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. (NASA / CXC / M. WEISS)

This Is Why There Are No Alternatives To The Big Bang

Not everyone is satisfied with the Big Bang. But every alternative is a disastrous failure.


It’s treated as though it’s an unassailable scientific truth: 13.8 billion years ago, the Universe as we know it emerged from a hot, dense state known as the Big Bang. While there were a number of serious alternatives considered for decades, throughout the 20th century, a scientific consensus emerged more than 50 years ago with the discovery of the Cosmic Microwave Background. Despite many attempts to revive a variety of the discredited ideas, as well as attempts to formulate new possibilities, all have fallen away under the burden of the full suite of astronomical data. The Big Bang reigns supreme as the only valid theory of our cosmic origins.

Here’s how we discovered our Universe started with a bang.

The expanding Universe, full of galaxies and the complex structure we observe today, arose from a smaller, hotter, denser, more uniform state. It took thousands of scientists working for hundreds of years for us to arrive at this picture, and yet the lack of viable alternatives isn’t a flaw, but a feature of how successful the Big Bang truly is. (C. FAUCHER-GIGUÈRE, A. LIDZ, AND L. HERNQUIST, SCIENCE 319, 5859 (47))

A suite of new discoveries in the early 20th century revolutionized our view of the Universe. In 1923, Edwin Hubble measured individual stars in spiral nebulae, measuring their variable periods and their observed brightness. Thanks to the work of Henrietta Leavitt in formulating Leavitt’s law, which related such a star’s variable period to its intrinsic brightness, we obtained distance measurements to the galaxies that housed them. These galaxies were well outside our own Milky Way, with most residing millions of light years away.

Hubble’s discovery of a Cepheid variable in Andromeda galaxy, M31, opened up the Universe to us, giving us the observational evidence we needed for galaxies beyond the Milky Way and leading to the expanding Universe. (E. HUBBLE, NASA, ESA, R. GENDLER, Z. LEVAY AND THE HUBBLE HERITAGE TEAM)

Combined with redshift measurements, we were able to discover an important relationship: the farther away a galaxy appeared to be from us, the greater its redshift was measured to be. A number of possible explanations were advanced, such as the light from these objects lost energy as they traveled through space, or the more distant galaxies were moving away faster than the nearer ones, as though they all originated from an explosion.

However, one explanation emerged as the most compelling: the Universe was expanding. This explanation was consistent with the predictions of General Relativity, as well as the observed large-scale smoothness observed in all directions and locations. As more galaxies at greater distances were discovered, this picture was validated further. The Universe was expanding.

The farther a galaxy is, the faster it expands away from us, and the more its light appears redshifted. A galaxy moving with the expanding Universe will be even a greater number of light years away, today, than the number of years (multiplied by the speed of light) that it took the light emitted from it to reach us. (LARRY MCNISH OF RASC CALGARY CENTER)

Again, multiple valid explanations, even in the context of General Relativity, emerged. Sure, if the Universe were expanding in all directions, then we’d see distant objects moving away from us, with the more distant objects appearing to recede more rapidly. But this could be:

  • because the objects also had large, unmeasurable transverse motions, as though the Universe were rotating as well,
  • or because the Universe was oscillating, and if we looked far enough, we would see the expansion reverse,
  • or because the expansion caused the slow creation of new matter, resulting in a Universe that appeared unchanging in time,
  • or because the Universe originated from a hot, dense state.

Only this last option represents the hot Big Bang.

As far back as humanity has ever seen in the Universe, just a few hundred million years after the Big Bang, we still know that the very first stars and galaxies should have existed before even that. Our picture of the Big Bang, General Relativity, the seeds of structure formation, and much more, all forms a consistent picture that tells us we are not quite yet at the beginning. (NASA, ESA, AND A. FEILD (STSCI))

But if the idea of the Big Bang were correct, there would be a slew of new predictions that should arise. The expanding Universe, in the context of General Relativity, was the first, but there were three other, major ones that would lead to different observable consequences from the alternatives.

Galaxies comparable to the present-day Milky Way are numerous, but younger galaxies that are Milky Way-like are inherently smaller, bluer, more chaotic, and richer in gas in general than the galaxies we see today. For the first galaxies of all, this ought to be taken to the extreme. (NASA AND ESA)

The first is that if the Universe originated from an arbitrarily hot, dense, and more uniform state to expand-and-cool to what we see today, then as we look farther away, we’re looking back in time, and we should see the Universe as it was when it was younger. We should see, therefore, galaxies that were smaller, less massive, and made up of younger, bluer stars at great distances, before arriving at a time where there were no stars or galaxies at all.

A Universe where electrons and protons are free and collide with photons transitions to a neutral one that’s transparent to photons as the Universe expands and cools. Shown here is the ionized plasma (L) before the CMB is emitted, followed by the transition to a neutral Universe (R) that’s transparent to photons. It’s the spectacular two-photon transition in a hydrogen atom which enables the Universe to become neutral exactly as we observe it. (AMANDA YOHO)

The second, extrapolating even farther back, would be that there should be a time when the Universe was so hot and energetic that not even neutral atoms could form. At some very early stage, therefore, the Universe transitioned from an ionized plasma to one filled with neutral atoms. Any radiation that was around at that early stage should simply stream to our eyes, affected only by the expansion of the Universe.

