Maybe the universe is eternal, through a multiverse or endless cycles, and never began

Why this is a live option

Physics hasn't spoken a final word on the very earliest moments. Our best theories break down near the Big Bang, and physicists have proposed many models in which the universe, or a larger reality, extends further back. Sean Carroll pressed this point in his 2014 debate with William Lane Craig, arguing that the Kalam premise is unsupported by modern cosmology.

The Borde-Guth-Vilenkin theorem

In 2003, physicists Arvind Borde, Alan Guth and Alexander Vilenkin published a result showing that any universe that has, on average, been expanding cannot be extended infinitely into the past. It must have a past boundary. This applies to many multiverse models too, including eternal inflation. Vilenkin has said publicly that it seems cosmologists can no longer hide behind the possibility of a past-eternal universe. The theorem isn't the final word, since it relies on classical spacetime, but it's a significant hurdle for eternal models.

The problem with cycles

Cyclic models, where the universe repeatedly expands and contracts, face difficulties from the second law of thermodynamics. Entropy, or disorder, builds up over each cycle. In an infinitely old cyclic universe, we would expect to have reached maximum entropy long ago. Some newer models try to avoid this, but they tend to need special assumptions, and often still require a beginning.

Philosophical problems with an infinite past

There are also philosophical arguments. If the past were infinite, an actually infinite number of days would have elapsed to reach today. Many philosophers think an actual infinite can't be built up one step at a time; you could never finish counting to infinity. This is debated, but it adds weight to the idea that time had a beginning.

Would an eternal universe disprove God?

Not at all. Thomas Aquinas thought you couldn't prove from reason alone that the universe had a beginning, and he still argued for God from the universe's contingency: why is there something rather than nothing? Even an eternal multiverse would need an explanation of why it exists and has the laws it does. So the question shifts but doesn't disappear.

A fair concession

Cosmology is a moving field. It would be unwise to rest belief in God on a single scientific model. The evidence for a beginning is strong but not beyond dispute.

How to explain this to a friend

You don't need to master cosmology to talk about this. A simple approach is to say that scientists are still debating the earliest moments, and that the most discussed models still seem to need a beginning. Then add the deeper point: even an eternal universe wouldn't explain why there is something rather than nothing. That keeps the conversation honest about the science while showing the question of God doesn't depend on one theory.

The bottom line

Most proposed eternal models still seem to need a beginning, according to the BGV theorem and thermodynamics. And even if the universe were eternal, the deeper question of why anything exists remains. Read more at The Kalam cosmological argument.