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Of all the questions humanity has ever pondered, perhaps the most profound one we’ve ever asked is, “Where did all of this come from?” For generations, we told one another tales of our own invention and chose the narrative that sounded best to us. The idea that we could find the answers by examining the Universe itself was foreign until recently, when scientific measurements began to provide answers to longstanding puzzles that had stymied philosophers, theologians, and all sorts of thinkers alike. By opening up the Universe itself to scientific scrutiny, and letting evidence rather than ideology guide the way, profound advances were made as never before.
The 20th century brought us general relativity, quantum physics, and the Big Bang: all accompanied by spectacular observational and experimental successes, while alternative ideas that failed to match the data fell away. These frameworks enabled us to make theoretical predictions that we then went out and tested, and they passed with flying colors, which is why they’ve continued to persist for so long.
But — at least for the Big Bang — some unexplained problems remained that required us to look further than that initial framework could provide. When we looked beyond the confines of the Big Bang, we reached an uncomfortable conclusion that we’re still reckoning with today: any information about the beginning of the Universe is no longer contained within our observable cosmos. Instead, it was wiped out by the very process that gave rise to the Big Bang itself. Here’s the disconcerting story you might not have heard before.
The stars and galaxies we see today didn’t always exist, and the farther back we go, the closer to an apparent singularity the Universe gets, as we go to hotter, denser, and more uniform states. However, there is a limit to that extrapolation, as going all the way back to a singularity creates puzzles we cannot answer.
Credit : NASA, ESA, and A. Feild (STScI)
In the 1920s, just under a century ago, our conception of the Universe changed forever as two sets of observations came together in perfect harmony. For the past few years, scientists led by Vesto Slipher had begun to measure spectral lines — emission and absorption features — of a variety of stars and nebulae, including the spiral and elliptical nebulae seen away from the plane of the Milky Way. Because atoms are the same everywhere in the Universe, the electrons within each species of atom always make the same transitions at the same energies: they have the same absorption and emission spectra.
A few of these nebulae, exclusively among the spirals and ellipticals, had those spectral lines shifted toward the red part of the spectrum by extremely large amounts: redshifts that corresponded not only to high recession speeds, but to speeds that were faster than anything found within our galaxy.
Practically contemporaneously, beginning in 1923, Edwin Hubble and Milton Humason began measuring individual stars in these distant spiral (and later, elliptical) nebulae, enabling a determination of their distances. Those distances turned out to be much greater than anything found within our own Milky Way: millions of light-years away in most instances.
When you combined the distance and redshift measurements together, it all pointed to one inescapable conclusion that was also theoretically supported by Einstein’s general theory of relativity: the Universe was expanding. The farther away a galaxy is, the faster it appears to recede from us.
Edwin Hubble’s original plot of galaxy distances versus redshift (left), establishing the expanding universe, versus a more modern counterpart from approximately 70 years later (right). In agreement with both observation and theory, the universe is expanding.
Credit : Edwin Hubble (L), Robert Kirshner (R)
If the Universe is expanding today, that allows us to draw several different conclusions, immediately, about what’s occurring across the Universe. In particular, all three of the following must be true:
The Universe is getting less dense, as the (fixed amount of) matter in it begins to occupy larger and larger volumes as more time elapses.
The Universe is cooling over time, as the light within it gets stretched to longer wavelengths as time marches on.
And galaxies that aren’t gravitationally bound together must be getting farther apart from one another over time, as the expansion drives them apart.
Each one of those facts is remarkable and mind-bending in its own way: a stark contrast to the static-and-stable, eternally constant Universe that was the prevailing picture prior to these observations. Based on these facts, we swiftly became able to extrapolate what’s going to happen to the Universe as time marches inexorably forward.
But there’s more to the story, too. Those same laws of physics that tell us what’s going to happen in the future can also allow us to extrapolate backward to what happened in the past, and even the Universe itself is no exception to that. If the Universe is expanding, cooling, and getting less dense today, that means it must have been smaller, hotter, and denser long ago, in the distant past.
While matter and radiation become less dense as the Universe expands, owing to its increasing volume, dark energy (and also the field energy during inflation) is a form of energy inherent to space itself. As new space gets created in the expanding Universe, the dark energy density remains constant, even as the densities of other components drop.
Credit : E. Siegel/Beyond the Galaxy
This realization led to the origin of one of the biggest ideas of the 20th century: the hot Big Bang. By extrapolating this hotter, denser, more uniform early state as far back as possible, a series of remarkable predictions could be extracted that would lead to profound observable consequences. Four of the biggest ones include the recognition that:
More distant galaxies should be smaller, more numerous, lower in mass, and richer in hot, blue stars than their modern-day counterparts, leading to specific predictions about galaxy clustering and evolution over time.
There should be fewer and fewer heavy elements as we look backward in time, down to a minimum set by the light elements that were produced in the early crucible of the hot, dense Big Bang.
