Is the universe really one big black hole?

Below is a passage from our Lost in Space-Time newsletter. Every month, we invite a physicist or mathematician to share intriguing concepts from their field. You can 
sign up for Lost in Space-Time here
.

“You wrote a book about black holes?”

A stranger sips his drink as we mingle at a gathering. I respond with a polite nod while stirring my piña colada.

“So tell me,” he asks, fixing his gaze upon me, “is it really true that the entire universe is a black hole?”

I’m not taken aback. This query often arises when I share that I’ve dedicated years to discussing black holes with scientists and exploring observatories to understand our current knowledge surrounding these cosmic giants.

It’s no surprise that people are intrigued. Headlines frequently suggest that the sparkling galaxies we observe in space could be ensconced within a massive black hole. Videos on YouTube delve into such speculations, attracting millions of viewers. While it might resemble something from a science fiction narrative, the investigation into this hypothesis has roots stretching back to at least 1972, when physicist Raj Kumar Pathria published a letter in the journal Nature entitled “The Universe as a Black Hole.” Since then, the astonishing assertion resurfaces periodically.

So, is there truth to it?

Creating a Black Hole

In simple terms, a black hole is a zone in space where gravitational forces are so tremendous that nothing, not even light, can break free.

These mysterious entities were initially uncovered through mathematics by astronomer Karl Schwarzschild during World War I. Amid the turmoil of battles on the French-German front, he explored what Albert Einstein’s freshly released equations of general relativity indicated about the motion of celestial bodies and the configuration of stars.

Schwarzschild derived a formula that illustrates how space and time can act in ways that deviate radically from our everyday experiences, folding in on themselves and spawning the type of inescapable region we now term a black hole.

This pivotal discovery contributed to a deeper understanding of black hole mechanics. Imagine taking a specific mass, such as a star, a planet, or even a human body. Squeeze it into a volume defined by Schwarzschild’s formula, et voilà! A black hole emerges.

The required volume for this transformation hinges on the object’s mass. For a human, it’s astonishingly minuscule: a hundred times smaller than a proton. For Earth, it’s roughly the size of a golf ball, whereas for the sun, that volume approximates the dimensions of downtown Los Angeles (about 6 kilometers, or just under 4 miles, across).

As evident, generating black holes is quite challenging. Typically, matter resists being compressed into such ultra-high densities. Only the most catastrophic cosmic events—like the explosion of very massive stars in supernovae—can compel matter to collapse and form a black hole.

Yet there’s an interesting twist in the black hole generation narrative. While those created by dying stars originate from exceptionally dense matter, their much larger supermassive counterparts at the centers of most galaxies exhibit relatively low densities. Schwarzschild’s formula indicates that the larger a black hole grows, the more emptiness it contains, resulting in a reduced average density (albeit in a somewhat abstract sense; defining the density of a complex space-time entity like a black hole is not straightforward). Consequently, the largest observed black holes boast an average density lower than that of air!

So what about the universe? Given its primarily empty nature, is it conceivable that its strikingly low density could still align with that of a black hole?

Assessing the Universe’s Size

Armed with Schwarzschild’s formula, astronomers can ascertain if an object qualifies as a black hole: first, assess its mass; then, determine its volume. If the mass is confined within a volume smaller than that indicated by Schwarzschild’s formula, it is classified as a black hole.

Now, let’s apply this principle to the universe as a whole. To accomplish this, we need its mass and volume. Unfortunately, traversing the cosmos with a measuring tape to gauge its full expanse is unfeasible; we can only detect light and particles from the distant parts of space.

The oldest light we observe originates from the cosmic microwave background, created just 380,000 years following the big bang. Due to the universe’s expansion, those light sources now reside at vast distances from us. The maximum distance that light could have traveled since the big bang defines the observable universe, which spans a diameter of 93 billion light years.

Thanks to meticulous measurements conducted over numerous years, astronomers estimate the mass within this volume to be about 10⁵⁴ kg (1 followed by 54 zeros, or one septendecillion).

