Science’s best answer to “where did the Universe come from?”

We’ve all had a moment, at some point in our lives, where we began to wonder about things greater than ourselves. What were things like before we came into existence? What were they like before our parents, grandparents, or even any human came into existence? Was there a time before life on Earth or even planet Earth itself existed? What about the Sun? What about any stars or planets at all? And what about the entire Universe itself: matter, energy, space, time, and even the underlying laws of nature themselves?

It’s possible, and perhaps even likely, that curious humans have been asking questions such as these for as long as there have been humans: for hundreds of thousands of years. For nearly all of that time, our scientific knowledge was far too primitive to draw any such conclusions. We didn’t know about the history of life on Earth, about the geological and fossil evidence for the enormous timescales required for evolution, or about the nature of the planets and stars found all throughout the Universe, as well as the science of astrophysics.

But here, in 2025, the advances of the 20th and 21st centuries have firmly brought those questions into the realm of science, giving us answers that are rooted in factual reality, rather than in our own emotional satisfaction. In a long, more than two hour interview with Big Think, I got the opportunity to unpack all of this. Have a listen, and follow along here for a (condensed) set of highlights.

The long history of Earth, and of life on Earth, was something that humans began to discover long before we began to understand the Universe: back in the 1800s. Charles Darwin, famed nowadays for his discovery of the mechanism of evolution — inherited traits, coupled with random mutations, plus the effects of natural selection — wasn’t just a naturalist who looked at organisms and studied inheritance, but was a natural scientist who was trying to make sense of the world.

One of the things he got to study was the Wealden Dome in southern England: an eroded layer of uplifted sedimentary rock that had characteristic chalk-rich deposits on both sides. Embedded within those layers were fossils from a very long time ago: hundreds of millions of years in Earth’s past. Based on the timescales needed for:

• the biological evolution of the past-and-present organisms seen in the fossils,
• and the geological creation, deposition, and erosion of the layers in the sedimentary rock,

Darwin recognized that the Earth itself needed to be old: hundreds of millions, and perhaps even billions, of years old. Although there were no known mechanisms that could power the Sun for such long time periods in the 19th century, the evidence for this “old Earth” persisted, and was difficult to ignore.

A cross-section of the Wealden Dome, in the south of England, which required hundreds of millions of years just to explain the erosion features observed, with fossils of past life found in the different layers. The chalk deposits on either side, absent in the center, provide evidence for an incredibly long geological timescale required to produce this structure: longer than any contemporary explanation for the Sun’s energy could have provided in the late 19th century. This was noted by none other than Charles Darwin in the mid-1800s, and would present a puzzle that would not be resolved until the process powering the Sun, nuclear fusion, became understood.

Astronomy would eventually catch up, however, and began doing so in earnest in the 1910s and 1920s. A series of profound discoveries happened in those years.

• Einstein put forth his general theory of relativity in 1915, overthrowing Newtonian gravitation and giving us a Universe where spacetime was a fabric, and where matter and energy’s presence and distribution determined the curvature and evolution of that spacetime.
• Vesto Slipher, examining the spiral and elliptical nebulae in the sky throughout the 1910s, found evidence that their light was redshifted, or indicative that they were moving away from us, the farther away they appeared to be.
• Alexander Friedmann, working with Einstein’s equations in 1922, determined that a Universe that was uniformly filled with any species of energy — matter, radiation, a cosmological constant, spatial curvature, or anything else — could not be both static and stable, but would be compelled to either expand or contract (and hence, to evolve) over time.

And then, the big advance came in 1923: when Edwin Hubble was observing the great nebula in Andromeda. There were bright “flares” that appeared in there, as lights appeared, brightened, and then faded away. After observing three separate flares over the span of just a couple of nights, assuming they were novae, Hubble got very, very excited to find a fourth: right in the same location as the first. Excitedly, he crossed out the “N” he had previously made, for nova, and wrote “VAR!” in red, capital letters instead.

Perhaps the most famous photographic plate in all of history, this image from October of 1923 features the great nebula (now galaxy) in Andromeda along with the three novae that Hubble observed within them. When a fourth brightening event happened in the same location as the first, Hubble recognized this was no nova, but a Cepheid variable star. The “VAR!” written in red pen was Hubble having a spectacular realization: this meant Andromeda was an extragalactic object, located far beyond the Milky Way.

You see, novae are stellar events that take a long time to recharge: years, decades, centuries, or millennia, at minimum. But variable stars brighten and fainten in days or even hours, and the rapid appearance and disappearance of this light meant that this wasn’t a nova at all, but must be a variable star.

For a variable star to appear this faint, however, it must be extremely far away: not hundreds or thousands or even tens of thousands of light-years away, but more like a million light-years or so. (Today, we know the distance to Andromeda is more like 2.5 million light-years.) Hubble recognized that Andromeda must be an extragalactic object — what was originally known as an “island Universe” — and that the other spiral and elliptical nebulae, like Andromeda, were likely entire galaxies unto themselves as well.

