How Gravitational Waves Could Let Us See the First Moments After the Big Bang

Cosmology has had several ground-breaking discoveries over the last 100+ years since Einstein developed his theory of relativity. Two of the most prominent were the discovery of the Cosmic Microwave Background (CMB) in 1968 and the confirmation of gravitational waves in 2015. Each utilized different tools, but both lent credence to the Big Bang Theory, which relates to the universe’s formation. However, we still don’t understand a vital part of that formation, and a new review paper by Rishav Roshan and Graham White at the University of Southampton suggests that we might be able to make some headway on our one-second “gap” in knowledge by using our newfound understanding of gravitational waves.

First, let’s look at what problem physicists are trying to solve. The Big Bang theory of cosmology is currently the one most widely accepted by scientists. There are different stages in it, including the earliest stage, known as “inflation,” and a stage where atoms begin to form, known as Big Bang Nucleosynthesis (BBN). However, there was a one-second gap between the end of inflation and the beginning of BBN that scientists have been unable to see into.

It’s difficult to see what happened in that second because that specific period was opaque to electromagnetic waves, such as the CMB that helped prove the Big Bang theory in the first place. While one second may not seem like a whole lot of time, the universe might have dropped something like twenty-two orders of magnitude in temperature in that one second. How that temperature change played out is critical to understanding what happened in BBN and, therefore, in creating the universe as we know it today.

Luckily, gravitational waves are here to save the day. They could permeate even that one-second gap, allowing cosmologists for the first time to peer into the previously mysterious time and try to glean any information they could about the imbalance between matter and antimatter or the expansion rate of the universe itself at that point. But to do so, they need a new set of tools.

Now that gravitational waves have officially been found, after a search that lasted more than 100 years, scientists have plenty of new ideas for novel ways to detect them. The paper breaks down three methods, each of which could find waves of different frequencies.

First are more advanced systems similar to LIGO that detected the first wave. Known as interferometers, these precise tools use synced lasers to detect any minute differences between two locations that gravitational waves might have caused. Scientists have drawn up plans for the future, including more enormous interferometers based on the ground and some based in space, which wouldn’t be subject to disturbances like earthquakes. These solutions promise to look into gravitational waves in the microhertz to kilohertz range of frequencies.

Astrometry and pulsar timing array are the two other techniques. Both are useful in other parts of cosmology but can also detect lower-frequency gravitational waves if the instruments monitoring them are sensitive enough. Astrometry is more commonly used for exoplanet detection, whereas pulsar timing arrays are a typical measure of distance in cosmology. However, both could be affected by slow gravitational waves that could be detectable by the same instruments already used to monitor them.

These different techniques will search for the Stochastic Gravitational-Wave Background (SGWB). This operates similarly to the CMB in that it’s a leftover remnant of the beginning phase of the universe. Still, in this case, the SGWB comprises gravitational waves that would allow astronomers to see back to the very beginning of the universe.

If these new instruments do detect it, they could potentially detect some massively energetic events that happened during that one-second temperature de-escalation. One of the most commonly considered creation theories for gravitational waves is an “acoustic” source. This isn’t sound as we would traditionally consider it, but it describes massive shockwaves that would happen from two “sound shells” surrounding early, hot matter running into each other. Creation theories like this are commonly grouped in the paper as “cosmic phase transitions.”

Another grouping in the paper surrounds events known as “topological defects.” Topology is a common theme in physics, and a “defect,” in this case, represents an actual break in space-time as we know it. Those events could have obvious gravitational implications, some of which should be detectable in the frequencies tracked by the new detectors.

A final set of events that could induce gravitational waves is called “scalars.” Instead of representing a “break,” like the defects mentioned above, these events are just giant-scale versions of known physics. Gravitational waves can be caused by large masses moving together, though equations better describe such a”scalar” event than words.

Other, even more exotic events could form gravitational waves during this time period, but detecting them would require higher-frequency detectors than are currently available. Designs for some that could detect high-frequency gravitational waves are currently on the drawing board, but no solid commitment or experimental proof of their efficacy is forthcoming at the time of writing.

Cosmologists will undoubtedly have enough to chew on even without detecting higher-frequency GWs. We’ve talked before about the coming age of gravitational wave astronomy – and every day, it’s getting closer to reality—papers like those from Drs. Roshan and White are what help light the way.

Learn More:
Roshan & White – Using gravitational waves to see the first second of the Universe
UT – Future Gravitational Wave Observatories Could See the Earliest Black Hole Mergers in the Universe
UT – Gravitational Waves Could Show us the First Minute of the Universe
UT – Gravitational Wave Observatories Could Detect Primordial Black Holes Speeding Through the Solar System

Lead Image:
Representation of gravitational waves in the CMB.
Credit – Harvard-Smithsonian Center for Astrophysics

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