Next, let us calculate the theoretical size of a black hole weighing one septendecillion kilograms. Inputting this value into Schwarzschild’s formula and following through the calculations reveals a staggering conclusion: such a black hole would stretch 300 billion light years across, roughly tripling the observable universe’s size. In essence, by examining the mass and dimensions contained within the observable universe, it aligns with the criteria of being classified as a black hole.

“Incredible,” the curious individual at the cocktail event exclaims, “so does that mean the universe is indeed a black hole?”

“Hold your horses,” I reply. To truly answer that query, we must delve deeper into the inner workings of a black hole.

Diving into the Abyss

Black holes are peculiar entities. One of their numerous strange characteristics is that, externally, they appear to occupy a fixed size, yet internally, they are constantly shifting. According to Schwarzschild’s formula, the space within them stretches in one direction while simultaneously constricting in the other two. (If the black hole is rotating, its inner workings become even more bizarre, but that’s a topic for another newsletter.)

Cosmologists label this kind of structure anisotropic. Tropos means ‘direction’, iso represents ‘equal’, and the an indicates negation. The anisotropic dynamics within a black hole imply that out of the three spatial dimensions, one will expand while the other two contract—akin to a rubber sheet being elongated into a fine thread. This distortion correlates closely with the tidal stretching of infalling matter, which Stephen Hawking famously termed spaghettification.

In contrast to black holes, the expansion of the universe occurs isotropically (meaning it expands uniformly in all directions). That doesn’t resemble the inner structure of a black hole, does it?

However, this doesn’t dismiss the idea of a universe behaving like a black hole. After all, black holes share two striking attributes with our universe, both of which seem familiar: an event horizon and a singularity.

The event horizon designates a boundary beyond which no light can escape. For black holes, it signifies the point of no return, where matter can no longer elude. In the context of the universe, it arises from the rapid expansion of space, which ensures that light from very distant galaxies can never catch up with us.

This cosmic event horizon resembles an inverted version of the black hole event horizon: the latter obscures our view into the depths of the black hole’s darkness, while the former obstructs our visibility to the universe’s furthest reaches.

This peculiar relationship also applies to the daunting singularity—the point where matter density and space-time curvature surge to an infinite level. According to Schwarzschild’s findings, the singularity constitutes a future point in time that any unfortunate astronauts entering a black hole must confront post-event horizon. Conversely, our cosmological framework contains a singularity in the past. As we trace the universe’s expansion backward, all points in space converge, leading to increasingly higher densities. As these densities approach infinity, the cryptic inaugural moment of the big bang culminates in a singularity. Therefore, while in black holes the singularity is mathematically fixed in the future, for our expanding universe, it is positioned in the past. In both cases, the singularities present in our models underscore our incomplete grasp of what transpires at these incomprehensibly dense points.

Putting all these aspects together—the dissimilarities in expansion, event horizons, and singularities—illustrates a compelling argument that our universe is not a black hole. It simply doesn’t present itself as one!

“But wait,” the stranger interjects, a hint of disappointment in his tone, “I thought we just calculated that our universe fulfills the criteria for being a black hole. This seems contradictory!”

“While the calculation holds true,” I respond, “it turns out that a similar mathematical relationship as Schwarzschild’s is also embedded within our model for an expanding universe. It is not exclusively a property of black holes.”

We may not yet fully understand the peculiarities present on the grandest cosmic scales, beyond what our telescopes can reveal. However, based on our fundamental models of expanding universes and non-rotating black holes, our universe does not exhibit the characteristics of being ensconced within a black hole. What should we take from this? Personally, I view it as a testament to gravity’s adaptability, giving rise to wondrous phenomena such as gravity-constricting black holes and a rapidly expanding universe simultaneously.

Jonas Enander, a Swedish science writer with a PhD in physics, has recently published the book Facing Infinity: Black holes and our place on Earth (Atlantic Books/The Experiment, 2025), which delves into the influence of black holes both on the cosmos and humanity. To express these concepts, he produced a video narrating the story through watercolor illustrations.