While Hubble went on to measure the stars within (and hence, the distances to) many other galaxies, combining his data with Slipher’s data led others to the conclusion that the Universe was expanding, as the farther away a distant object was, the faster it appeared to recede from our perspective. The first to put these pieces together was Georges Lemaître in 1927, but others would soon follow. By the 1930s, Hubble, Einstein, and many other influential astrophysicists accepted this conclusion. The Universe was expanding and getting less dense, and that meant that long ago, in the distant past, it was denser, things were closer together, and its volume was smaller.

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.

Well, if the Universe was smaller and denser long ago, then it must have been hotter long ago as well.

Why is that? Because the Universe isn’t just full of matter — i.e., the stuff that stars and planets are made out of — but also full of radiation, or photons: quantum particles of light. Photons each have a specific energy to them, and that energy is defined by their wavelength. As the Universe expands, the wavelength of every photon traveling through that expanding Universe gets stretched (i.e., expands) as well, lengthening it and bringing these photons down to lower energies.

That’s what happens when we go forward in time. So what happens if we look backward, and ask, “What was the Universe doing in the past?” If the Universe was smaller and denser in the past, and the distances between objects were shorter in the past, then the wavelength of photons in the Universe must have been shorter as well. If photons had shorter wavelengths earlier on, then that implies that they were more energetic in the past, and that the Universe must have been hotter. And this idea, of a smaller, hotter, denser past to our Universe, of a Universe that began hot and dense but that has expanded and cooled to become the Universe we inhabit today, is the core idea of what we now know as the hot Big Bang.

As a balloon inflates, any coins glued to its surface will appear to recede away from one another, with “more distant” coins receding more rapidly than the less distant ones. Any light will redshift, as its wavelength ‘stretches’ to longer values as the balloon’s fabric expands. This visualization solidly explains cosmological redshift within the context of the expanding Universe. If the Universe is expanding today, that implies a past where it was smaller, hotter, denser, and more uniform: leading to the picture of the hot Big Bang.

A small, hot, dense past must also have been very close to perfectly uniform, and therefore, as the Universe has expanded and cooled over time, it must also have gravitated and clumped and clustered together. This implies that, as we go back in time and look farther and farther away into the distant past:

• we’d find a time where galaxies were smaller, lower in mass, and less evolved than they are today,
• that there were fewer stars in the distant past than there are today,
• that if you go back early enough, you will find a time with no stars or galaxies, as they haven’t formed yet,
• that even before that, the Universe would’ve been hot enough to prevent the formation of neutral atoms,
• that even before that, the Universe would’ve been hot enough to prevent the formation of stable atomic nuclei,
• that at still earlier times, we could have created matter-antimatter pairs of nearly any species of particle,
• and that even before that, it would’ve been too hot, dense, and energetic to even form protons and neutrons.

These are some of the main predictions of the hot Big Bang. In addition to an expanding Universe, we should see evidence for the emergence, growth, and evolution of structure. A younger Universe should be less enriched in the types of heavy elements formed in stars: carbon, oxygen, silicon, sulfur, iron, and more, eventually revealing, at the earliest times, only the elements forged in the fires of the hot Big Bang itself. And a sign of the leftover relic radiation from the Big Bang, or a cosmic background of radiation, should persist even today, just a few degrees above absolute zero.

According to the original observations of Penzias and Wilson, the galactic plane emitted some astrophysical sources of radiation (center), but above and below, all that remained was a near-perfect, uniform background of radiation. The temperature and spectrum of this radiation has now been measured, and the agreement with the Big Bang’s predictions are extraordinary. If we could see microwave light with our eyes, the entire night sky would look like the green oval shown.

The evidence for this came in almost complete reverse order. In the 1950s, it was realized that most of the heavy elements in our cosmos weren’t formed in the early stages of the hot Big Bang, but rather were built up in the cores of stars through nuclear fusion; it wouldn’t be until the 1970s, in earnest, that the evidence showed that the light elements and their isotopes only were forged in the hot Big Bang. The large-scale structure and evolution of the Universe, from galaxy evolution to the growth and distribution of galaxy clusters and the large-scale cosmic web, would elude us in a scientific manner until the 1980s and even the 1990s; this was no easy task.

But the leftover glow from the Big Bang — originally called the primeval fireball and now known as the cosmic microwave background — was discovered quite by serendipitous accident in the mid-1960s, when Arno Penzias and Robert Wilson did it with the Holmdel Horn Antenna in New Jersey. We’ve since measured:

• the spectrum of this radiation,
• the variations in temperature across the sky of this radiation,
• and the wavelength-dependence of this radiation.

