Did the universe need a cause?

The kalam cosmological argument, named for the medieval Islamic theologians who developed it and revived by the philosopher William Lane Craig, runs in three steps: whatever begins to exist has a cause of its beginning; the universe began to exist; therefore the universe has a cause of its beginning^{[source 1]}. Craig presents it largely in the form given by the Muslim theologian al-Ghazali (1058–1111), whose reasoning he follows closely^{[source 1][source 2]}.

Craig defends the first premise on three grounds: something cannot come into being from nothing; if it could, there would be no reason why anything and everything does not; and apparent exceptions, such as particles produced in quantum decay, do not come from nothing^{[source 1]}. For the second premise he offers philosophical arguments against an infinite past, and scientific evidence: the expansion of the universe traced back to the Big Bang, the second law of thermodynamics, and a 2003 theorem by Arvind Borde, Alan Guth and Alexander Vilenkin showing that any region of the universe that has on average been expanding must have a boundary in the past, so its history cannot be extended infinitely back^{[source 1][source 4]}. He then argues that a cause of all space and time must itself be beyond them: timeless, immaterial, enormously powerful and personal^{[source 1][source 2]}.

Craig’s philosophical case for the second premise turns on the difference between a potential infinite, a series that can always be extended but is never complete, and an actual infinite, a completed infinite collection. Following the medieval argument, he holds that an actual infinite cannot exist in reality and cannot be built up one moment at a time, so the series of past events must be finite and the universe must have begun^{[source 1]}.

Craig also argues that the cause must be personal. A timeless, unchanging cause that produced its effect automatically would produce it from eternity, so the universe would be as old as its cause. Only a free agent, he argues, can bring about an effect with a beginning from a cause that has no beginning, by freely choosing to create^{[source 1]}.

Critics challenge the first premise. Our experience of causes comes from things beginning within the universe, and whether it applies to the universe as a whole is the point in question. Some appeal to quantum physics, in which individual events such as quantum decays have no deterministic cause^{[source 1][source 2]}. Craig replies that quantum events still have causal conditions, and that a quantum vacuum is not nothing^{[source 1][source 2]}.

Others accept that the past might be finite but question the reasoning against an infinite one. Modern mathematics treats actual infinities as consistent, and critics argue that the paradoxes Craig uses, such as Hilbert’s Hotel, a hotel with infinitely many rooms that is full yet can take new guests, are strange but not contradictory^{[source 1][source 2]}. Craig replies that set theory shows actual infinities can be discussed consistently, not that they can exist in reality^{[source 1]}.

Others challenge what a beginning would mean. If time itself began at the Big Bang, there was no earlier moment at which a cause could act; Adolf Grünbaum argued on this ground that asking for a cause of the Big Bang is a pseudo-question^{[source 2][source 6]}. Craig replies that God, timeless without the universe, enters time with the act of creation, so the cause and its effect can begin together^{[source 1]}. The Big Bang model also describes the universe’s early expansion rather than necessarily an absolute beginning, since general relativity is expected to break down at the earliest times, and physicists have proposed models without a first moment^{[source 2][source 5]}.

A related dispute concerns the nature of time. Craig’s argument assumes a tensed view, sometimes called the A-theory, on which past events have genuinely ceased and the series of events grows one moment at a time. Grünbaum and others argued that on a tenseless view, the B-theory, on which past, present and future events are equally real, the reasoning about building up an infinite past one moment at a time does not get started; Craig replies by defending the tensed view^{[source 2]}.

Sean Carroll argues that cosmologists take both “beginning” and “eternal” models seriously, and that which is true should be settled by which models best fit the evidence, not by metaphysical principles about causes^{[source 5]}.

Some critics accept a first cause but question the identification with God. Even granting the argument, they say, it shows at most a cause of the universe, and further argument is needed for that cause to be one, personal, or the God of any particular religion^{[source 2]}. Craig replies that analyzing what a cause of space and time would have to be yields a personal, timeless, immaterial and enormously powerful Creator^{[source 1]}.

Cosmologists agree that the observable universe has expanded from a hot, dense early state. Whether that was an absolute beginning is open: the theorem Craig cites rules out some eternal models but not all, and a full theory of quantum gravity may change the picture^{[source 2][source 3]}. Vilenkin and Audrey Mithani, examining the main eternal scenarios, concluded that the answer is “probably yes”^{[source 3]}.

The first premise, and the step from a first cause to a personal God, are philosophical questions, and philosophers remain divided over both^{[source 2]}. The kalam remains one of the most discussed arguments in contemporary philosophy of religion, with defenders and critics still publishing on both premises^{[source 2]}.