There is no evidence for a Universe before the Big Bang

Nobel Laureate Roger Penrose, famed for his work on black holes, claims we've seen evidence from a prior Universe. Only, we haven't.

The original Big Bang has since been modified to include an early inflationary phase, pushing whatever came before inflation to an unobservable place. When inflation ends, the hot Big Bang ensues, and we can see evidence from the final tiny fraction-of-a-second of inflation imprinted on our observable Universe. However, we can't see anything from before that time. Despite the assertions of one of the most famous living physicists, there's no evidence for a Universe prior to that.

Penrose's idea of a conformal cyclic cosmology hypothesizes that our Universe arose from a pre-existing Universe that would leave imprints on our cosmos today. This is a fascinating and imaginative alternative to inflation, but the data doesn't support it, despite Penrose's dubious claims that it does.
(Credit: SkyDivePhil/YouTube)

One of the greatest scientific successes of the past century was the theory of the hot Big Bang: the idea that the Universe, as we observe it and exist within it today, emerged from a hotter, denser, more uniform past. Originally proposed as a serious alternative to some of the more mainstream explanations for the expanding Universe, it was shockingly confirmed in the mid-1960s with the discovery of the “primeval fireball” that remained from that early, hot-and-dense state: today known as the Cosmic Microwave Background.

For more than 50 years, the Big Bang has reigned supreme as the theory describing our cosmic origins, with an early, inflationary period preceding it and setting it up. Both cosmic inflation and the Big Bang have been continually challenged by astronomers and astrophysicists, but the alternatives have fallen away each time that new, critical observations have come in. Even 2020 Nobel Laureate Roger Penrose’s attempted alternative, Conformal Cyclic Cosmology, cannot match the inflationary Big Bang’s successes. Contrary to many years of headlines and Penrose’s continued assertions, we see no evidence of “a Universe before the Big Bang.”

(Credit: E. Siegel; ESA/Planck and the DOE/NASA/NSF Interagency Task Force on CMB research)
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.

The Big Bang is commonly presented as though it were the beginning of everything: space, time, and the origin of matter and energy. From a certain archaic point of view, this makes sense. If the Universe we see is expanding and getting less dense today, then that means it was smaller and denser in the past. If radiation — things like photons — is present in that Universe, then the wavelength of that radiation will stretch as the Universe expands, meaning it cools as time goes on and was hotter in the past.

At some point, if you extrapolate back far enough, you’ll achieve densities, temperatures, and energies that are so great that you’ll create the conditions for a singularity. If your distance scales are too small, your timescales are too short, or your energy scales are too high, the laws of physics cease to make sense. If we run the clock backward some 13.8 billion years toward the mythical “0” mark, those laws of physics break down at a time of ~10-43 seconds: the Planck time.

(Credit: NASA/CXC/M. Weiss)

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.

If this were an accurate depiction of the Universe — that it began hot and dense and then expanded and cooled — we’d expect a large number of transitions to occur in our past history.

All the possible particles and antiparticles would get created in great numbers, with the excess annihilating away to radiation when it gets too cool to continually create them.
The electroweak and Higgs symmetries break when the Universe cools below the energy at which those symmetries are restored, creating four fundamental forces and particles with non-zero rest masses.

Quarks and gluons condense to form composite particles like protons and neutrons.
Neutrinos stop interacting efficiently with the surviving particles.
Protons and neutrons fuse to form the light nuclei: deuterium, helium-3, helium-4, and lithium-7.
Gravitation works to grow the overdense regions, while radiation pressure works to expand them when they get too dense, creating a set of oscillatory, scale-dependent imprints.
And approximately 380,000 years after the Big Bang, it becomes cool enough to form neutral, stable atoms without them being instantly blasted apart.
When this last stage occurs, the photons permeating the Universe, which had previously scattered off of the free electrons, simply travel in a straight line, lengthening in wavelength and diluting in number as the Universe expands.