There should come a time when the Universe was too hot to form neutral atoms, leading to a leftover bath of now-cold radiation that exists as a relic from that time.
And as a result of that early, hot, dense past, there should have been a time where atomic nuclei were blasted apart by the ultra-energetic radiation, leaving a relic mix of hydrogen and helium isotopes as the survivors in the aftermath of that phase.
All four of these predictions were observationally confirmed at various times in the 20th century, with that leftover bath of radiation — originally known as the “primeval fireball” and now called the cosmic microwave background — discovered in the mid-1960s often referred to as the “smoking gun” piece of evidence in favor of the Big Bang.
This image shows Arno Penzias and Robert Wilson, co-discoverers of the cosmic microwave background (CMB), with the Holmdel Horn Antenna used to discover it. Although many sources can produce low-energy radiation backgrounds, the properties of the CMB, including its perfectly blackbody nature and uniform temperature in all directions, confirm its cosmic origin. As time goes on and the leftover glow from the Big Bang continues to redshift, larger telescopes sensitive to longer wavelengths and smaller number densities of photons will be required to continue to detect it.
Credit : NASA, restored by Bammesk/Wikimedia Commons
You might think — as many did at the time — that we can therefore extrapolate the Big Bang all the way back, arbitrarily far into the past, until all the matter and energy in the Universe was concentrated into a single spacetime point at one particular instant. The Universe would, at that instant, reach infinitely high temperatures and densities, creating a physical condition known as a singularity. A singularity is a pathology under the general theory of relativity, representing an event where the laws of physics as we know them give predictions that no longer make sense and cannot be valid anymore. From that initial, singular moment, the entire Universe, as well as everything in it, ultimately arose.
At last! After millennia of searching, we had it: an ultimate origin story, rooted in science, for the entire Universe itself.
The Universe began with a Big Bang some finite time ago, corresponding to the birth of space and time, and everything we’ve ever observed has been a product of the aftermath of that event. For the first time, we had a scientific answer that truly indicated not only that the Universe had a beginning, but when that beginning occurred. In the words of Georges Lemaitre, the first person to put together the physics of the expanding Universe, it was “a day without yesterday.”
A visual history of the expanding Universe includes the hot, dense state known as the Big Bang and the growth and formation of structure subsequently. The full suite of data, including the observations of the light elements and the cosmic microwave background, leaves only the Big Bang as a valid explanation for all we see. As the Universe expands, it also cools, enabling ions, neutral atoms, and eventually molecules, gas clouds, stars, and finally galaxies to form.
Credit : NASA/CXC/M. Weiss
Only, there were a number of unresolved puzzles that the Big Bang posed, pointed out and popularized by people like Bob Dicke and Jim Peebles in the 1960s and 1970s.
Why did regions that were causally disconnected — i.e., had no time to exchange information, even at the speed of light — have the same temperatures as one another?
Why were the initial expansion rate of the Universe (which works to expand things) and the total amount of energy in the Universe (which gravitates and fights the expansion) perfectly balanced early on: to more than 50 decimal places?
And why, if we reached these ultra-high temperatures and densities early on, are there no leftover relic remnants from those times in our Universe today?
Despite the large number of brilliant minds that thought about these puzzles, a solution was hard to come by. That’s because, in the context of the Big Bang alone, there is no solution. You simply need to foist these properties off onto what we call initial conditions : asserting that “the Universe must have been born this way.” Throughout the 1970s, many of the top physicists and astrophysicists in the world pored over these problems, theorizing about possible solutions to these puzzles. Then, in late 1979, a young theorist named Alan Guth had a spectacular realization that changed history.
In the top panel, our modern Universe has the same properties (including temperature) everywhere because they originated from a region possessing the same properties. In the middle panel, the space that could have had any arbitrary curvature is inflated to the point where we cannot observe any curvature today, solving the flatness problem. And in the bottom panel, pre-existing high-energy relics are inflated away, providing a solution to the high-energy relic problem. This is how inflation solves the three great puzzles that the Big Bang cannot account for on its own.
Credit : E. Siegel/Beyond the Galaxy
The new theory was known as cosmic inflation, and postulated that perhaps the idea of the Big Bang was only a good extrapolation back to a certain point in time: a time that corresponded to a finite volume, a finite temperature, and a finite density. Instead of extrapolating back further, to a hotter, denser, smaller state, the hot Big Bang would have instead been preceded (and set up) by this inflationary state.
Instead of reaching arbitrary high temperatures, densities, and energies, inflation states that:
the Universe was no longer filled with matter and radiation,
but instead possessed a large amount of energy intrinsic to the fabric of space itself,
which caused the Universe to expand exponentially (where the expansion rate doesn’t change over time), doubling and doubling again with each fraction-of-a-second that elapsed,
which drives the Universe to achieve a flat, empty, almost-perfectly uniform state,
right up until the moment that inflation comes to an end. When it ends, the energy that was inherent to space itself — the energy that’s the same everywhere, except for the quantum fluctuations imprinted atop it — gets converted into matter and energy, resulting in a hot Big Bang.
