13.8 billion years ago, the universe as we know it ⁠— full of matter and radiation, expanding and cooling and gravitating ⁠— came into existence with the onset of the hot Big Bang. Today, we can see for enormous cosmic distances, measuring the signals that come to us from across the universe’s history, successfully reconstructing the story of how we originated and came to be. But as time continues to pass, a novel form of energy in our universe, dark energy, comes to further and further dominate the expansion of space. As dark energy takes over, it causes the universe’s expansion to accelerate, which gradually removes the key information needed to draw the conclusions we’ve reached today. It’s enough to make one wonder if, were we born far into the future instead of today, we’d be able to learn about the Big Bang at all? That’s what Patreon supporter Aaron Weiss wants to know, asking:

“[A]t some point in the future, all objects not gravitationally bound to us will recede away. [T]he only points of light in the night sky will be objects in our local group. At that point in time, will there be any evidence of the universe’s expansion that might suggest to future astronomers that there are/were stars and galaxies beyond what would be visible to them? Would they have lines-of-site that lead to nothing but the CMB?”

It’s a scary but important question to ponder: are we only reaching the conclusions we do about the universe because of when and where we happen to exist in cosmic history? Let’s look to the far future to find out.

The cosmic microwave background appears very different to observers at different redshifts, because they’re seeing it as it was earlier in time. In the far future, this radiation will shift into the radio and its density will drop rapidly, but it will never disappear entirely. (Credit: NASA/BlueEarth; ESO/S. Brunier; NASA/WMAP)

Today, there are four major pieces of evidence that we typically consider as the cornerstones of the hot Big Bang. The whole reason we consider the Big Bang as the unchallenged scientific consensus is because it’s the only framework, consistent with the laws of physics (like Einstein’s General Relativity), that explains the following four observations.

  1. the expanding universe, discovered through the redshift-distance relation for galaxies,
  2. the abundance of the light elements, as measured through various gas clouds, nebulae, and stellar populations across the universe,
  3. the leftover glow from the Big Bang, today’s cosmic microwave background, as directly detected via microwave and radio observatories,
  4. and the growth of large-scale structure in the universe, as revealed by galaxy evolution and their clumping and clustering patterns seen across cosmic time.

It’s important to remember that cosmology, like all branches of the astronomical sciences, is fundamentally observationally driven. Whatever it is that our theories predict, we can only compare them with what the universe gives us to observe. The way we discovered each of these phenomena in our universe has its own remarkable story, but it’s a story that won’t be around permanently for us to always observe.

The growth of the cosmic web and the large-scale structure in the Universe, shown here with the expansion itself scaled out, results in the Universe becoming more clustered and clumpier as time goes on. Initially small density fluctuations will grow to form a cosmic web with great voids separating them. However, once the nearest galaxies recede to too-great distances, we will have extraordinary difficulty in reconstructing the evolutionary history of our cosmos. (Credit: Volker Springel)

The reason is straightforward: the conclusions that we draw are informed by the light that we can observe. When we look out at the universe with our best modern tools, we see lots of objects within our own galaxy — the Milky Way — as well as many objects whose light originates from far beyond our own cosmic backyard. Although this is something we take for granted, perhaps it shouldn’t be. After all, the conditions that exist in our universe today won’t be the same as the conditions that exist in the far future.

Today, our home galaxy extends for a little over 100,000 light-years in diameter, and possesses somewhere around ~400 billion stars within it, as well as copious amounts of gas, dust, and dark matter, with a wide variety of stellar populations: old and young, red and blue, low-mass and high-mass, and containing both small and large fractions of heavy elements within them. Beyond that, we have perhaps 60 other galaxies within the local group (within about ~3 million light-years), and somewhere around 2 trillion galaxies littered throughout the visible universe. By looking at objects farther away in space, we’re actually measuring them over cosmic time, enabling us to reconstruct the history of the universe.

Fewer galaxies are seen nearby and at great distances than at intermediate ones, but that’s due to a combination of galaxy mergers and evolution and also being unable to see the ultra-distant, ultra-faint galaxies themselves. Many different effects are at play when it comes to understanding how the light from the distant Universe gets redshifted. (Credit: NASA / ESA)

The problem, however, is that the universe isn’t merely expanding, but that the expansion is accelerating owing to the existence and properties of dark energy. We understand that the universe is a struggle — a race, of sorts — between two main players:

  1. the initial expansion rate that the universe was “born” with at the onset of the hot Big Bang,
  2. and the sum total of all the various forms of matter and energy within the universe.

