The Reality of the Multiverse

If one embraces the principles of cosmic inflation and quantum mechanics, the conclusion is unavoidable: the multiverse is a reality.

No matter how advanced our technology may become, searching the universe will never reveal an edge. The cosmos extends infinitely in every direction, filled consistently with matter and radiation. Observations indicate an expanding universe characterized by remnants from an intensely hot beginning, evolving galaxies, and changing elemental abundances as stars undergo their life cycles.

However, what exists beyond our visible universe? Is it merely an expanse of nothingness past the limits of light since the Big Bang? Does more of the universe exist beyond our observational capabilities? Or is there a multiverse—enigmatic and perpetually hidden from our view?

Assuming our understanding of the universe isn't fundamentally flawed, the multiverse appears to be the logical resolution. Here’s why.

The notion of the multiverse is contentious yet fundamentally straightforward. Just as Earth, the Sun, and the Milky Way don’t occupy unique places within the cosmos, the multiverse posits that nothing about our entire observable universe is exceptional.

The multiverse suggests that our universe, along with everything within it, forms merely a small segment of a larger framework. This overarching structure includes our observable universe, extending far beyond what we can detect. This entire entity—the unobservable universe—might itself be a part of a larger spacetime that contains numerous other, isolated universes, which may or may not resemble our own.

If this is the interpretation of the multiverse, one could understandably be skeptical about our ability to ascertain its existence. Physics and astronomy are disciplines grounded in measurable, experimental, or observational validation. Seeking evidence for something that exists beyond our visible universe and leaves no discernible trace within it seems to suggest that the multiverse is fundamentally untestable.

Nonetheless, there are many phenomena we cannot observe but know to be true. For instance, years before gravitational waves were directly detected, we inferred their existence from observable effects. Binary pulsars—neutron stars in mutual orbit—exhibited decreasing revolution periods, implying energy loss consistent with predictions of gravitational waves.

Although the confirmations provided by LIGO and Virgo through direct detection were welcome, the indirect evidence had already established the necessity of gravitational waves. Some may dispute that indirect evidence suffices to indicate their existence; LIGO and Virgo, for instance, did not detect gravitational waves from the binary pulsars we have observed.

So, if the multiverse cannot be directly observed, what indirect evidence supports its existence? How can we conclude that a vast unobservable universe lies beyond what we can currently perceive, and that our universe is likely just one among many in a multiverse?

We examine the universe itself and draw inferences based on what our observations reveal.

When we observe the edge of the observable universe, we find that light from the earliest moments—specifically, the Cosmic Microwave Background—displays distinct patterns in the sky. These patterns unveil not only the density and temperature variations present at the universe's inception but also provide insight into its matter and energy composition and the geometry of space itself.

From these observations, we can conclude that space is neither positively nor negatively curved but is spatially flat. This suggests that the unobservable universe likely extends far beyond what we can access—it does not curve back on itself, repeat, or contain empty voids. If any curvature exists, its diameter surpasses hundreds of times the distance we can currently observe.

As time progresses, more of the universe akin to ours continues to be unveiled, consistent with this understanding.

This observation hints at the existence of a broader unobservable universe beyond our reach but does not definitively prove it, nor does it substantiate the multiverse hypothesis. However, two fundamental concepts in physics are established with considerable certainty: cosmic inflation and quantum mechanics.

Cosmic inflation, the theory that spawned the hot Big Bang, asserts that instead of commencing with a singularity, there exists a physical limit to how hot and dense the universe could have been in its early stages. Had temperatures reached arbitrarily high levels in the past, we would observe specific signatures, which are currently absent:

• Significant temperature fluctuations in the early universe,
• Density fluctuations confined by the scale of the cosmic horizon,
• High-energy relics from the early universe, such as magnetic monopoles.

These signatures are notably lacking. The temperature fluctuations exist at a mere 0.003% level; density fluctuations exceed cosmic horizon scales; and stringent limits on monopoles and other relics are in place. The absence of these signatures carries immense implications: the universe did not reach those exceedingly high temperatures. Something must have preceded the hot Big Bang to establish its conditions.

This leads us to the concept of cosmic inflation. Introduced in the early 1980s, it was crafted to resolve several puzzles surrounding the Big Bang while generating measurable, testable predictions for observable phenomena in our universe.

We observe the anticipated lack of spatial curvature; we witness an adiabatic nature in the universe's initial fluctuations; and we have detected a spectrum and magnitude of fluctuations aligning with inflation's predictions. We have also identified superhorizon fluctuations as predicted by inflationary theory.

While our understanding of inflation isn't exhaustive, we possess a robust body of evidence supporting the occurrence of a period of inflation in the early universe. This phase set the stage for the Big Bang and predicted a range of fluctuations that seeded the cosmic structures we observe today. Thus far, inflation is the only theory that provides predictions aligning with our observations.

One might argue, “So what? You merely expanded a small region of space into a vast volume, containing our observable universe. Even if this is correct, it only indicates that our unobservable universe extends beyond our visible portion. It doesn't confirm the multiverse.”

This reasoning is valid. However, we must consider one more crucial element: quantum mechanics.

Inflation is treated as a field, akin to all known quanta in the universe, adhering to the principles of quantum field theory. In the quantum realm, myriad counterintuitive rules apply, the most pertinent being that of quantum uncertainty.

While we typically regard uncertainty as arising between two variables—momentum and position, energy and time, etc.—there exists an intrinsic uncertainty in the value of a quantum field. As time progresses, a value that was once definite becomes increasingly uncertain; we can only assign probabilities to it.

In essence, the value of any quantum field disperses over time.

Let’s synthesize this: we have an inflating universe on one side and quantum mechanics on the other. We can visualize inflation as a ball slowly rolling atop a flat hill. As long as the ball stays on the hill, inflation persists. Once it reaches the end, it descends into the valley, converting the inflationary field's energy into matter and radiation.

This transition marks the conclusion of cosmic inflation through a process termed reheating, resulting in the familiar hot Big Bang. However, when inflation occurs, the field value changes gradually. In various inflating regions, this field value spreads out in distinct, random amounts and directions. In some areas, inflation ceases quickly; in others, it persists longer.

This key observation explains why a multiverse is not just a possibility but a necessity! Where inflation concludes promptly, we witness a hot Big Bang and a vast universe, where a small segment may resemble our observable universe. In regions where inflation continues for an extended period, new hot Big Bangs can arise, leading to even larger universes.

These other regions are not only inflating but also expanding. The rate at which these inflating regions grow can be calculated and compared to the rate at which new universes and hot Big Bangs occur. In every scenario where inflation produces predictions aligning with the observed universe, new universes and inflating regions emerge faster than inflation can conclude.

This depiction of vast universes, far exceeding our limited observable universe, being continuously generated throughout this exponentially inflating space encapsulates the essence of the multiverse. It's not merely a new, verifiable scientific hypothesis but rather an unavoidable theoretical conclusion based on our current understanding of physics. Whether the laws of physics are consistent across these other universes remains unknown.

In a universe governed by inflationary theory and quantum mechanics, a multiverse is an inescapable outcome. As always, we are diligently collecting new and compelling evidence to enhance our understanding of the cosmos. It’s possible that inflation or quantum mechanics could be flawed, or that our interpretation of these principles might contain fundamental errors. However, thus far, everything aligns. Unless a significant misunderstanding exists, the multiverse is inevitable, and the universe we inhabit is merely a tiny fraction of it.

Starts With A Bang is now featured on Forbes and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, "Beyond The Galaxy" and "Treknology: The Science of Star Trek from Tricorders to Warp Drive."