From inflation to the hot Big Bang, to the birth and death of stars, galaxies, and black holes, all the way to our ultimate dark energy fate, we know that entropy never decreases with time. But we still don’t understand why time itself flows forward. However, we’re pretty certain that entropy, and the thermodynamic arrow of time, cannot be the answer.
Credit : E. Siegel; ESA/Planck and the DOE/NASA/NSF Interagency Task Force on CMB research
Theoretically, this was a brilliant leap, because it offered a plausible physical explanation for the observed properties the Big Bang alone could not account for. Causally disconnected regions would have the same temperature as one another because they all arose from the same inflationary “patch” of space. The expansion rate and the energy density were perfectly balanced because inflation imparted that particular expansion rate and energy density to the Universe, everywhere, during the period prior to the Big Bang. And there were no left over, high-energy remnants because the Universe only reached a finite temperature after inflation ended: an insufficiently high temperature to produce those relic remnants.
In fact, inflation also made a series of novel predictions that differed from that of the non-inflationary Big Bang, meaning we could go out and test this idea. As of today, in 2026, we’ve collected data that puts four of those predictions to the test :
The Universe should have a maximum, non-infinite upper limit to the temperatures reached during the hot Big Bang.
Inflation should possess quantum fluctuations that become density imperfections in the Universe that are 100% adiabatic (with constant entropy).
Some fluctuations should be on super-horizon scales: fluctuations on scales larger than light could have traveled since the hot Big Bang.
Those fluctuations should be almost, but not perfectly, scale-invariant, with slightly greater magnitudes on large scales than small ones.
If one wants to investigate the signals within the observable Universe for unambiguous evidence of super-horizon fluctuations, one needs to look at super-horizon scales at the TE cross-correlation spectrum of the CMB. With the final (2018) Planck data now in hand, the evidence is overwhelmingly in favor of their existence, validating an extraordinary prediction of inflation and flying in the face of a prediction that, without inflation, such fluctuations shouldn’t exist.
Credit : ESA and the Planck collaboration; annotations by E. Siegel
With data from satellites like COBE, WMAP, and Planck, as well as ground-based observatories like the South Pole Telescope, the Atacama Cosmology Telescope, and the Simons Observatory, we’ve now tested all four of those predictions. Only inflation (and not the non-inflationary hot Big Bang) yields predictions that are in line with what we’ve observed.
But this, too, is profound: it means that the Big Bang wasn’t the very beginning of everything. Instead, it was only the beginning of the Universe as we’re familiar with it. Prior to the hot Big Bang, there was a state known as cosmic inflation that eventually ended and gave rise to the hot Big Bang, and we can observe the imprints of cosmic inflation on the Universe today.
However, the observable imprints that persist only correspond to the final tiny, minuscule fraction of a second of inflation. Only, perhaps, for the final ~10 -32 seconds of it (or thereabouts) can we observe the imprints that inflation left on our Universe. It’s possible that inflation lasted for only that duration, or for far, far longer. It’s possible that the inflationary state was eternal to the past (although there are theoretical objections to that scenario ), or that inflation was transient, arising from something other than the predecessor state. It’s possible that the pre-inflationary Universe did begin with a singularity, or arose as part of a cycle, or has always existed. But that information doesn’t exist in our Universe. Inflation — by its very nature — erases whatever existed in the pre-inflationary Universe, and only those final 10 -32 seconds of inflation leave any observable imprint on our cosmos at all.
The quantum fluctuations that occur during inflation do indeed get stretched across the Universe, and later, smaller-scale fluctuations get superimposed atop the older, larger-scale ones. These field fluctuations cause density imperfections in the early Universe, which then lead to the temperature fluctuations we measure in the cosmic microwave background, after all the interactions between dark matter, normal matter, and radiation occur prior to the formation of the first stable, neutral atoms.
Credit : E. Siegel/Beyond the Galaxy
In many ways, inflation is like pressing the cosmic “reset” button. Whatever existed prior to the inflationary state, if anything, gets expanded away so rapidly and thoroughly that all we’re left with is empty, uniform space with the quantum fluctuations that inflation creates superimposed atop it. When inflation ends, only a tiny volume of that space — somewhere between the size of a human being and a city block — will become our observable Universe. Everything else, including any of the information that would enable us to reconstruct what happened earlier in our Universe’s past, now lies forever beyond our reach.
It’s one of the most remarkable achievements of science: that we can go back billions of years in time and understand when and how our Universe, as we know it, came to be this way. But like many adventures, revealing those answers has only raised more questions. The puzzles that have arisen this time, however, may truly never be solved. If that information is no longer present in our Universe, it will take a revolution to solve the greatest puzzle of all: where did all this, originally, come from ?
This article was first published in December of 2022. It was updated in July of 2026.
This article Forget seeing the Universe’s beginning. It’s already been erased is featured on Big Think .
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