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. (NASA / WMAP SCIENCE TEAM)

Based on the temperature at which atoms become neutral vs. ionized, we expect this radiation to be just a few degrees above absolute zero, shifting it into the microwave portion of the spectrum today. This is where the term Cosmic Microwave Background comes from. Furthermore, because it had a thermal origin but redshifted with the expanding Universe, we also expect it to exhibit a particular shape to its spectrum: a blackbody spectrum. The radiation background was initially detected at right around 3 K, and has since had measurements refined so that we not only know it to be 2.7255 K, but that its spectrum is definitively blackbody and not consistent with an explanation of reflected starlight. (Which could be accommodated by one of the alternative explanations.)

Long before the data from BOOMERanG came back, the measurement of the spectrum of the CMB, from COBE, demonstrated that the leftover glow from the Big Bang was a perfect blackbody in a way that reflected starlight, as the quasi-steady-state model predicted, could not explain what we saw. (E. SIEGEL / BEYOND THE GALAXY)

Finally, there’s a third prediction: that based on the early history of the Universe, elements should have been forged by nuclear fusion in particular ratios. Today, this should mean that before any stars were formed, the Universe should have been about:

  • 75% hydrogen (by mass),
  • 25% helium-4,
  • 0.01% deuterium,
  • 0.01% helium-3, and
  • 1-part-in-a-billion lithium-7.

That’s it; there should have been no elements heavier than that. Hydrogen, helium, a little bit of isotopes of each, and a tiny bit of lithium.

The predicted abundances of helium-4, deuterium, helium-3 and lithium-7 as predicted by Big Bang Nucleosynthesis, with observations shown in the red circles. The Universe is 75–76% hydrogen, 24–25% helium, a little bit of deuterium and helium-3, and a trace amount of lithium by mass. After tritium and beryllium decay away, this is what we’re left with, and this remains unchanged until stars form. (NASA / WMAP SCIENCE TEAM)

Observationally, this has been confirmed as well. Distant light, either from early galaxies or distant quasars, gets absorbed by intervening clouds of gas, allowing us to probe the contents of that gas. In 2011, we discovered two pristine clouds of gas, detecting hydrogen and helium in the exact, predicted ratios, and discovering (for the first time) a population of gas that had no oxygen or carbon: the first products of newly-formed stars.

The absorption spectra of different populations of gas (L) allow us to derive the relative abundances of elements and isotopes (center). In 2011, two distant gas clouds containing no heavy elements and a pristine deuterium-to-hydrogen ratio (R) were discovered for the first time. (MICHELE FUMAGALLI, JOHN M. O’MEARA, AND J. XAVIER PROCHASKA, VIA ARXIV.ORG/ABS/1111.2334)

The only way to arrive at the Cosmic Microwave Background with the uniformity, spectrum, and temperature it possesses is to posit a hot, thermal origin for it in the context of the expanding Universe. This was conjectured back in the 1940s by George Gamow and his collaborators, first observed in the 1960s by Arno Penzias and Bob Wilson, and had its spectrum definitively proven to be blackbody in the 1990s with the COBE satellite.

The large-scale structure of the Universe has been determined through all-sky surveys and deep field measurements with ground-and-space-based observatories, and has revealed a Universe consistent with the Big Bang and not with the alternatives. And the evolution of the elemental abundances, from metal-free early stages to metal-poor intermediate stages to the late-time, metal-rich stages that we observe today, all demonstrate the validity of the Big Bang.

There are now many independent observations of pristine gas from shortly after the Big Bang, showcasing the sensitive deuterium quantities relative to hydrogen. The agreement between observation and the theoretical predictions of the Big Bang is another victory for our best model of the Universe’s origin. (S. RIEMER-SØRENSEN AND E. S. JENSSEN, UNIVERSE 2017, 3(2), 44)

If you can come up with an alternative explanation for these four observations, you will have the start of a viable alternative to the Big Bang. Explain the observed expansion of the Universe, the large-scale structure and the evolution of galaxies, the Cosmic Microwave Background along with its temperature and spectral properties, and the relative abundances and evolution of the elements in the Universe, and you’ll challenge the theory of our cosmic beginnings.

After the Big Bang, the Universe was almost perfectly uniform, and full of matter, energy and radiation in a rapidly expanding state. As time goes on, the Universe not only forms elements, atoms, and clumps and clusters together that lead to stars and galaxies, but expands and cools the entire time. No alternative can match it. (NASA / GSFC)

For more than 50 years, no alternative has been able to deliver on all four counts. No alternative can even deliver the Cosmic Microwave Background as we see it today. It isn’t for lack of trying or a lack of good ideas; it’s because this is what the data indicates. Scientists don’t believe in the Big Bang; they conclude it based on the full suite of observations. The last adherents to the ancient, discredited alternatives are at last dying away. The Big Bang is no longer a revolutionary endpoint of the scientific enterprise; it’s the solid foundation we build upon. It’s predictive successes have been overwhelming, and no alternative has yet stepped up to the challenge of matching its scientific accuracy in describing the Universe.


Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.

Like what you read? Give Ethan Siegel a round of applause.

From a quick cheer to a standing ovation, clap to show how much you enjoyed this story.