Ask Ethan: Does uneven cosmic expansion change our fate?

One of the greatest achievements of the past 100 years is learning our place in the Universe: where it came from, how it got to be the way it is today, and what its ultimate fate will be. A century ago, we didn’t even know that the Universe was expanding; today, we know:

• how fast it’s expanding,
• how fast it was expanding all throughout cosmic history,
• what the Universe is made of (dark energy, dark matter, normal matter, and a little bit of radiation and neutrinos),
• that it began with the inflation-triggeres hot Big Bang,
• and how the Universe should end: in a heat death, driven by our dark energy-dominant present and future.

It’s a remarkable set of accomplishments that we arrived at slowly, over decades, as our observational data and theoretical understanding of what it means both advanced together, significantly, over the mid-to-late 20th century and the early 21st century as well.

But is all of this a certainty? Or could some of the foundational assumptions that led to our current picture be proven wrong by recent data? And if so, could our cosmic fate be different than what’s so frequently assumed? That’s what Tony Segura wants to know about, writing in to ask:

“I’ve been wondering how the universe ends, I’ve seen articles about how the universe isn’t expanding evenly so that makes the Big Rip less likely and Heat Death might look a little strange (I’m not of that anyway) and the Big Crunch seems less likely, so how do you think it’s going to end?”

There actually is some evidence that the Universe may not be truly isotropic, and there’s also some evidence that our traditionally-accepted fate — of a cosmic heat death — may not actually be what we’re in store for. Here’s what we learn when we put all the different pieces of the story together.

The foundational cosmological principle

There is an underlying assumption behind our standard view of how the Universe works: the assumption that, on the largest of all cosmic scales, if you look at any region of space at a given moment in time, that on average, every region is roughly the same as every other region. The assumption that any large-volume region of space is the same as every other region of the same volume is known as homogeneity, and the assumption that any direction you can look in is the same as every other direction you can look in is known as isotropy. We assumed, long before we were able to measure it, that the Universe is likely to be both of those things: isotropic and homogeneous.

This was only ever going to be true:

• on the largest of cosmic scales (because you’re going to have small-scale inhomogeneities in order to form galaxies, stars, planets, and intelligent observers),
• at any particular moment in time (because you’re going to need the Universe to evolve, and the way that an isotropic and homogeneous Universe evolves is either by expanding or contracting),
• and on average (because every individual region has its own specific properties, like a specific set of density imperfections, and that necessarily includes both high-density and low-density regions that deviate to some degree from the cosmic average).

In many ways, the cosmological principle is only an assumption, but it’s an assumption that’s powerful, useful, and most importantly, largely validated as we’ve come to observe and discover the Universe.

The power of assuming the Universe is isotropic and homogeneous is that — if you apply those assumptions to the underlying theory of Einstein’s general relativity — you discover that the Universe cannot be static and stable, but instead must expand or contract. If you begin with an isotropic, homogeneous Universe that expands or contracts, then the composition of the Universe (e.g., matter, radiation, dark energy, etc.) tells you how that expansion or contraction will evolve, with no uncertainties, on the largest scales of all.

The usefulness of assuming the Universe is isotropic and homogeneous is that if you can measure the distances to astronomical objects and the rate at which they either recede away from us (for an expanding case) or appear headed towards us (for a contracting case), you can extrapolate what you observe and measure to both earlier times and later times, potentially teaching us about the origin and fate of the Universe.

And the important observational facts that the Universe has been borne out to:

• have the same number and number density of galaxies,
• with the same overall amount of mass,
• with their light being shifted in a way that indicates they are receding away from us at a speed that’s proportional to their distance from us,
• in an omnidirectional fashion,

validates this view of cosmic history. Looking backwards and extrapolating into the past, the cosmological principle and the observations together lead to the Big Bang, while looking forward and extrapolating into the future, leads to a determination of our ultimate fate.

Our fate in the standard “ΛCDM” picture

Our observations have progressed to the point where we can determine not only how quickly the Universe is expanding today, but how the expansion rate was different in the past. Because an isotropic, homogeneous Universe that’s expanding will have its rate of expansion change over time in a fashion that depends on what’s in it, then measuring the expansion rate of the Universe at all different epochs in cosmic history will allow us to determine what our Universe’s ingredients are, and in what amounts as well.