The initial expansion compels the fabric of space to expand, stretching all unbound objects farther and farther away from one another. Gravitation, based on the total energy density of the universe, works to counteract that expansion. As a result, you can imagine three possible fates for the universe:

  • one where the expansion wins, where there isn’t enough gravitation in all the “stuff” that’s present to counteract the initial large expansion and everything expands forever,
  • one where gravitation wins, where the universe expands to a maximum size and then recollapses,
  • and one right on the border between the two, where the expansion rate asymptotes to zero, but never reverses itself.

That was what we expected, but it turns out that the universe is doing a fourth, rather unexpected thing.

dark energy
The different possible fates of the Universe, with our actual, accelerating fate shown at the right. After enough time goes by, the acceleration will leave every bound galactic or supergalactic structure completely isolated in the Universe, as all the other structures accelerate irrevocably away. We can only look to the past to infer dark energy’s presence and properties, which require at least one constant, but its implications are larger for the future. (Credit: NASA & ESA)

It’s as though, for the first few billion years of our cosmic history, it appeared as though we were right on the border between expansion forever and an eventual recontraction. Distant galaxies, if you were to observe them over time, would have each continued to recede from us, but their inferred recession speed, as determined from their measured redshifts, appeared to slow down over time: just what you’d expect for a matter-rich universe that was expanding.

But about six billion years ago, those same galaxies all of a sudden started to recede from us more quickly. In fact, the inferred recession speed of every object that isn’t already gravitationally bound to us — i.e., that’s outside of our local group — has been increasing over time, something confirmed by a wide suite of independent observations.

The culprit? There must be a new form of energy permeating the universe, inherent to the fabric of space, which doesn’t dilute but rather maintains a constant energy density as time goes on. This dark energy has come to dominate the energy budget of the universe, and will take over entirely in the far future. As the universe continues to expand, matter and radiation get less dense, but dark energy’s density remains constant.

dark energy
While matter (both normal and dark) and radiation become less dense as the Universe expands owing to its increasing volume, dark energy is a form of energy inherent to space itself. As new space gets created in the expanding Universe, the dark energy density remains constant. In the far future, dark energy will be the only component of the universe important for detemining our cosmic fate. (Credit: E. Siegel/Beyond the Galaxy)

This will have many effects, but one of the more fascinating things that will occur is that our local group will remain gravitationally bound together, while all of the other galaxies, galaxy groups, galaxy clusters, and any larger structures will all accelerate away from us. If we had come into existence at a later date after the Big Bang — 100 billion or even a few trillion years after the Big Bang, as opposed to 13.8 billion years — most of the evidence that we presently use to infer the Big Bang would, by then, be completely removed from our view of the universe.

Our first hint of the expanding universe came from measuring the distance to and redshifts of the nearest galaxies beyond our own. Today, those galaxies are only a few million to a few tens-of-millions of light-years away. They’re bright and luminous, easily revealed with the smallest telescopes or even a pair of binoculars. But in the far future, the galaxies of the local group will all merge together, while even the closest galaxies beyond our local group will have receded away to tremendously large distances and incredible faintnesses. Once enough time passes, even today’s most powerful telescopes, even if they were to observe the abyss of empty space for weeks on end, would reveal not a single galaxy beyond our own.

Looking back through cosmic time in the Hubble Ultra Deep Field, ALMA traced the presence of carbon monoxide gas. This enabled astronomers to create a 3-D image of the star-forming potential of the cosmos, with gas-rich galaxies are shown in orange. In the far future, larger, longer-wavelength observatories will be required to reveal even the closest galaxies. (Credit: R. Decarli (MPIA); ALMA (ESO/NAOJ/NRAO))

This accelerated expansion, brought on by the dominance of dark energy, would also steal from us critical information about the other cornerstones of the Big Bang.

  • Without any other galaxies or clusters/groups of galaxies to observe beyond our own, there’s no way to measure the large-scale structure of the universe, and infer how matter clumped, clustered, and evolved throughout it.
  • Without populations of gas and dust outside of our own galaxy, particularly with different abundances of heavy elements, there’s no way to reconstruct the early, initial abundance of the lightest elements before the formation of stars.
  • And after a tremendous amount of time, there will be no cosmic microwave background anymore, as that leftover radiation from the Big Bang will become so sparse and low-energy, stretched and rarified by the expansion of the universe, that it will no longer be detectable.

And so, at least on the surface, it appears that with all four of today’s cornerstones gone, we’d be completely unable to learn about our true cosmic history and the early, hot, dense stage that gave rise to the universe as we know it. Instead, we’d see that whatever our local group becomes — likely an evolved, gas-free, potentially elliptical galaxy — it would appear that we were all alone in an otherwise empty universe.