(Credit: Amanda Yoho for Starts With A Bang)

In the hot, early Universe, prior to the formation of neutral atoms, photons scatter off of electrons (and to a lesser extent, protons) at a very high rate, transferring momentum when they do. After neutral atoms form, owing to the Universe cooling to below a certain, critical threshold, the photons simply travel in a straight line, affected only in wavelength by the expansion of space.

Back in the mid-1960s, this background of cosmic radiation was first detected, catapulting the Big Bang from one of a few viable options for our Universe’s origin to the only one consistent with the data. While most astronomers and astrophysicists immediately accepted the Big Bang, the strongest proponents of the leading alternative Steady-State theory — people like Fred Hoyle — came up with progressively more and more absurd contentions to defend their discredited idea in the face of overwhelming data.

But each idea failed spectacularly. It couldn’t have been tired starlight, nor reflected light, nor dust that was heated up and radiating. Each and every explanation that was tried was refuted by the data: the spectrum of this cosmic afterglow was too perfect a blackbody, too equal in all directions, and too uncorrelated with the matter in the Universe to line up with these alternative explanations. While science moved on to the Big Bang becoming part of the consensus, i.e., a sensible starting point for future science, Hoyle and his ideological allies worked to hold back the progress of science by advocating for scientifically untenable alternatives.

(Credit: Sch/Wikimedia Commons (L); COBE/FIRAS, NASA/JPL-Caltech (R))

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.

Ultimately, science moved on while the contrarians became more and more irrelevant, with their trivially incorrect work fading into obscurity and their research program eventually ceasing upon their deaths.

In the meantime, from the 1960s up through the 2000s, the sciences of astronomy and astrophysics — and particularly the sub-field of cosmology, which focuses on the history, growth, evolution, and fate of the Universe — grew spectacularly.

We mapped out the large-scale structure of the Universe, discovering a great cosmic web.
We discovered how galaxies grew and evolved, and how their stellar populations inside changed with time.
We learned that all the known forms of matter and energy in the Universe were insufficient to explain everything we observe: some form of dark matter and some form of dark energy are required.

And we were able to further verify additional predictions of the Big Bang, such as the predicted abundances of the light elements, the presence of a population of primordial neutrinos, and the discovery of density imperfections of exactly the necessary type to grow into the large-scale structure of the Universe we observe today.

(Credit: E.M. Huff, SDSS-III/South Pole Telescope, Zosia Rostomian)

The Universe doesn’t just expand uniformly, but has tiny density imperfections within it, which enable us to form stars, galaxies, and clusters of galaxies as time goes on. Adding density inhomogeneities on top of a homogeneous background is the starting point for understanding what the Universe looks like today.

At the same time, there were observations that were no doubt true, but that the Big Bang had no predictive power to explain. The Universe allegedly reached these arbitrarily high temperatures and high energies at the earliest times, and yet there are no exotic leftover relics that we can see today: no magnetic monopoles, no particles from grand unification, no topological defects, etc. Theoretically, something else beyond what is known must be out there to explain the Universe we see, but if they ever existed, they’ve been hidden from us.

The Universe, in order to exist with the properties we see, must have been born with a very specific expansion rate: one that balanced the total energy density exactly, to more than 50 significant digits. The Big Bang has no explanation for why this should be the case.

And the only way different regions of space would have the same exact temperature is if they’re in thermal equilibrium: if they have time to interact and exchange energy. Yet the Universe is too big and has expanded in such a way that we have many causally disconnected regions. Even at the speed of light, those interactions couldn’t have taken place.

(Credit: ESA and the Planck Collaboration)

The leftover glow from the Big Bang, the CMB, isn’t uniform, but has tiny imperfections and temperature fluctuations on the scale of a few hundred microkelvin. While this plays a big role at late times, after gravitational growth, it’s important to remember that the early Universe, and the large-scale Universe today, is only non-uniform at a level that’s less than 0.01%. Planck has detected and measured these fluctuations to better precision than ever before.

This presents a tremendous challenge for cosmology and for science in general. In science, when we see some phenomena that our theories cannot explain, we have two options.

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