Once you know those two things — the expansion rate and the contents of the Universe — you can then do something remarkable: you can determine the fate of your Universe as well. The reason why is straightforward: the total density of matter-and-energy, in a homogeneous and isotropic Universe, is what determines how the expansion rate changes from one moment to the next. And, in turn, as the Universe expands, the matter-and-energy density changes, as an expanding Universe increase the volume of space while keeping the number of particles the same, causing the density to dilute.

Only if there’s a form of energy that’s inherent to space itself, like dark energy, will (that one component of) the energy density remain constant; for all other forms of energy — normal matter, dark matter, photons, neutrinos — the energy density drops as the Universe expands. Knowing all of this, it leads to several different potential fates of the Universe, depending on what the expansion rate is and what the matter-and-energy density is. Those fates include:

• A Big Crunch, which arises in a Universe dominated by matter and/or radiation, where the matter-and-radiation density is great enough that gravitation’s effects can overcome the effects of expansion: creating a scenario where the Universe expands to a maximum size, stops expanding, and then begins contracting.
• A Heat Death (or Big Freeze), which arises if the Universe doesn’t have enough matter and/or radiation to reverse the expansion, and instead the Universe expands forever, with the matter-and-energy density dropping lower and lower as time continues to progress.
• A “Goldilocks” scenario (or critical Universe), where the Universe — filled with matter and radiation (and not dark energy) — would recollapse if there were only one more atom in it, but because that atom isn’t there, the expansion rate asymptotes to zero, never quite reaching it but always expanding ever slower as time goes on.
• A Big Rip, where the Universe doesn’t just become dominated by dark energy, but where the dark energy density actually increases with time, becoming more and more powerful. If that were to occur, the Universe’s expansion rate would increase to greater and greater values over time, and at high enough values, the fabric of space, as well as everything that’s ever formed in the Universe, would tear itself apart.

In the standard dark energy-dominated (ΛCDM) Universe, where dark energy has a constant, unchanging energy density and the other ingredients are present but no longer dominant, the Heat Death/Big Freeze scenario is the one we can expect to occur. But that conclusion, of course, is predicated on the underlying assumptions that the Universe really is isotropic and homogeneous.

Evidence for anisotropy?

There are frequently studies that come out that claim to have discovered large-scale anisotropies or inhomogeneities in the Universe: places where the Universe is a little different, in some way, from the standard predictions that arise in a cosmos that is both isotropic and homogeneous. Examples include:

• indications that there are more spiral galaxies rotating in one direction (clockwise) than in the opposite direction (counterclockwise) from our perspective,
• teams claiming they’ve discovered structures, through quasar alignments or clusters of gamma-ray bursts, that are too large to form in an isotropic, homogeneous, ΛCDM Universe with the age and expansion rate we infer,
• the announcement that there are hemispherical asymmetries in the Universe, with either different expansion rates or different cosmic microwave background properties from one another,
• a large-scale anisotropy appearing in galaxy clustering data, with galaxy clustering statistics appearing different from one another in different regions of the sky,
• and differences in the observed motions from galaxy clusters found in different directions, as measured in X-ray light.

However, a large number of lines of evidence that seem to contradict our fundamental assumptions doesn’t always translate to the assumptions being wrong.

Many of these claims turn out to have only a low confidence to them. Many of them lack reproducibility; many of them rely on assumptions themselves that have not been validated; many of them actually contain errors that undermine the asserted conclusion.

• Studies claiming to find an asymmetry in “clockwise spirals” versus “counterclockwise spirals” always use small numbers of statistics. When all of the known galaxies taken with the largest-scale survey we have are include, no asymmetry appears.
• Large-scale quasar and gamma-ray burst groupings have been discovered, but there is no underlying map of galaxies or overall mass that shows these are actually physical structures linking them together, rather than just a chance alignment of rare signals that appear with incomplete data.
• The recent claim of large-scale anisotropies appearing in galaxy clustering data turned out to be spurious, as there were two key errors in the 2026 study, using DESI data, that led to a claim that has since been thoroughly refuted once those errors were corrected.