We’ve determined that it is, in fact, blackbody in nature: just as the hot Big Bang predicts. We’ve determined it is the same temperature in all regions of the sky to ~1-part-in-30,000, and that the imperfections in the overall temperature are Gaussian (or follow a normal distribution) in nature. It’s just as the Big Bang predicted, validated perfectly by observations.

The Sun’s actual light (yellow curve, left) versus a perfect blackbody (in gray), showing that the Sun is more of a series of blackbodies due to the thickness of its photosphere; at right is the actual perfect blackbody of the CMB as measured by the COBE satellite. Note that the “error bars” on the right are an astounding 400 sigma. The agreement between theory and observation here is historic, and the peak of the observed spectrum determines the leftover temperature of the cosmic microwave background: 2.73 K.

This is how the Big Bang, originally formulated in the 1920s by Georges Lemaitre, expanded upon and developed in the 1940s by George Gamow, and then had its most essential prediction confirmed (refuting several prominent alternatives) in the 1960s by Penzias and Wilson, provided us with our first scientific answer to the question of “where did all this come from?” For the first time in human history, we had an answer that the Universe itself provided to the biggest existential question affecting all of humanity.

But then, on the other hand, there were also puzzles that the Big Bang framework, on its own, couldn’t explain.

• Why was the Universe exactly the same temperature to such a severe degree, even in regions that haven’t had enough time to come into causal contact, achieve thermodynamic equilibrium, or exchange information?
• Why was the Universe perfectly spatially flat, and why did the matter-and-energy density balance with the expansion rate so pristinely, even after billions of years of cosmic evolution?
• And why, if the Universe could be extrapolated back to an arbitrarily hot, dense state, did we see no evidence for leftover high-energy relics, as predicted by theoretical physics, in our late-time, modern Universe?

It was by pondering these questions, and by searching for a mechanism to provide a solution to them while simultaneously reproducing all the successes of the hot Big Bang model of our early Universe, that led scientists in the late 1970s and early 1980s to a powerful theoretical extension to our description of cosmic history: a period of cosmic inflation that preceded and set up the hot Big Bang.

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.

Inflation, originally theoretically developed in the 1980s, went on to make a series of profound predictions about what should be in our Universe that differed significantly from the predictions of the old school, non-inflationary hot Big Bang. Those predictions include:

• a nearly, but not perfectly, scale invariant spectrum of initial density/temperature fluctuations,
• including fluctuations that exist on scales larger than the size of the cosmic horizon (e.g., super-horizon fluctuations),
• in a Universe that reached a maximum temperature that’s well below the energy scale at which physics breaks down (the Planck scale),
• whose fluctuations are 100% adiabatic and 0% isocurvature (the only allowable alternative) in nature.

Observationally, those four predictions have now been robustly tested, and inflation is 4-for-4, while the non-inflationary hot Big Bang is 0-for-4. That cements the inflationary hot Big Bang as our best theory, at present, for the cosmic origins of our Universe.

But huge unknowns, and open questions, still remain. We may have figured out what origin story best fits the full suite of data that we have — a fit with no major holes, gaps, or unexplained observations to the story — but there are a great many aspects to the cosmic story that we’re still ignorant of.

The quantum fluctuations inherent to space, stretched across the Universe during cosmic inflation, gave rise to the density fluctuations imprinted in the cosmic microwave background, which in turn gave rise to the stars, galaxies, and other large-scale structures in the Universe today. This is the best picture we have of how the entire Universe behaves, where inflation precedes and sets up the Big Bang. Unfortunately, we can only access the information contained inside our cosmic horizon, which is all part of the same fraction of one region where inflation ended some 13.8 billion years ago.

For example: inflation predicts the existence of tensor modes, or gravitational wave fluctuations, imprinted all across the Universe. Inflation can tell you what the spectrum of those fluctuations should be, but it can’t tell you the amplitude; we only have upper limits on how large they can be as we attempt to make those critical measurements. How did the inflationary state arise, and how long did it endure for? We know, theoretically, that it couldn’t have been eternal to the past, but did it arise from:

• an original singularity that then gave rise to inflation,
• a non-singular state that transitioned to have inflation begin somewhere,
• or a past-eternal state that triggered inflation to begin at some point in some location?

How long did inflation last: a fraction of a second, or many many times the current age of the Universe, or anywhere in between? Are there any observations we can make that will shed a light on the specific type, or flavor, of inflation that occurred in our past? Can we model inflation successfully by a single scalar field, or will some more complex model (eventually) be necessary?

As is always the case with science, the answers we’ve found so far don’t represent the end of the story, but rather the foundation for the next steps we’re working to uncover the answers to at present. For all the generations of humanity prior to that of our grandparents in the 20th century, the question of “where did the Universe come from?” was one that we only had stories about; it’s only since the mid-1960s that we have scientific answers to them. We can now say a lot that’s quite meaningful and information-rich about our cosmic origins. The next steps, and the answers to the next round of questions, bring us to where the frontiers of modern science are today.