The universe likely started at some point, but until we can seethe BB, we won't know for sure that it's not another expansion after a Big Crunch, maybe a super massive black hole sucks up everything in the universe and blows up... You just have no imagination and are desperate for it all to have been poofed into being by your invisible magician.
Okay, Taz, since you brought up the Big Crunch hypothesis, let's start with the basics and then systematically survey the alternative models that have been proposed over the years in an attempt to explain the absurdity of an eternally existing universe. . . .
First, absorb the following excerpt from my article:
We may now see the synthesis reduced to its essential integrants: inflationary theory and the Big Bang model.
Inflationary Theory
While it necessarily presupposes certain initial conditions relative to the three major problems listed in the above, the calculi of inflationary theory smoothly correlate with the extant conditions observed in the universe. In other words, while cosmic inflation "resolves these problems" in an ad hoc fashion, it, nevertheless, accounts for the absence of magnetic monopoles in and the flatness of the extant universe, as well as the homogeneity and isotropy of the extant universe, negating the horizon problem. And while Guth initially proposed cosmic inflation in 1980 to fix the standard Big Bang model relative to extant conditions, these conditions have since been additionally affirmed by the detailed observations of the cosmic microwave background (CMB). Additionally, significant, indirect evidence—not bottomed on ad hoc calculi of initial conditions relative to extant conditions—supports cosmic inflation. For example, the spectral radiance of the CMB features a very distinct pattern (small irregularities of varying sizes) consistent with what would be expected if a uniformly distributed hot gas were effectuated by quantum fluctuations.
Big Bang Model (State)
The model exquisitely describes how the universe expanded from a very high-density and high-temperature state after the rapid, exponential expansion of space, including a wide range of phenomena: chemical diversity and the abundance of vital light elements; large scale, astronomical structures and systems; Hubble's law regarding relative galactic motion; the afterglow of radiation permeating the universe (CMB).
Bottom line: the Big Bang model most certainly does recount an absolute beginning of our universe in the finite past!
To which atheists routinely respond: "scientists are talking about the visible universe."
By this they presumably don't mean to preclude the wider, observable universe of today (approximately 93 billion light years in diameter) or preclude the continuously expanding universe beyond as if the visible universe were something other than our universe. Rather, they're unwittingly alluding to the earliest visible image of the universe, which dates back to the time when it was approximately 380,000 years old and 42 million light years in radius. Technically, the visible universe denotes that of the recombination epoch, the earliest point in cosmological history at which we can look back via the CMB. Before this epoch, the universe was too hot and dense—a sea of opaque plasma containing baryonic matter and electromagnetic radiation—to create the CMB. The universe cooled enough for electrons and primordial atomic nuclei (bounded protons and neutrons) to recombine into neutral atoms. From that point on photons were free to travel, and the universe became increasingly transparent. Immediately following on the heels of cosmic inflation, the initial stage of the Big Bang state is the quark epoch, which is extrapolated backwards from the CMB. The CMB is a snapshot of the final stage of the Big Bang state, which is the recombination epoch.
The referents observable universe and visible universe are often used interchangeably, but for obvious reasons, one should be aware of the technical distinction. In any event, the point the atheist thinks he's ultimately making is that the calculi of the Big Bang model do not necessarily preclude the potentiality of an eternally self-subsistent cosmos of some kind in which the Big Bang only obtains to the beginning of our universe. As we shall see this is nonsense. Our universe may not be the only universe within a multiverse or world ensemble, but all of the evidence refutes a past-eternal cosmos.