The galaxy shown at the center of the image here, MCG+01-02-015, is a barred spiral galaxy located inside a great cosmic void. It is so isolated that if humanity were located in this galaxy instead of our own and developed astronomy at the same rate, we wouldn’t have detected the first galaxy beyond our own until we reached technology levels only achieved in the 1960s. In the far future, every inhabitant in the universe will have an even more difficult time reconstructing our cosmic history. (Credit: ESA/Hubble & NASA, N. Gorin (STScI), Acknowledgement: Judy Schmidt)

But that doesn’t mean we’ll have no signals at all that could lead us to conclusions concerning our cosmic origins. Many clues would still remain, both theoretically and observationally, and with a clever enough species investigating them, they might yet draw correct inferences about the hot Big Bang, inferences that could then be borne out through the process of scientific investigation.

Here’s how a species from the far future could figure it all out.

Theoretically, once we discovered the present law of gravity — Einstein’s General Relativity — we could apply it to the entire universe, arriving at the same early solutions that we discovered here on Earth through the 1910s and the 1920s, including the solution for an isotropic and homogeneous universe. We would discover that a static universe that was filled with “stuff” was unstable, and must be expanding or contracting. Mathematically, we would work out the consequences of an expanding universe as a toy model, but on the surface, the universe would appear to be exhibiting a steady-state solution. However, observational clues would still exist.

The cluster Terzan 5 has many older, lower-mass stars present within (faint, and in red), but also hotter, younger, higher-mass stars, some of which will generate iron and even heavier elements. It contains a mix of Population I and Population II stars, indicating that this cluster underwent multiple episodes of star formation. The different properties of different generations can lead us to draw conclusions about the initial abundances of the light elements. (Credit: NASA/ESA/Hubble/F. Ferraro)

First off, stellar populations within our own galaxy would still come in tremendous varieties. The longest-lived stars in the universe can persist for many trillions of years, and new episodes of star formation, although they’d become somewhat rare, should still occur so long as our local group’s gas doesn’t become totally depleted. This means, through the science of stellar astronomy, we’d still be able to determine not only the age of various stars, but their metallicities: the abundances of the heavy elements they were born with. Just as we do today, we’d be able to extrapolate back to “before the first stars formed, how abundant were the various elements,” and we would find the same abundances of helium3, helium-4, and deuterium that the science of Big Bang Nucleosynthesis yields today.

We could then look for three specific signals.

  1. The severely-redshifted leftover glow from the Big Bang, with just a few extremely long-wavelength radio-frequency photons arriving, but arriving from all over the sky. A large, ultra-cool radio observatory in space could find it, but we’d have to know to build it.
  2. An even more severe and obscure signal would arise from very early times: the 21-cm spin-flip transition of hydrogen. When you form a hydrogen atom from protons and electrons, 50% of the atoms have aligned spins and 50% have anti-aligned spins. Over timescales of around ~10 million years, the aligned atoms will “flip” their spins, emitting radiation of a very specific wavelength that gets redshifted. If we knew in what wavelength range to look and with what sensitivity we needed to look, we could detect this background as well.
  3. And finally, we could brute-force our way to the edge of the universe: building a telescope large enough and in the proper wavelength band to actually detect the ultra-distant, ultra-faint galaxies that never fully disappear from our view. We’d just have to know enough to justify building something so resource-intensive to look to such great distances, despite not having any direct evidence of such objects nearby.
This artist’s rendering shows a night view of the Extremely Large Telescope in operation on Cerro Armazones in northern Chile. The telescope is shown using lasers to create artificial stars high in the atmosphere. A larger, longer-wavelength observatory, most probably in space, will be required to reveal even the nearest galaxies in the far future. Credit: ESO/L. Calçada.)

It’s an incredibly tall order to imagine the universe as it will be in the far future, when all of the evidence that led us to our present conclusions is no longer accessible to us. Instead, we have to think about what will be present and observable — both obviously and only if you figure out to search for it — and then imagine a path towards discovery. Even though the task will be more difficult hundreds of billions or even trillions of years from now, a civilization that was smart enough and savvy enough would be able to create their own “four cornerstones” of cosmology that led them to the Big Bang.

The strongest clues would come from the same theoretical considerations we applied back in the early days of Einstein’s General Relativity and the observational science of stellar astronomy, and in particular an extrapolation to the primordial abundances of the light elements. From those pieces of evidence, we could figure out how to predict the existence and properties of the leftover glow from the Big Bang, the spin-flip transition of neutral hydrogen, and eventually, the ultra-distant, ultra-faint galaxies that can still be observed. It won’t be an easy task, but if uncovering the nature of reality is at all important to a far future civilization, it can be done. Whether they succeed or not, however, is entirely up to how much they’re willing to invest.

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

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