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: 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 far distant fates of the Universe offer a number of possibilities, but if dark energy is truly a constant, as the data best indicates, it will continue to follow the red curve, leading to the long-term scenario frequently described on Starts With A Bang: of the eventual heat death of the Universe. If dark energy can strengthen, weaken, or reverse sign over time, however, all bets are off, and alternative possibilities, like a Big Crunch or a Big Rip, suddenly abound.
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, 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.
Get the fall print issue, shipping in October, to members only. 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. Our view of a small region of the Universe near the northern galactic cap, where each pixel in the image represents a mapped galaxy.
On the largest scales, the Universe is the same in all directions and at all measurable locations, with the major difference being that distant galaxies appear smaller, younger, denser, and less evolved than the ones we find nearby: evidence for cosmic evolution with time, but no changes in isotropy or homogeneity. 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.
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