the reheating process at the end of inflation. The researchers found that different models of reheating led to very distinct and potentially detectable signatures in the primordial gravitational waves.

For example, we don't know if reheating was slow and gentle or more abrupt. These two hypothetical scenarios lead to very different signatures of gravitational waves.

These primordial gravitational waves are far too low-frequency to be observed with our current detectors, like the Laser Interferometer Gravitational-Wave Observatory in the U.S. and the Virgo interferometer in Europe. But future space-based observatories, such as the European Space Agency's proposed Laser Interferometer Space Antenna and Big Bang Observer, will be specifically designed to hunt for primordial gravitational waves. 

Seeing these primordial gravitational waves directly would provide ironclad evidence for the process of inflation and help us understand the physics that went into it. And according to the new work, we'll also get hints about reheating, the end of inflation and the detailed physics that gave rise to the universe as we know it today.

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