I recently responded to an article by astrophysicist Ethan Siegel on the website Big Think titled “Surprise: the Big Bang isn’t the beginning of the universe anymore.” I mentioned that Siegel is an atheist, so he understandably wishes to avoid the conclusion that the universe is not eternal since a cosmic beginning points to a Creator. I described how Siegel argued that the universe might not have had a beginning based on the cosmological model known as eternal chaotic inflation. I concluded by explaining how Siegel’s argument fails since the Borde-Guth-Vilenkin (BGV) theorem proves that universes modeled by eternal inflationary dynamics also must have an absolute beginning. Siegel responded to my critique with a second Big Think article titled “Does modern cosmology prove the existence of God?” In this article, he explicitly reveals his atheistic motivations, and he again presents arguments that Stephen Meyer has already fully addressed. 

Quantum Mechanics and Causality

Siegel begins his piece by outlining the Kalam cosmological argument for God that Meyer detailed in Return of the God Hypothesis:

• “Whatever begins to exist has a cause.
• The Universe began to exist.
• Therefore, the Universe has a cause to its existence.”

Siegel then attempts to challenge the first premise by arguing that quantum phenomena appear to occur without causes:

…there is no cause for the phenomenon of when this atom will decay. It is as though the Universe has some sort of random, acausal nature to it that renders certain phenomena fundamentally indeterminate and unknowable. In fact, there are many other quantum phenomena that display this same type of randomness, including entangled spins, the rest masses of unstable particles, the position of a particle that’s passed through a double slit, and so on.

This claim is highly misleading since it confuses determinism with causality. Quantum mechanics is not deterministic since it describes only the probabilities that certain events could occur such as the paths a photon could take in the double slit experiment. But the laws of quantum mechanics act in our universe as the causal agent for all such events. Quantum theorist Giacomo Mauro D’Ariano comments in his article “Causality re-established”:

The notion of causality is logically completely independent of the misidentified concept of ‘determinism’, and, being a consequence of quantum theory, is ubiquitous in physics.

In contrast, before the universe existed, neither quantum mechanical processes nor any other physical laws were active. Therefore, they could not have brought our universe into existence as Siegel suggests. 

Avoiding the Cosmic Beginning

Siegel challenges the second premise by again asserting that the universe might not have had a beginning. As in his first article, he appeals to inflationary theory, but here he takes a different tack. He argues that the volume of an inflating universe grows exponentially, and an exponential curve moving backward in time never drops entirely to zero. Therefore, the universe could have been expanding eternally without a beginning. This argument also fails since it ignores the fact that inflationary models break down when the volume of the universe drops below the Planck scale.

In this regime, only quantum gravitational models apply. One class of quantum cosmological models presuppose a beginning such as those constructed by Stephen Hawking and Alexander Vilenkin (here, here). Other classes of models assume that the universe can only contract to a minimal size corresponding to a bounce that transitions a contracting universe to an expanding universe. Examples include those based on loop quantum gravity (here, here). These cyclical models also require a beginning due to the constant increase of entropy. No realistic quantum models allow for an eternal universe. 

Borde-Guth-Vilenkin Theorem

Siegel than attempts to refute the claim that the Borde-Guth-Vilenkin (BGV) theorem decisively points to our universe having an absolute beginning:

It’s true that, about 20 years ago, there was a theorem published — the Borde-Guth-Vilenkin theorem — that demonstrated that a Universe that always expands cannot have done so infinitely to the past. (It’s another way of expressing past-timelike incompleteness.) However, there is nothing that demands that the inflating Universe be preceded by a phase that was also expanding. There are numerous loopholes in this theorem as well: if you reverse the arrow of time, the theorem fails; if you replace the law of gravity with a specific set of quantum gravitational phenomena, the theorem fails; if you construct an eternally inflating steady-state Universe, the theorem fails.

He specifically references the steady-state eternal inflation model advanced by astrophysicists Anthony Aguirre and Steven Gratton. The proposed universe centers on a special state of extremely low entropy and compacted space that represents the Big Bang event. The universe expands both forward in time and backward in time. Since the universe would have been expanding indefinitely into the past, it circumvents the BGV theorem and requires no beginning. 

Here again, Siegel presents an argument that Stephen Meyer has already thoroughly addressed. Meyer states in his book: 

The BGV theorem applies to any universe that meets very general conditions, including those implied by inflationary cosmological models. As Alexander Vilenkin explained, “A remarkable thing about this theorem is its sweeping generality. … The only assumption that we made was that the expansion rate of the universe never gets below some nonzero value, no matter how small.”

Return of the God Hypothesis, pp. 125-126

And in his extended research notes under Note 6c, Meyer explains why the Aguirre and Gratton model is completely unrealistic. It requires an unimaginable level of fine-tuning in the infinite past for the universe to have contracted to such a special low-entropy, compact state at the transition from contraction to expansion. If the model were even plausible, the level of required fine-tuning would represent even greater evidence of design than it was intended to avoid by removing the beginning. By appealing to it, Siegel is proverbially jumping from the philosophical frying pan into the fire.