It’s vital to keep in mind that even a very large number of low-quality, dubious claims are not enough to refute a theory. As Einstein quipped when the book “100 against Einstein” was published, where 100 scientists made 100 different claims that relativity was wrong, “Why so many? If the claim were wrong, one would be enough.”

Is anything else “non-standard” about our Universe?

There are plenty of anomalies that do appear, however, and some of them may wind up holding true despite attempts to find fault with the analysis, data, or conclusions. One stubborn example involves the use of X-ray data to measure the expansion rate of the Universe in different directions, and the data indicates that the expansion rate was indeed significantly different in multiple different regions of the sky. That would seem to violate isotropy directly: in a straightforward fashion.

That result was initially reported with an overwhelming significance: of 5.4σ, which rises above the “gold standard threshold” for statistical significance in astrophysics and cosmology. However, even the authors of that study, despite its high quality and its robustness to reanalysis, had to admit that:

• there could be “bulk flows,” or local inhomogeneities of small enough magnitude and on small enough scales, that arise due to the gravitationally-induced motions of the measured galaxy clusters,
• and that if you include the effect of cosmic variance — where we only have one Universe to observe but different “sample universes” with similar properties would have a percentage of such universes that did match our own — the significance plummets well below that “gold standard” for discovery: down to 3.6σ.

This requires us to remain skeptical about any claims about anisotropy. As revolutionary as it would be, if it’s not a real effect, we must take care not to fool ourselves about what the data is actually indicating.

Putting it all together

Still, our standard picture of the Universe — the hot Big Bang, our ΛCDM cosmology, and the expected Heat Death fate for our cosmos — are all contingent on the assumptions of cosmic isotropy and homogeneity. What if those assumptions are wrong? What if one of the many “weak” lines of evidence turns out to strengthen into a claim that could refute the core assumption of isotropy in the Universe? What would that mean for cosmology, and for the fate of the Universe in particular?

To be completely honest, it depends on several factors. It depends on the magnitude of any anisotropy that exists: anisotropy at the 1-part-in-10,000 level could be insignificant, and consistent with cosmic variance, but anisotropy at the 10% or even the 1% level would be quite significant. It depends on the scale of an anisotropy: anisotropies on the scale of 1.4 billion light-years or less is wholly expected, but anisotropies on larger scales shouldn’t be present. It depends on the type of anisotropy that exists: a large dipole anisotropy could simply be due to the relative motion of one part of the Universe (containing us) relative to the rest of the Universe, but an anisotropy that’s consistent with a large bulk rotation of the Universe could render the inflationary Big Bang entirely inconsistent with observations.

But for the specific case of an anisotropy that indicates different rates of expansion in different directions, it could lead to a major change in our expected fate, even with the presence of dark energy.

• It could mean that even as many parts of the Universe experience a Heat Death, other parts — including parts on extremely large scales — remain gravitationally bound and may even recollapse completely, even arbitrarily far into the future.
• It could imply the existence of unexpected bulk flows, which could potentially alter our currently existing conclusions about dark energy, which might lead to a wildly inhomogeneous Universe that doesn’t necessarily experience a Heat Death.
• It could be an indication of anisotropically evolving dark energy, where perhaps certain directions experience an accelerated expansion but others may not.

But the biggest threat to our current cosmological picture is if the underlying assumptions of isotropy and homogeneity that govern the expanding Universe turn out to be incorrect. Instead of the Friedmann-Lemaître-Robertson-Walker metric describing the underlying spacetime of our Universe, perhaps something else, like a Kasner metric, better describes our spacetime.

The expanding Universe, the Big Bang, and dark energy’s existence and properties are so thoroughly well-established that these final options seem unlikely; it would take extraordinary evidence to overturn current thought. But that’s why we keep investigating even the flimsiest of clues. After all, the greatest strength of science is that we continuously attempt to disprove and discredit even our most cherished scientific ideas. Only the ones that can survive each and every challenge will persist; the rest are relegated to the dustbin of historical also-rans.

Send in your Ask Ethan questions to startswithabang at gmail dot com!

This article is featured on Big Think.